The stratospheric response to extratropical torques and its relationship with the annular mode

Transcription

1 The stratospheric response to extratropical torques and its relationship with the annular mode Peter Watson 1, Lesley Gray 1,2 1. Atmospheric, Oceanic and Planetary Physics, Oxford University 2. National Centre for Atmospheric Science

2 The stratospheric polar vortex During winter, the pole cools by radiation. Thermal wind balance then implies a strong westerly jet will form around the pole. In the NH, planetary waves cause this jet to be very variable ( stratospheric sudden warmings (SSWs)). SSWs are thought to impact the troposphere. Image courtesy of Dann Mitchell Baldwin and Dunkerton (2001)

3 Similarity between the stratospheric annular mode and the response to external forcings NAM signature in GPH at 50hPa in ERA-40: Equatorial quasi-biennial oscillation (e.g. Watson and Gray, 2014) ENSO (e.g. Sassi et al. 2004) Solar cycle (e.g. Labitzke, 2005) Volcanic eruptions (e.g. Stenchikov et al., 2006)

4 Forced response often resembles leading EOF in other systems Lorenz (1963) butterfly attractor Palmer and Weisheimer (2011) time-averaged response of Lorenz 1963 system to a constant forcing is very nearly parallel to the leading EOF, for all forcings in the xy plane. Ring and Plumb (2007, 2008) response of tropospheric model to applied torques and heating resembles NAM

5 Motivations and basic method Question Is the stratospheric circulation response to arbitrary forcings annular modelike? Experiment examine the response to a variety of simple zonal torques in a stratospheric model (Can also help understanding of how the stratosphere responds to gravity wave drag.)

6 Stratosphere-Mesosphere model Global primitive equation model of middle atmosphere (16-80km). Resolution: 5 x5 x2km. 16km GPH specified to represent the tropospheric state from output of HadGEM2 GCM. Shown in many studies to represent stratospheric variability well. Expected to simulate the large-scale dynamical response to torques qualitatively reasonably. Experiments performed both with perpetual January conditions and with a seasonal cycle. Runs with applied torques compared to control runs without.

7 Control runs - u PerJan u climatology SeasCyc Jan u climatology u climatologies (contours) and differences from ERA-40 (colours) Stronger, reasonably realistic vortex

8 The applied zonal torques Torques are zonally symmetric and of the form: f φ, z = f 0 φ 0, z 0 exp φ φ 0 δφ 2 z z 0 δz 2 z (φ 0, z 0 ) Latitude f 0 chosen to make zonal momentum added by the torque the same in each experiment (except when the torque magnitude is varied).

9 PerJan u response The steady-state response to applied torques is NAM-like. A positive torque can give a negative u response! PerJan NAM u Colours u responses Contours torques Numbers anomaly correlations with NAM signature

10 PerJan transient u response

11 PerJan planetary wave response Torque creates curvature in u. The meridional potential vorticity gradient is increased, encouraging greater upward planetary wave propagation. Waves cause deceleration of u (Eliassen- Palm flux convergence increases).

12 SeasCyc transient u response Response in SC runs does not become NAM-like. How have the wave feedbacks changed?

13 SeasCyc wave response Qualitative nature of wave feedbacks is similar to that in PerJan runs and this seems robust, but feedbacks are weaker. Consistent with ray theory and the stronger vortex in these runs.

14 Concluding remarks Applied westerly torques in the wintertime extratropical stratosphere have two principal effects: 1. Directly accelerating the zonal mean flow. 2. Increasing the meridional PV gradient so that Rossby wave propagation into the stratosphere increases, producing opposite acceleration of u. The wave feedback is large and can dominate the response. When the wave feedback is strong, the response is NAM-like (PerJan runs), but not when it is weaker (SeasCyc runs). Supports the idea that arbitrary forcings will tend to produce a NAM-like response, if feedbacks are strong enough. May explain why NH extratropical response to various natural forcings is NAM-like in stratosphere. u climatology is important for determining the strength of wave feedbacks and response to forcings.