What maintains the subtropical jet?

Lachmy Orli and Nili Harnik, 2014: The Transition to a Subtropical Jet Regime and Its Maintenance. J. Atmos. Sci., 71, 1389–1409

In observations, the zonal mean winter jet is subtropical – at the edge of the Hadley cell, while the zonal mean summer jet is eddy driven or merged- in midlatitudes in the Ferrel cell (Figure 1 below). Thus the bserved subtropical jet occurs during times when the waves are strong.

Figure 1: The climatological zonal mean zonal winds (shading) and the meridional stream function (contours) using NCEP reanalysis.

In the MQG model, a subtropical jet forms when the eddies are weak. This is because the eddy momentum flux convergence is zero at the edge of the Hadley cell, thus the eddies necessarily act to shift the subtropical jet poleward. If the eddies are strong enough they succeed, and the jet shifts to the peak in eddy momentum flux convergence, inside the Ferrel cell.

Why are eddies weak in the subtropical jet regime even though the vertical shear of the jet is strong?

Because at subtropical latitudes, β is strong enough to make the lower level PV gradients positive. Baroclinic instability is thus between midlatitude lower level waves and upper level subtropical waves and is inefficient. In fact, the most unstable waves in the subtropical jet regime are high latitude, slowly westward propagating planetary scale waves. For these waves the eddy momentum flux convergence maximizes too poleward to strongly affect the subtropical jet.

What happens in observations?

A closer look at the zonally varying flow (figure 2 below) shows that eddies are strongest at longitudes where the jet is in midlatitudes, while they are weakest where the jet is subtropical.

The observations are thus consistent with the MQG model in localized zonal sectors: a subtropical jet forms when the eddies are weak enough to allow it to be sustained.

Figure 2: Climatological Jun-Aug mean 300hPa zonal wind (contours) and synoptic 850hPa poleward heat flux (shading, m/sec K) using NCEP reanalysis for years 1969–2011.