Publication: Estudio de la variabilidad interna de la baroclinicidad y consistencia con los mecanismos de realimentación de los modos anulares
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2016-02-22
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Universidad Complutense de Madrid
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Abstract
Los modos anulares son el primer patrón de variabilidad de la altura geopotencial y la presión en superficie en los extratrópicos, patrón que es robusto para un amplio rango de escalas de tiempo. Este patrón tiene gran simetría zonal y estructura dipolar en latitud, la cual se asocia al intercambio de masa de aire entre latitudes medias y altas y al desplazamiento norte-sur del frente polar y de la corriente en chorro extratropical. Dado que la posición del frente polar determina en gran medida las trayectorias preferentes de las borrascas, y por tanto la distribución de la precipitación extratropical, existe un gran interés en entender la dinámica implicada en esta variabilidad. Este trabajo estudia la dinámica de la variabilidad interna anular atmosférica desde la perspectiva del viento zonal en base al concepto del índice zonal. Éste se define como la componente principal del primer modo de variabilidad del viento medio zonal, el cual consiste en una migración latitudinal del chorro alrededor de su posición climatológica. Además de tener la misma estructura que las anomalías de viento de los modos anulares, las series temporales que describen la variabilidad de ambos patrones se encuentran altamente correlacionadas. El uso de una métrica basada en el viento zonal ofrece una visión particularmente simple de la variabilidad anular porque el viento zonal promediado zonalmente e integrado verticalmente es únicamente forzado por el flujo eddy de momento y amortiguado por la fricción. Desde esta perspectiva, la persistencia de la variabilidad anular viene determinada fundamentalmente por las propiedades del flujo eddy de momento. Lorenz and Hartmann [2001; 2003] mostraron que aunque el forzamiento eddy está dominado por su rápida componente aleatoria, también tiene una débil componente de baja frecuencia que depende de las anomalías del viento zonal. Este acoplamiento, o realimentación, hace que las anomalías del viento zonal decaigan en escalas de tiempo más largas que la fricción...
The annular mode is the leading pattern of variability of extratropical geopotential height and surface pressure, a pattern that is robust for a wide range of timescales. This pattern has great zonal symmetry and a dipolar structure in latitude, associated with air mass exchange between middle and high latitudes and with meridional displacements of the polar front and extratropical jet stream. Since the polar front latitude strongly constrains the preferred storm tracks, and hence the extratropical precipitation patterns, there is much interest in understanding the dynamics of this variability. This work studies the dynamics of the internal atmospheric annular variability from a zonal wind perspective using the zonal index concept. This is defined as the principal component of the leading variability pattern of zonal-mean zonal wind, which consists of a latitudinal migration of the jet about its climatological position. This pattern agrees well with the structure of the zonal wind anomalies associated with the annular variability, and the corresponding time series are also highly correlated. The use of the zonal wind metric provides a particularly simple framework to analyze the annular variability because the zonally- and vertically-integrated zonal wind is forced by the eddy momentum flux alone and damped by friction. From this perspective, the persistence of annular variability is essentially determined by the eddy momentum flux properties. Lorenz and Hartmann [2001; 2003] showed that although the eddy forcing is dominated by its fast, random component, it also has a weak low frequency component that depends on the zonal wind anomalies. This coupling, or feedback, makes zonal wind anomalies decay in time scales longer than friction.
The annular mode is the leading pattern of variability of extratropical geopotential height and surface pressure, a pattern that is robust for a wide range of timescales. This pattern has great zonal symmetry and a dipolar structure in latitude, associated with air mass exchange between middle and high latitudes and with meridional displacements of the polar front and extratropical jet stream. Since the polar front latitude strongly constrains the preferred storm tracks, and hence the extratropical precipitation patterns, there is much interest in understanding the dynamics of this variability. This work studies the dynamics of the internal atmospheric annular variability from a zonal wind perspective using the zonal index concept. This is defined as the principal component of the leading variability pattern of zonal-mean zonal wind, which consists of a latitudinal migration of the jet about its climatological position. This pattern agrees well with the structure of the zonal wind anomalies associated with the annular variability, and the corresponding time series are also highly correlated. The use of the zonal wind metric provides a particularly simple framework to analyze the annular variability because the zonally- and vertically-integrated zonal wind is forced by the eddy momentum flux alone and damped by friction. From this perspective, the persistence of annular variability is essentially determined by the eddy momentum flux properties. Lorenz and Hartmann [2001; 2003] showed that although the eddy forcing is dominated by its fast, random component, it also has a weak low frequency component that depends on the zonal wind anomalies. This coupling, or feedback, makes zonal wind anomalies decay in time scales longer than friction.
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Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de la Tierra, Astronomía y Astrofísica I, leída el 18-01-2015