Structure of Large-Scale Convective Anomalies over Tropical Oceans

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  • 1 Atmospheric and Environmental Research, Inc., Cambridge, Massachusetts
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Abstract

Geographical variations in the variance and cross-correlation of monthly mean sea surface temperature (SST), outgoing longwave radiation (OLR, a proxy for deep convection and vertical motion), and convergence of winds at the surface and at 850 mb across the tropical Indian and Pacific oceans are examined. Within about 10° of the equator at most longitudes the variance of these quantities associated with the seasonal cycle is less than the variance associated with anomalies from the seasonal cycle. Largest variances in the SST and surface convergence data occur across the eastern near-equatorial Pacific, whereas OLR and 850 mb convergence variances are largest across the western Pacific. OLR anomalies are significantly correlated with collocated SST and surface convergence anomalies from the date line eastward to the South American coast, but are uncorrelated west of the. date line. The OLR and 850 mb convergence anomalies are significantly correlated from about 120°W westward but are uncorrelated east of that longitude. The near-surface convergence field thus contains a complicated vertical structure that may not be adequately represented in models with a single lower layer.

These calculations suggest that the relative effectiveness of different mechanisms for large ocean ocean-atmosphere coupling varies considerably across the near-equatorial oceans. Direct thermodynamic linkage between SST and convection anomalies is consistent with the results only across the eastern near-equatorial Pacific. Surface gradients of SST are most effective at forcing low-level atmospheric circulation anomalies over the eastern Pacific, where SST and surface convergence have large variances and OLR and 850 mb convergence anomalies are small and not well correlated. West of the date line, forcing by midtropospheric latent heating seems most consistent with the observed relationships.

Abstract

Geographical variations in the variance and cross-correlation of monthly mean sea surface temperature (SST), outgoing longwave radiation (OLR, a proxy for deep convection and vertical motion), and convergence of winds at the surface and at 850 mb across the tropical Indian and Pacific oceans are examined. Within about 10° of the equator at most longitudes the variance of these quantities associated with the seasonal cycle is less than the variance associated with anomalies from the seasonal cycle. Largest variances in the SST and surface convergence data occur across the eastern near-equatorial Pacific, whereas OLR and 850 mb convergence variances are largest across the western Pacific. OLR anomalies are significantly correlated with collocated SST and surface convergence anomalies from the date line eastward to the South American coast, but are uncorrelated west of the. date line. The OLR and 850 mb convergence anomalies are significantly correlated from about 120°W westward but are uncorrelated east of that longitude. The near-surface convergence field thus contains a complicated vertical structure that may not be adequately represented in models with a single lower layer.

These calculations suggest that the relative effectiveness of different mechanisms for large ocean ocean-atmosphere coupling varies considerably across the near-equatorial oceans. Direct thermodynamic linkage between SST and convection anomalies is consistent with the results only across the eastern near-equatorial Pacific. Surface gradients of SST are most effective at forcing low-level atmospheric circulation anomalies over the eastern Pacific, where SST and surface convergence have large variances and OLR and 850 mb convergence anomalies are small and not well correlated. West of the date line, forcing by midtropospheric latent heating seems most consistent with the observed relationships.

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