clitnate variability in the Antarctic region and global change

The cha racte ri stics of, and th e mechanisms causing, multi-decadal variability are currently rece iving much attention. This undertaking is pa rti cula rl y challenging in the sub-Antarctic region because of the paucity of data, and the complexit y of the governing physical processes. In this paper we report on aspects of high-southern-latitude variability which appear in the European Centre for Medium-range vVeather Forecasts twice-daily analyses for the period I J anuary 1980 to 31 August 1996 and in the results of global climate model experiments. We show that the number of cyclone positions in the 5070° S latitude band exhibits considerable interannual variabilit y, as well as changes on longer time-scales. The seasonal distribution of cyclones is linked with the "semi-annual osci llation". We show that the variabi lity of this phenomenon in a 1000 year run of the GFDL coupled model shows "red" characteristics (and on decadal time-scales is simil ar to that displayed in the available observations ). The interaction with the ocean and sea ice is stressed as an important factor in determining the nature of climate variability in sub-Anta rctic latitudes. INTRODUCTION VARIABILITY AND TRENDS IN PRESSURE OVER THE SUB-ANTARCTIC The 15 yea r CLIVAR resea rch programme (WCRP, 1995) has now begun. It has as one of its aims " to describe and understa nd the physical processes responsible for climate variability and predictability on seasonal, interannual, decadal, and centennial time-scales, through the co llection and analysis of observations and the development and app lication of models of the coupled climate system". Until recently, most studies of variability on these time-scales were focused on the tropics, but it is now clear that the Antarctic region exhibits great variability in these temporal windows, though we still have no comprehensive picture of the mechanisms driving this variability. Some of thi s variability can be traced back to the tropics (e.g. remote effects of the El Nino-Southern Oscillation (Smith and Stearns, 1993; Simmonds and J acka, 1995; Karoly and others, 1996)), while other as pec ts appear to be related to the high-latitude interactions of atmosphere, ice, ocean and bottom water formation (e.g. Manabe and Stouffer, 1996). In keeping with the spirit of CLIVAR, in this paper we discuss some observational and model analyses pertaining to multi-decadal variability in the sub-Antarctic region. The low-frequency variations in surface pressure, cyclone behaviour, sea ice and the semi-annual osci llat ion (SAO ) are connected by complex physical processes, and aspects of these associations are addressed in this paper. We also report on some model experiments which were undertaken to reveal the natural variability displayed by the SAO in a long simulation, and its sensitivity to the removal of Antarctic sea ice. I n exploring this topic, we have made use of the daily global meteorological analyses performed by the European Centre }1g. 1. Trends in summer ( Decem berFebruQ1]') AlSLP determinedJrom the ECAITVF datasetJor the period 198096 The contour interval is lhPa per decade, the zero line is bold, and negative contours are dashed. Regions over which the trends dijJer significantly from zero ( at the 95% corifidel/ce level) are indicated by stljJpling.