22.09.2015: FB1-Seminar

11:00 h, Hörsaal, Düsternbrooker Weg 20

Abstract:

The North Hemisphere polar stratospheric response to Central Pacific El Niño (CP El Niño) events is still uncertain. Some studies found a stronger and colder polar stratosphere while others show a weaker and warmer polar vortex. Indeed, the observed signal depends on the CP El Niño index and the number of events considered. Then, there is no consensus on its resemblance with the canonical East Pacific El Niño (EP El Niño).

Our results, based on reanalysis data, show that Stratospheric Sudden Warmings (SSWs) occurrence dominates the CP El Niño signal in the North Hemisphere polar stratosphere. CP winters without SSWs are characterized by a significant cooling from November to January. In contrast, during CP winters with SSWs significant warm anomalies are observed, related in fact to SSWs impact. Therefore, CP winters with and without SSWs show significant and opposite responses and accordingly, in the absence of SSWs the stratospheric responses to EP and CP are distinguishable in early winter. EP and CP different responses are investigated in High-top Coupled Model Intercomparison Project phase 5 (CMIP5) historical simulations. Results highlight the importance of analysing the seasonal evolution of the signals. We found an earlier enhancement of the Pacific-North American pattern for EP El Niño and a delayed anomalous upward wave propagation for CP El Niño. Resulting in a non-significant polar stratospheric response for CP winters, while for EP events the characteristic warmer and weaker polar vortex is observed. SSWs play a relevant role in connecting the tropospheric tropical EP El Niño signal with the North-Atlantic European (NAE) region. On the contrary, for CP events CMIP5 models do not show such stratospheric pathway. Consequently, for the seasonal prediction in the NAE region only EP El Niño winters, together with SSWs, should be considered.

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