A Satellite Era Warming Hole in the Equatorial Atlantic Ocean

Abstract

Observations during the satellite era 1979-2018 only depict small sea surface temperature (SST) trends over the Equatorial Atlantic cold tongue region in boreal summer. This lack of surface warming of the cold tongue, termed warming hole here, denotes an 11% amplification of the mean SST annual cycle. The warming hole is driven by a shoaling of the equatorial thermocline, linked to increased wind stress forcing, and damped by the surface turbulent heat fluxes. The satellite era warming deficit is not unusual during the twentieth century-similar weak trends were also observed during the 1890s-1910s and 1940s-1960s. The tendency for surface cooling appears to reflect an interaction of external forcing, which controls the timing and magnitude of the cooling, with the intrinsic variability of the climate system. The hypothesis for externally forced modulation of internal variability is supported by climate model simulations forced by the observed time-varying concentrations of atmospheric greenhouse gases and natural aerosols. These show that increased greenhouse forcing warmed the cold tongue and aerosols cooled it during the satellite era. However, internal variability, as derived from control integrations with fixed, preindustrial values of greenhouse gases and aerosols, can potentially cause larger cooling than observed during the satellite era. Large uncertainties remain on the relative roles of external forcing and intrinsic variability in both observations and coupled climate models.Plain Language Summary The Atlantic cold tongue is a region of locally cooler ocean surface waters that develops just south of the equator in boreal summer, partly reflecting the upwelling of deep cold waters by the action of the southeasterly trade winds. Although there has been considerable global warming since the beginning of global satellite measurements in 1979, there is hardly any surface warming in the Atlantic cold tongue region during 1979-2018. This warming hole is most pronounced in boreal summer. Observations suggest that despite strong heat transfer from the atmosphere to the ocean, the upper ocean may have cooled. Climate model simulations show that variations in external forcing associated with greenhouse gases can cause warming of the cold tongue and aerosols cooling of it. However, model simulations that exclude variations in these external forcing show that the mechanisms internal to the climate system can potentially cause larger cooling than observed during the satellite era. Thus, we attribute the warming hole to a combination of internal variability and external forcing. It must be stressed, however, that understanding the relative roles of internal variability versus external forcing is greatly hampered by large errors in both the observational data sets and climate models.

Origin of Weakened Interannual Sea Surface Temperature Variability in the Southeastern Tropical Atlantic Ocean

“…However, multidecadal variability is large. Nnamchi et al (2020) found that SSTs during the satellite era 1979-2018 exhibit a warming hole over the equatorial Atlantic cold tongue region in boreal summer. This lack of surface warming denotes an 11% amplification of the mean SST annual cycle in that region.…”

Response of the Equatorial Atlantic Cold Tongue to Stratospheric Aerosol Geoengineering

“…As mentioned above (Sect. 1), WEA wind changes could affect the D20 through equatorial waves and lead to SST changes in the EACT region (Planton et al 2018;Marin et al 2009;Nnamchi et al 2020). Under RCP8.5, we analyze the correlation between wind changes in the WEA and D20 changes and found that there is a weak correlation (r = 0.38 not significantly at the 95% significance level) between these two terms (Fig.…”

“…An intensification of winds in the WEA could induce upwelling Kelvin waves that propagate eastward and trigger the rising of the thermocline (represented by the 20 °C isotherm or D20) to the East, and vice versa for a weakening of winds (e.g., Castaño-Tierno et al 2018). This movement of the thermocline leads to a reduction (augmentation) in the sea surface height (SSH) and SST in the EACT region (Planton et al 2018;Marin et al 2009;Nnamchi et al 2020). Thus, as the EACT region is largely related to the West African summer monsoon precipitation which is of key importance for agriculture productivity and water availability, it is essential to study the changes of SST under GLENS in this region.…”

Impact of model’s resolution on the sea surface temperature climate change signal in the South Eastern Tropical Atlantic

“…These processes can also affect the interannual variability (Prodhomme et al, 2019) and affect the uncertainty of the simulated climate change signal (Tokinaga and Xie, 2011;Nnamchi et al, 2020).…”