Understanding Hadley Cell Expansion versus Contraction: Insights from Simplified Models and Implications for Recent Observations

Abstract

This study seeks a deeper understanding of the causes of Hadley Cell (HC) expansion, as projected under global warming, and HC contraction, as observed under El Niño. Using an idealized general circulation model, the authors show that a thermal forcing applied to a narrow region around the equator produces "El Nino-like" HC contraction, while a forcing with wider meridional extent produces "global warming-like" HC expansion. These circulation responses are sensitive primarily to the thermal forcing's meridional structure and are less sensitive to its vertical structure. If the thermal forcing is confined to the midlatitudes, the amount of HC expansion is more than three times that of a forcing of comparable amplitude that is spread over the tropics. This finding may be relevant to recently observed trends of rapid tropical widening.The shift of the HC edge is explained using a very simple model in which the transformed Eulerian mean (TEM) circulation acts to diffuse heat meridionally. In this context, the HC edge is defined as the downward maximum of residual vertical velocity in the upper troposphere iiJ*^^; this corresponds well with the conventional Eulerian definition of the HC edge. In response to a positive thermal forcing, there is anomalous diabatic cooling, and hence anomalous TEM descent, on the poleward flank of the thermal forcing. This causes the HC edge (í5j,gx) to shift toward the descending anomaly, so that a narrow forcing causes HC contraction and a wide forcing causes HC expansion.

Atmospheric circulation and hydroclimate impacts of alternative warming scenarios for the Eocene

“…Such tropical heating anomalies are known to robustly induce cyclonic circulation anomalies in the extratropical North Pacific (Alexander et al, 2002). Furthermore, it is well known from both observations (Caballero, 2007) and model studies (Tandon et al, 2013) that El Niño events and their associated tropical heating anomalies drive an equatorward shift in the extratropical jets and storm tracks, in agreement with what we find here (Fig. 2h).…”

Equilibrium temperature distribution and Hadley circulation in an axisymmetric model

“…3, that the energy input is not as important as the forcing distribution. Our results are consistent with those obtained both by Hou and Lindzen (1992) (hereafter HL92) and recently by Tandon et al (2013), who performed experiments similar to those described here. The conclusions will be drawn in Sect.…”

“…(6). Tandon et al (2013) Greenwood and Wing, 1995) where roughly equal temperatures are present throughout the world. During those geological ages, the temperature was generally higher everywhere, but adding a constant to the temperature does not change the response of this kind of model.…”

Changes in the width of the tropical belt due to simple radiative forcing changes in the GeoMIP simulations

“…For these reasons we will focus exclusively on changes in subtropical static stability. The Held (2000) scaling theory has been used to study tropical expansion in models ranging from dry dynamical cores to fully coupled climate models Lu et al, 2007Lu et al, , 2008, although modified scaling theories that relax the angular momentum conservation constraint (Kang and Lu, 2012), as well as theories based on other criteria (Lu et al, 2008;Korty and Schneider, 2008;Tandon et al, 2013;Levine and Schneider, 2015), may be more realistic. Similar to Levine and Schneider (2015), we evaluate the gross static stability, hereafter "subtropical static stability", at the tropical belt edge latitude.…”

“…Similar to Tandon et al (2011), Maycock et al (2013 found that idealized increases in stratospheric water vapor drove enhanced stratospheric cooling and a poleward shift of the tropospheric jets. Warming in the troposphere alone can also drive an expansion of the Hadley cells Tandon et al, 2013). Thus, stratospheric cooling and tropospheric warming can both drive poleward shifts in the circulation.…”