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During austral summer, the majority of precipitation over the Pacific Ocean is concentrated in the South Pacific Convergence Zone (SPCZ). The surface boundary conditions required to support the diagonally (northwest-southeast) oriented SPCZ are determined through a series of experiments with an atmospheric general circulation model. Continental configuration and orography do not have a significant influence on SPCZ orientation and strength. The key necessary boundary condition is the zonally asymmetric component of the sea surface temperature (SST) distribution. This leads to a strong subtropical anticyclone over the southeast Pacific that, on its western flank, transports warm moist air from the equator into the SPCZ region. This moisture then intensifies (diagonal) bands of convection that are initiated by regions of ascent and reduced static stability ahead of the cyclonic vorticity in Rossby waves that are refracted toward the westerly duct over the equatorial Pacific. The climatological SPCZ is comprised of the superposition of these diagonal bands of convection. When the zonally asymmetric SST component is reduced or removed, the subtropical anticyclone and its associated moisture source is weakened. Despite the presence of Rossby waves, significant moist convection is no longer triggered; the SPCZ disappears. The diagonal SPCZ is robust to large changes (up to ±6 °C) in absolute SST (i.e. where the SST asymmetry is preserved). Extreme cooling (change <−6 °C) results in a weaker and more zonal SPCZ, due to decreasing atmospheric temperature, moisture content and convective available potential energy. [ABSTRACT FROM AUTHOR]

Abstract: Variations in the sea surface temperature of the Eastern Equatorial Pacific, usually associated with the El Niño Southern Oscillation, have a global impact. Previous work has shown that the stratosphere modulates the Northern Hemisphere response to these temperature anomalies. Here, we show that the stratosphere also plays a major role in modulating the response to tropical sea surface temperature anomalies representative of the mean state of the Pliocene era, approximately four million years ago. Key to the response is a predominance of sudden stratospheric warmings during boreal winter, which cause a large anomalous surface pressure response over the North Atlantic Ocean. The stratospheric modulation results in relatively cold winter/spring Eurasian surface temperatures and relatively warm surface temperatures in Greenland and North America. The implications of these results for simulating and understanding the climate of the Pliocene era are discussed. [Copyright &y& Elsevier]