Intraseasonal variability of tropical Atlantic sea-surface temperature: air-sea interaction over upwelling fronts
Résumé
Tropical Atlantic Sea Surface Temperatures (SSTs) maximum Intra-Seasonal Variability (ISV) and their interaction with local surface winds are investigated applying statistical analysis to observations and to a recent coupled reanalysis over the 2000–2009 decade. Five cores of strong ISV emerge, with standard deviation reaching about 1°C in frontal areas of the three main upwelling systems: equatorial, Angola-Benguela and Senegal-Mauritania (the southern side of the Canary upwelling). West of 10°W along the equator, a 20-60-day peak caused by tropical instability waves is shown to generate surface wind anomalies through the adjustment of the horizontal surface pressure gradient in addition to the modification of near-surface atmospheric stratification. East of 10°W along the equator, an intense biweekly oscillation increases the ocean and atmosphere ISV. In the two coastal upwelling fronts, intraseasonal SST anomalies resemble each other. They are shown to be influenced by coastal Kelvin waves in addition to large-scale wind forcing. Over the Angola-Benguela upwelling, coastal wind bursts controlling the SST ISV are associated with anomalously strong pressure patterns related to the Madden-Julian Oscillation, the St. Helena anticyclone, and the Antarctic Oscillation. In the Senegal-Mauritania upwelling, the wind anomalies mainly linked to the Azores anticyclone in the southern front during November to May appear to be connected to the Saharan heat low in the northern front from June to September. In all five regions and as expected for such upwelling regimes, vertical oceanic mixing represents the dominant term in the mixed-layer heat budget. In the equatorial band, as found in previous studies, horizontal advection is equally important, while it appears surprisingly weak in coastal fronts. Finally, a striking result is the general lack of surface wind signal related to the SST ISV in the coastal upwellings.