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A Dynamic Pathway by which Northern Hemisphere Extratropical Cooling Elicits a Tropical Response
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  • Matthew Luongo,
  • Shang-Ping Xie,
  • Ian Eisenman,
  • Yen-Ting Hwang,
  • Hung-Yi Tseng
Matthew Luongo
University of California, San Diego

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Shang-Ping Xie
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Ian Eisenman
UC San Diego
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Yen-Ting Hwang
National Taiwan University
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Hung-Yi Tseng
National Taiwan University
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Previous studies have found that Northern Hemisphere aerosol-like cooling induces a La Nina-like quasi-equilibrium response in the tropical Indo-Pacific. Here, we explore a coupled atmosphere-ocean feedback pathway by which this response is communicated. We override ocean surface wind stress in a comprehensive climate model to decompose the total ocean-atmosphere response to forced extratropical cooling into the response of surface buoyancy forcing alone and surface momentum forcing alone. In the subtropics, the buoyancy-forced response dominates: the positive low cloud feedback amplifies sea surface temperature (SST) anomalies which are then communicated to the tropics via wind-driven evaporative cooling. In the deep tropics, the momentum-driven Bjerknes feedback creates zonally asymmetric SST patterns in the Indian and Pacific basins. Although subtropical cloud feedbacks are model-dependent, our results suggest this feedback pathway is robust across a suite of models such that models with a stronger subtropical low cloud response exhibit a stronger La Nina response.