Climate dynamics

Abstract:

We have studied the characterization of the 11-year solar cycle (SC) signals in the Southern Hemisphere (SH) during the winter and spring using European Centre for Medium-Range Weather Forecasts (ECMWF) daily and monthly data from 1979 to 2009. By separating the response into high (<6 months) and low (>36 months) frequency domains, we have found that spatially different 11-year SC signals exist for high- and low-frequency domains. In the stratosphere, the high- and low-frequency responses tend to enhance each other near the Equator and Subtropics, while they oppose one another at high latitudes. The high-frequency response is marked by a strengthened stratospheric jet during winter and the response is not static but tracks with the centre of the polar vortex. In the lower stratosphere, the positive response of temperature to the 11-year SC is dominated by its low-frequency component, which extends from the North Pole to the South Pole. The low-frequency tropospheric response is latitudinally symmetrical about the Equator and consistent with the modelled responses to temperature perturbation in the lower stratosphere. The signals are found to be sensitive to contamination from the 2002 sudden stratospheric warming event and major volcanic eruptions but the general spatial pattern of the responses remains similar. A significant projection of the 11-year SC onto the Southern Annular Mode (SAM) can only be detected in the stratosphere and in the high-frequency component. The signature is marked by a strengthening of the stratospheric SAM during winter and a weakening of the SAM in the uppermost stratosphere during spring. © 2011 Royal Meteorological Society.

Abstract:

The mean state, variability, and extreme variability of the stratospheric polar vortices, with an emphasis on the Northern Hemisphere (NH) vortex, are examined using two-dimensional moment analysis and extreme value theory (EVT). The use of moments as an analysis tool gives rise to information about the vortex area, centroid latitude, aspect ratio, and kurtosis. The application of EVT to these moment-derived quantities allows the extreme variability of the vortex to be assessed. The data used for this study are 40-yr ECMWFRe-Analysis (ERA-40) potential vorticity fields on interpolated isentropic surfaces that range from 450 to 1450 K. Analyses show that the most extreme vortex variability occurs most commonly in late January and early February, consistent with when most planetary wave driving from the troposphere is observed. Composites around sudden stratospheric warming (SSW) events reveal that the moment diagnostics evolve in statistically different ways between vortex splitting events and vortex displacement events, in contrast to the traditional diagnostics. Histograms of the vortex diagnostics on the 850-K (~10 hPa) surface over the 1958-2001 period are fitted with parametric distributions and show that SSW events constitute the majority of data in the tails of the distributions. The distribution of each diagnostic is computed on various surfaces throughout the depth of the stratosphere; it shows that in general the vortex becomes more circular with higher filamentation at the upper levels. The Northern and Southern Hemisphere (SH) vortices are also compared through the analysis of their respective vortex diagnostics, confirming that the SH vortex is less variable and lacks extreme events compared to the NH vortex. Finally, extreme value theory is used to statistically model the vortex diagnostics and make inferences about the underlying dynamics of the polar vortices. © 2011 American Meteorological Society.