EXTRATROPICAL FORCING OF TROPICAL CONVECTION IN A NORTHERN WINTER SIMULATION WITH THE UGAMP GCM

The role of the extratropics in determining the distribution of tropic al convection and its inherent time-scales has been investigated, usin g results from a perpetual January integration of the UK Universities Global Atmospheric Modelling Programme (UGAMP) General Circulation Mod el. This study has concentrated on the influence of upper tropospheric extratropical troughs in the Pacific waveguide, and of the east Asian cold surge events. The mechanisms through which the extratropics infl uence the model's tropical convection have been identified, and the va lidity of the model's behaviour has been assessed by comparing the res ults with previous observational studies. The model has considerable s kill in representing the extratropical forcing of tropical convection. The results have shown that in the tropical east Pacific convection i s initiated by the ascent and decreased static stability ahead of an u pper tropospheric extratropical trough, which has penetrated the deep tropics in the region of upper-level equatorial westerlies. Over the m aritime continent, convection is enhanced by cold-surge events, in whi ch the increased near-surface northerlies substantially enhance the ai r-sea interaction over the South China Sea. These cold surges are trig gered by the movement of the Siberian anticyclone in association with the passage of midlatitude weather systems. An important result has be en the apparent coherent relationship between convective activity over different parts of the tropical Pacific Ocean, as a consequence of a well defined sequence of events involving tropical-extratropical inter actions. This emphasises that it would be seriously misleading to cons ider the tropics in isolation from the extratropics, and has highlight ed the importance of the extratropics in determining the distribution and temporal scales of tropical convection during the northern winter. Two factors have been identified as important for a good simulation o f extratropical forcing of tropical convection: firstly, the simulated mean climate should be sufficiently accurate to allow wave energy fro m the extratropics to influence the tropics; secondly, the convective parametrization should be formulated in such a way as to recognize and respond to extratropical phenomena.