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WES feedback and the Atlantic Meridional Mode: observations and CMIP5 comparisons

Abstract

The Atlantic Meridional Mode (AMM) is the dominant mode of tropical SST/wind coupled variability. Modeling studies have implicated wind-evaporation-SST (WES) feedback as the primary driver of the AMM’s evolution across the Atlantic basin; however, a robust coupling of the SST and winds has not been shown in observations. This study examines observed AMM growth, propagation, and decay as a result of WES interactions. Investigation of an extended maximum covariance analysis shows that boreal wintertime atmospheric forcing generates positive SST anomalies (SSTA) through a reduction of surface evaporative cooling. When the AMM peaks in magnitude during spring and summer, upward latent heat flux anomalies occur over the warmest SSTs and act to dampen the initial forcing. In contrast, on the southwestern edge of the SSTA, SST-forced cross-equatorial flow reduces the strength of the climatological trade winds and provides an anomalous latent heat flux into the ocean, which causes southwestward propagation of the initial atmosphere-forced SSTA through WES dynamics. Additionally, the lead-lag relationship of the ocean and atmosphere indicates a transition from an atmosphere-forcing-ocean regime in the northern subtropics to a highly coupled regime in the northern tropics that is not observed in the southern hemisphere. CMIP5 models poorly simulate the latitudinal transition from a one-way interaction to a two-way feedback, which may explain why they also struggle to reproduce spatially coherent interactions between tropical Atlantic SST and winds. This analysis provides valuable insight on how meridional modes act as links between extratropical and tropical variability and focuses future research aimed at improving climate model simulations.

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Acknowledgements

This material is based upon work supported in part by the National Science Foundation Graduate Research Fellowship (NSF; DGE-1144086). M.J.D is supported by the Jet Propulsion Laboratory at the California Institute of Technology, under a contract with the National Aeronautics and Space Administration. A.J.M is supported by the NSF (OCE1419306) and the National Oceanic and Atmospheric Administration (NOAA; NA14OAR4310276). S-P.X. is funded by the NSF Climate and Large Scale Dynamics Program 1305719, and NOAA Climate Program Office NA10OAR4310250. We would like to thank Daniel J. Vimont for his helpful comments during the course of our study. We also thank Yu Kosaka for downloading and processing the CMIP5 data presented in Table 1. ERSSTv3b data set is freely available and maintained by NOAA’s National Climate Data Center. NCEP Reanalysis data was provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their web site at http://www.esrl.noaa.gov/psd/. We also express our gratitude to the World Climate Research Programme’s Working Group on Coupled Modelling, which maintains CMIP. Additionally, we thank the climate modeling groups listed in Table 1 for producing their model output and making it available. Finally, we thank two anonymous reviewers for comments that improved the quality of this paper.

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Correspondence to Dillon J. Amaya.

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Amaya, D.J., DeFlorio, M.J., Miller, A.J. et al. WES feedback and the Atlantic Meridional Mode: observations and CMIP5 comparisons. Clim Dyn 49, 1665–1679 (2017). https://doi.org/10.1007/s00382-016-3411-1

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  • DOI: https://doi.org/10.1007/s00382-016-3411-1

Keywords

  • Atlantic Meridional Mode
  • WES feedback
  • CMIP5
  • Air-sea interactions
  • Maximum covariance analysis