Michiel R. van den Broeke

Spatial and temporal variation of sublimation on Antarctica: Results of a high‐resolution general circulation model

In this paper we use output of a high-resolution general circulation model (ECHAM-3 T106, resolution 1.1° x 1.1°) to study the spatial and temporal variation of sublimation on Antarctica. First, we compare model results with available observations of sublimation rates. The yearly cycle, with small latent heat fluxes during the winter, is well reproduced, and the agreement with sparsely available spot observations is fair. The model results suggest that a significant 10-15% of the annual precipitation over Antarctica is lost through sublimation and that sublimation plays an important role in the formation of blue ice areas. A preliminary analysis of the atmospheric boundary layer moisture budget shows that the spatial variation of sublimation in the coastal zone of East Antarctica can be explained by variations of horizontal advection of dry air. Dry air advection, and thus surface sublimation, is enhanced in areas where katabatic winds are strong and have a large downslope component and where the Antarctic topography drops suddenly from the plateau to the coastal zone. In areas where horizontal advection is small, like the plateau and the large ice shelves, special conditions must be met to make significant sublimation at the surface possible.

THE SEMI-ANNUAL OSCILLATION AND ANTARCTIC CLIMATE. PART 1: INFLUENCE ON NEAR SURFACE TEMPERATURES (1957-79)

We studied the influence of the semi-annual oscillation (SAO) on near-surface temperatures in Antarctica, using observations of 27 stations that were operational during (part of) the period 1957-79. For the annual cycle of surface pressure, the second harmonic explains 17-36% of the total variance on the Antarctic Plateau, 3668% along the East Antarctic coast and almost 80% on the west coast of the Peninsula, and decreases further to the north. As a result of the amplification of the wave-3 structure of the circulation around Antarctica, a significant modification of the seasonal cooling is observed at many stations. The magnitude of this modification is largely determined by the strength of the temperature inversion at the surface: the percentage of the variance explained by the second harmonic of the annual temperature cycle is then largest on the Antarctic Plateau (1 1-18%), followed by the large ice shelves and coastal East Antarctica (6-12%) and stations at or close to the Peninsula (0-5%). A significant coupling between the half-yearly wave in surface pressure and that in surface temperature is found for coastal East Antarctica, which can be directly explained by the changes in meridional circulation brought about by the SAO. We show that the coupling of Antarctic temperatures to the meridional circulation is not only valid on the seasonal time scale ofthe SAO, but probably also on daily and interannual time scales. This has important implications for the interpretation of time series of Antarctic temperatures, a problem that will be addressed in part 2 of this paper.

A new surface accumulation map for western Dronning Maud Land, Antarctica, from interpolation of point measurements

As a result of intensive field activities carried out by several nations over the past 15 years, a set of accumulation measurements for western Dronning Maud Land, Antarctica, was collected, based on firn-core drilling and snow-pit sampling. This new information was supplemented by earlier data taken from the literature, resulting in 111 accumulation values. Using Geographical Information Systems software, a first region-wide mean annual snow-accumulation field was derived. In order to define suitable interpolation criteria, the accumulation records were analyzed with respect to their spatial autocorrelation and statistical properties. The resulting accumulation pattern resembles well- known characteristics such as a relatively wet coastal area with a sharp transition to the dry interior, but also reveals complex topographic effects. Furthermore, this work identifies new high-return shallow- drilling sites by uncovering areas of insufficient sampling density.

The semi-annual oscillation and Antarctic climate. Part 2: recent changes

Following a weakening of the semi-annual oscillation (SAO) since the mid- 1970s, the half-yearly pressure wave in the Southern Hemisphere has become less significant. As a result, May/June temperatures have decreased in East Antarctica, which has moderated Antarctic warming. Spectral analysis of 87 years of pressure data at Orcadas suggest that the recent weakening of the SAO is part of the natural variability of the Southern Hemisphere circulation on decadal timescales. We interpret the time series of composite Antarctic temperature in terms of the historical strengthening and weakening of the SAO. If the dominant oscillations that occurred in the past prove to be persistent, an accelerated East Antarctic warming trend is expected for the coming decades. There are indications that the strength of the SAO is linked to the Southern Oscillation, in the sense that warm phases of the Southern Oscillation coincide with strong westerlies, a weakly developed SAO and below-average temperatures in East Antarctica. Temperatures on the west coast of the Antarctic Peninsula show strongly deviant patterns, which can not be explained by the same mechanism that applies to East Antarctica.

Representation of Antarctic Katabatic Winds in a High-Resolution GCM and a Note on Their Climate Sensitivity

A high-resolution GCM (ECHAM-3 T106, resolution 1.1 83 1.18) is found to simulate many characteristic features of the Antarctic climate. The position and depth of the circumpolar storm belt, the semiannual cycle of the midlatitude westerlies, and the temperature and wind field over the higher parts of the ice sheet are well simulated. However, the strength of the westerlies is overestimated, the annual latitudinal shift of the storm belt is suppressed, and the wintertime temperature and wind speed in the coastal areas are underestimated. These errors are caused by the imperfect simulation of the position of the subtropical ridge, the prescribed sea ice characteristics, and the smoothened model topography in the coastal regions. Ice shelves in the model are erroneously treated as sea ice, which leads to a serious overestimation of the wintertime surface temperature in these areas. In spite of these deficiencies, the model results show much improvement over earlier simulations. In a climate run, the model was forced to a new equilibrium state under enhanced greenhouse conditions (IPCC scenario A, doubled CO2), which enables us to cast a preliminary look at the climate sensitivity of Antarctic katabatic winds. Summertime katabatic winds show a decrease of up to 15% in the lower parts of the ice sheet, as a result of the destruction of the surface inversion by increased absorption of solar radiation (temperature‐ albedo feedback). On the other hand, wintertime near-surface winds increase by up to 10% owing to a deepening of the circumpolar trough. As a result, the model predicts that the annual mean wind speed remains within 10% of its present value in a doubled CO2 climate, but with an increased amplitude of the annual cycle.