What forms the subtropical maximum in mid-tropospheric warming?

Climate observations and predictions present the following warming patterns in mid-troposphere: A tropical minimum centered on the Indo-Pacific warm pool and a few peaks in the subtropics. Given the fact that the subtropical warming peaks nearly collocate with the subsidence centers, we propose the following mechanism in favor of mean vertical advection (VA) of the warmed air. As the tropospheric warming follows moist adiabats, anomalous warm advection is generated by climatological subsidence in the subtropics, and vice versa in the convective equatorial regions.

Diagnostics with global climate models' output show that in convective regions, the cooling effect of VA nearly balances latent heating. In the subtropics, the warming effect of VA is the leading term to force the response of longwave cooling (a sign of enhanced warming) and weakening of vertical motion. Experiments with a linear baroclinic model illustrate that VA is the major former of the subtropical warming peaks and slow down of Hadley circulation. Following the thermal wind balance, it reduces the subtropical wind shear.

Enhanced subtropical warming is equivalent to the expansion of Hadley circulation. Our theory gives a thermodynamic explanation for both phenomena and links it to vertical warming gradient. From the energy balance point of view, the efficiency of Hadley cell to transport heat from equator to mid-latitude increases due to larger vertical gradient of potential temperature. Thus, tropical meridional temperature gradient is flattened, tropics expand and wind shear weakens.