Coupling between the Atlantic cold tongue and the West African monsoon in boreal spring and summer

Abstract

[1] The formation of the Atlantic cold tongue (ACT) is the dominant seasonal sea surface temperature signal in the eastern equatorial Atlantic (EEA). A comprehensive analysis of variability in its spatial extent, temperature, and onset is presented. Then, the physical mechanisms which initiate ACT onset, as well as the feedbacks from the ACT to the maritime boundary layer, and how the ACT influences the onset of the West African monsoon (WAM) are discussed. We argue that in the EEA, the air-sea coupling between the ACT and WAM occurs in two phases. From March to mid-June, the ACT results from the intensification of the southeastern trades associated with the St. Helena anticyclone. Steering of surface winds by the basin shape of the EEA imparts optimal wind stress for generating the maximum upwelling south of the equator. During the second phase (mid-June-August), wind speeds north of the equator increase as a result of the northward progression of the intensifying trades and as a result of significant surface heat flux gradients produced by the differential cooling between the ACT and the tropical waters circulating in the Gulf of Guinea (GG). It is anticipated that the atmospheric divergence induced at low levels north of the equator reduces convection over the GG and that increased northward winds shift convection over land. Correlations between the ACT and the WAM onset dates over the last 26 years measure as much as 0.8. This suggests that the ACT plays a key role in the WAM onset.

Diapycnal heat flux and mixed layer heat budget within the Atlantic Cold Tongue

“…In particular, inter-annual SST variability within the Atlantic cold tongue (ACT) is known to influence the onset and strength of the West African Monsoon (Marin et al 2009;Brandt et al 2011a;Caniaux et al 2011). Hence, in order to improve predictions of rainfall variability, SST variability and its causes need to be understood.…”

Sea surface temperature patterns in the Tropical Atlantic: Principal component analysis and nonlinear principal component analysis

“…The first mode pattern captures the zonal mode pattern in the tropical Atlantic (Zebiak 1993;Carton and Huang 1994;Huang and Shukla 1997;Handoh and Bigg 2000;Ruiz-Barradas et al 2000). By multiplying EOF1 with its related PC, we observe some typical ACT cold years, such as 1990such as , 1992such as , 1997such as , 2005such as , and warm years 1984such as , 1987such as (Caniaux et al 2011. At the end of 1964 and again in 1967, there was a general cooling over most of the Atlantic and particularly strong in the east.…”

“…This is the primary seasonal SST signal in the equatorial Atlantic Ocean (Merle et al 1980;Bruno 1983;Xie and Carton 2004). This Atlantic signal appears almost all years in the East Equatorial Atlantic (EEA) and positions itself south of the Equator with a longitudinal extent to almost 20°W, centered and located a few degrees south of the Equator in the eastern part of the basin and slightly shifted equatorward (Caniaux et al 2011). However, the detailed processes responsible for its variability are still the subject of considerable debate (e.g., Jouanno et al 2011a, b).…”

“…Figure 5 is a plot of the normalized time series of NLPC, which bears a strong resemblance to the ACT time series. This "ACT index" is obtained by computing the inter-annual mean of SSTA in the ACT zone [in the box from 5°S -2°N and from the coast to 20°W during the ACT period (June -August) Caniaux et al (2011)]. Normalization is obtained by subtracting the mean and dividing by the standard deviation.…”

“…Due to the ACT effect on the WAM, suggesting potential rainfall predictability, the problem of understanding the inter-relationship between the ACT and the WAM remains at the center of many research endeavors (Okumura and Xie 2004;Hagos and Cook 2009;Druyan and Fulakeza 2015). Caniaux et al (2011) shows that there is interannual variability in the ACT spatial pattern in GoG, which is not fully understood. Picaut et al (1984) compared the seasonal and interannual SST variability in the tropical Atlantic.…”