Principal Investigator Janette A Lindesay Project s13

Geography Department, Machine VP

School of Resource and Environmental Management

Co-Investigator Simon J Mason

Climatology Research Group, University of the Witwatersrand, Johannesburg

Modelling the Atmospheric Response to Midlatitude Forcing During ENSO

The El Nino Southern Oscillation (ENSO) phenomenon, which is an irregularly-occurring large-scale alteration in interactions between the ocean and atmosphere across the tropical Pacific Ocean, has been linked in numerous studies with well-defined rainfall and atmospheric circulation anomalies in many parts of the world. In eastern and south-eastern Australia and much of southern Africa, amongst other areas, an ENSO event is associated with drought (as is the case in 1994/5). There are some ENSO years, however, when rainfall is NOT below average. Diagnostic studies have suggested that the cause for these atypical events should be sought in sea surface temperature anomalies, particularly in the middle latitudes, and their impact on the large-scale atmospheric circulation. Understanding of the possible mechanisms responsible for these observations would enhance our insight into the variability of both ENSO teleconnections and the climate system. This project aims to develop such understanding through modelling the responses of the large-scale atmospheric circulation to observed sea surface temperature anomaly fields.

What are the basic questions addressed?

The project investigates the links between sea surface temperature anomalies, particularly in the middle latitudes, and the large-scale atmospheric circulation during years in which the ENSO phenomenon failed to produce the expected anomalies in rainfall in southern Africa and Australia.

What are the results to date and the future of the work?

This project began in late July 1994. By the end of 1994 a suitable version of the CSIRO 9-level R21 GCM (coupled to a slab ocean) had been implemented on the VP2200 with the assistance of CSIRO Division of Atmospheric Research. Test runs had been performed and the first stage of analysis completed (the compilation of a stable 10-year monthly climatology against which to compare the results of the perturbation experiments). Preliminary visualisation tools had been developed using NCAR Graphics. The perturbation experiments using specific sea-surface temperature anomalies will be performed early in 1995, and the project is expected to be completed by mid-1995.

What computational techniques are used and why is a supercomputer required?

GCMs are based on the iterative solving of the numerous equations that describe the physics of the atmosphere and its interactions with the cryosphere, biosphere, hydrosphere and geosphere, on a grid covering the globe (at rhomboidal truncation 21 in this case) and at numerous levels through the atmosphere (in this case 9). For climatological work the timesteps simulated are of the order of 30 minutes, and results are integrated on daily and monthly bases. Any meaningful experiment requires simulations of several years' data for each case under investigation. The task is sufficiently computationally intensive that it would not be feasible without the use of a supercomputer.