Hjem
Geofysisk institutt

GFI/BCCR Seminar: On the Relation Among SST, deep convection and surface winds in the equatorial Atlantic

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Ingo Richter (JAMSTEC, Yokohama Institute for Earth Sciences, Japan):

On the Relation Among SST, deep convection and surface winds in the equatorial Atlantic

 

Abstract
Surface winds are a key factor controlling the annual cycle and interannual variability of the equatorial Atlantic Ocean. In terms of the annual cycle, the rapid intensification of the equatorial easterlies in late spring and early summer is crucial to Atlantic cold tongue development, which in turn influences the onset phase of the West African monsoon. On interannual time scales, the equatorial easterlies are subject to pronounced variability in March-April-May (MAM), which gives rise to warm or cold SST events in the central and eastern equatorial Atlantic.

In the present study we apply a simple diagnostic model of the boundary layer winds to CMIP5 model output to examine the factors controlling equatorial Atlantic winds in MAM. We find that the prevailing surface easterlies flow against the attendant pressure gradient. This counter-gradient flow is maintained by vertical transport of zonal momentum from the free troposphere and, to a lesser extent, meridional advection of momentum. Both factors are linked to the distribution of deep convection, which supports previous findings that GCM precipitation biases are one of the root causes for the persistent westerly wind bias in GCMs.

Vertical momentum transport also dominates the interannual variability of surface winds in boreal spring. GCM experiments with prescribed climatological sea-surface temperatures (SSTs) indicate that these free tropospheric influences are mostly associated with internal atmospheric variability. Since the surface wind anomalies in boreal spring are crucial to the development of warm SST events (Atlantic Niños), the results imply that interannual variability in the region may rely far less on coupled air-sea feedbacks than is the case in the tropical Pacific.