Tropical Diabatic Heating and Its Influence on the Extratropical Jet during Winter

Abstract:

The extratropical jets moderate the connection between the tropics and mid-to-high latitudes by acting as waveguides and by influencing the development of transient and quasi-stationary Rossby waves. Tropical diabatic heating (thermal driving) helps set the mean strength and variability of the jets by driving the thermally direct Hadley circulation, which transports momentum out of the tropics. Understanding the relative importance of thermal driving and its relationship to the jets is challenging because of geographic differences from sector to sector, and variability over a wide range of time scales. Using the ERA-Interim reanalysis, we calculate a thermal driving index to quantify the relationship between diabatic heating and wintertime jet variability in the Northern Hemisphere. ERA-Interim is restricted to the satellite interval, which is an important consideration for accurately capturing tropical variability. We document clarified results relative to previous analyses based on the ERA-40 reanalysis.

Regressions of zonal mean zonal wind anomalies onto the thermal driving index show a stronger relationship between diabatic heating and the jet in the North Pacific sector than in the North Atlantic sector on these time scales. We find evidence of the expected link via the Hadley cell: the tropical Pacific thermal driving index is associated with a stronger signature in the meridional overturning stream function than the tropical Atlantic thermal driving index. Absolute values of diabatic heating are stronger in the North Pacific than the North Atlantic in all winter months, and the seasonality of the Pacific thermal driving is much more prominent, such that the Pacific dominates the hemispheric zonal mean picture.

Ongoing work will better quantify the influence of thermal driving on the extratopical jets at subseasonal to interannual time scales, and investigate the robustness of these relationships in other reanalysis products and in global climate models.