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

Abstract

Abstract. Several recent studies have shown the width of the tropical belt has increased over the last several decades. The mechanisms driving tropical expansion are not well known and the recent expansion is underpredicted by state-of-the art GCMs. We use the CAM3 GCM to investigate how tropical width responds to idealized atmospheric heat sources, focusing on zonal displacement of the tropospheric jets. The heat sources include global and zonally restricted lowertropospheric warmings and stratospheric coolings, which coarsely represent possible impacts of ozone or aerosol changes. Similar to prior studies with simplified GCMs, we find that stratospheric cooling -particularly at high-latitudes -shifts jets poleward and excites Northern and Southern Annular Mode (NAM/SAM)-type responses. We also find, however, that modest heating of the midlatitude boundary layer drives a similar response; heating at high latitudes provokes a weaker, equatorward shift and tropical heating produces no shift. Over 70 % of the variance in annual mean jet displacements across 27 experiments is accounted for by a newly proposed "Expansion Index", which compares mid-latitude tropospheric warming to that at other latitudes. We find that previously proposed factors, including tropopause height and tropospheric stability, do not fully explain the results. Results suggest recently observed tropical expansion could have been driven not only by stratospheric cooling, but also by mid-latitude heating sources due for example to ozone or aerosol changes.

Black carbon aerosol‐induced Northern Hemisphere tropical expansion

“…We quantify tropical width using several measures: (1) Mean Meridional Circulation (MMC), which is determined as the latitude where the MMC at 500 hPa becomes zero poleward of the subtropical maxima [ Johanson and Fu , ]; (2) latitude of the tropospheric zonal wind maxima (JET) using the percentile method [ Allen et al ., ]; and (3) the latitude where the zonal mean precipitation minus evaporation ( P − E ) becomes zero on the poleward side of the subtropical minima [ Lu et al ., ]. We follow Allen et al .…”

“…The maximum BC‐induced warming occurs in the NH midlatitudes (Figure a). Recent studies suggest that the heating of the NH midlatitudes can shift the maximum meridional temperature gradient poleward, resulting in the shift of the tropospheric jet as meridional temperature gradient is related to midlatitude baroclinicity through the thermal wind relationship [ Fu et al ., ; Allen et al ., , ]. Figure b shows the poleward displacement of the maximum meridional temperature gradient in the NH for 10XBC experiment.…”

“…Allen et al . [] performed several simulations using a range of idealized heating profiles and found that while midlatitude heating produced a poleward jet shift and expansion of the HC, heating in the tropics resulted in little change.…”

Direct and semi‐direct aerosol effects in the NASA GEOS‐5 AGCM: aerosol‐climate interactions due to prognostic versus prescribed aerosols

“…Frierson et al ., ; Lu et al ., ]. Recent evidence suggests that tropospheric heating due to black carbon aerosol is a likely driver of the NH tropical expansion seen in the observational record [ Allen et al ., ]. Aerosols also affect mid‐ and high‐latitude winds near the surface, a measure of the storm tracks (see Supplementary Fig.…”

Future aerosol reductions and widening of the northern tropical belt

“…Thus, both models suggest that the NH midlatitudes may warm the most in response to future aerosol decreases. Recent studies show that such midlatitude warming amplification can displace the tropical edge poleward [ Allen et al , , ].…”