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The Tibetan Plateau (TP) affects its surrounding regions through thermal and dynamic processes. Hydrological cycles in TP are experiencing dramatic changes under the warming climate. As one of the most important circulations, the Indian summer monsoon (ISM) affects precipitation changes and therefore has a major effect on TP water resources. In this paper, the widths of ISM are defined as extents perpendicular to the moisture trajectories. They were calculated using ground-based measurements and verified with reanalysis datasets. The directions of maximum summer precipitation decreases (Dirmin) were selected using the cluster proxy of directions of daily maximum winds (Wd) and the slope aspects derived from digital elevation models (DEMs) at various resolutions. The major findings were that (1) similar southwestern Dirmin were found based on the Wd and the slope aspects derived from DEMs with resolutions of 100 and 200 km; (2) Reanalysis data from China Meteorological Forcing Dataset verified that a high percentage (~ 90%) of cells showing precipitation decreases also had slope aspects (derived from 100- and 200-km resolution DEMs) in the range of 112.5°–247.5°; (3) The possible widths of ISM trajectories in TP were approximately 100–200 km. [ABSTRACT FROM AUTHOR]

Despite the emerging influence of anthropogenic climate change on the global water cycle, at regional scales the combination of observational uncertainty, large internal variability, and modeling uncertainty undermine robust statements regarding the human influence on precipitation. Here, we use output from global climate models in a perfect-data sense to develop a framework for conducting regional detection and attribution (D&A) for precipitation, starting with the contiguous United States (CONUS) where observational uncertainty is lower than in other regions. Our unified approach can simultaneously detect systematic trends in mean and extreme precipitation, attribute trends to anthropogenic forcings, compute the effects of forcings as a function of time, and map the effects of individual forcings. Model output is used to conduct a set of tests that yield a parsimonious representation for characterizing seasonal precipitation over the CONUS for the historical record (1900 to present day), which ensures our D&A is insensitive to structural uncertainty. Our framework is developed using synthetic data in a Pearl-causal perspective wherein causality can be identified using intervention-based simulations. While the hypothesis-based framework and accompanying generalized D&A formula we develop should be widely applicable, we include a strong caution that the hypothesis-guided simplification of the formula for the historical climatic record of CONUS as described in this paper will likely fail to hold in other geographic regions and under future warming. [ABSTRACT FROM AUTHOR]

Previous studies in the literature show that the annual cycle of surface air temperature (SAT) is changing in both amplitude and phase, and the SAT departures from the annual cycle are long-term correlated. However, the classical definition of temperature anomalies is based on the assumption that the annual cycle is constant, which contradicts the fact of changing annual cycle. How to quantify the impact of the changing annual cycle on the long-term correlation of temperature anomaly variability still remains open. In this paper, a recently developed data adaptive analysis tool, the nonlinear mode decomposition (NMD), is used to extract and remove time-varying annual cycle to reach the new defined temperature anomalies in which time-dependent amplitude of annual cycle has been considered. By means of detrended fluctuation analysis, the impact induced by inter-annual variability from the time-dependent amplitude of annual cycle has been quantified on the estimation of long-term correlation of long historical temperature anomalies in Europe. The results show that the classical climatology annual cycle is supposed to lack inter-annual fluctuation which will lead to a maximum artificial deviation centering around 600 days. This maximum artificial deviation is crucial to defining the scaling range and estimating the long-term persistence exponent accurately. Selecting different scaling range could lead to an overestimation or underestimation of the long-term persistence exponent. By using NMD method to extract the inter-annual fluctuations of annual cycle, this artificial crossover can be weakened to extend a wider scaling range with fewer uncertainties. [ABSTRACT FROM AUTHOR]

Understanding the processes responsible for precipitation and its future change is important to develop plausible and sustainable climate change adaptation strategies, especially in regions with few available observed data like Congo Basin (CB). This paper investigates the atmospheric circulation processes associated with climate model biases in CB rainfall, and explores drivers of projected rainfall changes. Here we use an ensemble of simulations from the Swedish Regional Climate Model (RCM) RCA4, driven by eight General Circulation Models (GCMs) from the Coupled Model Intercomparison Project Phase 5 (CMIP5), for the 1.5 ∘ C and 2 ∘ C global warming levels (GWLs), and under the representative concentration pathways (RCPs) 4.5 and 8.5. RCA4 captures reasonably well the observed patterns of CB rainfall seasonality, but shows dry biases independent of seasons and large scale driving atmospheric conditions. While simulations mimic observed peaks in transition seasons (March–May and September–November), the rain-belt is misplaced southward (northward) in December–February (June–August), reducing the latitudinal extent of rainfall. Moreover, ERA-Interim reanalysis driven RCM simulation and RCM–GCM combinations show similar results, indicating the dominance of systematic biases. Modelled dry biases are associated with dry upper-tropospheric layers, resulting from a western outflow stronger than the eastern inflow and related to the northern component of African Easterly Jet. From the analysis of the climate change signal, we found that regional scale responses to anthropogenic forcings vary across GWLs and seasons. Changes of rainfall and moisture divergence are correlated, with values higher in March–May than in September–November, and larger for global warming of 2.0 ∘ C than at 1.5 ∘ C . There is an increase of zonal moisture divergence fluxes in upper atmospheric layers ( > 700 hPa ) under RCP8.5 compared to RCP4.5. Moreover, it is found that additional warming of 0.5 ∘ C will change the hydrological cycle and water availability in the CB, with potential to cause challenges to water resource management, agriculture, hydro-power generation, sanitation and ecosystems. [ABSTRACT FROM AUTHOR]

This paper analyzes the diurnal temperature range (DTR) over land in simulations of the recent past and in future projections by 20 models participating in the Coupled Model Intercomparison Project phase 5 (CMIP5). The annually averaged DTR is evaluated for the present-day climate using two gridded datasets (HadGHCND and CRU). The DTR varies substantially between different CMIP5 models, particularly in the subtropics, and is generally underestimated. In future projections of the high emission scenario RCP8.5, the models disagree on both the sign and the magnitude of the change in DTR. Still, a majority of the models project a globally averaged reduction in the DTR, with an increase over Europe, a decrease over the Sahara desert and a substantial decrease in DTR at high latitudes in winter. The general DTR reduction is partly linked to the enhancement of the downwelling clear sky longwave radiation due to greenhouse gases. At high latitudes in winter, the decrease in DTR seems to be enforced by an increase in cloudiness, but in most other regions counteracted by decreases in cloud fraction. Changes in the hydrological cycle and in the clear sky shortwave radiation also impact the DTR. The DTR integrates many processes and neither the model differences in the DTR nor in the change in DTR can be attributed to a single parameter. Which variables that impact the model discrepancies vary both regionally and seasonally. However, clouds seem to matter in most regions and seasons and the evaporative fraction is important in summer. [ABSTRACT FROM AUTHOR]

Understanding real uncertainties in projections is needed to support actions of decision and policy makers. In this study, two state-of-the-art regional climate models with ~ 25 km horizontal resolution forced with three different global climate models are employed to investigate the response of the West African Monsoon (WAM) system under the low (RCP2.6) and high (RCP8.5) emission scenarios. We make a step towards assessing the plausibility of rainfall change (ΔPr) patterns based on the models' ability to realistically represent the WAM system under the historical period, and by analyzing changes in mechanisms associated with ΔPr patterns. Under global warming, experiments exhibit diverse ΔPr patterns underpinned by different combinations of mechanisms operating simultaneously. A dipole-like surface pressure change between the Sahara Desert and the Guinea Coast appears to be a differentiating factor between experiments featuring homogeneous or increased rainfall over all or a part of the Sahel from those simulating heterogeneous or reduced rainfall over the same domain. This dipole acts by modulating the amount and the extent of deepening of the monsoon flux inland, and hence the latitudinal positioning of the monsoon convective system. This process contributes to moistening the whole or central and eastern Sahel in some experiments while drying the Guinea Coast. The West African Westerly Jet (WAWJ) is responsible for strengthening and moistening the western Sahel under RCP2.6. However, the WAMJ becomes much stronger under RCP8.5 and may have contributed to the drying of the western Sahel by shifting moisture eastwards in addition to the mid-tropospheric moisture divergence through enhanced African Easterly Jet. Furthermore, for experiments expecting wetting (drying) of the Sahel, the strengthening (weakening) of the tropical easterly jet may have contributed to the projected intensification (reduction) of WAM rainfall by favoring uplift (downlift) above 600 hPa. It also emerges that the ΔPr patterns from REMO2015 experiments are likely more plausible than those from RegCM4-v7. The impacts of strong warming on the regional hydrological cycle are further investigated and we found that changes gradually intensify with the level of radiative forcing, highlighting the importance of mitigating warming as recommended by the Paris agreement. [ABSTRACT FROM AUTHOR]

This is the second of the two-part paper series on the analysis and evaluation of the Fifth phase of Coupled Model Intercomparison Project (CMIP5) simulation of contemporary climate as well as IPCC, AR5 Representative Concentrations Pathways (RCP), 4.5 and 8.5 scenarios projections of the Greater Horn of Africa (GHA) Climate. In the first part (Otieno and Anyah in Clim Dyn, ) we focused on the historical simulations, whereas this second part primarily focuses on future projections based on the two scenarios. Six Earth System Models (ESMs) from CMIP5 archive have been used to characterize projected changes in seasonal and annual mean precipitation, temperature and the hydrological cycle by the middle of twenty-first century over the GHA region, based on IPCC, 5th Assessment Report (AR5) RCP4.5 and RCP8.5 scenarios. Nearly all the models outputs analyzed reproduce the correct mean annual cycle of precipitation, with some biases among the models in capturing the correct peak of precipitation cycle, more so, March-April-May (MAM) seasonal rainfall over the equatorial GHA region. However, there is significant variation among models in projected precipitation anomalies, with some models projecting an average increase as others project a decrease in precipitation during different seasons. The ensemble mean of the ESMs indicates that the GHA region has been experiencing a steady increase in both precipitation and temperature beginning in the early 1980s and 1970s respectively in both RCP4.5 and RCP8.5 scenarios. Going by the ensemble means, temperatures are projected to steadily increase uniformly in all the seasons at a rate of 0.3/0.5 °C/decade under RCP4.5/8.5 scenarios over northern GHA region leading to an approximate temperature increase of 2/3 °C by the middle of the century. On the other hand, temperatures will likely increase at a rate of 0.3/0.4 °C/decade under RCP4.5/8.5 scenarios in both equatorial and southern GHA region leading to an approximate temperature increase of 2/2.5 °C by the middle of twenty-first century. Nonetheless, projected precipitation increase varied across seasons and sub-regions. With the exception of the equatorial region, that is projected to experience precipitation increase during DJF season, most sub-regions are projected to experience precipitation increase within their peak seasons, with the highest rate of increase experienced during DJF and OND seasons over southern and equatorial GHA regions respectively. Notably, as precipitation increases, the deficit (E < P) between evaporation (E) and precipitation (P) increased over the years, with a negatively skewed distribution. This generally implies that there is a high likelihood of an increased deficit in local moisture supply. This remarkable change in the general hydrological cycle (i.e. deficit in local moisture) is projected to be also coincident with intensified westerly anomaly influx from the Congo basin into the region. However, better understanding of the detailed changes in hydrological cycle will require comprehensive water budget analyses that require daily or sub-daily variables, and was not a specific focus of the present study. [ABSTRACT FROM AUTHOR]

The CNRM atmospheric general circulation model Arpege-Climat is relaxed towards atmospheric reanalyses outside the 10°S-32°N 30°W-50°E domain in order to disentangle the regional versus large-scale sources of climatological biases and interannual variability of the West African monsoon (WAM). On the one hand, the main climatological features of the monsoon, including the spatial distribution of summer precipitation, are only weakly improved by the nudging, thereby suggesting the regional origin of the Arpege-Climat biases. On the other hand, the nudging technique is relatively efficient to control the interannual variability of the WAM dynamics, though the impact on rainfall variability is less clear. Additional sensitivity experiments focusing on the strong 1994 summer monsoon suggest that the weak sensitivity of the model biases is not an artifact of the nudging design, but the evidence that regional physical processes are the main limiting factors for a realistic simulation of monsoon circulation and precipitation in the Arpege-Climat model. Sensitivity experiments to soil moisture boundary conditions are also conducted and highlight the relevance of land-atmosphere coupling for the amplification of precipitation biases. Nevertheless, the land surface hydrology is not the main explanation for the model errors that are rather due to deficiencies in the atmospheric physics. The intraseasonal timescale and the model internal variability are discussed in a companion paper. [ABSTRACT FROM AUTHOR]

Recent studies have shown that changes in global mean precipitation are larger for solar forcing than for CO2 forcing of similar magnitude. In this paper, we use an atmospheric general circulation model to show that the differences originate from differing fast responses of the climate system. We estimate the adjusted radiative forcing and fast response using Hansen’s “fixed-SST forcing” method. Total climate system response is calculated using mixed layer simulations using the same model. Our analysis shows that the fast response is almost 40% of the total response for few key variables like precipitation and evaporation. We further demonstrate that the hydrologic sensitivity, defined as the change in global mean precipitation per unit warming, is the same for the two forcings when the fast responses are excluded from the definition of hydrologic sensitivity, suggesting that the slow response (feedback) of the hydrological cycle is independent of the forcing mechanism. Based on our results, we recommend that the fast and slow response be compared separately in multi-model intercomparisons to discover and understand robust responses in hydrologic cycle. The significance of this study to geoengineering is discussed. [ABSTRACT FROM AUTHOR]

Using the latest Coupled Model Intercomparison Projects phase 6 abrupt-4xCO2 scenario, this study investigates the sea surface salinity (SSS) and hydrological cycle changes in response to global warming in the tropical Atlantic and tropical eastern Pacific. The analysis results reveal the enhancement of the global water cycle and the effect of El Niño-like sea surface temperature (SST) warming. Under global warming, inter-basin water vapor transports increase by about 0.11 Sv (19%) from the tropical Atlantic to the Pacific in response to the rise of the freshwater flux gradient between the two basins. The increase of specific humidity dominates the enhancement of moisture transport (contribution of about 85%), resulting in an SSS decrease in the tropical Pacific and an increase in the tropical Atlantic. On seasonal scales, the increase of the trans-basin SST gradient leads to the enhancement and westward shift of the Walker Circulation in summer and autumn (contribution of 31% in August to 47% in November), further resulting in the precipitation increase and the salinity decrease in the tropical Pacific. Furthermore, the El Niño-like SST warming induces a wind-evaporation-SST feedback in the tropical eastern Pacific. The reduced SST meridional gradient weakens the atmospheric circulation, redistributing the trans-basin water vapor transport. Correspondingly, precipitation (salinity) decreases (increases) in the northeastern Pacific and increases (decreases) in the southeastern Pacific, following the "warmer-get-wetter" mechanism. [ABSTRACT FROM AUTHOR]

Atmospheric circulations bring moisture from above the ocean to the high mountains of the western Tibet Plateau (TP), thus wind variability is of great importance to the water cycle centered on the western TP. This study therefore examines the leading modes of the wind fields over the western TP. We use the multivariate empirical orthogonal function (MV-EOF) analysis method to detect the dominant wind patterns above the western TP. This method extracts the leading modes of combined meridional and zonal wind variability at 200-hPa in the region of 22° N–50° N, 50° E–92° E. We find the first leading mode of the combined zonal and meridional wind field in the annual mean and in most seasons (spring, summer and autumn) over the western TP shows high similarity to the Western Tibetan Vortex (WTV), a large-scale atmospheric vortex-like pattern recently recognized over the western TP. In winter, the WTV, however, is closer to the second leading mode. We estimate the sensitivity of our results by changing the domain of the MV-EOF analysis region surrounding the western. We find the influence of the WTV is less sensitive to analysis location around the western TP. In short, the WTV generally represents the first leading mode of the wind field in most seasons over the western TP. This study augments our knowledge on wind variability over the western TP. [ABSTRACT FROM AUTHOR]

Pliocene warmth has been used as an analogue for predicting climate response to the rapid atmospheric CO2 increase in the future. Pedogenic carbonates provide an essential archive to reconstruct terrestrial paleoenvironment in a warm world. The stable carbon isotopic compositions of pedogenic carbonate ( δ 13 C carb ) are principally used to reconstruct the history of C4 plants along with coexisting organic matter δ 13 C SOM . However, a growing body of evidence shows spatial and temporal decoupling between δ 13 C carb and δ 13 C SOM especially in arid regions, raising concerns when using δ 13 C carb as a proxy for photosynthetic pathway. Here, we report paired δ 13 C carb and δ 13 C SOM data from Pliocene to early Pleistocene loess-paleosols in Chinese Loess Plateau (CLP), middle latitude East Asia. The model sensitivity analysis suggests the dominant control of soil respiration flux (SRF) on δ 13 C carb values in this region. SRF is then reconstructed using the Monte Carlo simulation method. Our results show that the SRF was enhanced during Pliocene warmth, associated with increased vegetation density and East Asian Summer Monsoon (EASM) precipitation. An overall higher SRF across the CLP during the mid-Pliocene, compared with the Holocene and Last Glacial Maximum (LGM) period, suggests there was a northwestward shift of the EASM rain belt and vegetation under warmer climate. Our results suggest that global temperature has dominated the middle latitude ecosystem and soil variations through its influence on the hydrological cycle, providing insights into future ecological and environmental response to global warming in East Asia. [ABSTRACT FROM AUTHOR]

Precipitation changes over the Tibetan Plateau (TP), "Asian Water Tower", greatly affect water resources of the surrounding countries. A dipole pattern in summer precipitation trend over the TP is observed for 1961–2013, increasing over the north TP (NTP) and decreasing over the south TP (STP). Since the TP is one of the most vulnerable regions to climate change in the world, it is of great importance to investigate the role of anthropogenic forcing in the TP precipitation changes. Moisture budget analysis based on reanalysis reveals that the precipitation trends over NTP and STP are dominated by thermodynamic and dynamic terms of vertical moisture advection, respectively. Southward shift in the summer subtropical Asian westerly jet (AWJ) over the west of TP and northward shift in AWJ over the east of TP collaboratively result in the increase and decrease in dynamic component over NTP and STP, respectively. Here, we successfully detect the anthropogenic contribution to the NTP summer precipitation trends using the attribution experiments from the CLIVAR Climate of the 20th Century Plus Project (C20C+). The drying trends over STP are mainly controlled by internal variability, and the anthropogenic forcing is favorable for the drying trends. The impacts of anthropogenic forcing are twofold. First, human activity induced warming increases the specific humidity over TP, increasing the thermodynamic component of moisture budget. Second, the warming trend caused by anthropogenic forcing is unevenly distributed over the Eurasian continent, leading to the southward (northward) shift of AWJ over the west (east) of TP, favoring more precipitation over the NTP but less precipitation over the STP. This study provides evidence of the importance of anthropogenic forcing on the water cycle changes over the TP. [ABSTRACT FROM AUTHOR]

The Mediterranean-Black Sea system consists of two semi-enclosed basins connected by the Turkish Straits. In turn, the Mediterranean Sea is connected to the Atlantic Ocean through the narrow Strait of Gibraltar. The hydrological cycle of the system is driven by fresh water exchanges between the atmosphere, continents and oceans, and by salty water mass exchange among the ocean basins. Monitoring such water fluxes, especially its time evolution, is essential to understand the water cycle in the region, which is very sensitive to global climate changes and influences the variability of the Atlantic Meridional Overturning Circulation (AMOC), which in turn affects the global climate. In this study, we have estimated the hydrological cycle of the Mediterranean-Black Sea system from the time-variable gravity observations performed by the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On satellites, and precipitation and evaporation from ERA5 atmospheric reanalysis data for the period 2002–2020. In the Black Sea, rivers introduce an average water volume of 391 ± 12 km3/year, one third of which escape through the atmosphere and two thirds go to the Mediterranean Sea. In the latter, 1787 ± 23 km3/year are lost via net evaporation. The rivers runoff (502 ± 27 km3/year), and the inflow of Atlantic waters (1020 ± 56 km3/year; 0.0323 ± 0.0018 Sv), finally restore the Mediterranean water budget. The balance is not reached instantaneously, and this delay introduces a seasonal variability in all the fluxes. In particular, the net water flux from the Atlantic Ocean increases up to 2660 ± 111 km3/year in August/September, and reverses to − 407 ± 140 km3/year in April/May. On top of the climatology, the mean annual Atlantic water flux varies significantly between 706 and 1262 km3/year. [ABSTRACT FROM AUTHOR]

The partitioning of rainfall at the land surface into interception, infiltration and surface runoff plays an important role in the water cycle as it controls the time scale at which water returns to the atmosphere. Rainfall intensity is of crucial importance to this partition. High resolution convection permitting models significantly improve simulated sub-daily rainfall intensity distributions, in particular those associated with convective rainfall in the tropics. Here we compare the land surface hydrological response in a pair of 10-year simulations over an African domain performed using the Met Office Unified Model: a typical configuration using parameterised convection operating at 25 km and the second a high resolution convection permitting simulation at 4.5 km with the parametrized convection switched off. Overall pan-African interception in the convection permitting scheme is 70% lower, whilst surface runoff is 43% higher than the parameterized convection model. These changes are driven by less frequent, but more intense rainfall with a 25% increase in rainfall above 20 mm h−1 in the 4.5 km model. The parameterised scheme has a ~ 50% canopy water contribution to evaporative fraction which is negligible in the convection permitting scheme. Conversely, the convection permitting scheme has higher throughfall and infiltration leading to higher soil moisture in the weeks following rain resulting in a 30–50% decrease in the daytime sensible heat flux. We examine how important the sub-grid rainfall parameterisation in the model is for the differences between the two configurations. We show how, switching a convective parameterisation off can substantially impact land surface behaviour. [ABSTRACT FROM AUTHOR]

The intimate link between soil moisture and precipitation makes it a "chicken-and-egg situation" that challenges climate studies of the continental water cycle. This association is particularly acute over the Mediterranean, increasingly exposed to droughts with climate change. This study aims at deciphering the impact of spring soil moisture state in the Mediterranean on subsequent warm season precipitation. In an idealized setup, two distinct climate models are used to generate extreme dry or wet soil conditions, and run climate simulations initialized and/or forced by these conditions. Changes in precipitation distribution and persistence are analyzed and where applicable compared to composites from a reanalysis. Spring soil moisture anomalies are found to be very persistent, but the precipitation response is largely model dependent. Overall, dry soils lead to a reduction of precipitation for early summer months and conversely for wet soils although with a fainter and less robust signal. On the other hand, wet soils tend to favor the persistence of precipitation throughout summer over several sub-regions. Our results highlight the stringent need to reduce the wide array of uncertainties associated to soil moisture, land-atmosphere coupling and convection in climate models, before ascertaining that soil moisture initialization could provide more skillful sub-seasonal to seasonal precipitation prediction. [ABSTRACT FROM AUTHOR]

In atmospheric models, the partitioning of precipitation between infiltration and runoff has a major influence on the terrestrial water budget, and thereby on the simulated weather or climate. River routing models are now available to convert the simulated runoff into river discharge, offering a good opportunity to validate land surface models at the regional scale. However, given the low resolution of global atmospheric models, the quality of the hydrological simulations is much dependent on various processes occurring on unresolved spatial scales. This paper focuses on the parameterization of sub-grid hydrological processes within the ISBA land surface model. Five off-line simulations are performed over the French Rhône river basin, including various sets of parameterizations related to the sub-grid variability of topography, precipitation, maximum infiltration capacity and land surface properties. Parallel experiments are conducted at a high (8 km by 8 km) and low (1° by 1°) resolution, in order to test the robustness of the simulated water budget. Additional simulations are performed using the whole package of sub-grid parameterizations plus an exponential profile with depth of saturated hydraulic conductivity, in order to investigate the interaction between the vertical soil physics and the horizontal heterogeneities. All simulations are validated against a dense network of gauging measurements, after the simulated runoff is converted into discharge using the MODCOU river routing model. Generally speaking, the new version of ISBA, with both the sub-grid hydrology and the modified hydraulic conductivity, shows a better simulation of river discharge, as well as a weaker sensitivity to model resolution. The positive impact of each individual sub-grid parameterization on the simulated discharges is more obvious at the low resolution, whereas the high-resolution simulations are more sensitive to the exponential profile with depth of saturated hydraulic conductivity. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP) plays an important role in regulating the global hydrologic cycle. Using a fully coupled climate model, we conduct sensitivity experiments to quantify the impact of the TP on North Africa precipitation. Removing the TP in the model can enhance North African precipitation. Specifically, North Africa precipitation increases substantially during the rainy season (from May to October) though it remains unchanged during the dry season (from November to April). During the rainy season, the TP's absence in the model causes an anomalous moisture transport from the Indian Ocean and tropical Atlantic to North Africa, which enhances the moisture convergence over North Africa and increases precipitation there. Later on, the change in the Atlantic, that is, cooling (warming) in the North (South) Atlantic forces a southward cross-equatorial moisture transport anomaly from North Africa to the equatorial Atlantic, decreasing the moisture convergence over North Africa and thus precipitation. In general, the moisture convergence is strengthened in most regions of North Africa due to the TP removal, so the resultant precipitation is increased. During the dry season, atmospheric convection center over the Africa continent is located mainly south of the equator, and there is almost no anomalous moisture transport toward North Africa in response to the TP removal. These results suggest that the uplift of the TP may have led to North African aridity. [ABSTRACT FROM AUTHOR]

The South Atlantic subtropical underwater (STUW) is a high-salinity water mass formed by subduction within the subtropical gyre. It is a major component of the subtropical cell and affects stratification in the downstream direction due to its high salinity characteristics. Understanding the interannual variability in STUW subduction is essential for quantifying the impact of subtropical variability on the tropical Atlantic. Using the output from the ocean state estimate of the Consortium for Estimating the Circulation and Climate of the Ocean (ECCO), this study investigates the interannual variability in STUW subduction from 1992 to 2016. We find that heat fluxes, wind stress, and wind stress curl cause interannual variability in the subduction rate. Heat fluxes over the subduction area modulate the sea surface buoyancy and regulate the mixed layer depth (MLD) during its deepening and shoaling phases. Additionally, the wind stress curl and zonal wind stress can modulate the size of the subduction area by regulating the probability of particles entrained into the mixed layer within 1 year of tracing. This analysis evaluates the influence of subtropical wind patterns on the South Atlantic subsurface high-salinity water mass, highlighting the impact of heat and wind on the interannual changes in the oceanic component of the hydrological cycle. [ABSTRACT FROM AUTHOR]

The tropical Atlantic Ocean receives an important freshwater supply from river runoff and from precipitation in the intertropical convergence zone. It results in a strong salinity stratification that may influence vertical mixing, and thus sea surface temperature (SST) and air–sea fluxes. The aim of this study is to assess the impact of salinity stratification on the tropical Atlantic surface variables. This is achieved through comparison among regional 1/4 ∘ coupled ocean–atmosphere simulations for which the contribution of salinity stratification in the vertical mixing scheme is included or discarded. The analysis reveals that the strong salinity stratification in the northwestern tropical Atlantic induces a significant increase of SST (0.2 ∘ C–0.5 ∘ C) and rainfall (+ 19%) in summer, hereby intensifying the ocean–atmosphere water cycle, despite a negative atmospheric feedback. Indeed, the atmosphere dampens the oceanic response through an increase in latent heat loss and a reduction of shortwave radiation reaching the ocean surface. In winter, the impacts of salinity stratification are much weaker, most probably because of a deeper mixed layer at this time. In the equatorial region, we found that salinity stratification induces a year-round shoaling of the thermocline, reinforcing the cold tongue cool anomaly in summer. The concept of barrier layer has not been identified as relevant to explain the SST response to salinity stratification in our region of interest. [ABSTRACT FROM AUTHOR]

The precipitation recycling (PR) ratio is an important indicator that quantifies the land-atmosphere interaction strength in the Earth system's water cycle. To better understand how the heterogeneous land surface in the Tibetan Plateau (TP) contributes to precipitation, we used the water-vapor tracer (WVT) method coupled with the Weather Research and Forecasting (WRF) regional climate model. The goals were to quantify the PR ratio, in terms of annual mean, seasonal variability and diurnal cycle, and to address the relationships of the PR ratio with lake treatments and precipitation amount. Simulations showed that the PR ratio increases from 0.1 in winter to 0.4 in summer when averaged over the TP with the maxima centered at the headwaters of three major rivers (Yangtze, Yellow and Mekong). For the central TP, the highest PR ratio rose to over 0.8 in August, indicating that most of the precipitation was recycled via local evapotranspiration in summer. The larger daily mean and standard deviation of the PR ratio in summer suggested a stronger effect of land-atmosphere interactions on precipitation in summer than in winter. Despite the relatively small spatial extent of inland lakes, the treatment of lakes in WRF significantly impacted the calculation of the PR ratio over the TP, and correcting lake temperature substantially improved both precipitation and PR ratio simulations. There was no clear relationship between PR ratio and precipitation amount; however, a significant positive correlation between PR and convective precipitation was revealed. This study is beneficial for the understanding of land-atmosphere interaction over high mountain regions. [ABSTRACT FROM AUTHOR]

Some 180 million people depend on the Indus River as a key water resource, fed largely by precipitation falling over the western Himalaya. However, the projected response of western Himalayan precipitation to climate change is currently not well constrained: CMIP5 GCMs project a reduced frequency and vorticity of synoptic-scale systems impacting the area, but such systems would exist in a considerably moister atmosphere. In this study, a convection-permitting (4 km horizontal resolution) setup of the Weather Research and Forecasting (WRF) model is used to examine 40 cases of these synoptic-scale systems, known as western disturbances (WDs), as they interact with the western Himalaya. In addition to a present-day control run, three experiments are performed by perturbing the boundary and initial conditions to reflect pre-industrial, RCP4.5 and RCP8.5 background climates respectively. It is found that in spite of the weakening intensity of WDs, net precipitation associated with them in future climate scenarios increases significantly; conversely there is no net change in precipitation between the pre-industrial and control experiments despite a significant conversion of snowfall in the pre-industrial experiment to rainfall in the control experiment, consistent with the changes seen in historical observations. This shift from snowfall to rainfall has profound consequences on water resource management in the Indus Valley, where irrigation is dependent on spring meltwater. Flux decomposition shows that the increase in future precipitation follows directly from the projected moistening of the tropical atmosphere (which increases the moisture flux incident on the western Himalaya by 28%) overpowering the weakened dynamics (which decreases it by 20%). Changes to extreme rainfall events are also examined: it is found that such events may increase significantly in frequency in both future scenarios examined. Two-hour maxima rainfall events that currently occur in 1-in-8 WDs are projected to increase tenfold in frequency in the RCP8.5 scenario; more prolonged (1-week maxima) events are projected to increase fiftyfold. [ABSTRACT FROM AUTHOR]

Tropical floodplains have a significant impact on the regional water cycle and on land-atmosphere interaction but are not always considered in Land Surface Models (LSMs) or in Earth system models. This study evaluates the importance of representing tropical floodplains in an LSM to provide realistic river discharges, evapotranspiration fluxes and other variables crucial for land's interaction with the ocean and atmosphere. Off-line simulations of the world's largest tropical wetland, the Pantanal are conducted with ORCHIDEE, the LSM of IPSL's regional and global Earth system model. We analyse the period 1961–2000, which includes both dry and wet decades. Atmospheric uncertainty is considered through the utilization of three forcing data sets, each one in two versions: the original dataset and a regionally bias adjusted version. The activation of the floodplain module leads to a systematic improvement of intra-annual variability and extremes of river discharge. Temporal evolution and spatial distribution of flooded area are coherent with satellite estimations, although the model, due to the coarse resolution of the topography, underestimates the extent of the area. Considering floodplains in ORCHIDEE enhance the evapotranspiration since it permits the water from the upstream region to evaporate in the plains. This have strong consequences on the water balance and on the spatial pattern of surface fluxes. The simulations allow us to perform a model-guided residual estimation of evapotranspiration through a water balance obtaining an annual evapotranspiration over Pantanal of 1220 mm while precipitation is estimated to be 1250 mm with an uncertainty of 180 mm. [ABSTRACT FROM AUTHOR]

State-of-the-art climate change projections of the CMIP5 simulations suggest a fairly complex pattern of global precipitation changes, with regions of reduced and enhanced precipitation. Conceptual understanding of these projected precipitation changes is difficult if only based on coupled general circulation model (CGCM) simulations, due to the complexity of these models. In this study we describe a simple deconstruction of the ensemble mean CMIP5 projections based on sensitivity simulations with the globally resolved energy balance (GREB) model. In a series of sensitivity experiments we force the GREB model with four different CMIP5 ensemble mean changes in: surface temperature, evaporation and the vertical atmospheric velocities mean and its standard deviation. The resulting response in the precipitation of the GREB model is very close to the CMIP5 ensemble mean response, suggesting that the precipitation changes can be well represented by a linear combination of these four forcings. The results further provide good insights into the drivers of precipitation change. The GREB model suggests that not one forcing alone can be seen as the main driver, but only the combination of all four changes results in the complex response pattern. However, the dominant forcings are the changes in the large-scale circulation, rather than the pure thermodynamic warming effect. Here, it is interesting to note that changes in high-frequency atmospheric variability of vertical air motion (weather), that are partly independent of the changes in the mean circulation, have a control on the pattern of the time-mean global precipitation changes. The approach presented here provides a powerful basis on which the hydrological cycles of CGCM simulations can be analysed. [ABSTRACT FROM AUTHOR]

Convection-permitting models (CPM) with at least 4 km horizontal grid spacing enable the cumulus parameterization to be switched off and thus simulate convective processes more realistically than coarse resolution models. This study investigates if a North American scale CPM can reproduce the observed warm season precipitation diurnal cycle on a climate scale. Potential changes in the precipitation diurnal cycle characteristics at the end of the twenty first century are also investigated using the pseudo global warming approach under a high-end anthropogenic emission scenario (RCP8.5). Simulations are performed with the Advanced Research Weather Research and Forecasting (ARW-WRF) model with 4-km horizontal grid spacing. Results from the WRF historical run (2001–2013) are evaluated against hourly precipitation from 2903 weather stations and a gridded hourly precipitation product in the U.S. The magnitude and timing of the diurnal cycle peak are realistically simulated in most of the U.S. and southern Canada. The model also captures the transition from afternoon precipitation peaks eastward of the Rocky Mountains to night peaks in the central U.S., which is related to propagating mesoscale convective systems. However, the historical climate simulation does not capture the observed early morning peaks in the central U.S. and overestimates the magnitude of the diurnal precipitation peak in the southeast region. In the simulation of the future climate, both the precipitation amount of the diurnal cycle and precipitation intensity increase throughout the domain, along with an increase in precipitation frequency in the northern region of the domain in May. These increases indicate a clear intensification of the hydrologic cycle during the warm season with potential impacts on future water resources, agriculture, and flooding. [ABSTRACT FROM AUTHOR]

Soil temperature is an important variable of earth's surface system, and studying it is of great benefit to understanding the energy and hydrological cycles and climate change. Based on soil temperature data at a depth of 3.2 m from 113 stations in China from 1980 to 2017, the spatial distribution and temporal variation of the soil temperatures in China were analyzed by using a fuzzy C-means algorithm (FCM), linear fitting and the Mann–Kendall method. The results show that the average annual soil temperature at 3.2 m in China gradually increased from north to south, with the lowest temperatures in the northeast and Qinghai–Tibetan Plateau ( T 3.2 m ≤ 5 ∘ C ) and the highest in South China ( T 3.2 m ≥ 25 ∘ C ). The country is divided into 10 regions using the FCM method. The soil temperature anomalies show a continuous increase over time, but there are significant regional differences in the warming speeds. The northeast and northwest regions exhibit the largest temperature increases, approximately 0.57 ° C / 10 a , North China has the second highest increases, with approximately 0.36 ° C / 10 a , and the values in other areas are less than 0.3 ° C / 10 a . Over the 38-year period, the soil temperature increased by approximately 1.4 ° C , and the temperature increase mainly occurred during the last 20 years. The positive center of the annual soil temperature anomaly at 3.2 m in the Qinghai–Tibetan Plateau during the last 2 decades of the twentieth century became cooling to a negative soil temperature anomaly at 3.2 m during the first 2 decades of the twenty-first century. The negative centers for the annual soil temperature anomaly at 3.2 m in the south of Shaanxi and the middle reaches of the Yangtze River during the last 2 decades of the twentieth century became warming to positive soil temperature anomalies at 3.2 m in the first 2 decades of the twenty-first century. In the other regions, different turning points of the positive–negative temperature anomaly are exhibited at 3.2 m. [ABSTRACT FROM AUTHOR]

We examine the dynamics of thresholds of the Atlantic Meridional Overturning Circulation (AMOC) in an Atmosphere–Ocean General Circulation Model (AOGCM) and a simple box model. We show that AMOC thresholds in the AOGCM are controlled by low-order dynamics encapsulated in the box model. In both models, AMOC collapse is primarily initiated by the development of a strong salinity advection feedback in the North Atlantic. The box model parameters are potentially observable properties of the unperturbed (present day) ocean state, and when calibrated to a range of AOGCM states predict (within some error bars) the critical rate of fresh water input (Hcrit) needed to turn off the AMOC in the AOGCM. In contrast, the meridional fresh water transport by the MOC (MOV, a widely-used diagnostic of AMOC bi-stability) on its own is a poor predictor of Hcrit. When the AOGCM is run with increased atmospheric carbon dioxide, Hcrit increases. We use the dynamical understanding from the box model to show that this increase is due partly to intensification of the global hydrological cycle and heat penetration into the near-surface ocean, both robust features of climate change projections. However changes in the gyre fresh water transport efficiency (a less robustly modelled process) are also important. [ABSTRACT FROM AUTHOR]

Middle America is affected by tropical cyclones (TCs) from the Eastern Pacific and the North Atlantic Oceans. We characterize the regional climatology (1998–2016) of the TC contributions to the atmospheric branch of the hydrological cycle, from May to December. TC contributions to rainfall are quantified using Tropical Rainfall Measuring Mission (TRMM) Multi-satellite Precipitation Analysis (TMPA) product 3B42 and TC tracks derived from three sources: the International Best Track Archive for Climate Stewardship (IBTrACS), and an objective feature tracking method applied to the Japanese 55-year and ERA-Interim reanalyses. From July to October, TCs contribute 10–30% of rainfall over the west and east coast of Mexico and central Mexico, with the largest monthly contribution during September over the Baja California Peninsula (up to 90%). TCs are associated with 40–60% of daily extreme rainfall (above the 95th percentile) over the coasts of Mexico. IBTrACS and reanalyses agree on TC contributions over the Atlantic Ocean but disagree over the Eastern Pacific Ocean and continent; differences over the continent are mainly attributed to discrepancies in TC tracks in proximity to the coast and TC lifetime. Reanalysis estimates of TC moisture transports show that TCs are an important moisture source for the regional water budget. TC vertically integrated moisture flux (VIMF) convergence can turn regions of weak VIMF divergence by the mean circulation into regions of weak VIMF convergence. We discuss deficiencies in the observed and reanalysis TC tracks, which limit our ability to quantify robustly the contribution of TCs to the regional hydrological cycle. [ABSTRACT FROM AUTHOR]

Anthropogenic aerosols (AA) can affect cloud and precipitation through aerosol–radiation interaction (ARI) and aerosol–cloud interaction (ACI). Over the past few decades, anthropogenic aerosol emissions have exhibited remarkable changes in the magnitude and in spatial pattern. The most significant changes are the increased emissions over both South Asia and East Asia. In this study, the atmospheric component of a state-of-the-art climate model that includes eight species of tropospheric aerosols, coupled to a multi-level mixed-layer ocean model, has been used to investigate the impacts of Asian anthropogenic aerosol precursor emission changes from 1970s to 2000s on large scale circulation and precipitation in boreal summer over East Asia. Results reveal significant changes in circulation and clouds over East Asia and over the tropical and western North Pacific (WNP). Increased Asian AA emissions lead to anomalous cyclonic circulation over the Maritime continent (MC) and anomalous anticyclonic circulation over the WNP, resulting in anomalous moisture transport convergence over the MC and therefore increased precipitation. They also lead to anomalous moisture flux divergence over both the WNP and large land areas of East Asia, especially over northern China, and therefore decreased precipitation there. These large scale circulation anomalies over the adjacent oceans are related to aerosol change induced ocean feedbacks, predominantly through ACI. It is the slow responses over the adjacent oceans (e.g., SST changes) through coupled atmosphere–ocean interaction in pre-monsoon seasons and summer that shape the changes of the East Asian summer monsoon and local precipitation. The results in this study suggest that increased Asian AA emissions from 1970s to 2000s may have played an important role for the observed southward shift of the Pacific intertropical convergence zone and precipitation belt, weakening of East Asian summer monsoon and reduced precipitation over northern China in East Asia during the latter half of the twentieth century. [ABSTRACT FROM AUTHOR]

Sub-grid orographic drag directly acts on wind and impacts the regional water cycle through control of atmospheric water vapor (AWV) transport. The effect of turbulent orographic form drag (TOFD) on wind and precipitation is investigated in this study using the WRF model for a winter month over the western Tibetan Plateau (TP), where solid precipitation supplies large amounts of water resources. The diurnal cycle of wind components and atmospheric circulation simulated with TOFD are consistent with observations and ERA-Interim data, whereas stronger westerlies exist in the simulation without the TOFD scheme. The latter results in more zonal AWV transport from the west and more precipitation over the western TP and surroundings. The implementation of the TOFD scheme leads to reduced biases, when evaluated with two observation-based precipitation products. It is therefore concluded that this scheme has a clear dynamical control on the regional atmospheric water recharge and thus the parameterization of the small-scale orographic drag in the model helps to improve the prediction of wintertime precipitation in the western TP region. [ABSTRACT FROM AUTHOR]

This study undertakes a multi-model comparison with the aim to describe and quantify systematic changes of the global energy and water budgets when the horizontal resolution of atmospheric models is increased and to identify common factors of these changes among models. To do so, we analyse an ensemble of twelve atmosphere-only and six coupled GCMs, with different model formulations and with resolutions spanning those of state-of-the-art coupled GCMs, i.e. from resolutions coarser than 100 km to resolutions finer than 25 km. The main changes in the global energy budget with resolution are a systematic increase in outgoing longwave radiation and decrease in outgoing shortwave radiation due to changes in cloud properties, and a systematic increase in surface latent heat flux; when resolution is increased from 100 to 25 km, the magnitude of the change of those fluxes can be as large as 5 W m−2. Moreover, all but one atmosphere-only model simulate a decrease of the poleward energy transport at higher resolution, mainly explained by a reduction of the equator-to-pole tropospheric temperature gradient. Regarding hydrological processes, our results are the following: (1) there is an increase of global precipitation with increasing resolution in all models (up to 40 × 103 km3 year−1) but the partitioning between land and ocean varies among models; (2) the fraction of total precipitation that falls on land is on average 10% larger at higher resolution in grid point models, but it is smaller at higher resolution in spectral models; (3) grid points models simulate an increase of the fraction of land precipitation due to moisture convergence twice as large as in spectral models; (4) grid point models, which have a better resolved orography, show an increase of orographic precipitation of up to 13 × 103 km3 year−1 which explains most of the change in land precipitation; (5) at the regional scale, precipitation pattern and amplitude are improved with increased resolution due to a better simulated seasonal mean circulation. We discuss our results against several observational estimates of the Earth's energy budget and hydrological cycle and show that they support recent high estimates of global precipitation. [ABSTRACT FROM AUTHOR]

The influence of El Niño–Southern Oscillation (ENSO) on wintertime fog days in eastern China was investigated through the analysis of fog station data in China and the National Centers for Environmental Prediction reanalysis data during the period of 1954–2007. A composite analysis shows distinctive anomaly patterns of fog days in early and late winter during ENSO. More frequent fogs appear in Central and North China (around 30°–40°N) in November–December (ND), while more frequent fogs occur over Middle Reach of Yangtze River (27°–32°N) and less frequent fogs appear over 32°–40°N in January–February (JF) during El Niño. The cause of this distinctive sub-seasonal evolution feature is discussed as the following. Actually we analyzed all meteorological fields during El Niño and La Niña, and found that they are approximately in a mirror image. To reduce the number of figures, we presented the difference fields in most of cases. We found that anomalous southwesterlies associated with El Niño penetrate into North China (up to 40°N) in ND. The northward moisture transport leads to near-surface air saturation over the region. Moreover, atmosphere becomes convectively unstable in South China (20°–30°N), which leads to a positive precipitation anomaly there. The local convergence leads to an anomalous divergence and subsidence north of 30°N. Both the saturated air and stable stratification lead to more frequent fog days in 30°–40°N. Compared to ND, anomalous southwesterly flow in JF is weaker, and can only transport moisture up to Central China (near 30°N). As maximum precipitation anomaly is confined near the southeastern coast of China, anomalous divergence and subsidence appear north of 27°N. This stable stratification together with the anomalous moisture transport causes the increase of fog over Central China but decrease of fog in North China (33°–38°N) due to the subsidence induced dryness. [ABSTRACT FROM AUTHOR]

Global surface evapotranspiration is one of the most significant components of the response of the water cycle to a warming climate. However, trends in surface evapotranspiration differ considerably from the trend in climate warming according to recent studies, with some studies even showing an opposing trend. The reason for this difference in the response of surface evapotranspiration to climate warming is still not completely understood. We validated the gridded FLUXNET evapotranspiration dataset and the Global Land Surface Assimilation Dataset (GLDAS) against evapotranspiration data observed in northern China using the eddy covariance system. The response of surface evapotranspiration to an increase in temperature varied with the type of climate (classified by the amount of precipitation) and the trend of surface evapotranspiration with warming showed similar features to the transitions between these climate types. The climate type with precipitation in the range of 250–350 mm was the most sensitive to the effects of warming on evapotranspiration. In more humid climates, surface evapotranspiration increased with increasing temperature, whereas in drier climates surface evapotranspiration decreased with increasing temperature. A similar response of evapotranspiration to increasing temperatures was also observed elsewhere in regions of climate transition. There are two main mechanisms of evapotranspiration: (1) an increase in temperature resulting in a direct increase in potential evapotranspiration; and (2) an increase in temperature resulting in a loss of soil moisture due to the increase in evapotranspiration, which in turn will indirectly suppress surface evapotranspiration due to the loss of vegetation. [ABSTRACT FROM AUTHOR]

Climate models simulate an increase in global precipitation at a rate of approximately 1–3% per Kelvin of global surface warming. This change is often interpreted through the lens of the atmospheric energy budget, in which the increase in global precipitation is mostly offset by an increase in net radiative cooling. Other studies have provided different interpretations from the perspective of the surface, where evaporation represents the turbulent transfer of latent heat to the atmosphere. Expanding on this surface perspective, here we derive a version of the Penman–Monteith equation that allows the change in ocean evaporation to be partitioned into a thermodynamic response to surface warming, and additional diagnostic contributions from changes in surface radiation, ocean heat uptake, and boundary-layer dynamics/relative humidity. In this framework, temperature is found to be the primary control on the rate of increase in global precipitation within model simulations of greenhouse gas warming, while the contributions from changes in surface radiation and ocean heat uptake are found to be secondary. The temperature contribution also dominates the spatial pattern of global evaporation change, leading to the largest fractional increases at high latitudes. In the surface energy budget, the thermodynamic increase in evaporation comes at the expense of the sensible heat flux, while radiative changes cause the sensible heat flux to increase. These tendencies on the sensible heat flux partly offset each other, resulting in a relatively small change in the global mean, and contributing to an impression that global precipitation is radiatively constrained. [ABSTRACT FROM AUTHOR]

We examine long-term trends in the historical record of extreme precipitation events occurring over the French Mediterranean area. Extreme events are considered in terms of their intensity, frequency, extent and precipitated volume. Changes in intensity are analysed via an original statistical approach where the annual maximum rainfall amounts observed at each measurement station are aggregated into a univariate time-series according to their dependence. The mean intensity increase is significant and estimated at + 22% (+ 7 to + 39% at the 90% confidence level) over the 1961-2015 period. Given the observed warming over the considered area, this increase is consistent with a rate of about one to three times that implied by the Clausius-Clapeyron relationship. Changes in frequency and other spatial features are investigated through a Generalised Linear Model. Changes in frequency for events exceeding high thresholds (about 200 mm in 1 day) are found to be significant, typically near a doubling of the frequency, but with large uncertainties in this change ratio. The area affected by severe events and the water volume precipitated during those events also exhibit significant trends, with an increase by a factor of about 4 for a 200 mm threshold, again with large uncertainties. All diagnoses consistently point toward an intensification of the most extreme events over the last decades. We argue that it is difficult to explain the diagnosed trends without invoking the human influence on climate. [ABSTRACT FROM AUTHOR]

Slowing variability in climate system is an important source of climate predictability. However, it is still challenging for current dynamical models to fully capture the variability as well as its impacts on future climate. In this study, instead of simulating the internal multi-scale oscillations in dynamical models, we discussed the effects of internal variability in terms of climate memory. By decomposing climate state x(t) at a certain time point t into memory part M(t) and non-memory part ε(t), climate memory effects from the past 30 years on climate prediction are quantified. For variables with strong climate memory, high variance (over 20%) in x(t) is explained by the memory part M(t), and the effects of climate memory are non-negligible for most climate variables, but the precipitation. Regarding of multi-steps climate prediction, a power law decay of the explained variance was found, indicating long-lasting climate memory effects. The explained variances by climate memory can remain to be higher than 10% for more than 10 time steps. Accordingly, past climate conditions can affect both short (monthly) and long-term (interannual, decadal, or even multidecadal) climate predictions. With the memory part M(t) precisely calculated from Fractional Integral Statistical Model, one only needs to focus on the non-memory part ε(t), which is an important quantity that determines climate predictive skills. [ABSTRACT FROM AUTHOR]

In recent decades the water resource of Nile river basin (NRB) has suffered from increasing demands from competing users, and global warming impact that exacerbate occurrences of hydrologic extremes. To investigate climate change impact on the future precipitation and temperature of NRB, a regional climate model, weather research and forecasting (WRF) was set up at one domain, 36-km resolution to dynamically downscale the base period (1976-2005), RCP4.5 and RCP8.5 climate scenarios of four General Circulation Model (GCMs) of CMIP5 over NRB for 2050s and 2080s. For base periods, WRF simulated surface temperature and precipitation that agree well with observed CRU and GPCC data of NRB, respectively. Under downscaled RCP4.5 and RCP8.5 climate scenarios, the annual precipitation of Blue Nile, Atbara, and Sobat river basin, Bahar El Ghazal and Lake Victoria regions are projected to change by about [− 7, 14.2], [− 19, 25.3], [− 7, 39], [− 5.9, 23], and [3.6, 27] % in the 2050s, and [− 14, 25], [− 22.5, 39], [− 4.7, 60.4], [− 11, 31], and [11.8, 41] % in the 2080s, respectively. The mean annual air temperature for sub-basins of NRB is projected to increase by 1.67-2 °C in the 2050s, 2-2.5 °C in the 2080s under RCP4.5, and by 2.5-3 °C in the 2050s and 3.9-4.6 °C in the 2080s under RCP8.5, respectively. Most precipitation extreme indices investigated are projected to increase, which implies that NRB could experience more severe and frequent extreme precipitation in future. Furthermore, extreme temperature indices of NRB are projected to decrease in cold, but increase in warm night/days. [ABSTRACT FROM AUTHOR]

The Atlantic multidecadal variability (AMV) is a coherent pattern of variability of the North Atlantic sea surface temperature field affecting several components of the climate system in the Atlantic region and the surrounding areas. The relatively short observational record severely limits our understanding of the physical mechanisms leading to the AMV. The present study shows that the spatial and temporal characteristics of the AMV, as assessed from the historical records, should also be considered as highly uncertain. Using 11 multi-century preindustrial climate simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) database, we show that the AMV characteristics are not constant along the simulation when assessed from different 200-year-long periods to match the observed period length. An objective method is proposed to test whether the variations of the AMV characteristics are consistent with stochastic internal variability. For 7 out of the 11 models analysed, the results indicate a non-stationary behaviour for the AMV time series. However, the possibility that the non-stationarity arises from sampling errors can be excluded with high confidence only for one of the 7 models. Therefore, longer time series are needed to robustly assess the AMV characteristics. In addition to any changes imposed to the AMV by external forcings, the detected dependence on the time interval identified in most models suggests that the character of the observed AMV may undergo significant changes in the future. [ABSTRACT FROM AUTHOR]

Regional climate models (RCMs) have been used to dynamically downscale global climate projections at high spatial and temporal resolution in order to analyse the atmospheric water cycle. In southern Africa, precipitation pattern were strongly affected by the moisture transport from the southeast Atlantic and southwest Indian Ocean and, consequently, by their sea surface temperatures (SSTs). However, global ocean models often have deficiencies in resolving regional to local scale ocean currents, e.g. in ocean areas offshore the South African continent. By downscaling global climate projections using RCMs, the biased SSTs from the global forcing data were introduced to the RCMs and affected the results of regional climate projections. In this work, the impact of the SST bias correction on precipitation, evaporation and moisture transport were analysed over southern Africa. For this analysis, several experiments were conducted with the regional climate model REMO using corrected and uncorrected SSTs. In these experiments, a global MPI-ESM-LR historical simulation was downscaled with the regional climate model REMO to a high spatial resolution of 50 × 50 km2 and of 25 × 25 km2 for southern Africa using a double-nesting method. The results showed a distinct impact of the corrected SST on the moisture transport, the meridional vertical circulation and on the precipitation pattern in southern Africa. Furthermore, it was found that the experiment with the corrected SST led to a reduction of the wet bias over southern Africa and to a better agreement with observations as without SST bias corrections. [ABSTRACT FROM AUTHOR]

We investigate the climate change impact on snowmelt-driven runoff (SDR) over the Alpine region using the output from two Med-CORDEX and two EURO-CORDEX regional climate model projections (RCP8.5 scenario) at two resolutions (12, 50 km) driven by a sub-set of the CMIP5 GCMs. Comparison with the European Water Archive observed runoff dataset (242 stations) over the Alps shows a good performance by the higher resolution models in representing present day SDR, with the lower resolution simulations being less accurate in capturing the SDR timing. In the future projections all the models show a temperature increase of up to 4° by the end of the 21st century throughout the Alps and this leads to an anticipation of SDR timing throughout the year that can span from 1 to 3 months depending on the model horizontal resolution. These timing changes are associated with changes in snow cover modulated by the complex Alpine topography. In fact, model resolution plays a critical role in regulating the magnitude, timing and spatial distribution of the response of snow cover and SDR to warming. We find that the accurate simulation of changes in runoff timing requires a high resolution representation of the Alpine topography, and can be important for water storage regulations concerning energy production, agriculture and domestic use. [ABSTRACT FROM AUTHOR]

We examine the global water cycle characteristics in the Accelerated Climate Modeling for Energy v0.3 model (a close relative to version 5.3 of the Community Atmosphere Model) in atmosphere-only simulations spanning the years 1980-2005. We evaluate the simulations using a broad range of observational and reanalysis datasets, examine how the simulations change when the horizontal resolution is increased from 1° to 0.25∘, and compare the simulations against models participating in the the Atmosphere Model Intercomparison Project of the 5th Coupled Model Intercomparison Project (CMIP5). Particular effort has been made to evaluate the model using the best available observational estimates and verifying model biases with additional datasets when differences are known to exist among the observations. Regardless of resolution, the model exhibits several biases: global-mean precipitation, evaporation, and precipitable water are too high, light precipitation occurs too frequently, and the atmospheric residence time of water is too short. Many of these biases are shared by the multi-model mean climate of models participating in CMIP5. The reasons behind regional biases in precipitation are discussed by examining how different fields, such as local evaporation and transport of water vapor, contribute to the bias. Although increasing the horizontal resolution does not drastically change the water cycle, it does lead to a few differences: an increase in global mean precipitation rate, an increase in the fraction of total precipitation that falls over land, more frequent heavy precipitation (>30 mm day-1), and a decrease in precipitable water. One of the most notable changes is the shift of precipitation produced by the convective parameterization to that produced by the large-scale microphysics parameterization. We analyze how changes in moisture and circulation with resolution contribute to this shift in the precipitation partitioning. Because changing horizontal resolution requires some re-tuning, the effect of that tuning was evaluated by performing an additional simulation at 1∘ but using the tunings from the 0.25∘ simulation. The evaluation shows that the more frequent heavy precipitation, the decrease in precipitable water, and the shift from convective to large-scale precipitation are predominantly due to resolution changes, while tuning changes have a major influence on the global mean precipitation and the land/ocean partitioning of precipitation. [ABSTRACT FROM AUTHOR]

We use a regionally coupled ocean-sea ice-atmosphere-hydrological discharge model to investigate the influence of changes in the atmospheric large-scale circulation on the interannual variability of the Arctic freshwater (FW) components. This model includes all sinks and sources of FW and allows for the analysis of a closed FW cycle in the Arctic. We show that few atmospheric winter modes explain large parts of the interannual variability of the Arctic FW cycle. A strong Icelandic low causing anomalous strong westerlies over the North Atlantic leads to warmer and wetter conditions over Eurasia. The ocean circulation is then characterized by a strong transpolar drift leading to increased export of FW in liquid and solid form into the North Atlantic. In contrast to this, a weaker than usual Icelandic low and a strong Siberian high is associated with a strong Beaufort Gyre and thus an accumulation of FW within the Arctic Ocean. Not only specific winter conditions but also increased precipitation in late spring and summer, caused by enhanced cyclone activity over land, lead to increased Eurasian runoff, which is responsible for most of the variability in Arctic river runoff. [ABSTRACT FROM AUTHOR]

On seasonal and inter-annual time scales, vertically integrated moisture divergence provides a useful measure of the tropical atmospheric hydrological cycle. It reflects the combined dynamical and thermodynamical effects, and is not subject to the limitations that afflict observations of evaporation minus precipitation. An empirical orthogonal function (EOF) analysis of the tropical Pacific moisture divergence fields calculated from the ERA-Interim reanalysis reveals the dominant effects of the El Niño-Southern Oscillation (ENSO) on inter-annual time scales. Two EOFs are necessary to capture the ENSO signature, and regression relationships between their Principal Components and indices of equatorial Pacific sea surface temperature (SST) demonstrate that the transition from strong La Niña through to extreme El Niño events is not a linear one. The largest deviation from linearity is for the strongest El Niños, and we interpret that this arises at least partly because the EOF analysis cannot easily separate different patterns of responses that are not orthogonal to each other. To overcome the orthogonality constraints, a self-organizing map (SOM) analysis of the same moisture divergence fields was performed. The SOM analysis captures the range of responses to ENSO, including the distinction between the moderate and strong El Niños identified by the EOF analysis. The work demonstrates the potential for the application of SOM to large scale climatic analysis, by virtue of its easier interpretation, relaxation of orthogonality constraints and its versatility for serving as an alternative classification method. Both the EOF and SOM analyses suggest a classification of 'moderate' and 'extreme' El Niños by their differences in the magnitudes of the hydrological cycle responses, spatial patterns and evolutionary paths. Classification from the moisture divergence point of view shows consistency with results based on other physical variables such as SST. [ABSTRACT FROM AUTHOR]

A homogeneous monthly data set of sea surface temperature (SST) and pseudo wind stress based on in situ observations is used to investigate the climatic trends over the tropical Atlantic during the last five decades (1964-2012). After a decrease of SST by about 1 °C during 1964-1975, most apparent in the northern tropical region, the entire tropical basin warmed up. That warming was the most substantial (>1 °C) in the eastern tropical ocean and in the longitudinal band of the intertropical convergence zone. Surprisingly, the trade wind system also strengthened over the peirod 1964-2012. Complementary information extracted from other observational data sources confirms the simultaneity of SST warming and the strengthening of the surface winds. Examining data sets of surface heat flux during the last few decades for the same region, we find that the SST warming was not a consequence of atmospheric heat flux forcing. Conversely, we suggest that long-term SST warming drives changes in atmosphere parameters at the sea surface, most notably an increase in latent heat flux, and that an acceleration of the hydrological cycle induces a strengthening of the trade winds and an acceleration of the Hadley circulation. These trends are also accompanied by rising sea levels and upper ocean heat content over similar multi-decadal time scales in the tropical Atlantic. Though more work is needed to fully understand these long term trends, especially what happens from the mid-1970's, it is likely that changes in ocean circulation involving some combination of the Atlantic meridional overtuning circulation and the subtropical cells are required to explain the observations. [ABSTRACT FROM AUTHOR]

The response of the hydrological cycle to climate variability and change is a critical open question, where model reliability is still unsatisfactory, yet upon which past climate history can shed some light. Sea ice is a key player in the climate system and in the hydrological cycle, due to its strong albedo effect and its insulating effect on local evaporation and air-sea heat flux. Using an atmospheric general circulation model with specified sea surface temperature and sea-ice distribution, the role of sea ice in the hydrological cycle is investigated under last glacial maximum (LGM) and present day conditions, and by studying its contribution to the 'temperature-precipitation feedback'. By conducting a set of sensitivity experiments in which the albedo and thickness of the sea ice are varied, the various effects of sea ice in the hydrological cycle are isolated. It is demonstrated that for a cold LGM like state, a warmer climate (as a result of reduced sea-ice cover) leads to an increase in snow precipitation over the ice sheets. The insulating effect of the sea ice on the hydrological cycle is found to be larger than the albedo effect. These two effects interact in a nonlinear way and their total effect is not equal to summing their separate contribution. [ABSTRACT FROM AUTHOR]

Global hydrographic and air-sea freshwater flux datasets are used to investigate ocean salinity changes over 1950-2010 in relation to surface freshwater flux. On multi-decadal timescales, surface salinity increases (decreases) in evaporation (precipitation) dominated regions, the Atlantic-Pacific salinity contrast increases, and the upper thermocline salinity maximum increases while the salinity minimum of intermediate waters decreases. Potential trends in E-P are examined for 1950-2010 (using two reanalyses) and 1979-2010 (using four reanalyses and two blended products). Large differences in the 1950-2010 E-P trend patterns are evident in several regions, particularly the North Atlantic. For 1979-2010 some coherency in the spatial change patterns is evident but there is still a large spread in trend magnitude and sign between the six E-P products. However, a robust pattern of increased E-P in the southern hemisphere subtropical gyres is seen in all products. There is also some evidence in the tropical Pacific for a link between the spatial change patterns of salinity and E-P associated with ENSO. The water cycle amplification rate over specific regions is subsequently inferred from the observed 3-D salinity change field using a salt conservation equation in variable isopycnal volumes, implicitly accounting for the migration of isopycnal surfaces. Inferred global changes of E-P over 1950-2010 amount to an increase of 1 ± 0.6 % in net evaporation across the subtropics and an increase of 4.2 ± 2 % in net precipitation across subpolar latitudes. Amplification rates are approximately doubled over 1979-2010, consistent with accelerated broad-scale warming but also coincident with much improved salinity sampling over the latter period. [ABSTRACT FROM AUTHOR]

The Twentieth Century Reanalysis (20CR) holds the distinction of having the longest record length (140-year; 1871-2010) of any existing global atmospheric reanalysis. If the record can be shown to be homogenous, then it would be the first reanalysis suitable for long-term trend assessments, including those of the regional hydrologic cycle. On the other hand, if discontinuities exist, then their detection and attribution-either to artificial observational shocks or climate change-is critical to their proper treatment. Previous research suggested that the quintupling of 20CR's assimilated observation counts over the central United States was the primary cause of inhomogeneities for that region. The same work also revealed that, depending on the season, the complete record could be considered homogenous. In this study, we apply the Bai-Perron structural change point test to extend these analyses globally. A rigorous evaluation of 20CR's (in)homogeneity is performed, composed of detailed quantitative analyses on regional, seasonal, inter-variable, and intra-ensemble bases. The 20CR record is shown to be homogenous (natural) for 69 (89) years at 50 % of land grids, based on analysis of the July 2 m air temperature. On average 54 % (41 %) of the grids between 60°S and 60°N are free from artificial inhomogenetites in their February (July) time series. Of the more than 853,376 abrupt shifts detected in 26 variable fields over two monthly time series, approximately 72 % are non-climate in origin; 25 % exceed 1.8 standard deviations of the preceding time series. The knock-on effect of inhomogeneities in 20CR's boundary forcing and surface pressure data inputs to its surface analysis fields is implicated. In the future, reassessing these inhomogeneities will be imperative to achieving a more definitive attribution of 20CR's abrupt shifts. [ABSTRACT FROM AUTHOR]

The reversibility of the Atlantic meridional overturning circulation (AMOC) is investigated in multi-model experiments using global climate models (GCMs) where CO concentrations are increased by 1 or 2 % per annum to 2× or 4× preindustrial conditions. After a period of stabilisation the CO is decreased back to preindustrial conditions. In most experiments when the CO decreases, the AMOC recovers before becoming anomalously strong. This "overshoot" is up to an extra 18.2Sv or 104 % of its preindustrial strength, and the period with an anomalously strong AMOC can last for several hundred years. The magnitude of this overshoot is shown to be related to the build up of salinity in the subtropical Atlantic during the previous period of high CO levels. The magnitude of this build up is partly related to anthropogenic changes in the hydrological cycle. The mechanisms linking the subtropical salinity increase to the subsequent overshoot are analysed, supporting the relationship found. This understanding is used to explain differences seen in some models and scenarios. In one experiment there is no overshoot because there is little salinity build up, partly as a result of model differences in the hydrological cycle response to increased CO levels and partly because of a less aggressive scenario. Another experiment has a delayed overshoot, possibly as a result of a very weak AMOC in that GCM when CO is high. This study identifies aspects of overshoot behaviour that are robust across a multi-model and multi-scenario ensemble, and those that differ between experiments. These results could inform an assessment of the real-world AMOC response to decreasing CO. [ABSTRACT FROM AUTHOR]

Changes in subtropical precipitation and the Hadley circulation (HC) are inextricably linked. The original Halley-Hadley model cannot explain certain aspects of the Earth's meridional circulation in the tropics and is of limited use in understanding regional changes in precipitation. Here, we expand on previous work on the regional and seasonal aspects of the HC, in particular how land-sea temperature contrasts contribute to the strength and width of the HC. We show that the Earth's HC is well described by three regionally distinct cells along the eastern edges of the major ocean basins with opposite circulations elsewhere. Moreover, comparable summertime hemisphere cells emerge in each region. While it has been recognized that continents modify the meridional pressure gradient, we demonstrate that a substantial part of the Earth's HC is driven by zonal pressure gradients (ZPGs) that only exist due to continental heating and air-sea interaction. Projected changes in land-sea temperature contrasts in a warming climate due to the relatively low thermal capacity of land will also affect ZPGs and thus HC strength and width, with implications for extremes in hydroclimate and freshwater resources across the increasingly vulnerable subtropics. [ABSTRACT FROM AUTHOR]

The role of atmospheric general circulation model (AGCM) horizontal resolution in representing the global energy budget and hydrological cycle is assessed, with the aim of improving the understanding of model uncertainties in simulating the hydrological cycle. We use two AGCMs from the UK Met Office Hadley Centre: HadGEM1-A at resolutions ranging from 270 to 60 km, and HadGEM3-A ranging from 135 to 25 km. The models exhibit a stable hydrological cycle, although too intense compared to reanalyses and observations. This over-intensity is explained by excess surface shortwave radiation, a common error in general circulation models (GCMs). This result is insensitive to resolution. However, as resolution is increased, precipitation decreases over the ocean and increases over the land. This is associated with an increase in atmospheric moisture transport from ocean to land, which changes the partitioning of moisture fluxes that contribute to precipitation over land from less local to more non-local moisture sources. The results start to converge at 60-km resolution, which underlines the excessive reliance of the mean hydrological cycle on physical parametrization (local unresolved processes) versus model dynamics (large-scale resolved processes) in coarser HadGEM1 and HadGEM3 GCMs. This finding may be valid for other GCMs, showing the necessity to analyze other chains of GCMs that may become available in the future with such a range of horizontal resolutions. Our finding supports the hypothesis that heterogeneity in model parametrization is one of the underlying causes of model disagreement in the Coupled Model Intercomparison Project (CMIP) exercises. [ABSTRACT FROM AUTHOR]