Atmospheric response to sea‐surface temperature in the eastern equatorial Atlantic at quasi‐biweekly time‐scales

Abstract

International audienceThe surface wind response to SST and SST meridional gradient is investigated in the Gulf of Guinea by using daily observations and reanalyses in the 2000-2009 decade, with a focus on boreal spring and summer months (May to August), where quasi-biweekly fluctuations in the position of the northern front of the equatorial cold tongue induce quasi-biweekly equatorial sea surface temperature (SST) anomalies. Following a large-scale wind acceleration (deceleration), an equatorial SST cold (warm) anomaly is created within a few days. In order to explain the local atmospheric response to this SST anomaly, the two following mechanisms are invoked: first, a colder (warmer) ocean decreases (increases) the vertical stability in the marine atmospheric boundary layer, which favors a weaker (stronger) surface wind; and second, a negative (positive) anomaly of SST meridional gradient induces a positive (negative) anomaly of sea level pressure meridional gradient, which decelerate (accelerate) the surface wind. The first mechanism has an immediate effect in the equatorial belt between 1°S-1°N (and to a lesser extent between 3°S and 1°S), while the second takes one or two days to adjust and damps anomalous southeasterlies up to 800 hPa in the low troposphere between 7°S and 1°N, through reversed anomalies of meridional SST and pressure gradient. This negative feedback leads to weaker (stronger) winds in the southeastern Tropical Atlantic, which forces the opposite phase of the oscillation within about a week. Around the equator, where the amplitude of the oscillation is found maximal, both mechanisms combine to maximize the wind response to the front fluctuations. Between the equator and the coast, a low-level secondary atmospheric circulation takes control of the surface wind acceleration or deceleration around 3°N, which reduces the influence of the SST front fluctuations

Seasonal Mixed Layer Temperature Balance in the Southeastern Tropical Atlantic

“…As SSTs warm, atmospheric stratification erodes allowing more efficient mixing between the boundary layer and the free atmosphere, causing the southeasterly trades to intensify (Hayes et al, ). These wind field variations in relation to SST have been observed in the cold tongue regions of the equatorial Pacific (Wallace et al, ) and Atlantic (De Coëtlogon et al, ) and are evident in the relation of wind speed (Figure a) to the seasonal cycle of SST (Figure c) at 6°S, 8°E.…”

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

“…Reanalysis from the Climate Forecast System Reanalysis (CFSR: Saha et al , ) are retrieved from the National Centers for Environmental Prediction (NCEP) site (http://www.ncep.noaa.gov). A distinctive characteristic of CFSR is that it is performed with a coupled ocean–atmosphere model, which is shown to better describe the air–sea interaction in the eastern tropical Atlantic than the ERA‐Interim (European Centre for Medium‐Range Weather Forecasts Re‐Analysis: de Coëtlogon et al , ). The data are available on a 0.5°×0.5° horizontal grid, with vertical atmospheric profiles retrieved on 27 levels from 1000 to 100 hPa.…”

“…Indeed, as the meridional component dominates the mean wind field in this region, its response to SSTAs follows a southward (northward) acceleration over colder (warmer) water, causing divergence (convergence) over the negative (positive) equatorial SSTAs. Also, locally averaged observed SST and surface wind projection on the NWCTI (Figure (a)) clearly shows that wind anomalies vary in phase with SSTAs, meaning that the delay of wind response to SSTAs is very short (less than 1 day), which is coherent with the SW mechanism (de Coëtlogon et al , ).…”

“…After lag −3, the wind anomaly weakens because the SST cools down, probably through both LN and SW mechanisms (de Coëtlogon et al , ). This reduces the horizontal advection and the residual after a few days.…”

Interactions of atmospheric gases and aerosols with the monsoon dynamics over the Sudano-Guinean region during AMMA

“…Between these two scales, an additional meridional atmospheric cell is suspected in the low atmosphere, enhancing convergence at the coast (Leduc- Leballeur et al, 2013). This cell results from a gradient of wind speed due to the meridional gradient of sea surface temperature (de Coëtlogon et al, 2014). The recent research program "Dynamics-aerosol-chemistry-cloud interactions in West Africa program" (DACCIWA) has been dedicated to the study of land-sea-atmosphere interactions in West Africa.…”