Mechanisms Controlling Variability of the Interhemispheric Sea Surface Temperature Gradient in the Tropical Atlantic

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  • 1 Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison, Madison, Wisconsin
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Abstract

The seasonal evolution of sea surface temperature (SST) fields in the tropical Atlantic is explored for composites of extremely STRONG and WEAK northward SST gradients, because these are known to control the basinwide pressure gradient, latitude position of the intertopical convergence zone (ITCZ), and regional rainfall. Aimed at the origin and maintenance of anomalies in the interhemispheric SST gradient, and using surface and subsurface marine observations, differences in forcing of SST patterns from surface heat fluxes and entrainment are scaled by calendar-monthly mixed layer depth, and compared to the observed evolution of SST. For years with a STRONG as compared to WEAK gradient in boreal winter, a substantial portion of anomalous SST evolution from September to February can be attributed to departures in SST forcing by latent heat transfer: in the North Atlantic, strengthened northeast trades associated with a stronger North Atlantic high promote enhanced evaporative cooling, while weakened southeast trades reduce latent heat loss, intensifying seasonal warming. During March to August, Ekman upwelling in the north contributes less cooling to anomalously cold surface waters, while in the south autumn entrainment cools the upper layer more rapidly. The resulting decay of both cold anomalies in the north and warm anomalies in the south reduces the anomalously strong SST gradient. A pronounced decadal-scale trend toward a stronger northward SST gradient in January is reflected in an enhancement of Northeast Brazil rainfall, which is strongly correlated to the March–April interhemispheric SST gradient; both Pacific SST and the Atlantic SST gradient in January are associated with variability in the March–April gradient; however, SST in the Pacific and the SST gradient in the Atlantic are uncorrelated in January.

Abstract

The seasonal evolution of sea surface temperature (SST) fields in the tropical Atlantic is explored for composites of extremely STRONG and WEAK northward SST gradients, because these are known to control the basinwide pressure gradient, latitude position of the intertopical convergence zone (ITCZ), and regional rainfall. Aimed at the origin and maintenance of anomalies in the interhemispheric SST gradient, and using surface and subsurface marine observations, differences in forcing of SST patterns from surface heat fluxes and entrainment are scaled by calendar-monthly mixed layer depth, and compared to the observed evolution of SST. For years with a STRONG as compared to WEAK gradient in boreal winter, a substantial portion of anomalous SST evolution from September to February can be attributed to departures in SST forcing by latent heat transfer: in the North Atlantic, strengthened northeast trades associated with a stronger North Atlantic high promote enhanced evaporative cooling, while weakened southeast trades reduce latent heat loss, intensifying seasonal warming. During March to August, Ekman upwelling in the north contributes less cooling to anomalously cold surface waters, while in the south autumn entrainment cools the upper layer more rapidly. The resulting decay of both cold anomalies in the north and warm anomalies in the south reduces the anomalously strong SST gradient. A pronounced decadal-scale trend toward a stronger northward SST gradient in January is reflected in an enhancement of Northeast Brazil rainfall, which is strongly correlated to the March–April interhemispheric SST gradient; both Pacific SST and the Atlantic SST gradient in January are associated with variability in the March–April gradient; however, SST in the Pacific and the SST gradient in the Atlantic are uncorrelated in January.

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