Sudden stratospheric warmings during El Niño and La Niña: sensitivity to atmospheric model biases

Abstract

The number of sudden stratospheric warmings (SSWs) per year is affected by the phase of the El Niño–Southern Oscillation (ENSO), yet there are discrepancies between the observed and modelled relationship. We investigate how systematic model biases in atmospheric winds and temperatures may affect the ENSO–SSW connection. A two-step bias correction process is applied to the troposphere, stratosphere, or full atmosphere of an atmospheric general circulation model. ENSO-type sensitivity experiments are then performed by adding El Niño and La Niña sea surface temperature (SST) anomalies to the model's prescribed SSTs, to reveal the impact of differing climatologies on the ENSO–SSW teleconnection. The number of SSWs per year is overestimated in the control run, and this statistic is improved when biases are reduced in both the stratosphere and troposphere. The seasonal cycle of SSWs is also improved by the bias corrections. The composite SSW responses in the stratospheric zonal wind, geopotential height, and surface response are well represented in both the control and bias-corrected runs. The model response of SSWs to ENSO phase is more linear than in observations, in line with previous modelling studies, and this is not changed by the reduced biases. However, the ratio of wave 1 events to wave 2 events as well as the tendency to have more wave 1 events during El Niño years than La Niña years is improved in the bias-corrected runs.