Changes in Antarctic snowfall over the past 50 years
- This page is part of the topic Antarctic climate and environment change in the instrumental period
General spatial and temporal characteristics of Antarctic snowfall
Snowfall accumulation, referred to as the surface mass balance (SMB), is the primary mass input to the Antarctic ice sheets, and is the net result of precipitation, sublimation/vapour deposition, drifting snow processes, and melt. Precipitation, which primarily occurs as snowfall, is dominant among these components (Bromwich 1988) and establishing its spatial and temporal variability is necessary to assess ice sheet surface mass balance. Comprehensive studies of snowfall characteristics over Antarctica are given by Bromwich (1988), Turner et al. (1999), Genthon and Krinner (2001), van Lipzig et al. (2002), Bromwich et al. (2004a), van de Berg et al. (2005), and Monaghan et al. (2006a). Snowfall is influenced to first order by the Antarctic topography, and ground-penetrating radar has shown that its spatial distribution is highly variable. Most of the snowfall occurs along the steep coastal margins (Figure 4.11) and is caused by orographic lifting of relatively warm, moist air associated with the many transient, synoptic-scale cyclones that encircle the continent (e.g. Bromwich et al. 1995; Genthon and Krinner, 1998). The synoptic activity decreases inward from the coast, and over the highest, coldest reaches of the continent the primary mode of snowfall is due to cooling of moist air just above the surface-based temperature inversion (Schwerdtfeger, 1970). This extremely cold air has little capacity to hold moisture, and thus the interior of the East Antarctic Ice Sheet is a polar desert, with a large area that receives less than 5 cm water equivalent of snowfall each year (e.g. Vaughan et al., 1999; Giovinetto and Zwally, 2000). Large-scale atmospheric influences on Antarctic snowfall include the ENSO (Cullather et al., 1996) and the SAM (Genthon and Cosme, 2003; van den Broeke and van Lipzig; 2004). ENSO has an intermittent teleconnection with Antarctica (Genthon and Cosme, 2003) that especially impacts snowfall variability in West Antarctica (Cullather et al., 1996; Bromwich et al., 2000; 2004b; Guo et al., 2004). The response of Antarctic snowfall to SAM forcing is complex (Genthon et al., 2003), but may be linked to near-surface wind flow and temperature anomalies that are associated with the SAM (van den Broeke and van Lipzig, 2004).
Long-term Antarctic snowfall accumulation estimates
In recent decades, estimates of SMB over the Antarctic ice sheets have been made by three techniques: in-situ observations, remote sensing, and atmospheric modeling. Constructing a reliable data set of SMB over Antarctica for a long time period from these methods has been difficult for numerous reasons, including for example a sparse surface observational network (e.g. Giovinetto and Bentley, 1985); difficulties distinguishing between clouds and the Antarctic ice surface in satellite radiances (Xie and Arkin, 1998); and incomplete parameterizations of polar cloud microphysics and precipitation in atmospheric models (Guo et al., 2003). Considering the limitations of the techniques, it is not surprising that the long-term-averaged continent-wide maps of SMB over Antarctica yield a broad envelope of results. The long-term estimates of SMB from several studies range from +119 mm/yr (van de Berg et al., 2005) to +197 mm/yr (Ohmura et al., 1996) water equivalent (weq) for the grounded ice sheets (estimates for the conterminous ice sheets, which include the ice shelves, are generally ~10% higher). The large range of long-term SMB estimates has contributed to uncertainty in calculations of the total mass balance of the Antarctic ice sheets (e.g. van den Broeke et al., 2006), and thus an important future endeavour will be to narrow the gap between estimates of SMB. In general, the studies employing glaciological data are considered the most reliable; the study of Vaughan et al. (1999) approximates SMB as 149 mm/yr for the grounded ice sheets, although a recent study (van de Berg et al., 2006) shows evidence that the Vaughan et al. (1999) dataset may underestimate coastal accumulation, and gives an updated value of 171 mm/yr. Considering the large spread between estimates, it is not surprising that calculated temporal trends vary widely (Monaghan et al., 2006a).
Recent trends in Antarctic snowfall
On average, about 6 mm global sea level equivalent falls as snow on Antarctica each year (Budd and Simmonds, 1991). Thus, it is important to assess trends in Antarctic SMB, as even small changes can have considerable impacts on the global sea level budget. The latest studies employing global and regional atmospheric models to evaluate changes in Antarctic SMB show that no statistically significant increase has occurred since ~1980 over the entire grounded ice sheet, WAIS, or the East Antarctic Ice Sheet (EAIS) (Monaghan et al., 2006a; van de Berg et al., 2005; van den Broeke et al., 2006). A validation of the modeled-versus-observed changes (Monaghan et al., 2006a) suggests that the recent model records are more reliable than the earlier global model records that inferred an upward trend in Antarctic SMB since 1979 (Bromwich et al., 2004a). The new studies also clearly show that interannual SMB variability is considerable; yearly snowfall fluctuations of +/-20 mm/yr weq, i.e., +/-0.69 mm/yr GSL (global sea level) equivalent, are common (Monaghan et al., 2006a), and might easily mask underlying trends over the short record.
In contrast to modeling studies, satellite altimetry measurements by Davis et al. (2005) suggest that increased snowfall has recently caused the EAIS to thicken, mitigating sea level rise by about 0.12 mm/yr between 1992-2003. Zwally et al. (2005) also found a thickening over EAIS from satellite altimetry for a similar period, but it was a factor of three smaller than the Davis study. Zwally et al. (2005) argued that their method more accurately accounts for firn compaction and the interannual variability of surface height fluctuations. The difference between the positive trends from the satellite altimetry studies and the zero trends in the modeling studies may be in part due to different temporal and spatial coverage (satellite altimetry does not extend past 81.6°S and has limitations along the steep coastal margins).
To extend the length of the Antarctic SMB record, Monaghan et al. (2006b) used the spatial information provided by atmospheric model precipitation fields from ERA-40 to extrapolate a suite of ice core SMB records in space and time. The resulting spatially resolved SMB dataset spans 1955-2004, approximately doubling the length of the existing model-based records. An updated version of the dataset (Monaghan and Bromwich, 2008), now adjusted to reflect the ERA-40 snowfall variance at interannual timescales, indicates that the 1955-2004 continent-averaged trend is positive and statistically insignificant (0.19 ± 32 mm/yr), and is characterized by upward trends through the mid-1990s and downward trends thereafter (Figure 4.12). The shape of the time series in Figure 4.12 suggests that a cyclic signal with a period of about 50 years may be impacting Antarctic snowfall, but is inconclusive without a longer time series. The continent-averaged trend is the net result of both positive and negative regional trends, which in some drainage basins are weakly (p<0.10) statistically significant (Figure 4.13). The positive SMB trends on the western side of the Antarctic Peninsula have been attributed to a deepening of the circumpolar pressure trough, which has enhanced ascent in the region (Turner et al., 2005b).
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