Title: Climate in West Antarctica over the last century
Author: Steig, E.J., White, J.W. and Ding, Q.
Periodical: American Geophysical Union, Fall Meeting 2011, abstract #PP32B-03

There have been significant changes in atmospheric circulation over West Antarctica in the last few decades. These changes have been linked to ozone depletion in the stratosphere, greenhouse gases in the troposphere, and rising temperatures in the equatorial Pacific. The circulation changes have resulted in sea ice declines and increased temperatures on the West Antarctic ice sheet, and may also explain recent oceanographic changes linked to ice shelf thinning (Steig et al, 2011). It remains an open question whether these are exceptional changes to Antarctic climate, or fall within the range of unforced variability. Instrumental climate records are too short and too sparse in the Antarctic to address this question. Ice core records obtained from the West Antarctic Ice Sheet (WAIS) and Antarctic Peninsula provide an opportunity to extend the observational record of West Antarctic climate into the past. We present δ18O records from the WAIS Divide site in central West Antarctica, along with both new and previously published data from shallow records from the U.S. "ITASE" program in West Antarctica (Steig et al., 2005; Schneider et al., 2006) and British Antarctic Survey records from the Peninsula (Thomas et al., 2009). The West Antarctic δ18O records are highly correlated with temperature -- explaining about 80% of the decadal variance -- and capture unequivocally the warming trend of the last few decades across most of West Antarctica. Centennial variations are also captured, with the same scaling (about 0.8%/°C) (Fegyveresi et al., 2011). The covariance of δ18O and temperature occurs because both are increased under conditions of anomalously strong meridional flow. The δ18O signal is further amplified by the presence of open water near the Antarctic coastline, due to reduced sea ice divergence from the same northerly flow. Extremes in δ18O are frequently associated with strong El Ni??o events. The response is particularly strong when warming occurs in the west central tropical Pacific (Ding et al., 2011). Interannual δ18O variations among the West Antarctic Ice Sheet records are normally distributed, allowing estimates of significance levels of large excursions. During the decades of the 1990s through 2000s, precipitation isotope ratios were as high or higher than at any time in the last century, with a significance level of p<0.01. For Ellsworth Land (eastern sector of West Antarctica at the base of the Antarctic Peninsula) the 1990s are anomalous in the last 200 years (p<.05). The only two periods that compare with the 1990s are the 1940s and 1880s. Sea surface temperatures in the central tropical Pacific in the 1990s were consistently at or above the highest temperatures of the 1940s and 1880s. Isotope-enabled general circulation model experiments show that tropical Pacific warming -- which tracks overall global warming -- is both sufficient and necessary to explain the observed decadal-scale ??18O anomalies. Thus, the anomalous δ18O of the 1990s is a consequence of the exceptional warmth of the central tropical Pacific. Recent climate changes in West Antarctica cannot be ascribed to regional decadal-scale variability, but appear to reflect the large-scale warming of the planet

Year: 2011