Seasonal and Interannual Mixed‐Layer Heat Budget Variability in the Western Tropical Atlantic From Argo Floats (2007–2012)

Abstract

Oceanic and atmospheric processes were investigated in order to explore the causes of seasonal and interannual variability of sea surface temperatures (SST) in the western tropical Atlantic (WTA; 20°S–20°N, 15°W–60°W). A mixed‐layer (ML) heat budget was performed by using Argo profiles and supplementary data sets based on satellite and atmospheric products during the period 2007–2012. The WTA is divided into four boxes which represent the main temporal and spatial heterogeneities of this region. An analysis of error of each term pointed out that the mean net surface heat fluxes are systematically underestimated by 20 W m−2. A correction of this term provides realistic estimates of the vertical mixing which was obtained as residual term. In agreement with previous studies, the results show that surface flux is the most important process that governs the seasonal cycle of the heat content. Changes in shortwave radiation and latent heat fluxes dictate the oceanic response to the meridional migration of the ITCZ. Along the equator, surface fluxes modulate the annual cycle of ML temperature, but are strongly balanced by horizontal advection. The entrainment term proves a small contribution to the cooling of the ML. On an interannual time scale, the strong positive (negative) SST anomalies observed in 2010 (2012) were generated during the previous winter in both years, mainly north of 10°N, during which the wind anomalies were at the origin of intense heat loss anomalies. Horizontal advection may contributes to the maintaining of these SST anomalies in the equatorial zone and south Atlantic.

Diabatic heating governs the seasonality of the Atlantic Niño

“…The net surface heat flux ( Q net ) constitutes a known source of observational uncertainty in the equatorial Atlantic 4 , 39 , 40 , and the reanalysis systems (especially of the ocean) tend to overestimate Q net damping (Supplementary Fig. 8 ).…”

The Tropical Atlantic Observing System

“…Progress has been made identifying processes responsible for the seasonal cycle of vertical turbulent mixing such as shear from the background currents, intraseasonal tropical instability waves (TIWs) and wind-driven waves, and the diurnal cycle in the mixed layer (Foltz et al, 2003;Jouanno et al, 2011;Giordani et al, 2013;Hummels et al, 2013;Wenegrat and McPhaden, 2015;Scannell and McPhaden, 2018). Away from the equator, surface heat fluxes play a more dominant role (Nobre et al, 2012;Foltz et al, 2013Foltz et al, , 2018Cintra et al, 2015;Nogueira Neto et al, 2018). However, there remain significant seasonal variations in the heat budget residuals (i.e., changes in mixed layer heat content that cannot be explained by the net surface heat flux) at some off-equatorial locations, implying that vertical mixing and other processes may be important ( Figure 6; Foltz et al, 2018).…”

Vertical Turbulent Cooling of the Mixed Layer in the Atlantic ITCZ and Trade Wind Regions

“…The tropical Atlantic experiences a strong seasonal cycle of sea surface temperature (SST) and weaker, but impactful, interannual to multidecadal variability (Carton & Huang, 1994;Carton & Zhou, 1997;Chiang et al, 2002;Foltz et al, 2019;Keenlyside & Latif, 2007;Martín-Rey et al, 2018;Mitchell & Wallace, 1992;Nobre & Shukla, 1996). Outside of the Atlantic equatorial and Intertropical Convergence Zone (ITCZ) regions (together spanning ∼5 • S to 10 • N) the seasonal cycle of SST has been shown to be forced to a large extent by local surface heat fluxes (Carton & Zhou, 1997;Cintra et al, 2015;Foltz et al, 2003Foltz et al, , 2013Nogueira Neto et al, 2018;Yu et al, 2006). Close to the equator, upwelling and vertical turbulent mixing are thought to contribute significantly to SST variability (Foltz et al, 2003(Foltz et al, , 2013Giordani et al, 2013;Hummels et al, 2013Hummels et al, , 2014Jouanno et al, 2011;Peter et al, 2006;Rhein & Dengler, 2010;Wade et al, 2011;Yu et al, 2006).…”