Atmospheric moisture budget over Antarctica and the Southern Ocean based on the ERA‐40 reanalysis

Abstract

ABSTRACT:The atmospheric moisture budget over Antarctica and the Southern Ocean was analysed for the period 1979-2001 on the basis of the ERA-40 reanalysis of the European Centre for Medium-Range Weather Forecasts. Meridional transport by transient eddies makes the largest contribution to the southward water vapour transport. The mean meridional circulation contributes to the northward transport in the Antarctic coastal areas, but this effect is compensated by the southward transport by stationary eddies. The convergence of meridional water vapour transport is at its largest at 64-68°S, while the convergence of zonal transport is regionally important in areas of high cyclolysis. Inter-annual variations in water vapour transport are related to the southern annular mode (SAM). The eastward transport has a significant (95% confidence level) positive correlation with the SAM index, while the northward transport has a significant negative correlation with SAM near 60°S. Hydrological balance is well-achieved in the ERA-40 reanalysis: the difference between the water vapour flux convergence (based on analysis) and the net precipitation (precipitation minus evaporation, based on 24-h forecasts) is only 13 mm yr −1 (3%) over the Southern Ocean and −8 mm yr −1 (5%) over the continental ice sheet. Over the open ocean, the analysis methodology favours the accuracy of the flux convergence. For the whole study region, the annual mean flux convergence exceeded net precipitation by 11 mm yr −1 (3%). The ERA-40 result for the mean precipitation over the Antarctic continental ice sheet in 1979-2001 is 177 ± 8 mm yr −1 , while previous estimates range from 173 to 215 mm yr −1 . For the period 1979-2001, the ERA-40 data do not show any statistically significant trend in precipitation over the Antarctic grounded ice sheet and ice shelves. From the ERA-40 data, the annual average net evaporation (evaporation minus condensation) is positive over the whole continent.

Spatial and temporal variability in summer snow pack in Dronning Maud Land, Antarctica

“…An analogous gradient in potential temperature perpendicular to the coast was also observed in a nearby area in Dronning Maud Land by Van den Broeke et al (1999). Granberg et al (2009) suggested that strong sublimation of wind-blown snow associated with katabatic winds is an important factor in generating this gradient, but did not give any quantitative estimates of the sublimation.…”

“…The effect of transient cyclones that travel eastward along the coast decreases inland, whereas the effect The Cryosphere, 5, [187][188][189][190][191][192][193][194][195][196][197][198][199][200][201]2011 www.the-cryosphere.net/5/187/2011/ T. Vihma et al: Spatial and temporal variability in summer snow pack 189 of katabatic winds is strongest over the steepest slopes. On the basis of previous observations (Van den Broeke et al, 1999Reijmer and Oerlemans, 2002;Reijmer and Van den Broeke, 2003;Granberg et al, 2009), our study region can be divided into two climatic zones: the ice shelf and the sloping escarpment region (consisting of Escarpment 1 between Basen and Fossilryggen, and Escarpment 2 between Fossilryggen and Svea; Fig. 1).…”

“…According to Van den Broeke et al (1999), farther west in Dronning Maud Land (from 3 to 15 • E), the snow density of the uppermost 1 m generally decreased with increasing distance from the coast. The data of Kärkäs et al (2005) and our data presented here did not show any significant correlation between the density and the distance from the coast.…”

“…We note, however, that in the ECMWF analyses the annual mean surface potential temperature strongly decreases towards the ice shelf (0.03 • C km −1 in our study region) although the model does not include any parameterization of sublimation from blowing/drifting snow, but just from the surface. Other arguments against the conclusion of Granberg et al (2009) are that (a) the katabatic winds strongly attenuate over the almost flat ice shelves (Van den Broeke and van Lipzig, 2003;Renfrew, 2004) and (b) due to the vicinity of the sea and the larger cyclone activity, the air humidity over the ice shelves is higher than in the interior (Tietäväinen and Vihma, 2008). These factors reduce sublimation related to katabatic winds over ice shelves.…”

“…The temporal variability of snow temperature decreases with depth. In winter the depthdependence of temperature is dominated by heat conduction, but in summer solar radiation and water vapour transport are also important (Brandt and Warren, 1997;Town et al, 2008).…”