The Stratospheric Pathway of La Niña

A Northern Hemisphere (NH) polar stratospheric pathway for La Nina events is established during wintertime based on reanalysis data for the 1958-2012 period. A robust polar stratospheric response is observed in the NH during strong La Nina events, characterized by a significantly stronger and cooler polar vortex. Significant wind anomalies reach the surface, and a robust impact on the North Atlantic-European (NAE) region is observed. A dynamical analysis reveals that the stronger polar stratospheric winds during La Nina winters are due to reduced upward planetary wave activity into the stratosphere. This finding is the result of destructive interference between the climatological and the anomalous La Nina tropospheric stationary eddies over the Pacific-North American region. In addition, the lack of a robust stratospheric signature during La Nina winters reported in previous studies is investigated. It is found that this is related to the lower threshold used to detect the events, which signature is consequently more prone to be obscured by the influence of other sources of variability. In particular, the occurrence of stratospheric sudden warmings (SSWs), partly linked to the phase of the quasi-biennial oscillation, modulates the observed stratospheric signal. In the case of La Nina winters defined by a lower threshold, a robust stratospheric cooling is found only in the absence of SSWs. Therefore, these results highlight the importance of using a relatively restrictive threshold to define La Nina events in order to obtain a robust surface response in the NAE region through the stratosphere

Role of Stratospheric Sudden Warmings on the response to Central Pacific El Niño

The Northern Hemisphere (NH) polar stratospheric response to Central Pacific El Niño (CP-El Niño) events remains unclear. Contradictory results have been reported depending on the definition and events considered. We show that this is due to the prominent role of Stratospheric Sudden Warmings (SSWs), whose signal dominates the NH winter polar stratospheric response to CP-El Niño. In fact, the CP-El Niño signal is robust when the events are classified according to the occurrence of SSWs and displays opposite response in winters with and without SSWs. In the absence of SSWs, polar stratospheric responses to Central Pacific and Eastern Pacific El Niño are clearly distinguishable in early winter, in relation to differences in the Pacific-North American pattern. Our results demonstrate that the occurrence of SSWs needs to be taken into account when studying the stratospheric response to CP-El Niño and explain why different responses to CP-El Niño have been reported previously

The stratospheric ENSO pathway: flavors and asymmetry

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Astrofísica y Ciencias de la Atmósfera, leída el 29-11-2017ENSO (El Niño-Southern Oscillation) es el principal modo de variabilidad interanual en los trópicos y muestra relevantes teleconexiones en los extratrópicos del hemisferio norte (HN) (Horel and Wallace 1981). Durante la fase cálida de ENSO, El Niño, su señal en la troposfera tropical se propaga hacia extratrópicos mediante ondas de Rossby (ej., García-Herrera et al. 2006), propiciando además un aumento de la actividad ascendente de las ondas hacia la estratosfera. (ej., Manzini et al. 2006). En consecuencia, el vórtice polar se debilita y la circulación meridiana media se intensifica generando anomalías cálidas en el polo, que se propagan hacia la troposfera, afectando el clima de las regiones del norte del Atlántico y Europa (NAE) (Ineson and Scaife 2009; Cagnazzo and Manzini 2009; Bell et al. 2009). Sin embargo, la respuesta estratosférica a un distinto tipo de El Niño, más centrado en el Pacífico (CP) que El Niño canónico en el este del Pacífico (EP), aún no está clara. Estudios previos han mostrado conclusiones contradictorias sobre el parecido de CP El Niño con EP El Niño en el HN y los resultados parecen depender del número de casos analizados y la definición utilizada (Garfinkel et al. 2013). La vía estratosférica para la fase fría de ENSO, La Niña, es también incierta, dado que en los registros observacionales su enfriamiento estratosférico parece ser débil o no significativo (Free and Seidel 2009; Mitchell et al. 2011)...El Niño-Southern Oscillation (ENSO) is the main mode of interannual variability in the tropics and has relevant teleconnections in the North Hemisphere (NH) extratropics (Horel and Wallace 1981). During the warm ENSO phase, El Niño, its tropical tropospheric signal propagates poleward by means of Rossby waves (e.g., García-Herrera et al. 2006), which leads to an increased upward wave activity into the stratosphere (e.g., Manzini et al. 2006). As a consequence, the polar vortex weakens and the mean meridional circulation enhances, with associated polar warm anomalies propagating downwards to the troposphere and impacting the North-Atlantic European (NAE) region climate (Ineson and Scaife 2009; Cagnazzo and Manzini 2009; Bell et al. 2009). Nonetheless, the stratospheric response to a different El Niño flavor, with the largest anomalies in the Central Pacific (CP) in comparison with the canonical eastern Pacific (EP) El Niño, remains unclear. Contradictory conclusions have been found on the resemblance of CP El Niño with EP El Niño and results seem to depend on the composite size and the definition used (Garfinkel et al. 2013). The stratospheric pathway for the cold ENSO phase, La Niña, is also uncertain, since its stratospheric cooling appears to be weak or non-significant in the observational record (Free and Seidel 2009; Mitchell et al. 2011)...Depto. de Física de la Tierra y AstrofísicaFac. de Ciencias FísicasTRUEunpu