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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 US coastal Northeast and mid-Atlantic region is a densely populated urban corridor that is susceptible to a variety of natural hazards, including hurricanes, nor'easters, and flooding. Climate change impacts, such as rising sea levels, further exacerbate the vulnerability of this region. Given the region's exposure to natural hazards and its importance as a population core, it is imperative to assess the vulnerability of the population that resides here. This paper examines the human vulnerability of 184 counties based on demographic, social and economic factors. Data from the American Community Survey (2005 to 2009) is used for analysis. Results indicate that 7 of the 15 most vulnerable counties have population sizes of more than half a million each. Most counties at the bottom end of vulnerability spectrum have low populations. The most vulnerable counties are located along the Atlantic coast. Economic and social conditions correlate more highly with vulnerability than does demographics. [ABSTRACT FROM AUTHOR]

Abstract: Although considerable progress towards reducing tropical climate biases in the tropical Pacific has been made in many current-generation of climate models over the past decades, reducing large biases and maintaining good agreement with the observations in the tropical Atlantic is still a major challenge and this deficiency may seriously degrade the credibility of the models in their simulation and projection of future climate change in the Atlantic sector. In this paper, we show that the bias in the eastern equatorial Atlantic has a major effect on sea-surface temperature (SST) response to a rapid change in the Atlantic Meridional Overturning Circulation (AMOC). By comparing identical water hosing experiments conducted with two different coupled general circulation models, we dissect oceanic mechanisms underlying the difference in models'' SST response. The results show that the different SST response is plausibly attributed to systematic differences in the simulated tropical Atlantic ocean circulation. Therefore, in order to accurately simulate past abrupt climate changes and project future changes, the bias in climate models must be reduced. [Copyright &y& Elsevier]