This annual report summarizes achievements in year 2 of the science programme set out in the UGAMP Science Plan for April 1997 to March 2002.
The atmospheric response to the 1997/98 El Nino event has been well simulated in ensemble integrations of the Unified Model; sub-seasonal weather systems have been shown to have been a factor in the rapid growth of the event, limiting its level of predictability. The important role of seasurface temperature anomalies in the Atlantic has been identified. These anomalies were shown to have had a significant impact on the climate of both the tropics and the extratropics. This impact included a contribution to a predictable signal in the weather patterns over Europe. It has therefore been shown that to develop systems for seasonal forecasting for Europe, it is essential that the state of the Atlantic Ocean is well simulated, as well as those of the Pacific and Indian Oceans.
A detailed quantitative picture has been obtained of the seasonal evolution, interhemispheric differences and year-to-year variability of water vapour in the vicinity of the tropopause. A mechanism has been identified - involving horizontal transport near the tropopause by weather systems - that leads to significant moistening of the polar lower stratosphere. Water vapour is the dominant greenhouse gas, and this work has contributed to narrowing the uncertainties in processes affecting its distribution. This achievement is a significant step towards a detailed quantitative analysis to be made of water vapour transport in climate models.
Through support for the Unified Model and the co-ordination of access to supercomputing services, the core programme has enabled the UGAMP-wide use of the model. Improvements have been made in the representation of physical processes in UGAMP's version of the Unified (climate) Model, and in the treatment of trace-gas transport in UGAMP models. (a) The radiation scheme developed at the Hadley Centre has been extended for use in the stratosphere and mesosphere, enabling more accurate simulations to be made of the climatic effects of ozone depletion. (b) From a sound theoretical foundation, a new parametrization has been developed of the effect of nonorographic gravity waves on the atmospheric circulation, superseding the artificial representation of the effect of such waves in the stratosphere and mesosphere of the Unified Model. (c) For the first time, an algorithm has been developed for adaptive mesh refinement on a sphere, enabling accurate, computationally efficient calculations to be made of the transport of trace gases, such as ozone, in the atmosphere.
UGAMP models of chemistry and transport have successfully simulated the long-term evolution of chemical compounds in the stratosphere, in particular the observed amounts of ozone over the Arctic and its interannual variability. A coupled model of dynamics, radiation and photochemistry has shown that a (positive) radiative feedback from ozone loss will enhance ozone loss if the stratosphere cools as a result of increasing loading of greenhouse gases. The amount of ozone in the troposphere arising from in-situ photochemistry has been calculated, together with the amount transported into the stratosphere. These achievements have advanced our ability to represent the impact of ozone changes on climate.
Alan O'Neill , Director