3 Coupling Between Tropical Width and Meridional Variations of MMTG
A fundamental driver of the atmospheric circulation is the meridional temperature gradient. Tropical expansion is associated with a poleward displacement of westerlies, jet streams, and storm tracks (Archer & Caldeira, 2008; Chen et al., 2008; Yin, 2005). These phenomena are all associated with the atmospheric meridional temperature gradients (Kaspi & Flierl, 2007; Lesieur et al., 2000; Sampe et al., 2010; Yang et al., 2019), raising the question of whether the displacement of the meridional temperature gradient is associated with the tropical width.
Given that the tropical width does not vary homogeneously over all ocean basins (Amaya et al., 2018), we examine the tropical width from a regional perspective. The annual‐mean tropical width over an individual ocean basin is calculated as a metric of the latitude where the near‐surface zonal wind changes from easterly to westerly in the subtropics (USF, see section 2). To get the corresponding pattern of SST gradient related to the wider tropics, we regress the USF indices of tropical width onto a field of absolute value of meridional SST gradients over their corresponding ocean basins (Figure 1). Both observations and model results from the CMIP5 exhibit a decrease of SST gradient over the latitude band between 25° and 40°, while an increase of SST gradient is found over the latitude band between 40° and 55°. Such features are notable across all ocean basins. The pattern illustrates that the wider tropics are linked to poleward displacement of the meridional SST gradient over the midlatitude area, that is, MMTG.
While the study corroborates John's assertion that the Hadley cell is expanding, it also acknowledges that it should be more prevalent across the southern hemisphere, where there is more ocean.
Interesting.