Search

Significant anomalies in frequency of summer extreme hot days (SEHDs) are broadly observed in the Asian monsoon region (AMR) in the post-ENSO summers. The delayed ENSO impacts are mainly conveyed by provoking the Indo-western Pacific Ocean capacitor (IPOC) effect that maintains the anomalous anticyclone in the western North Pacific. The related diabatic heating anomaly can trigger the westward-propagating Rossby wave to the Indian subcontinent, which increases the geopotential heights, reduces the cloud cover, and thus increases the seasonal surface temperature and SEHD frequency in the southern AMR. Besides, the reduced atmospheric moisture in the western North Pacific hinders the northward propagation of intraseasonal oscillation (ISO) and modulates the occurrence frequency of individual ISO phases, contributing to the significantly increased/decreased SEHDs in eastern China/Hokkaido, Japan, in the post–El Niño summers. The 25-model-ensemble mean of CMIP6 historical runs can reproduce well the observed SEHD anomalies in the southern AMR in the post-ENSO summers mainly due to the realistic simulation of ENSO impacts on the seasonal surface temperature, although a large intermodel spread exists due to different strengths of IPOC effect in each model owing to model biases in the mean state of the eastern tropical Pacific, the ENSO variance, and teleconnection to the Indian Ocean. Furthermore, future projections under the SSP5-8.5 scenario show that the delayed ENSO impacts on the southern AMR remain stable under global warming via a similar mechanism as in the observations and historical runs.

Understanding and predicting extreme weather and climate are ultimately important for adaptation and resilience. Here, we assess the prediction skills of frequency of summer extreme hot days (SEHDs) in the Asian monsoon region (AMR) by using the 1981–2014 hindcasts of the Predictive Ocean Atmosphere Model for Australia version 2 (POAMA-2) subseasonal-to-seasonal prediction system. Generally, the good prediction skills of SEHD frequency appear in the southern AMR south of 30°N including the Indian subcontinent, Indo-China peninsula, and the Philippines; the anomaly correlation coefficients between the observed and predicted region-mean SEHD anomalies are 0.72, 0.72, 0.70, 0.60, 0.61, and 0.61 at 1–6 months lead, respectively, all statistically significant at the 99.9% confidence level. The high prediction skills in POAMA-2 are due to its capacity on reproducing the observed delayed ENSO impacts on the large-scale atmospheric circulation, the seasonal surface air temperature, and thus the SEHD frequency in the southern AMR via provoking the Indo–western Pacific Ocean capacitor effect. Since POAMA-2 only provides the prediction at most half a year in advance, we also conduct the hindcasts in the SINTEX-F that can reproduce well the robust ENSO–SEHD relationship and has high prediction skill of ENSO itself. Results show that skillful prediction of the region-mean SEHD frequency can be up to 14 months in advance. These results are complementary to Part I of this study that ENSO can impact the SEHDs in the AMR and provide the sources of predictability.

The Indian Ocean Dipole (IOD) is one of the leading modes of interannual climate variability in the tropical Indian Ocean (IO). Paleoclimate provides real climate scenarios to examine IOD behaviors and the linkage to basic states. Based on 18 models from the Coupled Model Intercomparison Project phase 5 and 6 (CMIP5/6), the IOD change from the preindustrial period to mid-Holocene is investigated. The multimodel mean reveals that the IOD variability weakens by 14% as measured by the standard deviation of the Dipole Mode Index, which is defined using the zonal sea surface temperature (SST) difference. Such attenuation is dominated by the spatially consistent suppression in the western-pole SST variability, while the eastern pole contributes little due to the opposite-signed changes in its northwestern and southeastern portions. The primary reason for the aforementioned changes comes from the altered climatic background, which displays a positive IOD-like pattern during IOD growing seasons, with intensified westward currents along the equator and northwestward currents in the southeastern equatorial IO. Such changes in the mean-state currents modulate the strength of the IOD-related anomalous advection and subsequently cause alterations in the IOD variability. Further analyses show that the IOD attenuation in the mid-Holocene is unlikely caused by the concurrently subdued El Niño–Southern Oscillation in the tropical Pacific partially because of the diminished connections between the two oscillations themselves. The above simulated changes in both the IO mean climatology and IOD variability agree well with the available paleo-records in literature.

Eastern China (EC) suffered an extreme drought with long-lasting duration and record-breaking intensity in late summer–autumn 2019. Our diagnosed results show that the central Pacific (CP) El Niño, in tandem with warm sea surface temperature anomalies (SSTAs) over the Kuroshio Extension (KE) region, induces the meridionally elongated cyclonic circulation anomalies stretching from the western North Pacific (WNP) to the Yellow Sea. Its western flank corresponds to overwhelming low-level northerly wind anomalies over EC, which result in deficient moisture and anomalous descent over EC and hence cause the extreme drought in 2019. To investigate the relative contributions of SSTAs over different regions, we performed sensitivity experiments, and analyzed the relationship between extreme drought like what occurred in 2019 (a 2019Drought-like event) and the SSTAs in CMIP6 historical simulations. Modeling evidences reveal that both warm SSTAs over the central equatorial Pacific and the KE region are indispensable for shaping the meridionally elongated cyclone anomaly. Specifically, the cyclone anomaly over the WNP induced by CP El Niño aligns with the cyclone anomaly over the Yellow Sea induced by the warm SSTAs over the KE region, merging into a meridionally stretched cyclone anomaly to the east of EC. Consequently, the northerly anomalies stretch across EC, leading to unfavorable atmospheric conditions and the rainfall deficit there. Projection results show that the occurrence probability of a 2019Drought-like event will increase by 20% (decrease by 40%–50%) under a high (medium-low) emission scenario compared to present-day climate, indicating the nonlinear response of extreme drought to different emission scenarios and the urgency of carbon emission reduction. Significance Statement An extreme drought hit the Eastern China (EC) region in 2019 and caused tremendous losses. This study proposed that both the 2019 CP El Niño and the warm SST anomalies over the Kuroshio Extension (KE) region induce the meridionally elongated circulation anomalies and the resultant extreme drought. We showed that the typical circulation anomalies induced by central Pacific (CP) El Niño cannot totally explain the meridionally elongated circulation anomalies in 2019. Our modeling evidences confirmed the indispensable role of warm SST anomalies over KE region in the 2019 extreme drought’s formation. The projection results show that extreme drought like that in 2019 will occur more (less) frequently under a high (medium-low) emission scenario compared to modern-day level, indicating the urgency of carbon emission reduction.

North Indian Ocean (NIO) tropical cyclone activity is strongly influenced by aerosols and climate modes. In this study, we evaluated the impact of aerosols and climate modes on modulating tropical cyclone (TC) frequency over the NIO. A statistical generalized additive model based on Poisson regression was developed to assess their relative impacts. Aerosol optical depth for different compounds simulated by the Goddard Chemistry Aerosol Radiation and Transport model, sunspot number (SN) as solar variability, and eight climate modes—Atlantic meridional mode (AMM), El Niño–Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), Indian Ocean dipole (IOD), Pacific decadal oscillation (PDO), Pacific–North American teleconnection pattern (PNA), Arctic Oscillation (AO), and Antarctic Oscillation (AAO), all based on reanalysis datasets, were analyzed for the 40-yr period 1980–2019. A strong linkage was found between TC activity and the AMM, IOD, and ENSO over the NIO. In addition, black carbon, organic carbon, sea salt, and sulfate aerosols have a significant impact on the cyclone frequency. Among these factors, black carbon, organic carbon, sea salt, and AMM account for the most variance of TCs, and among the other climate modes, IOD contributes more than ENSO. This is the first attempt to have identified this ranked set of aerosols and climate indices according to their relative ability to impact NIO TCs. Possible linkages between the thermodynamic and dynamic effects of aerosols on the Indian monsoon environment and its modifications to the large-scale environmental parameters relevant to TC development, namely, sea surface temperature, vertical wind shear, relative vorticity, and relative humidity during different phases of the climate modes are discussed. Significance Statement Aerosols and climate modes have enormous impact on tropical cyclones (TCs). In this study, we evaluated the impact of aerosols and climate modes that modulate frequency of TCs over the north Indian Ocean. To assess the impact, a statistical generalized additive model based on Poisson regression was developed. A strong linkage was found between TC activity and Atlantic meridional mode, Indian Ocean dipole, and El Niño–Southern Oscillation, whereas other climate modes have no statistical significance. In addition, black carbon, organic carbon, sea salt, and SO4 aerosols have a strong linkage to cyclone frequency. The study postulates that most positive phases of these climate modes are associated with more TCs, while the negative phases are associated with fewer.

Given the climatic importance of the Madden–Julian oscillation (MJO), this study evaluates the capability of CMIP6 models in simulating MJO eastward propagation in comparison with their CMIP5 counterparts. To understand the representation of MJO simulation in models, a set of diagnostics with respect to MJO-associated dynamic and thermodynamic structures is applied, including large-scale zonal circulation, vertical structures of diabatic heating and equivalent potential temperature, moisture convergence at the planetary boundary layer (PBL), and the east–west asymmetry of moisture tendency relative to the MJO convection. The simulated propagation of the MJO in CMIP6 models shows an overall improvement in realistic speed and longer distance, which displays a robust linear regression relationship against the above-mentioned dynamic and thermodynamic structures. The improved MJO propagation in CMIP6 largely benefits from better representation of premoistening processes that is primarily contributed by improved PBL moisture convergence. In addition, the convergence of moisture and meridional advection of moisture prior to the MJO convection are enhanced in CMIP6, while the zonal advection of moisture is as weak as that in CMIP5. The increased convergence of moisture is a result of enhanced lower-tropospheric moisture and divergence, and the enhanced meridional advection of moisture can be caused by sharpened meridional gradient of mean lower-tropospheric moisture in the western Pacific. Further examination of the lower-tropospheric moisture budget reveals that the enhanced zonal asymmetry of the moisture tendency in CMIP6 is driven by the drying process to the west of the MJO convection, which is attributed to the negative vertical and zonal advections of moisture.

The Madden-Julian Oscillation (MJO) provides an important source of sub-seasonal to seasonal (S2S) predictability. Improved MJO prediction can be beneficial to S2S prediction of global climate and associated weather extremes. In this study, hindcasts based on an atmosphere-ocean coupled general circulation model (CGCM) are compared to those based on atmosphere general circulation models (AGCMs) to investigate influences of air-sea interactions on MJO prediction. Our results suggest that MJO prediction skill can be extended about one week longer in the CGCM hindcasts than AGCM-only experiments, particularly for boreal winter predictions.Further analysis suggests that improved MJO prediction in the CGCM is closely associated with improved representation of moistening processes. Compared to the AGCM experiments, the CGCM better predicts the boundary-layer moisture preconditioning to the east of MJO convection, which is generally considered crucial for triggering MJO deep convection. Meanwhile, the widely extended east-west asymmetric structure in free-tropospheric moisture tendency anomalies relative to the MJO convection center as seen in the observations is also well predicted in the CGCM. Improved prediction of MJO moisture processes in CGCM is closely associated with better representation of the zonal scale of MJO circulation and stronger Kelvin waves to the east of MJO convection, both of which have been recently suggested conductive for MJO eastward propagation. The above improvements by including air-sea coupling could be largely attributed to the realistic MJO-induced SST fluctuations through the convection-SST feedback. This study confirms a critical role of atmosphere-ocean coupling for the improvement of MJO prediction.

Dynamics of the East Asian spring rainband are investigated with a reanalysis dataset and station observations. Here, it is revealed that the rainband is anchored by external forcings. The midtropospheric jet core stays quasi-stationary around Japan. It has two branches in its entry region, which originate from the south and north flanks of the Tibetan Plateau and then run northeastward and southeastward, respectively. The southern branch advects warm air from the Tibetan–Hengduan Plateau northeastward, forming a rainband over southern China through causing adiabatic ascent motion and triggering diabatic feedback. The rainband is much stronger in spring than in autumn due to the stronger diabatic heating over the Tibetan–Hengduan Plateau, a more southward-displaced midtropospheric jet, and the resulting stronger warm advection over southern China. The northern jet branch forms a zonally elongated cold advection belt, which reaches a maximum around northern China, and then weakens and extends eastward to east of Japan. The westerly jet also steers strong disturbance activities roughly collocated with the cold advection belt via baroclinic instability. The high disturbance activities belt causes large cumulative warm advection (CWA) through drastically increasing extremely warm advection days on its eastern and south flanks, where weak cold advection prevails. CWA is more essential for monthly/seasonally rainfall than conventionally used time-average temperature advection because it is shown that strengthened warm advection can increase rainfall through positive diabatic feedback, while cold advection cannot cause negative rainfall. Thus, the rainband is collocated with the large CWA belt instead of the warm advection south of it. This rainband is jointed to the rainband over southern China, forming the long southwest–northeast-oriented East Asian spring rainband. Increasing moisture slightly displaces the rainband southeastward.

Under global warming, surface air temperature has risen rapidly and sea ice has decreased markedly in the Arctic. These drastic climate changes have brought about various severe impacts on the vulnerable environment and ecosystem there. Thus, accurate prediction of Arctic climate becomes more important than before. Here we examine the seasonal to interannual predictive skills of 2-m air temperature (2-m T) and sea ice cover (SIC) over the Arctic region (70°–90°N) during 1980–2014 with a high-resolution global coupled model called the Met Office Decadal Prediction System, version 3 (DePreSys3). The model captures well both the climatology and interannual variability of the Arctic 2-m T and SIC. Moreover, the anomaly correlation coefficient of Arctic-averaged 2-m T and SIC shows statistically significant skills at lead times up to 16 months. This is mainly due to the contribution of strong decadal trends. In addition, it is found that the peak warming trend of Arctic 2-m T lags the maximum decrease trend of SIC by 1 month, in association with the heat flux forcing from the ocean surface to lower atmosphere. While the predictive skill is generally much lower for the detrended variations, we find a close relationship between the tropical Pacific El Niño–Southern Oscillation and the Arctic detrended 2-m T anomalies. This indicates potential seasonal to interannual predictability of the Arctic natural variations.

Central Africa (CA) is identified as a location of a large positive trend of the occurrence of heat waves (HWs) during 1979–2016, appearing to result mostly from a regime shift around the year 2000. Therefore, we study the evolution of synoptic features associated with the occurrence of HW events in CA. It is found that the HW-related circulation is typically characterized by an anomalous convergence in the upper troposphere but there are important differences for HW events occurring in the south region of CA (CA_S) versus the north region (CA_N). For the occurrence of the HW events in CA_S, the anomalous subsidence associated with upper troposphere anomalous convergence is the dominant factor for their occurrence and magnitude: the strong subsidence leads to warming through greater solar insolation. The HW events in CA_S are also accompanied by an anomalous surface anticyclone in the north with anomalous northerly flow transporting heat into the CA_S region. In contrast, although the HW events in CA_N are also associated with upper troposphere anomalous convergence, the intensity of the convergence is weak with small solar insolation. Instead, the anomalous warm advection is the main factor for determining the magnitude of the HW events in CA_N, induced by the prevailing northerly winds acting on the anomalous temperature gradient. Thus, the synoptic features associated with HW events in the CA_N and CA_S are quite different despite their nearby locations. The discovered dominant factors for the HW events in CA can be used to improve the forecast skill.

Because of the seasonal northward migration of the East Asian summer monsoon, the mean-state atmospheric circulation in South China (SC) is remarkably different between the early (May–June) and late (July–August) rainy seasons. This study presents distinct teleconnections between the SC precipitation in the two periods and the sea surface temperatures (SSTs) in the tropical oceans. In the early rainy season when the major monsoon rain belt is located in SC, the increased local precipitation is related to the tropical Indian Ocean Basin warming. The basin warming induces an anomalous anticyclone in the South China Sea–western North Pacific (SCS-WNP). The related southwesterly anomalies transport more moisture to SC and lead to more moisture convergence and precipitation there. In the late rainy season when the major monsoon rain belt migrates northward to the Yangtze River valley, the precipitation increase in SC can be caused by the dipole SST anomalies in the tropical Pacific with the cold anomalies near the Maritime Continent and warm ones near the date line. The dipole SST anomalies generate an anomalous cyclone in the WNP with its center more northward than that of the anomalous anticyclone in the early rainy season. The related northeasterly anomalies along its northwestern flank reduce the climatological northward transport of moisture flux out of SC, and increase the moisture convergence and precipitation there. The distinct teleconnections between the SC precipitation and the tropical SSTs in the early and late rainy seasons can be well reproduced in the sensitivity experiments by an atmospheric general circulation model.

El Niño and La Niña, the warm and cold phases of El Niño–Southern Oscillation (ENSO), cause significant year-to-year disruptions in global climate, including in the atmosphere, oceans, and cryosphere. Australia is one of the countries where its climate, including droughts and flooding rains, is highly sensitive to the temporal and spatial variations of ENSO. The dramatic impacts of ENSO on the environment, society, health, and economies worldwide make the application of reliable ENSO predictions a powerful way to manage risks and resources. An improved understanding of ENSO dynamics in a changing climate has the potential to lead to more accurate and reliable ENSO predictions by facilitating improved forecast systems. This motivated an Australian national workshop on ENSO dynamics and prediction that was held in Sydney, Australia, in November 2017. This workshop followed the aftermath of the 2015/16 extreme El Niño, which exhibited different characteristics to previous extreme El Niños and whose early evolution since 2014 was challenging to predict. This essay summarizes the collective workshop perspective on recent progress and challenges in understanding ENSO dynamics and predictability and improving forecast systems. While this essay discusses key issues from an Australian perspective, many of the same issues are important for other ENSO-affected countries and for the international ENSO research community.

Future changes in the frequency of extreme drought events are of vital importance for risk assessment and relevant policy making. But a reliable estimation of their probability is intrinsically challenging due to limited available observations or simulations. Here, we use two large ensemble simulations, 50 members from CanESM2 and 40 members from CESM1 under the future RCP8.5 scenario, to elaborate a reliable projection of the 100-year drought events (once in a century) under different warming levels. It is however necessary to firstly remove systematic biases for the simulated temperature and precipitation through a bias-correction method based on quantile mapping. Droughts are diagnosed with the Standardized Precipitation Evapotranspiration Index (SPEI), which considers both precipitation and potential evapotranspiration (PET, involving temperature). The results show that the frequency of extreme droughts increases with the continued global warming. Some differences between the two ensembles are also observed, especially for high warming levels. The China-averaged probability of 100-year droughts that occur once in a century in the current climate increases by factors of 1.52 (1.90) and 1.44 (2.02) in the two ensembles under 1.5°C and 2°C warming levels, respectively. A simple statistical scheme shows that the increasing future risk of extreme droughts is mainly due to the increasing effect of PET on the occurrence of extreme drought events, while the effect of precipitation almost keeps constant with global warming.

The impacts of different types of El Niño–Southern Oscillation (ENSO) on the interannual negative correlation (seesaw) between the Somali cross-equatorial flow (CEF) and the Maritime Continent (MC) CEF during boreal summer (June–August) are investigated using the ECMWF twentieth-century reanalysis (ERA-20C) dataset and numerical experiments with a global atmospheric model [the Met Office Unified Model global atmosphere, version 6 (UM-GA6)]. The results suggest that ENSO plays a prominent role in governing the CEF-seesaw relation. A high positive correlation (0.86) exists between the MC CEF and Niño-3.4 index and also in the case of eastern Pacific (EP) El Niño, central Pacific (CP) El Niño, EP La Niña, and CP La Niña events. In contrast, a negative correlation (−0.35) exists between the Somali CEF and Niño-3.4 index, and this negative relation is significant only in the EP El Niño years. Further, the variation of the MC CEF is highly correlated with the local north–south sea surface temperature (SST) gradient, while the variation of the Somali CEF displays little relation with the local SST gradient. The Somali CEF may be remotely influenced by ENSO. The model results confirm that the EP El Niño plays a major role in causing the weakened Somali CEF via modifying the Walker cell. However, the impact of the EP El Niño on the Somali CEF differs with different seasonal background. It is also found that the interannual CEF seesaw displays a multidecadal change before and after the 1950s, which is linked with the multidecadal strengthening of the intensity of the EP ENSO.

The effects of declining anthropogenic aerosols in representative concentration pathway 4.5 (RCP4.5) are assessed in four models from phase 5 the Coupled Model Intercomparison Project (CMIP5), with a focus on annual, zonal-mean atmospheric temperature structure and zonal winds. For each model, the effect of declining aerosols is diagnosed from the difference between a projection forced by RCP4.5 for 2006–2100 and another that has identical forcing, except that anthropogenic aerosols are fixed at early twenty-first-century levels. The response to declining aerosols is interpreted in terms of the meridional structure of aerosol radiative forcing, which peaks near 40°N and vanishes at the South Pole. Increasing greenhouse gases cause amplified warming in the tropical upper troposphere and strengthening midlatitude jets in both hemispheres. However, for declining aerosols the vertically averaged tropospheric temperature response peaks near 40°N, rather than in the tropics. This implies that for declining aerosols the tropospheric meridional temperature gradient generally increases in the Southern Hemisphere (SH), but in the Northern Hemisphere (NH) it decreases in the tropics and subtropics. Consistent with thermal wind balance, the NH jet then strengthens on its poleward side and weakens on its equatorward side, whereas the SH jet strengthens more than the NH jet. The asymmetric response of the jets is thus consistent with the meridional structure of aerosol radiative forcing and the associated tropospheric warming: in the NH the latitude of maximum warming is roughly collocated with the jet, whereas in the SH warming is strongest in the tropics and weakest at high latitudes.

The prediction skill of dynamical downscaling is evaluated for climate forecasts over southern Africa using the Advanced Research Weather Research and Forecasting (WRF) model. As a case study, forecasts for the December–February (DJF) season of 2011/12 are evaluated. Initial and boundary conditions for the WRF model were taken from the seasonal forecasts of the Scale Interaction Experiment-Frontier Research Center for Global Change (SINTEX-F) coupled general circulation model. In addition to sea surface temperature (SST) forecasts generated by nine-member ensemble forecasts of SINTEX-F, the WRF was also configured to use SST generated by a simple mixed layer Price–Weller–Pinkel ocean model coupled to the WRF model. Analysis of the ensemble mean shows that the uncoupled WRF model significantly increases the biases (errors) in precipitation forecasted by SINTEX-F. When coupled to a simple mixed layer ocean model, the WRF model improves the spatial distribution of precipitation over southern Africa through a better representation of the moisture fluxes. Precipitation anomalies forecasted by the coupled WRF are seen to be significantly correlated with the observed precipitation anomalies over South Africa, Zimbabwe, southern Madagascar, and parts of Zambia and Angola. This is in contrast to the SINTEX-F global model precipitation anomaly forecasts that are closer to observations only for parts of Zimbabwe and South Africa. Therefore, the dynamical downscaling with the coupled WRF adds value to the SINTEX-F precipitation forecasts over southern Africa. However, the WRF model yields positive biases (>2°C) in surface air temperature forecasts over the southern African landmass in both the coupled and uncoupled configurations because of biases in the net heat fluxes.

In light of the growing recognition of the role of surface temperature variations in the Indian Ocean for driving global climate variability, the predictive skill of the sea surface temperature (SST) anomalies associated with the Indian Ocean dipole (IOD) is assessed using ensemble seasonal forecasts from a selection of contemporary coupled climate models that are routinely used to make seasonal climate predictions. The authors assess predictions from successive versions of the Australian Bureau of Meteorology Predictive Ocean–Atmosphere Model for Australia (POAMA 15b and 24), successive versions of the NCEP Climate Forecast System (CFSv1 and CFSv2), the ECMWF seasonal forecast System 3 (ECSys3), and the Frontier Research Centre for Global Change system (SINTEX-F) using seasonal hindcasts initialized each month from January 1982 to December 2006. The lead time for skillful prediction of SST in the western Indian Ocean is found to be about 5–6 months while in the eastern Indian Ocean it is only 3–4 months when all start months are considered. For the IOD events, which have maximum amplitude in the September–November (SON) season, skillful prediction is also limited to a lead time of about one season, although skillful prediction of large IOD events can be longer than this, perhaps up to about two seasons. However, the tendency for the models to overpredict the occurrence of large events limits the confidence of the predictions of these large events. Some common model errors, including a poor representation of the relationship between El Niño and the IOD, are identified indicating that the upper limit of predictive skill of the IOD has not been achieved.

Surface air temperature (SAT) over the globe, particularly the Northern Hemisphere continents, has rapidly risen over the last 2–3 decades, leading to an abrupt shift toward a warmer climate state after 1997/98. Whether the terrestrial warming might be caused by local response to increasing greenhouse gas (GHG) concentrations or by sea surface temperature (SST) rise is recently in dispute. The SST warming itself may be driven by both the increasing GHG forcing and slowly varying natural processes. Besides, whether the recent global warming might affect seasonal-to-interannual climate predictability is an important issue to be explored. Based on the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) climate prediction system in which only observed SSTs are assimilated for coupled model initialization, the present study shows that the historical SST rise plays a key role in driving the intensified terrestrial warming over the globe. The SST warming trend, while negligible for short lead predictions, has substantial impact on the climate predictability at long lead times (>1 yr) particularly in the extratropics. The tropical climate predictability, however, is little influenced by global warming. Given a perfect warming trend and/or a perfect model, global SAT and precipitation could be predicted beyond two years in advance with an anomaly correlation skill above ∼0.6. Without assimilating ocean subsurface observations, model initial conditions show a strong spurious cooling drift of subsurface temperature; this is caused by large negative surface heat flux damping arising from the SST-nudging initialization. The spurious subsurface cooling drift acts to weaken the initial SST warming trend during model forecasts, leading to even negative trends of global SAT and precipitation at long lead times and hence deteriorating the global climate predictability. Concerning the important influence of the subsurface temperature on the global SAT trend, future efforts are required to develop a good scheme for assimilating subsurface information particularly in the extratropical oceans.

Climate variability in the tropical Indo-Pacific sector has undergone dramatic changes under global ocean warming. Extreme Indian Ocean dipole (IOD) events occurred repeatedly in recent decades with an unprecedented series of three consecutive episodes during 2006–08, causing vast climate and socioeconomic effects worldwide and weakening the historic El Niño–Indian monsoon relationship. Major attention has been paid to the El Niño influence on the Indian Ocean, but how the IOD influences El Niño and its predictability remained an important issue to be understood. On the basis of various forecast experiments activating and suppressing air–sea coupling in the individual tropical ocean basins using a state-of-the-art coupled ocean–atmosphere model with demonstrated predictive capability, the present study shows that the extreme IOD plays a key role in driving the 1994 pseudo–El Niño, in contrast with traditional El Niño theory. The pseudo–El Niño is more frequently observed in recent decades, coincident with a weakened atmospheric Walker circulation in response to anthropogenic forcing. The study’s results suggest that extreme IOD may significantly enhance El Niño and its onset forecast, which has being a long-standing challenge, and El Niño in turn enhances IOD and its long-range predictability. The intrinsic El Niño–IOD interaction found here provides hope for enhanced prediction skill of both of these climate modes, and it sheds new light on the tropical climate variations and their changes under the influence of global warming.

This report describes observations of precipitation, temperature, and other climatology metrics from different global regions.

In this second part of a two-part paper, the mechanism for the amplitude asymmetry of SST anomalies (SSTA) between positive and negative Indian Ocean dipole (IOD) events is investigated through the diagnosis of coupled model simulations. Same as the observed in Part I, a significant negative skewness appears in the IOD east pole (IODE) in September–November (SON), whereas there is no significant skewness in the IOD west pole (IODW). A sensitivity experiment shows that the negative skewness in IODE appears even in the case when the ENSO is absent. The diagnosis of the model mixed layer heat budget reveals that the negative skewness is primarily induced by the nonlinear ocean temperature advection and the asymmetry of the cloud–radiation–SST feedback, consistent with the observation (Part I). However, the simulated latent heat flux anomaly is greatly underestimated in IODE during the IOD developing stage [June–September (JJAS)]. As a result, the net surface heat flux acts as strong thermal damping. The underestimation of the latent heat flux anomaly in the IODE is probably caused by the westward shift of along-coast wind anomalies off Sumatra.

The decadal variation in the tropical Indian Ocean is investigated using outputs from a 200-yr integration of the Scale Interaction Experiment-Frontier Research Center for Global Change (SINTEX-F1) ocean–atmosphere coupled model. The first EOF mode of the decadal bandpass- (9–35 yr) filtered sea surface temperature anomaly (SSTA) represents a basinwide mode and is closely related with the Pacific ENSO-like decadal variability. The second EOF mode shows a clear east–west SSTA dipole pattern similar to that of the interannual Indian Ocean dipole (IOD) and may be termed the decadal IOD. However, it is demonstrated that the decadal air–sea interaction in the Tropics can be a statistical artifact; it should be interpreted more correctly as decadal modulation of interannual IOD events (i.e., asymmetric or skewed occurrence of positive and negative events). Heat budget analysis has revealed that the occurrence of IOD events is governed by variations in the southward Ekman heat transport across 15°S and variations in the Indonesian Throughflow associated with the ENSO. The variations in the southward Ekman heat transport are related to the Mascarene high activities.

The Indian summer monsoon (ISM) is one of the main components of the Asian summer monsoon. It is well known that one of the starting mechanisms of a summer monsoon is the thermal contrast between land and ocean and that sea surface temperature (SST) and moisture are crucial factors for its evolution and intensity. The Indian Ocean, therefore, may play a very important role in the generation and evolution of the ISM itself. A coupled general circulation model, implemented with a high-resolution atmospheric component, appears to be able to simulate the Indian summer monsoon in a realistic way. In particular, the features of the simulated ISM variability are similar to the observations.In this study, the relationships between the ISM and tropical Indian Ocean (TIO) SST anomalies are investigated, as well as the ability of the coupled model to capture those connections. The recent discovery of the Indian Ocean dipole mode (IODM) may suggest new perspectives in the relationship between ISM and TIO SST. A new statistical technique, the coupled manifold, is used to investigate the TIO SST variability and its relation with the tropical Pacific Ocean (TPO). The analysis shows that the SST variability in the TIO contains a significant portion that is independent from the TPO variability. The same technique is used to estimate the amount of Indian rainfall variability that can be explained by the tropical Indian Ocean SST. Indian Ocean SST anomalies are separated in a part remotely forced from the tropical Pacific Ocean variability and a part independent from that. The relationships between the two SSTA components and the Indian monsoon variability are then investigated in detail.

Using 200 yr of coupled general circulation model (CGCM) results, causes for the termination of Indian Ocean dipole (IOD) events are investigated. The CGCM used here is the Scale Interaction Experiment-Frontier Research Center for Global Change (SINTEX-F1) model, which consists of a version of the European Community–Hamburg (ECHAM4.6) atmospheric model and a version of the Ocean Parallelise (OPA8.2) ocean general circulation model. This model reproduces reasonably well the present-day climatology and interannual signals of the Indian and Pacific Oceans. The main characteristics of the intraseasonal disturbances (ISDs)/oscillations are also fairly well captured by this model. However, the eastward propagation of ISDs in the model is relatively fast in the Indian Ocean and stationary in the Pacific compared to observations. A sudden reversal of equatorial zonal winds is observed, as a result of significant intraseasonal disturbances in the equatorial Indian Ocean in November–December of IOD events, which evolve independently of ENSO. A majority of these IOD events (15 out of 18) are terminated mainly because of the 20–40-day ISD activity in the equatorial zonal winds. Ocean heat budget analysis in the upper 50 m clearly shows that the initial warming after the peak of the IOD phenomenon is triggered by increased solar radiation owing to clear-sky conditions in the eastern Indian Ocean. Subsequently, the equatorial jets excited by the ISD deepen the thermocline in the southeastern equatorial Indian Ocean. This deepening of the thermocline inhibits the vertical entrainment of cool waters and therefore the IOD is terminated. IOD events that co-occur with ENSO are terminated owing to anomalous incoming solar radiation as a result of prevailing cloud-free skies. Further warming occurs seasonally through the vertical convergence of heat due to a monsoonal wind reversal along Sumatra–Java. On occasion, strong ISD activities in July–August terminated short-lived IOD events by triggering downwelling intraseasonal equatorial Kelvin waves.

An atmosphere–ocean coupled general circulation model known as the Scale Interaction Experiment Frontier version 1 (SINTEX-F1) model is used to understand the intrinsic variability of the Indian Ocean dipole (IOD). In addition to a globally coupled control experiment, a Pacific decoupled noENSO experiment has been conducted. In the latter, the El Niño–Southern Oscillation (ENSO) variability is suppressed by decoupling the tropical Pacific Ocean from the atmosphere. The ocean–atmosphere conditions related to the IOD are realistically simulated by both experiments including the characteristic east–west dipole in SST anomalies. This demonstrates that the dipole mode in the Indian Ocean is mainly determined by intrinsic processes within the basin. In the EOF analysis of SST anomalies from the noENSO experiment, the IOD takes the dominant seat instead of the basinwide monopole mode. Even the coupled feedback among anomalies of upper-ocean heat content, SST, wind, and Walker circulation over the Indian Ocean is reproduced.As in the observation, IOD peaks in boreal fall for both model experiments. In the absence of ENSO variability the interannual IOD variability is dominantly biennial. The ENSO variability is found to affect the periodicity, strength, and formation processes of the IOD in years of co-occurrences. The amplitudes of SST anomalies in the western pole of co-occurring IODs are aided by dynamical and thermodynamical modifications related to the ENSO-induced wind variability. Anomalous latent heat flux and vertical heat convergence associated with the modified Walker circulation contribute to the alteration of western anomalies. It is found that 42% of IOD events affected by changes in the Walker circulation are related to the tropical Pacific variabilities including ENSO. The formation is delayed until boreal summer for those IODs, which otherwise form in boreal spring as in the noENSO experiment.

The cold tongue in the tropical Pacific extends too far west in most current ocean–atmosphere coupled GCMs (CGCMs). This bias also exists in the relatively high-resolution SINTEX-F CGCM despite its remarkable performance of simulating ENSO variations. In terms of the importance of air–sea interactions to the climatology formation in the tropical Pacific, several sensitivity experiments with improved coupling physics have been performed in order to reduce the cold-tongue bias in CGCMs. By allowing for momentum transfer of the ocean surface current to the atmosphere [full coupled simulation (FCPL)] or merely reducing the wind stress by taking the surface current into account in the bulk formula [semicoupled simulation (semi-CPL)], the warm-pool/cold-tongue structure in the equatorial Pacific is simulated better than that of the control simulation (CTL) in which the movement of the ocean surface is ignored for wind stress calculation. The reduced surface zonal current and vertical entrainment owing to the reduced easterly wind stress tend to produce a warmer sea surface temperature (SST) in the western equatorial Pacific. Consequently, the dry bias there is much reduced. The warming tendency of the SST in the eastern Pacific, however, is largely suppressed by isopycnal diffusion and meridional advection of colder SST from south of the equator due to enhanced coastal upwelling near Peru. The ENSO signal in the western Pacific and its global teleconnection in the North Pacific are simulated more realistically. The approach as adopted in the FCPL run is able to generate a correct zonal SST slope and efficiently reduce the cold-tongue bias in the equatorial Pacific. The surface easterly wind itself in the FCPL run is weakened, reducing the easterly wind stress further. This is related with a weakened zonal Walker cell in the atmospheric boundary layer over the eastern Pacific and a new global angular momentum balance of the atmosphere associated with reduced westerly wind stress over the southern oceans.

In 2013, the vast majority of the monitored climate variables reported here maintained trends established in recent decades. ENSO was in a neutral state during the entire year, remaining mostly on the cool side of neutral with modest impacts on regional weather patterns around the world. This follows several years dominated by the effects of either La Niña or El Niño events. According to several independent analyses, 2013 was again among the 10 warmest years on record at the global scale, both at the Earths surface and through the troposphere. Some regions in the Southern Hemisphere had record or near-record high temperatures for the year. Australia observed its hottest year on record, while Argentina and New Zealand reported their second and third hottest years, respectively. In Antarctica, Amundsen-Scott South Pole Station reported its highest annual temperature since records began in 1957. At the opposite pole, the Arctic observed its seventh warmest year since records began in the early 20th century. At 20-m depth, record high temperatures were measured at some permafrost stations on the North Slope of Alaska and in the Brooks Range. In the Northern Hemisphere extratropics, anomalous meridional atmospheric circulation occurred throughout much of the year, leading to marked regional extremes of both temperature and precipitation. Cold temperature anomalies during winter across Eurasia were followed by warm spring temperature anomalies, which were linked to a new record low Eurasian snow cover extent in May. Minimum sea ice extent in the Arctic was the sixth lowest since satellite observations began in 1979. Including 2013, all seven lowest extents on record have occurred in the past seven years. Antarctica, on the other hand, had above-average sea ice extent throughout 2013, with 116 days of new daily high extent records, including a new daily maximum sea ice area of 19.57 million km2 reached on 1 October. ENSO-neutral conditions in the eastern central Pacific Ocean and a negative Pacific decadal oscillation pattern in the North Pacific had the largest impacts on the global sea surface temperature in 2013. The North Pacific reached a historic high temperature in 2013 and on balance the globally-averaged sea surface temperature was among the 10 highest on record. Overall, the salt content in nearsurface ocean waters increased while in intermediate waters it decreased. Global mean sea level continued to rise during 2013, on pace with a trend of 3.2 mm yr-1 over the past two decades. A portion of this trend (0.5 mm yr-1) has been attributed to natural variability associated with the Pacific decadal oscillation as well as to ongoing contributions from the melting of glaciers and ice sheets and ocean warming. Global tropical cyclone frequency during 2013 was slightly above average with a total of 94 storms, although the North Atlantic Basin had its quietest hurricane season since 1994. In the Western North Pacific Basin, Super Typhoon Haiyan, the deadliest tropical cyclone of 2013, had 1-minute sustained winds estimated to be 170 kt (87.5 m s-1) on 7 November, the highest wind speed ever assigned to a tropical cyclone. High storm surge was also associated with Haiyan as it made landfall over the central Philippines, an area where sea level is currently at historic highs, increasing by 200 mm since 1970. In the atmosphere, carbon dioxide, methane, and nitrous oxide all continued to increase in 2013. As in previous years, each of these major greenhouse gases once again reached historic high concentrations. In the Arctic, carbon dioxide and methane increased at the same rate as the global increase. These increases are likely due to export from lower latitudes rather than a consequence of increases in Arctic sources, such as thawing permafrost. At Mauna Loa, Hawaii, for the first time since measurements began in 1958, the daily average mixing ratio of carbon dioxide exceeded 400 ppm on 9 May. The state of these variables, along with dozens of others, and the 2013 climate conditions of regions around the world are discussed in further detail in this 24th edition of the State of the Climate series. © 2014, American Meteorological Society. All rights reserved.

This paper describes the mean ocean circulation and the tropical variability simulated by the Max Planck Institute for Meteorology (MPI-M) coupled atmosphere–ocean general circulation model (AOGCM). Results are presented from a version of the coupled model that served as a prototype for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) simulations. The model does not require flux adjustment to maintain a stable climate. A control simulation with present-day greenhouse gases is analyzed, and the simulation of key oceanic features, such as sea surface temperatures (SSTs), large-scale circulation, meridional heat and freshwater transports, and sea ice are compared with observations.A parameterization that accounts for the effect of ocean currents on surface wind stress is implemented in the model. The largest impact of this parameterization is in the tropical Pacific, where the mean state is significantly improved: the strength of the trade winds and the associated equatorial upwelling weaken, and there is a reduction of the model’s equatorial cold SST bias by more than 1 K. Equatorial SST variability also becomes more realistic. The strength of the variability is reduced by about 30% in the eastern equatorial Pacific and the extension of SST variability into the warm pool is significantly reduced. The dominant El Niño–Southern Oscillation (ENSO) period shifts from 3 to 4 yr. Without the parameterization an unrealistically strong westward propagation of SST anomalies is simulated. The reasons for the changes in variability are linked to changes in both the mean state and to a reduction in atmospheric sensitivity to SST changes and oceanic sensitivity to wind anomalies.