Intraseasonal variability of tropical Atlantic sea‐surface temperature: air–sea interaction over upwelling fronts

Abstract

International audienceTropical 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

The Tropical Atlantic Observing System

“…Both feedbacks involve the horizontal adjustment of the sea level pressure gradients to SST gradients (Lindzen and Nigam, 1987;Young, 1987). However, they are complicated by other factors such as the vertical adjustment of the planetary boundary layer to SST (Sweet et al, 1981;Hayes et al, 1989;Wallace et al, 1989) and the effects of higher atmospheric levels on surface pressure (Richter et al, 2014a;Diakhaté et al, 2016).…”

Global Perspectives on Observing Ocean Boundary Current Systems

“…The Canary Current Large Marine Ecosystem extends from the northern tip of the Iberian Peninsula (43 • N) to south of Senegal (12 • N), corresponding to the extent of the northeasterly trade winds in the northeastern Atlantic. Upwelling occurs yearround with meridional shifts in the trade winds leading to seasonality in the latitudinal range of upwelling, particularly in the south (Benazzouz et al, 2014;Faye et al, 2015), where strong intraseasonal to longer timescale variability is driven by internal or remotely forced pulsations of the trade winds, passages of African easterly waves, and oceanic coastally trapped waves (Polo et al, 2008;Diakhaté et al, 2016;Oettli et al, 2016). The ecosystem is broadly divided by the Strait of Gibraltar into the Iberian and the Northwest African areas, though strong subregional differences are observed due to variability in factors including coastal configuration, oxygen concentration, nutrient fertilization, and productivity (Arístegui et al, 2009).…”

Eastern Boundary Circulation and Hydrography Off Angola: Building Angolan Oceanographic Capacities

“…Across-front temperature differences range from 5° to 8°C. The front fluctuates in position and strength on time scales from intraseasonal (Kostianoy and Lutjeharms 1999;Mohrholz et al 2001;Goubanova et al 2013;Diakhaté et al 2016) to annual (Meeuwis and Lutjeharms 1990) to interannual. Interannual variability in the frontal region is often associated with severe warm and cold events during austral summer termed Benguela Niños and Benguela Niñas, respectively (Shannon et al 1986;Florenchie et al 2004;Rouault et al 2018).…”