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Abstract:

An influence of solar irradiance variations on Earth's surface climate has been repeatedly suggested, based on correlations between solar variability and meteorological variables1. Specifically, weaker westerly winds have been observed in winters with a less active sun, for example at the minimum phase of the 11-year sunspot cycle2-4. With some possible exceptions5,6, it has proved difficult for climate models to consistently reproduce this signal7,8. Spectral Irradiance Monitor satellite measurements indicate that variations in solar ultraviolet irradiance may be larger than previously thought9. Here we drive an ocean - atmosphere climate model with ultraviolet irradiance variations based on these observations. We find that the model responds to the solar minimum with patterns in surface pressure and temperature that resemble the negative phase of the North Atlantic or Arctic Oscillation, of similar magnitude to observations. In our model, the anomalies descend through the depth of the extratropical winter atmosphere. If the updated measurements of solar ultraviolet irradiance are correct, low solar activity, as observed during recent years, drives cold winters in northern Europe and the United States, and mild winters over southern Europe and Canada, with little direct change in globally averaged temperature. Given the quasiregularity of the 11-year solar cycle, our findings may help improve decadal climate predictions for highly populated extratropical regions. © 2011 Macmillan Publishers Limited. All rights reserved.

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.

Abstract:

Stratospheric sudden warmings (SSWs) are a major source of variability during Northern Hemisphere winter. The frequency of occurrence of SSWs is influenced by El Nio-Southern Oscillation (ENSO), the quasi-biennial oscillation (QBO), the 11 year solar cycle, and volcanic eruptions. This study investigates the role of ENSO and the QBO on the frequency of SSWs using the National Center for Atmospheric Research's Whole Atmosphere Community Climate Model, version 3.5 (WACCM3.5). In addition to a control simulation, WACCM3.5 simulations with different combinations of natural variability factors such as the QBO and variable sea surface temperatures (SSTs) are performed to investigate the role of QBO and ENSO. Removing only one forcing, variable SSTs or QBO, yields a SSW frequency similar to that in the control experiment; however, removing both forcings results in a significantly decreased SSW frequency. These results imply nonlinear interactions between ENSO and QBO signals in the polar stratosphere during Northern Hemisphere winter. This study also suggests that ENSO and QBO force SSWs differently. The QBO forces SSW events that are very intense and whose impact on the stratospheric temperature can be seen between December and June, whereas ENSO forces less intense SSWs whose response is primarily confined to the months of January, February, and March. The effects of SSWs on the stratospheric background climate is also addressed here. Copyright 2011 by the American Geophysical Union.