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Abstract:
A simple air-sea coupled model to assess feedbacks associated with the cross-equatorial sea surface temperature (SST) gradient variability is extended here to include the response to freshwater flux anomalies. An eigenanalysis reveals the presence of yet another feedback accompanying a cross-equatorial SST gradient, in addition to wind-evaporation-SST, wind-induced turbulence-mixed layer (ML)-SST, and wind-evaporation-ML-SST feedback. The feedback operates as follows: A cross-equatorial SST gradient anomaly forces surface wind anomalies to blow toward the warmer hemisphere, causing low-level convergence/divergence and hence a positive/negative precipitation anomaly in the warmer/cooler hemisphere. The positive/negative precipitation anomaly stratifies/destabilizes the near-surface ocean and results in a shallower/deeper ML, which enhances/reduces the warming by climatological shortwave radiation, and thus provides positive feedback to the initial SST gradient anomaly. The strength of this feedback is similar to the three known feedbacks in terms of stability. Sensitivity experiments with the coupled general circulation model MIROC6 reveal that the precipitation-induced buoyancy flux anomaly accounts for up to ~14% of the Atlantic meridional mode (AMM) amplitude in boreal spring through affecting the MLD anomaly in the deep tropics, which is consistent with the simple model results, supporting the existence of the feedback. In contrast, the evaporation-induced buoyancy anomaly contributes only marginally to the MLD and thus the SST anomalies. A ML temperature budget from MIROC6 confirms that sensitivity changes of the ML to the warming by climatological shortwave radiation due to the MLD anomaly are important in generating the SST anomalies associated with the AMM, which is consistent with previous observational studies.
