Stratospheric Influence during Tropical Upper-Tropospheric Warming in a Simple General Circulation Model

Abstract:

Comprehensive climate models show a large uncertainty of the polar vortex response to global warming which also affects the robustness of tropospheric circulation changes.
In order to investigate the stratospheric influence during tropical upper-tropospheric warming, three sets of experiments with different basic states in the stratosphere are performed using a dry dynamical-core model: one reference set with a polar vortex that is known to exhibit realistic stratosphere-troposphere coupling (REF), a second set with a polar vortex, but a cooler polar lower stratosphere compared to the REF experiment (CLS), and one set without a polar vortex (NPV).

In case of the NPV experiment, tropical warming leads to an upward shift of the critical layer above the tropospheric jet such that more Eliassen–Palm (EP) flux is able to enter the lower stratosphere.
As a consequence, the lower stratospheric Brewer–Dobson circulation (BDC) strengthens which counteracts the tropical heating by attenuating the meridional temperature gradient in the lower stratosphere.
In the experiments with polar vortex (REF and CLS), critical tropical warmings exist (2 K and 8 K, respectively) at which the tropospheric jet merges with the polar vortex in such a way that the resulting wave guide favors wave reflection.
Hence, the EP flux budget entering the stratosphere is abruptly decreased due to an emerging downward component and the lower stratospheric BDC weakens.
Consequentially, the previously negative feedback of the BDC on the meridional temperature gradient in the lower stratosphere is turned into a positive feedback leading to an abrupt strengthening of the polar vortex as well as to an accelerated poleward shift of the tropospheric jet compared to the NPV experiment.

Whether similar dynamical regimes exist also in comprehensive models, remains to be analyzed.
Nevertheless, the dependence of atmospheric circulation changes on the basic stratospheric state in this idealized model study might help to reconcile the results of comprehensive climate models.