Publication: The interannual variability of the tropical divergence tilt and its connection with the extratropical circulation
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2021-01
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American Meteorological Society
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Abstract
Previous theoretical work has suggested that the strength of the divergent eddy momentum flux in the deep tropics, due to correlations between rotational zonal velocities and divergent meridional velocities, increases with the meridional tilt of the large-scale divergence field. To test that idea, this work investigates the interannual variability of the divergent eddy momentum flux in reanalysis data. Consistent with the theory, it is found that the eddy momentum flux variability is driven by two main parameters: the amplitude of the tropical stationary wave and the tilt of the divergence field. Together, these two parameters account for 80% (90%) of the interannual eddy momentum flux variance during boreal (austral) winter. The interannual variability of these parameters is governed by the internal atmospheric dynamics. During boreal winter, interannual changes in MJO variability explain nearly half of the interannual variance in the stationary wave amplitude, depending on whether on average MJO anomalies interfere constructively or destructively with the stationary wave. The interannual variability of the divergence phase tilt is modulated by tropical–extratropical interactions in the Pacific. The tilt increases during the negative phase of the west Pacific Oscillation associated with a dipole of upper-level divergence (convergence) on the northern (southern) side of the Pacific jet exit region.
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© 2020 American Meteorological Society. This work benefited from the comments of three anonymous reviewers. We thank the European Centre for Medium-Range Weather Forecasts, the Japan Meteorological Agency, the National Center for Atmospheric Research, and the National Oceanic and Atmospheric Administration for producing/ providing the data used in this study. The ERA-Interim data were downloaded from the ECMWF website (https://www.ecmwf.int). The JRA-55 data were obtained from the Research Data Archive at the National Center for Atmospheric Research Computational and Information Systems Laboratory (https://doi.org/10.5065/ D6HH6H41). The NOAA ERSST data were provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, from their website (http://www.esrl.noaa.gov/psd/). The author acknowledges financial support by Grant CGL2015-72259-EXP by the State Research Agency of Spain and NSF funding (Grant AGS1733818) for a summer visit to Princeton that helped motivate this study.