Pranab Deb

Absence of 21st century warming on Antarctic Peninsula consistent with natural variability.

Since the 1950s, research stations on the Antarctic Peninsula have recorded some of the largest increases in near-surface air temperature in the Southern Hemisphere. This warming has contributed to the regional retreat of glaciers, disintegration of floating ice shelves and a ‘greening’ through the expansion in range of various flora. Several interlinked processes have been suggested as contributing to the warming, including stratospheric ozone depletion, local sea-ice loss, an increase in westerly winds, and changes in the strength and location of low–high-latitude atmospheric teleconnections. Here we use a stacked temperature record to show an absence of regional warming since the late 1990s. The annual mean temperature has decreased at a statistically significant rate, with the most rapid cooling during the Austral summer. Temperatures have decreased as a consequence of a greater frequency of cold, east-to-southeasterly winds, resulting from more cyclonic conditions in the northern Weddell Sea associated with a strengthening mid-latitude jet. These circulation changes have also increased the advection of sea ice towards the east coast of the peninsula, amplifying their effects. Our findings cover only 1% of the Antarctic continent and emphasize that decadal temperature changes in this region are not primarily associated with the drivers of global temperature change but, rather, reflect the extreme natural internal variability of the regional atmospheric circulation.

Unprecedented springtime retreat of Antarctic sea ice in 2016

During Austral spring 2016 Antarctic sea ice extent (SIE) decreased at a record rate of 75 × 103 km2 day-1, which was 46% faster than the mean rate and 18% faster than in any previous spring season during the satellite era. The decrease of sea ice area was also exceptional and 28% greater than the mean. Anomalous negative retreat occurred in all sectors of the Antarctic, but was greatest in the Weddell and Ross Seas. Record negative SIE anomalies for the day of year were recorded from 3 November 2016 until 9 April 2017. Rapid ice retreat in the Weddell Sea took place in strong northerly flow after an early maximum ice extent in late August. Rapid ice retreat occurred in November in the Ross Sea when surface pressure was at a record high level, with the Southern Annular Mode at its most negative for that month since 1968.

An assessment of the Polar Weather Research and Forecasting (WRF) model representation of near‐surface meteorological variables over West Antarctica

Despite the recent significant climatic changes observed over West Antarctica, which include large warming in central West Antarctica and accelerated ice loss, adequate validation of regional simulations of meteorological variables are rare for this region. To address this gap, results from a recent version of the Polar Weather Research and Forecasting model (Polar WRF) covering West Antarctica at a high horizontal resolution of 5 km were validated against near-surface meteorological observations. The model employed physics options that included the Mellor-Yamada-Nakanishi-Niino (MYNN) boundary layer scheme, the WRF Single Moment 5-Class cloud microphysics scheme, the new version of the Rapid Radiative Transfer Model for both shortwave and longwave radiation, and the Noah land surface model. Our evaluation finds this model to be a useful tool for realistically capturing the near-surface meteorological conditions. It showed high skill in simulating surface pressure (correlation ≥0.97), good skill for wind speed with better correlation at inland sites (0.7-0.8) compared to coastal sites (0.3-0.6), generally good representation of strong wind events, and good skill for temperature in winter (correlation ≥0.8). The main shortcomings of this configuration of Polar WRF are an occasional failure to properly represent transient cyclones and their influence on coastal winds, an amplified diurnal temperature cycle in summer, and a general tendency to underestimate the wind speed at inland sites in summer. Additional sensitivity studies were performed to quantify the impact of the choice of boundary layer scheme and surface boundary conditions. It is shown that the model is most sensitive to the choice of boundary layer scheme, with the representation of the temperature diurnal cycle in summer significantly improved by selecting the Mellor-Yamada-Janjic boundary layer scheme. By contrast, the model results showed little sensitivity to whether the horizontal resolution was 5 or 15 km.

Sensitivity of simulated summer monsoonal precipitation in Langtang Valley, Himalaya, to cloud microphysics schemes in WRF

A better understanding of regional-scale precipitation patterns in the Himalayan region is required to increase our knowledge of the impacts of climate change on downstream water availability. This study examines the impact of four cloud microphysical schemes (Thompson, Morrison, WRF Single-Moment 5-class, and WRF Double-Moment 6-class) on summer monsoon precipitation in the Langtang Valley in the central Nepalese Himalayas, as simulated by the Weather Research and Forecasting (WRF) model at 1-km grid spacing for a 10-day period in July 2012. The model results are evaluated through a comparison with surface precipitation and radiation measurements made at two observation sites. Additional understanding is gained from a detailed examination of the microphysical characteristics simulated by each scheme, which are compared with measurements using a spaceborne radar/lidar cloud product. Also examined are the roles of large and small-scale forcing. In general the schemes are able to capture the timing of surface precipitation better than the actual amounts in the Langtang Valley, which are predominately underestimated, with the Morrison scheme showing the best agreement with the measured values. The schemes all show a large positive bias in incoming radiation. Analysis of the radar/lidar cloud product and hydrometeors from each of the schemes suggests that ‘cold-rain’ processes are a key precipitation formation mechanism, which is also well represented by the Morrison scheme. As well as microphysical structure, both large-scale and localised forcing is also important.

Summer Drivers of Atmospheric Variability Affecting Ice Shelf Thinning in the Amundsen Sea Embayment, West Antarctica

Satellite data and a 35-year hindcast of the Amundsen Sea Embayment summer climate using the Weather Research and Forecasting model are used to understand how regional and large-scale atmospheric variability affects thinning of ice shelves in this sector of West Antarctica by melting from above and below (linked to intrusions of warm water caused by anomalous westerlies over the continental shelf edge). El Nino episodes are associated with an increase in surface melt but do not have a statistically significant impact on westerly winds over the continental shelf edge. The location of the Amundsen Sea Low and the polarity of the Southern Annular Mode (SAM) have negligible impact on surface melting, although a positive SAM and eastward shift of the Amundsen Sea Low cause anomalous westerlies over the continental shelf edge. The projected future increase in El Nino episodes and positive SAM could therefore increase the risk of disintegration of West Antarctic ice shelves.