A satellite era warming hole in the equatorial Atlantic Ocean

Although the globally averaged surface temperature of the Earth has considerably warmed since the beginning of global satellite measurements in 1979, a warming hole, with hardly any surface warming that is most pronounced in boreal summer, has been observed in the equatorial Atlantic region during this period. The warming hole occurs in an extended area of the equatorial Atlantic that includes the cold tongue, the 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. This lack of surface warming of the cold tongue denotes an 11% amplification of the mean annual cycle of the sea surface temperature during the satellite era. The warming hole is driven by an intensification of the equatorial upwelling of cold waters into the ocean surface layers and damped by the surface heat flux. In observations, the tendency for surface cooling appears to reflect intrinsic variability of the climate system and is not unusual during the instrumental period. The warming hole is associated with wind-induced ocean circulation changes to the south and north of the northward of the equator. Coupled model ensembles forced by the observed varying concentrations of atmospheric greenhouse gases and natural aerosols as well as unforced runs were analyzed. The ensembles suggest a strong role for atmospheric aerosols in the warming hole. However, although aerosols can cause a cooling of the cold tongue, intrinsic climate variability as represented in the unforced experiment can potentially cause larger cooling than has been observed during the satellite era. This study highlights the difficulty in reconciling observations and the climate models for the attribution of the warming hole.