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Many coupled general circulation models (CGCMs) suffer from serious model bias in the zonal gradient of sea surface temperature (SST) in the equatorial Atlantic. The bias of the equatorial Atlantic SST (EASST) may affect the interannual variability of the equatorial Atlantic, which in turn may influence the state of the tropical Pacific. In this paper we investigate the impact of the bias and the interannual variability of the EASST on the tropical Pacific in a CGCM. To determine the impact of the interannual variability of the EASST on the tropical Pacific, we compare a run in a fully coupled mode (CTL run) and a run in which the EASST is nudged toward the climatological monthly mean of the SST in the CTL run, but full air-sea coupling is allowed elsewhere (AT_m run). We find that, when the interannual variability of the EASST is excluded, the thermocline depth in the eastern equatorial Pacific is deepened, and the amplitude of the El Niño/Southern Oscillation is reduced by 30 % compared to the CTL run. The impact of the bias of the EASST on the tropical Pacific is investigated by comparing the AT_m run and a run in which the EASST is nudged toward the observed climatological monthly mean SST (AT_o run). It is found that, when the bias of the EASST is removed (i.e. AT_o run), the Gill-Matsuno type response to the warm SST anomalies in the western equatorial Atlantic induces low-level cyclonic anomalies in the eastern South Pacific, which leads to a deeper thermocline and colder SST in the South Pacific as compared to AT_m. The colder SST in the South Pacific reduces the precipitation along the South Pacific convergence zone. Our results of the model experiments demonstrate the importance of the EASST to the tropical Pacific climate. [ABSTRACT FROM AUTHOR]

This paper presents an evaluation of the multi-model simulations for the 4th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) in terms of their ability to simulate the ERA40 circulation types over the Euro-Atlantic region in winter season. Two classification schemes, k-means and SANDRA, have been considered to test the sensitivity of the evaluation results to the classification procedure. The assessment allows establishing different rankings attending spatial and temporal features of the circulation types. Regarding temporal characteristics, in general, all AR4 models tend to underestimate the frequency of occurrence. The best model simulating spatial characteristics is the UKMO-HadGEM1 whereas CCSM3, UKMO-HadGEM1 and CGCM3.1(T63) are the best simulating the temporal features, for both classification schemes. This result agrees with the AR4 models ranking obtained when having analysed the ability of the same AR4 models to simulate Euro-Atlantic variability modes. This study has proved the utility of applying such a synoptic climatology approach as a diagnostic tool for models' assessment. The ability of the models to properly reproduce the position of ridges and troughs and the frequency of synoptic patterns, will therefore improve our confidence in the response of models to future climate changes. [ABSTRACT FROM AUTHOR]

The main source of sea surface temperature (SST) variability in the Tropical Atlantic at interannual time scales is the Equatorial Mode or Atlantic El Niño. It has been shown to affect the adjacent continents and also remote regions, leading to a weakened Indian Monsoon and promoting La Niña-type anomalies over the Pacific. However, its effects in a warmer climate are unknown. This work analyses the impact of the Equatorial Mode at the end of the twenty first century by means of sensitivity experiments with an atmosphere general circulation model. The prescribed boundary conditions for the future climate are based on the outputs from models participating in the coupled model intercomparison project-phase V. Our results suggest that even if the characteristics of the Equatorial Mode at the end of the twenty first century remained equal to those of the twentieth century, there will be an eastward shift of the main rainfall positive anomalies in the Tropical Atlantic and a weakening of the negative rainfall anomalies over the Asian monsoon due to the change in climatological SSTs. We also show that extratropical surface temperature anomalies over land related to the mode will change in regions like Southwestern Europe, East Australia, Asia or North America due to the eastward shift of the sea level pressure systems and related surface winds. [ABSTRACT FROM AUTHOR]

Sea surface temperatures (SSTs) in the eastern tropical Atlantic are crucial for climate variability within the tropical belt. Despite this importance, state-of-the-art climate models show a large SST warm bias in this region. Knowledge about the seasonal mixed layer (ML) heat budget is a prerequisite for understanding SST mean state and its variability. Within this study all contributions to the seasonal ML heat budget are estimated at four locations within the Atlantic cold tongue (ACT) that are representative for the western (0°N, 23°W), central (0°N, 10°W) and eastern (0°N, 0°E) equatorial as well as the southern (10°S, 10°W) ACT. To estimate the contribution of the diapycnal heat flux due to turbulence an extensive data set of microstructure observations collected during ten research cruises between 2005 and 2012 is analyzed. The results for the equatorial ACT indicate that with the inclusion of the diapycnal heat flux the seasonal ML heat budget is balanced. Within the equatorial region, the diapycnal heat flux is essential for the development of the ACT. It dominates over all other cooling terms in the central and eastern equatorial ACT, while it is of similar size as the zonal advection in the western equatorial ACT. In contrast, the SST evolution in the southern ACT region can be explained entirely by air-sea heat fluxes. [ABSTRACT FROM AUTHOR]

One of the main sources of uncertainty in estimating climate projections affected by global warming is the choice of the global climate model (GCM). The aim of this study is to evaluate the skill of GCMs from CMIP3 and CMIP5 databases in the north-east Atlantic Ocean region. It is well known that the seasonal and interannual variability of surface inland variables (e.g. precipitation and snow) and ocean variables (e.g. wave height and storm surge) are linked to the atmospheric circulation patterns. Thus, an automatic synoptic classification, based on weather types, has been used to assess whether GCMs are able to reproduce spatial patterns and climate variability. Three important factors have been analyzed: the skill of GCMs to reproduce the synoptic situations, the skill of GCMs to reproduce the historical inter-annual variability and the consistency of GCMs experiments during twenty-first century projections. The results of this analysis indicate that the most skilled GCMs in the study region are UKMO-HadGEM2, ECHAM5/MPI-OM and MIROC3.2(hires) for CMIP3 scenarios and ACCESS1.0, EC-EARTH, HadGEM2-CC, HadGEM2-ES and CMCC-CM for CMIP5 scenarios. These models are therefore recommended for the estimation of future regional multi-model projections of surface variables driven by the atmospheric circulation in the north-east Atlantic Ocean region. [ABSTRACT FROM AUTHOR]

A multi-model analysis of Atlantic multidecadal variability is performed with the following aims: to investigate the similarities to observations; to assess the strength and relative importance of the different elements of the mechanism proposed by Delworth et al. (J Clim 6:1993-2011, 1993) (hereafter D93) among coupled general circulation models (CGCMs); and to relate model differences to mean systematic error. The analysis is performed with long control simulations from ten CGCMs, with lengths ranging between 500 and 3600 years. In most models the variations of sea surface temperature (SST) averaged over North Atlantic show considerable power on multidecadal time scales, but with different periodicity. The SST variations are largest in the mid-latitude region, consistent with the short instrumental record. Despite large differences in model configurations, we find quite some consistency among the models in terms of processes. In eight of the ten models the mid-latitude SST variations are significantly correlated with fluctuations in the Atlantic meridional overturning circulation (AMOC), suggesting a link to northward heat transport changes. Consistent with this link, the three models with the weakest AMOC have the largest cold SST bias in the North Atlantic. There is no linear relationship on decadal timescales between AMOC and North Atlantic Oscillation in the models. Analysis of the key elements of the D93 mechanisms revealed the following: Most models present strong evidence that high-latitude winter mixing precede AMOC changes. However, the regions of wintertime convection differ among models. In most models salinity-induced density anomalies in the convective region tend to lead AMOC, while temperature-induced density anomalies lead AMOC only in one model. However, analysis shows that salinity may play an overly important role in most models, because of cold temperature biases in their relevant convective regions. In most models subpolar gyre variations tend to lead AMOC changes, and this relation is strong in more than half of the models. [ABSTRACT FROM AUTHOR]

The Bay of Biscay, located in the Northeast Atlantic Ocean, is exposed to energetic waves coming from the open ocean that have crucial effects on the coast. Knowledge of the wave climate and trends in this region are critical to better understand the last decade's evolution of coastal hazards and morphology and to anticipate their potential future changes. This study aims to characterize the long-term trends of the present wave climate over the second half of the twentieth century in the Bay of Biscay through a robust and homogeneous intercomparison of five-wave datasets (Corrected ERA-40 (C-ERA-40), ECMWF Reanalysis Interim (ERA-Interim), Bay Of Biscay Wave Atlas (BOBWA-10kH), ANEMOC, and Bertin and Dodet )). The comparison of the quality of the datasets against offshore and nearshore measurements reveals that at offshore locations, global reanalyses slightly underestimate wave heights, while regional hindcasts overestimate wave heights, especially for the highest quantiles. At coastal locations, BOBWA-10kH is the dataset that compares the best with observations. Concerning long time-scale features, the comparison highlights that the main significant trends are similarly present in the five datasets, especially during summer for which there is an increase of significant wave heights and mean wave periods (up to +15 cm and +0.6 s over the period 1970-2001) as well as a southerly shift of wave directions (around −0.4° year). Over the same period, an increase of high quantiles of wave heights during the autumn season (around 3 cm year for 90th quantile of significant wave heights (SWH)) is also apparent. During winter, significant trends are much lower than during summer and autumn despite a slight increase of wave heights and periods during 1958-2001. These trends can be related to modifications in the wave-type occurrence. Finally, the trends common to the five datasets are discussed by analyzing the similarities with centennial trends issued from longer time-scale studies and exploring the various factors that could explain them. [ABSTRACT FROM AUTHOR]

There is evidence that the observed changes in winter North Atlantic Oscillation (NAO) drive a significant portion of Atlantic Multi Decadal Variability (AMV). However, whether the observed decadal NAO changes can be forced by the ocean is controversial. There is also evidence that artificially imposed multi-decadal stratospheric changes can impact the troposphere in winter. But the origins of such stratospheric changes are still unclear, especially in early to mid winter, where the radiative ozone-impact is negligible. Here we show, through observational analysis and atmospheric model experiments, that large-scale Atlantic warming associated with AMV drives high-latitude precursory stratospheric warming in early to mid winter that propagates downward resulting in a negative tropospheric NAO in late winter. The mechanism involves stratosphere/troposphere dynamical coupling, and can be simulated to a large extent, but only with a stratosphere resolving model (i.e., high-top). Further analysis shows that this precursory stratospheric response can be explained by the shift of the daily extremes toward more major stratospheric warming events. This shift cannot be simulated with the atmospheric (low-top) model configuration that poorly resolves the stratosphere and implements a sponge layer in upper model levels. While the potential role of the stratosphere in multi-decadal NAO and Atlantic meridional overturning circulation changes has been recognised, our results show that the stratosphere is an essential element of extra-tropical atmospheric response to ocean variability. Our findings suggest that the use of stratosphere resolving models should improve the simulation, prediction, and projection of extra-tropical climate, and lead to a better understanding of natural and anthropogenic climate change. [ABSTRACT FROM AUTHOR]

Existing multi-proxy climate reconstruction methods assume the suitably transformed proxy time series are linearly related to the target climate variable, which is likely a simplifying assumption for many proxy records. Furthermore, with a single exception, these methods face problems with varying temporal resolutions of the proxy data. Here we introduce a new reconstruction method that uses the ordering of all pairs of proxy observations within each record to arrive at a consensus time series that best agrees with all proxy records. The resulting unitless composite time series is subsequently calibrated to the instrumental record to provide an estimate of past climate. By considering only pairwise comparisons, this method, which we call PaiCo, facilitates the inclusion of records with differing temporal resolutions, and relaxes the assumption of linearity to the more general assumption of a monotonically increasing relationship between each proxy series and the target climate variable. We apply PaiCo to a newly assembled collection of high-quality proxy data to reconstruct the mean temperature of the Northernmost Atlantic region, which we call Arctic Atlantic, over the last 2,000 years. The Arctic Atlantic is a dynamically important region known to feature substantial temperature variability over recent millennia, and PaiCo allows for a more thorough investigation of the Arctic Atlantic regional climate as we include a diverse array of terrestrial and marine proxies with annual to multidecadal temporal resolutions. Comparisons of the PaiCo reconstruction to recent reconstructions covering larger areas indicate greater climatic variability in the Arctic Atlantic than for the Arctic as a whole. The Arctic Atlantic reconstruction features temperatures during the Roman Warm Period and Medieval Climate Anomaly that are comparable or even warmer than those of the twentieth century, and coldest temperatures in the middle of the nineteenth century, just prior to the onset of the recent warming trend. [ABSTRACT FROM AUTHOR]

The effects of diking on the photosynthetic rate, gross glucose production, biomass, and nutrient levels of the black mangrove Avicennia germinans L., were investigated during the major dry, the major wet and the intermediate season in completely diked mangrove areas, and compared to partially diked and non-diked biotopes. In the major dry and major wet seasons, the photosynthetic rate and gross glucose production were two to five times lower in the completely diked sites compared to the partially diked and non-diked habitats. The average dry weights of branches, flowers and leaves did not differ significantly between sites. The same held true for propagules from the major dry and major wet season. However, propagules from the transition period collected in the non-diked and partially diked biotopes yielded appreciable amounts of dry matter, while plants in the completely diked forests did not carry any propagules. No statistically significant differences were found among nutrient content in the leaves of plants from the different habitats. Our results suggest black mangrove growing in diked habitats may suffer a certain degree of environmental stress, which may affect important biological processes, such as photosynthesis, gross glucose production, and propagule formation. [ABSTRACT FROM AUTHOR]

The influence of changes in surface wind-stress on the properties (amplitude and period) and domain of existence of thermohaline millennial oscillations is studied by means of a coupled model of intermediate complexity set up in an idealized spherical sector geometry of the Atlantic basin. Using the atmospheric CO concentration as the control parameter, bifurcation diagrams of the model are built to show that the influence of wind-stress changes on glacial abrupt variability is threefold. First, millennial-scale oscillations are significantly amplified through wind-feedback-induced changes in both northern sea ice export and oceanic heat transport. Changes in surface wind-stress more than double the amplitude of the strong warming events that punctuate glacial abrupt variability obtained under prescribed winds in the model. Second, the average duration of both stadials and interstadials is significantly lengthened and the temporal structure of observed variability is better captured under interactive winds. Third, the generation of millennial-scale oscillations is shown to occur for significantly colder climates when wind-stress feedback is enabled. This behaviour results from the strengthening of the negative temperature-advection feedback associated with stronger northward oceanic heat transport under interactive winds. [ABSTRACT FROM AUTHOR]

Atlantic tropical cyclone activity, as measured by annual storm counts, reached anomalous levels over the past decade. The short nature of the historical record and potential issues with its reliability in earlier decades, however, has prompted an ongoing debate regarding the reality and significance of the recent rise. Here we place recent activity in a longer-term context by comparing two independent estimates of tropical cyclone activity over the past 1,500 years. The first estimate is based on a composite of regional sedimentary evidence of landfalling hurricanes, while the second estimate uses a previously published statistical model of Atlantic tropical cyclone activity driven by proxy reconstructions of past climate changes. Both approaches yield consistent evidence of a peak in Atlantic tropical cyclone activity during medieval times (around ad 1000) followed by a subsequent lull in activity. The statistical model indicates that the medieval peak, which rivals or even exceeds (within uncertainties) recent levels of activity, results from the reinforcing effects of La-Niña-like climate conditions and relative tropical Atlantic warmth. [ABSTRACT FROM AUTHOR]