The western Pacific subtropical high (WPSH) is closely related to Asian climate. Previous examination of changes in the WPSH found a westward extension since the late 1970s, which has contributed to the interdecadal transition of East Asian climate. The reason for the westward extension is unknown, however. The present study suggests that this significant change of WPSH is partly due to the atmosphere's response to the observed Indian Ocean-western Pacific (IWP) warming. Coordinated by a European Union's Sixth Framework Programme, Understanding the Dynamics of the Coupled Climate System (DYNAMITE), five AGCMs were forced by identical idealized sea surface temperature patterns representative of the IWP warming and cooling. The results of these numerical experiments suggest that the negative heating in the central and eastern tropical Pacific and increased convective heating in the equatorial Indian Ocean/Maritime Continent associated with IWP warming are in favor of the westward extension of WPSH. The SST changes in IWP influences the Walker circulation, with a subsequent reduction of convections in the tropical central and eastern Pacific, which then forces an ENSO/Gill-type response that modulates the WPSH. The monsoon diabatic heating mechanism proposed by Rodwell and Hoskins plays a secondary reinforcing role in the westward extension of WPSH. The low-level equatorial flank of WPSH is interpreted as a Kelvin response to monsoon condensational heating, while the intensified poleward flow along the western flank of WPSH is in accord with Sverdrup vorticity balance. The IWP warming has led to an expansion of the South Asian high in the upper troposphere, as seen in the reanalysis.

The influences of a substantial weakening of the Atlantic meridional overturning circulation (AMOC) on the tropical Pacific climate mean state, the annual cycle, and ENSO variability are studied using five different coupled general circulation models (CGCMs). In the CGCMs, a substantial weakening of the AMOC is induced by adding freshwater flux forcing in the northern North Atlantic. In response, the well-known surface temperature dipole in the low-latitude Atlantic is established, which reorganizes the large-scale tropical atmospheric circulation by increasing the northeasterly trade winds. This leads to a southward shift of the intertropical convergence zone (ITCZ) in the tropical Atlantic and also the eastern tropical Pacific. Because of evaporative fluxes, mixing, and changes in Ekman divergence, a meridional temperature anomaly is generated in the northeastern tropical Pacific, which leads to the development of a meridionally symmetric thermal background state. In four out of five CGCMs this leads to a substantial weakening of the annual cycle in the eastern equatorial Pacific and a subsequent intensification of ENSO variability due to nonlinear interactions. In one of the CGCM simulations, an ENSO intensification occurs as a result of a zonal mean thermocline shoaling.Analysis suggests that the atmospheric circulation changes forced by tropical Atlantic SSTs can easily influence the large-scale atmospheric circulation and hence tropical eastern Pacific climate. Furthermore, it is concluded that the existence of the present-day tropical Pacific cold tongue complex and the annual cycle in the eastern equatorial Pacific are partly controlled by the strength of the AMOC. The results may have important implications for the interpretation of global multidecadal variability and paleo-proxy data.

This study presents an overview of the El Niñ o-Southern Oscillation (ENSO) phenomenon and Pacific decadal variability (PDV) simulated in a multicentury preindustrial control integration of the NCAR Community Climate System Model version 4 (CCSM4) at nominal 18 latitude-longitude resolution. Several aspects of ENSO are improved in CCSM4 compared to its predecessor CCSM3, including the lengthened period (3-6 yr), the larger range of amplitude and frequency of events, and the longer duration of La Niñ a compared to El Niñ o. However, the overall magnitude of ENSO in CCSM4 is overestimated by ;30%. The simulated ENSO exhibits characteristics consistent with the delayed/recharge oscillator paradigm, including correspondence between the lengthened period and increased latitudinal width of the anomalous equatorial zonal wind stress. Global seasonal atmospheric teleconnections with accompanying impacts on precipitation and temperature are generally well simulated, although the wintertime deepening of the Aleutian low erroneously persists into spring. The vertical structure of the upper-ocean temperature response to ENSO in the north and south Pacific displays a realistic seasonal evolution, with notable asymmetries between warm and cold events. The model shows evidence of atmospheric circulation precursors over the North Pacific associated with the ''seasonal footprinting mechanism,'' similar to observations. Simulated PDV exhibits a significant spectral peak around 15 yr, with generally realistic spatial pattern and magnitude. However, PDV linkages between the tropics and extratropics are weaker than observed.

El Niño and La Niña are not a simple mirror image, but exhibit significant differences in their spatial structure and seasonal evolution. In particular, sea surface temperature (SST) anomalies over the equatorial Pacific cold tongue are larger in magnitude during El Niño compared to La Niña, resulting in positive skewness of interannual SST variations. The associated atmospheric deep convection anomalies are displaced eastward during El Niño compared to La Niña because of the nonlinear atmospheric response to SST. In addition to these well-known features, an analysis of observational data for the past century shows that there is a robust asymmetry in the duration of El Niño and La Niña. Most El Niños and La Niñas develop in late boreal spring/summer, when the climatological cold tongue is intensifying, and they peak near the end of the calendar year. After the mature phase, El Niños tend to decay rapidly by next summer, but many La Niñas persist through the following year and often reintensify in the subsequent winter. Throughout the analysis period, this asymmetric feature is evident for strong events in which Niño-3.4 SST anomalies exceed one standard deviation in December. Seasonally stratified composite analysis suggests that the eastward displacement of atmospheric deep convection anomalies during El Niño enables surface winds in the western equatorial Pacific to be more affected by remote forcing from the Indian Ocean, which acts to terminate the Pacific events.

Global mean sea surface temperature (SST) has risen steadily over the past century, but the overall pattern contains extensive and often uncertain spatial variations, with potentially important effects on regional precipitation. Observations suggest a slowdown of the zonal atmospheric overturning circulation above the tropical Pacific Ocean (the Walker circulation) over the twentieth century. Although this change has been attributed to a muted hydrological cycle forced by global warming, the effect of SST warming patterns has not been explored and quantified. Here we perform experiments using an atmospheric model, and find that SST warming patterns are the main cause of the weakened Walker circulation over the past six decades (1950-2009). The SST trend reconstructed from bucket-sampled SST and night-time marine surface air temperature features a reduced zonal gradient in the tropical Indo-Pacific Ocean, a change consistent with subsurface temperature observations. Model experiments with this trend pattern robustly simulate the observed changes, including the Walker circulation slowdown and the eastward shift of atmospheric convection from the Indonesian maritime continent to the central tropical Pacific. Our results cannot establish whether the observed changes are due to natural variability or anthropogenic global warming, but they do show that the observed slowdown in the Walker circulation is presumably driven by oceanic rather than atmospheric processes.

We synthesize variability and trends in multiple analyses of Antarctic near-surface temperature representing several independent source datasets and spatially complete reconstructions, and place these into the broader context of the behavior of other components of the climate system during the past 30-50 years. Along with an annual-mean trend during the past 50 years of about 0.1°C/decade averaged over Antarctica, there is a distinct seasonality to the trends, with insignificant change (and even some cooling) in austral summer and autumn in East Antarctica, contrasting with warming in austral winter and spring. Apart from the Peninsula, the seasonal warming is largest and most significant in West Antarctica in the austral spring since the late 1970s. Concurrent trends in sea ice are independent evidence of the observed warming over West Antarctic, with the decrease in sea ice area in the Amundsen and Bellingshausen Seas congruent with at least 50% of the inland warming of West Antarctica. Trends in near surface winds and geopotential heights over the high-latitude South Pacific are consistent with a role for atmospheric forcing of the sea ice and air temperature anomalies. Most of the circulation trend projects onto the two Pacific South American (PSA) modes of atmospheric circulation variability, while the Southern Annular Mode lacks a positive trend in spring that would otherwise cause a cooling tendency. The largest circulation trend is associated with the PSA-1 mode, a wave-train extending from the tropics to the high Southern latitudes. The PSA-1 mode is significantly correlated with SSTs in the southwestern tropical and subtropical Pacific. The increased SSTs in this region, together with the observed increase in rainfall, suggest that anomalous deep convection has strengthened or increased the occurrence of the Rossby wave-train associated with PSA-1. This hypothesis is supported by results from two ensembles of SST-forced atmospheric general circulation model simulations. Finally, the implications of the seasonality, timing, and spatial patterns of Antarctic temperature trends with respect to interpreting the relative roles of stratospheric ozone depletion, SSTs and increased atmospheric concentrations of greenhouse gasses are discussed.

The seasonal cycle of equatorial Atlantic sea surface temperature (SST) is characterized by a rapid cooling from April to July, coinciding with the onset of the West African summer monsoon and followed by a slow warming that lasts 3 times longer. Two ensemble simulations are carried out with an atmospheric general circulation model to investigate the mechanisms for the wind changes that cause this rapid oceanic cooling and its feedback onto the African monsoon. In the control simulation, SST is globally prescribed in its full climatological seasonal cycle, while in the second simulation, equatorial Atlantic SST is held constant in time from 15 April onward.Comparison of these simulations indicates that the equatorial cooling exerts a significant influence on the African monsoon, intensifying the southerly winds in the Gulf of Guinea and pushing the continental rainband inland away from the Guinean coast. The intensification of the cross-equatorial southerlies associated with the onset of the African monsoon, in turn, triggers the oceanic cooling in the east. The equatorial easterlies are also important for the seasonal cooling by inducing local upwelling and raising the thermocline in the east.Three mechanisms are identified for the easterly wind acceleration in the equatorial Atlantic in boreal summer. First, the monsoon rainfall distribution is such that it induces zonal sea level pressure gradients and easterly anomalies in the eastern Atlantic. Second, the strong cross-equatorial southerlies advect the easterly momentum from the south into the equator. Finally, zonal pressure gradients associated with the equatorial ocean cooling accelerate surface easterly winds in the middle and western Atlantic. This interaction of equatorial SST and zonal wind causes their westward copropagation, analogous to that in the equatorial Pacific.

Sudden changes of the Atlantic meridional overturning circulation (AMOC) are believed to have caused large, abrupt climate changes over many parts of the globe during the last glacial and deglacial period. This study investigates the mechanisms by which a large freshwater input to the subarctic North Atlantic and an attendant rapid weakening of the AMOC influence North Pacific climate by analyzing four different ocean–atmosphere coupled general circulation models (GCMs) under present-day or preindustrial boundary conditions. When the coupled GCMs are forced with a 1-Sv (Sv ≡ 106 m3 s−1) freshwater flux anomaly in the subarctic North Atlantic, the AMOC nearly shuts down and the North Atlantic cools significantly. The South Atlantic warms slightly, shifting the Atlantic intertropical convergence zone southward. In addition to this Atlantic ocean–atmosphere response, all of the models exhibit cooling of the North Pacific, especially along the oceanic frontal zone, consistent with paleoclimate reconstructions. The models also show deepening of the wintertime Aleutian low. Detailed analysis of one coupled GCM identifies both oceanic and atmospheric pathways from the Atlantic to the North Pacific. The oceanic teleconnection contributes a large part of the North Pacific cooling: the freshwater input to the North Atlantic raises sea level in the Arctic Ocean and reverses the Bering Strait throughflow, transporting colder, fresher water from the Arctic Ocean into the North Pacific. When the Bering Strait is closed, the cooling is greatly reduced, while the Aleutian low response is enhanced. Tropical SST anomalies in both the Atlantic and Pacific are found to be important for the equivalent barotropic response of the Aleutian low during boreal winter. The atmospheric bridge from the tropical North Atlantic is particularly important and quite sensitive to the mean state, which is poorly simulated in many coupled GCMs. The enhanced Aleutian low, in turn, cools the North Pacific by increasing surface heat fluxes and southward Ekman transport. The closure of the Bering Strait during the last glacial period suggests that the atmospheric bridge from the tropics and air–sea interaction in the North Pacific played a crucial role in the AMOC–North Pacific teleconnection.