Climate dynamics

Abstract:

The impact of solar irradiance changes on the winter polar stratosphere is investigated using a general circulation model in which the equatorial Quasi-Biennial Oscillation (QBO) is internally generated and self-sustaining. The model results compare favorably with observations, supporting previous findings that the equatorial zonal wind modulates the polar stratospheric response to solar irradiance changes. In the QBO easterly phase, Northern Hemisphere sudden stratospheric warmings are found to be more prevalent under solar minimum conditions than under solar maximum conditions. However, in the QBO westerly phase the reverse is true. The possible solar-modulation of the QBO period is also investigated. Although small changes are evident in the same sense as those observed, i.e. a lengthening of the period during solar minimum conditions, longer simulations would be required to verify the statistical significance of this result. Copyright 2005 by the American Geophysical Union.

Abstract:

ECMWF ERA-40 reanalysis zonal winds are very close to tropical rocketsonde and rawinsonde (radiosonde & radar wind soundings) observations up to 10 hPa. Above 10 hPa differences increase, although the ERA-40 data provide a good representation of tropical winds up to 2-3 hPa. The amplitudes of the quasi-biennial oscillation (QBO) and the semi-annual oscillation (SAO) derived from ERA-40 data match the rawinsonde and rocketsonde observations up to 2-3 hPa. We conclude that zonal-mean ERA-40 equatorial winds could be used, for most purposes, in place of rawinsonde station observations. Copyright 2005 by the American Geophysical Union.

Abstract:

The ERA-40 data set is used to examine the equatorial quasi-biennial oscillation (QBO). The data set extends from the ground to 0.1 hPa (∼65 km) and covers a 44-year period (January 1958 to December 2001), including 18.5 QBO cycles. Analysis of this data set of unprecedented spatial and temporal coverage has revealed a threefold structure in height in the QBO zonal wind anomalies at the equator. In addition to the well-known twofold structure in the lower and middle stratosphere, that is, easterlies overlying westerlies or vice versa, there is a third anomaly in the upper stratosphere. The sign of this upper stratospheric anomaly is the same as the lower stratospheric anomaly, thus forming anomalies of alternating sign throughout the depth of the equatorial stratosphere. The amplitude of this upper stratospheric anomaly is ∼10 m s-1, approximately one third of the amplitude of the lower stratospheric signal. The frequency and descent rates of the east and west QBO phases are analyzed in detail, with particular attention to any 11-year solar cycle influence. In addition to the observed solar modulation of the duration of the QBO west phase the analysis shows a solar modulation of the mean descent rate of the easterly shear zone. The mean time required for the easterly shear zone to descend from 20 to 44 hPa is 2 months less under solar maximum conditions than under solar minimum conditions (7.4 months versus 9.7 months). This rapid descent of the easterly shear zone cuts short the west phase of the QBO in the lower stratosphere during solar maximum periods. Copyright 2005 by the American Geophysical Union.

Abstract:

A multiple linear regression analysis of the ERA-40 dataset for the period 1979-2001 has been used to study the influence of the 11-yr solar cycle on atmospheric temperature and zonal winds. Volcanic, North Atlantic Oscillation (NAO), ENSO, and quasi-biennial oscillation (QBO) signatures are also presented. The solar signal is shown to be readily distinguishable from the volcanic signal. The main solar signal is a statistically significant positive response (i.e., warmer in solar maximum) of 1.75 K over the equator with peak values at 43 km and a reversed signal of similar magnitude at high latitudes that is seasonally dependent. Consistent with this is a statistically significant zonal wind response of up to 6 m s^-1 in the subtropical upper stratosphere/lower mesosphere that is also seasonally dependent. The wind anomalies are westerly/easterly in solar maximum /minimum. In addition, there is a statistically significant temperature response in the subtropical lower stratosphere that shows similarity in spatial structure to the QBO response, suggesting a possible interaction between the solar and QBO signals in this region. The solar response in tropospheric zonal winds is small but significant, confirming previous studies that indicate a possible modulation of the Hadley circulation. © 2005 American Meteorological Society.

Abstract:

Forty-year, time-varying GCM integrations have been run both with and without coupled chemistry. The results suggest that while computer resources preclude multi-member ensemble experiments with coupled chemistry models to achieve statistical significance, robust results can be achieved for some atmospheric parameters by running ensemble experiments with imposed ozone changes. Copyright © 2005 Royal Meteorological Society.