Wind–Mixed Layer–SST Feedbacks in a Tropical Air–Sea Coupled System: Application to the Atlantic

Abstract

The ocean–atmosphere feedback associated with the thermodynamic coupling among wind speed, evaporation, and sea surface temperature (SST), called the wind–evaporation–SST (WES) feedback, contributes to the cross-equatorial SST gradient over the tropical oceans. By conducting an eigenanalyses of simple linear air–sea coupled models, it is shown that two additional feedback processes are present when the variable oceanic mixed layer depth (MLD) is considered. The horizontal structures of the leading modes are similar to the WES mode, which shows a meridional dipole in the SST anomalies straddling the equator with cross-equatorial wind anomalies that represent the weakening/strengthening of the trade winds over the warm/cool SST anomalies. The coupling of the variable MLD with winds and SST more than doubles the growth rate of the WES mode and enhances the equatorward propagation of the coupled disturbances. The identified feedbacks operate as follows: The weaker winds associated with warm SST anomalies shoal the mixed layer through suppressed turbulent mixing, which causes the mixed layer to be more sensitive to the climatological shortwave radiation and amplifies the initial positive SST anomalies. Likewise, deepening of the mixed layer due to stronger winds acts to maintain the negative SST anomaly on the other side of the dipole. The MLD anomalies can also be generated by the buoyancy flux anomaly related to the wind-induced latent heat flux anomaly. The antiphase relationship between the SST and MLD anomalies seen in the simple model bears some resemblance to that which is observed in the observations and a state-of-the-art coupled model during the Atlantic meridional mode.

Summertime increases in upper-ocean stratification and mixed-layer depth

“…Changes in the surface and pycnocline layers can have widespread consequences for climate, as they may alter the rates at which exchanges occur between the surface and the deep ocean. For example, increased pycnocline stratification will expectedly weaken surface-to-depth exchanges as enhanced density gradients decouple surface and subsurface waters, act to shoal the surface mixed layer, and result in reduced air-sea gas transfer, deep-ocean ventilation and biological productivity 3,[13][14][15] . Detecting and understanding physical changes in the ocean's surface and pycnocline layers is thus essential to determine the role of the ocean in climate, and predict climate change and its ecosystem impacts.…”

Revisiting the Tropical Atlantic Influence on El Niño–Southern Oscillation

“…In the NTA region, 10-m wind speed anomalies in the reanalysis have their highest positive values in January (Fig. 2a), indicating increased upward latent heat flux and a deepening of the oceanic mixed layer (Kataoka et al 2019), which result in oceanic cooling. CTRL reproduces the peak in January, albeit at much weaker amplitude, which may be due to a relatively strong atmosphere-to-ocean forcing in the region; that is, the observed NTA SST anomalies are in part due to internal atmospheric variability, which is not constrained by the prescribed SST in our experiments.…”

Seasonal to Decadal Predictions With MIROC6: Description and Basic Evaluation

“…Regarding the mean state, biases in the tropical climate systems (e.g., underestimation of the summertime precipitation in the western Pacific and dry bias of the free troposphere) and the midlatitude atmospheric circulations both in the troposphere and the stratosphere are also significantly alleviated compared to those in MIROC5. The reader is referred to Tatebe et al (2019) for more detailed evaluation of MIROC6 (see also Kataoka et al, 2019, for the tropical Atlantic).…”