Higher tropical SSTs strengthen the tropical upwelling via deep convection

Recent observations show a distinct cooling of the tropical lower stratosphere, and chemistry-climate models (CCMs) suggest a link to a strengthening tropical upwelling, arguably related to increases in greenhouse gas concentrations from anthropogenic activity. The present study explores the strengthening of tropical upwelling by comparing ensemble realisations of two different transient scenarios with the CCM E39/C. Both scenarios share the same boundary conditions, including concentrations of ozone-depleting substances, but differ in their climate forcing via prescribed sea surface temperatures (SSTs) and well-mixed greenhouse gas concentrations. In the summer hemisphere tropics, higher SSTs amplify deep convection locally and hence the convective excitation of quasistationary waves. These waves propagate upward through the region of easterly winds while dissipating, but still carry enough of the signal into the low-latitude lower stratosphere to induce an anomalous low-latitude Brewer-Dobson (BD) cell. The transport change in turn increases the flux of ozone-poor tropospheric air into the tropical lower stratosphere

Determining the tropopause height from gridded data

Journal ArticleA method is presented to determine tropopause height from gridded temperature data with coarse vertical resolution. The algorithm uses a thermal definition of the tropopause, which is based on the concept of a threshold lapse-rate. Interpolation is performed to identify the pressure at which this threshold is reached and maintained for a prescribed vertical distance. The method is verified by comparing the heights calculated from analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF) with the observed heights at individual radiosonde stations. RMS errors in the calculated tropopause heights are generally small. They range from 30"40 hPa in the extratropics to 10 20 hPa in the tropics. The largest deviations occur in the subtropics, where the tropopause has strong meridional gradients that are not adequately resolved by the input data

Impact of prescribed SSTs on climatologies and long-term trends in CCM simulations

Chemistry-Climate Model (CCM) simulations are commonly used to project the past and future development of the dynamics and chemistry of the stratosphere, and in particular the ozone layer. So far, CCMs are usually not interactively coupled to an ocean model, so that sea surface temperatures (SSTs) and sea ice coverage are prescribed in the simulations. While for future integrations SSTs have to be taken from precalculated climate model projections, for CCM experiments resembling the past either modelled or observed SSTs can be used. This study addresses the question to which extent atmospheric climatologies and long-term trends for the recent past simulated in the CCM E39C-A differ when choosing either observed or modelled SSTs. Furthermore, the processes of how the SST signal is communicated to the atmosphere, and in particular to the stratosphere are examined. Two simulations that differ only with respect to the prescribed SSTs and that span years 1960 to 1999 are used. <br><br> Significant differences in temperature and ozone climatologies between the model simulations are found. The differences in ozone are attributed to differences in the meridional circulation, which are in turn driven by weaker wave forcing in the simulation with generally lower SSTs. The long-term trends over 40 years in annual mean temperature and ozone differ only in the troposphere, where temperatures are directly influenced by the local SST trends. Differences in temperature and ozone trends are only found on shorter time scales. The trends in tropical upwelling, as a measure of the strength of the Brewer-Dobson circulation (BDC), differ strongly between the simulations. A reverse from negative to positive trends is found in the late 1970s in the simulation using observed SSTs while trends are positive throughout the simulation when using modelled SSTs. The increase in the BDC is a robust feature of the simulations only after about 1980 and is evident mainly in the tropics in the lower stratosphere