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The equilibrium response to idealized thermal forcings in a comprehensive GCM: Implications for recent tropical expansion

by R. J. Allen, S. C. Sherwood, J. R. Norris, C. S. Zender
Atmospheric Chemistry and Physics ()
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Several recent studies have shown the width of the tropical belt has\nincreased over the last several decades. The mechanisms driving tropical\nexpansion are not well known and the recent expansion is underpredicted\nby state-of-the art GCMs. We use the CAM3 GCM to investigate how\ntropical width responds to idealized atmospheric heat sources, focusing\non zonal displacement of the tropospheric jets. The heat sources\ninclude global and zonally restricted lower-tropospheric warmings\nand stratospheric coolings, which coarsely represent possible impacts\nof ozone or aerosol changes. Similar to prior studies with simplified\nGCMs, we find that stratospheric cooling – particularly at high-latitudes\n– shifts jets poleward and excites Northern and Southern Annular\nMode (NAM/SAM)-type responses. We also find, however, that modest\nheating of the midlatitude boundary layer drives a similar response;\nheating at high latitudes provokes a weaker, equatorward shift and\ntropical heating produces no shift. Over 70 % of the variance in\nannual mean jet displacements across 27 experiments is accounted\nfor by a newly proposed "Expansion Index", which compares mid-latitude\ntropospheric warming to that at other latitudes. We find that previously\nproposed factors, including tropopause height and tropospheric stability,\ndo not fully explain the results. Results suggest recently observed\ntropical expansion could have been driven not only by stratospheric\ncooling, but also by mid-latitude heating sources due for example\nto ozone or aerosol changes.

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