Impact of Parametrized Nonorographic Gravity Wave Drag on Stratosphere‐Troposphere Coupling in the Northern and Southern Hemispheres

Inna Polichtchouk,Theodore G Shepherd,Nicholas J Byrne

doi:10.1029/2018gl078981

Abstract

AbstractThe impact of parametrized nonorographic gravity wave drag (NOGWD) on the stratosphere‐troposphere dynamical coupling in atmospheric models is relatively unexplored. Using the European Centre for Medium‐Range Weather Forecasts Integrated Forecast System, we find that changes in NOGWD strength have a substantial impact on the tropospheric eddy‐driven jet (EDJ) in both hemispheres, but the sense of the impact is opposite in the two hemispheres. In the Northern Hemisphere the impact occurs via changes in the amplitude and persistence of stratospheric anomalies. In the Southern Hemisphere it occurs instead via differences in the sensitivity of the EDJ to a given stratospheric anomaly, arising from changes in the seasonal cycle leading up to the polar vortex breakdown. Increasing NOGWD eliminates the springtime phase of the Southern Hemisphere tropospheric semiannual oscillation, resulting in a more equatorward annual‐mean EDJ and showing that the semiannual oscillation cannot be explained entirely from tropospheric mechanisms.

Highlights

A growing body of evidence suggests the importance of stratospheric polar vortex variability for tropospheric predictability on seasonal and subseasonal time scales in both hemispheres (e.g., Baldwin & Dunkerton, 2001; Byrne & Shepherd, 2018; Douville, 2009; Seviour et al, 2014; Sigmond et al, 2013; Son et al, 2013)
That the nature of the coupling itself is unchanged is shown by the fact that if stratospheric sudden warmings (SSWs) with similar northern annular mode (NAM) index in the middle to lower stratosphere are selected from the INCREASED and REDUCED cases, the surface impact is similar in both experiments
It should be noted that the variance of surface NAM anomalies is largely unchanged between the different cases (37 m for ERA-Interim, 40 m for CTRL, 38 m for INCREASED, and 40 m for REDUCED), such that a larger relative value for REDUCED in Figure 1 really does mean a larger impact on the troposphere

Summary

Introduction

A growing body of evidence suggests the importance of stratospheric polar vortex variability for tropospheric predictability on seasonal and subseasonal time scales in both hemispheres (e.g., Baldwin & Dunkerton, 2001; Byrne & Shepherd, 2018; Douville, 2009; Seviour et al, 2014; Sigmond et al, 2013; Son et al, 2013). In the Northern Hemisphere (NH), the main source of stratospheric polar vortex variability is stratospheric sudden warmings (SSWs), which typically precede an equatorward shift of the eddy-driven jets (EDJ) and have a signal in the tropospheric annular modes (e.g., Baldwin & Dunkerton, 2001; Hitchcock & Simpson, 2014; Karpechko et al, 2017). Polichtchouk et al (2018) have shown that it continues to be important even in a relatively high resolution model: Using the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) at T255L137 resolution, they found that decreasing NOGWD results in an increase in amplitude and persistence and a decrease in frequency of the SSWs in the NH, and a delay in the final warming date in the SH

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