Abstract
Abstract. The Goddard Earth Observing System Chemistry-Climate Model, Version 2 (GEOS V2 CCM) is used to investigate the response of the Antarctic stratosphere to (1) warm pool El Niño (WPEN) events and (2) the sensitivity of this response to the phase of the QBO. A new formulation of the GEOS V2 CCM includes an improved general circulation model and an internally generated quasi-biennial oscillation (QBO). Two 50-yr time-slice simulations are forced by repeating annual cycles of sea surface temperatures and sea ice concentrations composited from observed WPEN and neutral ENSO (ENSON) events. In these simulations, greenhouse gas and ozone-depleting substance concentrations represent the present-day climate. The modelled responses to WPEN, and to the phase of the QBO during WPEN, are compared with NASA's Modern Era Retrospective-Analysis for Research and Applications (MERRA) reanalysis. WPEN events enhance poleward tropospheric planetary wave activity in the central South Pacific region during austral spring, leading to relative warming of the Antarctic lower stratosphere in November/December. During the easterly phase of the QBO (QBO-E), the GEOS V2 CCM reproduces the observed 4–5 K warming of the polar region at 50 hPa, in the WPEN simulation relative to ENSON. In the recent past, the response to WPEN events was sensitive to the phase of the QBO: the enhancement in planetary wave driving and the lower stratospheric warming signal were mainly associated with WPEN events coincident with QBO-E. In the GEOS V2 CCM, however, the Antarctic response to WPEN events is insensitive to the phase of the QBO: the modelled response is always easterly QBO-like. The QBO signal does not extend far enough into the lower stratosphere and upper troposphere to modulate convection and thus planetary wave activity in the south central Pacific.
Highlights
Recent literature has identified two types of El Nino events
Conventional or “cold tongue” El Nino (CTEN) events are characterized by positive sea surface temperature (SST) anomalies in the eastern equatorial Pacific (Nino 3 region) (Rasmusson and Carpenter, 1982; Kug et al, 2009), while “warm pool” El Nino (WPEN) events are characterized by positive SST anomalies in the central equatorial Pacific (Nino 4 region) (Larkin and Harrison, 2005; Ashok et al, 2007; Kug et al, 2009)
For each year between 1979 and 2009, November/December mean zonal winds at 50 hPa, between 10◦ S and 10◦ N, form a quasibiennial oscillation (QBO) index: QBO easterly years (QBO-E) are identified when the QBO index is less than −2 m s−1; QBO westerly years (QBO-W) are identified when the QBO index is larger than 2 m s−1
Summary
Recent literature has identified two types of El Nino events. Conventional or “cold tongue” El Nino (CTEN) events are characterized by positive sea surface temperature (SST) anomalies in the eastern equatorial Pacific (Nino 3 region) (Rasmusson and Carpenter, 1982; Kug et al, 2009), while “warm pool” El Nino (WPEN) events are characterized by positive SST anomalies in the central equatorial Pacific (Nino 4 region) (Larkin and Harrison, 2005; Ashok et al, 2007; Kug et al, 2009). El Nino-related warming of the Arctic stratosphere has been identified in both observational (Bronnimann et al, 2004; Free and Seidel, 2009) and modelling studies (Sassi et al, 2004; Manzini et al, 2006; Cagnazzo et al, 2009) This warming is a response to increased planetary wave driving: Garfinkel and Hartmann. Hurwitz et al (2011) concluded that simulations forced by repeating WPEN and ENSON boundary conditions would greatly increase the sample size and strengthen their findings Such simulations require a model with a well-resolved stratosphere, a QBO, and because the stratospheric response to WPEN occurs in austral spring and summer, interactive polar ozone chemistry.
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