A number of climate models simulate / describe an Antarctic polar vortex that persists for 1–2 weeks longer in the springtime than in observations, being sometimes accompanied by a cold temperature bias. These biases have important implications on the stratospheric dynamics and chemistry, like more intense ozone destruction and systematic late seasonal ozone recovery over Antarctica, which affect simulated long-term ozone trends and Antarctic climate variability and evolution.
The general consensus to explain a simulated late final warming is that climate models lack sufficient parameterized gravity wave (GW) forcing in the southern stratosphere (specially around 60˚S), but its orographic or non-orographic origin is still controversial and somehow related to how GW parameterizations are usually constructed. On one hand, parameterized orographic gravity waves (OGWs) are mainly active in the stratosphere, which agrees with observations. Due to the misrepresentation of small islands and an absence of lateral propagation by construction, the parameterized drag presents an unphysical minimum around 60˚S, and thus OGWs are often seen as primarily responsible for the bias. On the other hand, non-orographic gravity waves (NGWs) are usually treated in parameterizations as small-amplitude waves, implying that they can propagate a long way up before dissipating and that they are poorly active in the stratosphere. This also constitutes an unphysical aspect of parameterizations: recent observational studies have emphasized the intermittent character of GW momentum fluxes (i.e. presence of sporadic, large momentum fluxes that tend to break and force the circulation at lower levels), and this intermittency is absent from nearly all parameterizations. Hence another possibility is that NGWs contribute to the missing drag.
In this presentation we will analyze the resolved and unresolved wave forcing during the final warming of the southern stratosphere in climate simulations with the LMDz general circulation model, and in (re-)analysis products. LMDz includes state-of-the-art stochastic parameterizations of NGWs tied to their tropospheric sources (i.e. convection and fronts/jets), which spontaneously generate intermittent, log-normally distributed momentum fluxes in agreement with observations. As a result, we will show that the contribution to the total GW drag in the stratosphere of nonorographic GWs is larger than that reported in previous studies with different GW parameterizations, and no significant bias on the final warming date is found in our model. We will also show that the ratio of OGW to NGW drag parameterized in LMDz is qualitatively realistic as compared to the high-resolution, gravity-resolving ECMWF operational analyses. Tying the wave amplitudes to source characteristics in parameterizations introduces realistic momentum-flux intermittency, which improves the simulation of the Antarctic stratospheric final warming.
*email: acamara@ucar.edu
*Preference: Oral