Scott Hosking

In recent decades there has been a marked strengthening of the Southern Hemisphere stratospheric polar vortex during spring, in response to ozone depletion. Observations show that zonal wind anomalies descend downward from the stratosphere to the ground, resulting in a significant strengthening of circumpolar westerlies around Antarctica during summer. This study uses a state-of-the-art GCM with high vertical resolution to

In contrast to the Arctic, total sea ice extent (SIE) across the Southern Ocean has increased since the late 1970s, with the annual mean increasing at a rate of 186×10(3) km(2) per decade (1.5% per decade; p<0.01) for 1979-2013. However, this overall increase masks larger regional variations, most notably an increase (decrease) over the Ross (Amundsen-Bellingshausen) Sea. Sea ice variability results from changes in atmospheric and oceanic conditions, although the former is thought to be more significant, since there is a high correlation between anomalies in the ice concentration and the near-surface wind field. The Southern Ocean SIE trend is dominated by the increase in the Ross Sea sector, where the SIE is significantly correlated with the depth of the Amundsen Sea Low (ASL), which has deepened since 1979. The depth of the ASL is influenced by a number of external factors, including tropical sea surface temperatures, but the low also has a large locally driven intrinsic variab...

Mean winter near-surface temperatures across West Antarctica have increased by around 0.8 K/decade over the years 1979 to 2009. By assessing the distribution of 30-year trends from all the available pre-industrial control runs of climate models in the IPCC AR4 data set, we find that this warming is at the upper bound of natural variability. We investigate the factors that influence these near-surface temperatures using meteorological reanalyses from ECMWF. The temperatures are positively correlated with geopotential height to high significance at 500 hPa and above - the troposphere acts as one system. This suggests that the trends are associated with factors that act throughout the troposphere such as increased greenhouse gases, or with factors that act from above such as possible increases in Polar Stratospheric Clouds or the loss of stratospheric ozone. However, climate model results indicate that increased greenhouse gases alone can only account for less than a third of the trend...

ABSTRACT During winter much of the Antarctic coast is susceptible to severe and hazardous strong-wind events (SWEs) associated with the enhancement of katabatic winds by synoptic weather systems. In this study a SWE that occurred at Mawson, East Antarctica, involving a hurricane force wind speed of ∼39 m s−1 is simulated by the Met Office Unified Model at high horizontal resolutions with grid lengths between 12 and 1.5 km. It is shown that all the simulations capture the qualitative evolution of the SWE but underestimate its peak wind speed. The extent of the underestimate is dependent on horizontal resolution, with the 4 and 1.5 km (12 km) models underforecasting the peak wind speed by around 15% (36%). In addition to a strengthening of the katabatic flow, the simulated low-level cyclonic winds associated with the depression responsible for the SWE caused the formation of a barrier-type jet parallel to the coast, resulting in strong wind convergence/interaction at the coastline and suggesting a strong topographic influence on the dynamics responsible for SWE formation. Moreover, it suggests that Mawson is influenced by small-scale gravity waves that formed in response to the stronger winds, and that representation of this was particularly sensitive to horizontal resolution. Additional experiments suggest that the Met Office Unified Model simulation of the SWE is most sensitive to the representation of turbulent mixing under stable conditions. This study is important to identify shortcomings in the performance of the Met Office Unified Model near Antarctica's coastal regions as well as to improve understanding of the processes responsible for SWEs.

ABSTRACT The influence of changes in winds over the Amundsen Sea has been shown to be a potentially key mechanism in explaining rapid loss of ice from major glaciers in West Antarctica, which is having a significant impact on global sea level. Here, Coupled Model Intercomparison Project Phase 5 (CMIP5) climate model data are used to assess twenty-first century projections in westerly winds over the Amundsen Sea (U AS ). The importance of model uncertainty and internal climate variability in RCP4.5 and RCP8.5 scenario projections are quantified and potential sources of model uncertainty are considered. For the decade 2090–2099 the CMIP5 models show an ensemble mean twenty-first century response in annual mean U AS of 0.3 and 0.7 m s−1 following the RCP4.5 and RCP8.5 scenarios respectively. However, as a consequence of large internal climate variability over the Amundsen Sea, it takes until around 2030 (2065) for the RCP8.5 response to exceed one (two) standard deviation(s) of decadal internal variability. In all scenarios and seasons the model uncertainty is large. However the present-day climatological zonal wind bias over the whole South Pacific, which is important for tropical teleconnections, is strongly related to inter-model differences in projected change in U AS (more skilful models show larger U AS increases). This relationship is significant in winter (r = −0.56) and spring (r = −0.65), when the influence of the tropics on the Amundsen Sea region is known to be important. Horizontal grid spacing and present day sea ice extent are not significant sources of inter-model spread.

In contrast to earlier studies, we describe the climatological deep low-pressure system that exists over the South Pacific sector of the Southern Ocean, referred to as the Amundsen-Bellingshausen Seas Low (ABSL), in terms of its relative (rather than actual) central pressure by removing the background area-averaged mean sea level pressure (MSLP). In doing so, we remove much of the influence of large-scale variability across the ABSL sector region (e.g., due to the Southern Annular Mode), allowing a clearer understanding of ABSL variability and its effect on the regional climate of West Antarctica. Using ERA-Interim reanalysis fields the annual cycle of the relative central pressure of the ABSL for the period 1979 to 2011 shows a minimum (maximum) during winter (summer), differing considerably from the earlier studies based on actual central pressure which suggests a semi-annual oscillation. The annual cycle of the longitudinal position of the ABSL is insensitive to the background pressure, and shows it shifting westwards from ~250° E to ~220° E between summer and winter, in agreement with earlier studies. We demonstrate that ABSL variability, and in particular its longitudinal position, plays an important role in controlling the surface climate of West Antarctica and the surrounding ocean by quantifying its influence on key meteorological parameters. Examination of the ABSL annual cycle in seventeen CMIP5 climate models run with historical forcing showed that the majority of them have definite biases, especially in terms of longitudinal position, and a correspondingly poor representation of West Antarctic climate.

We develop a climatology of the Amundsen Sea low (ASL) covering the period 1979–2008 using ECMWF operational and reanalysis fields. The depth of the ASL is strongly influenced by the phase of the Southern annular mode (SAM) with positive (negative) mean sea level pressure anomalies when the SAM is negative (positive). The zonal location of the ASL is linked to the phase of the mid-tropospheric planetary waves and the low moves west from close to 110°W in January to near 150°W in June as planetary waves 1 to 3 amplify and their phases shift westwards. The ASL is deeper by a small, but significant amount, during the La Niña phase of El Niño-Southern Oscillation (ENSO) compared to El Niño. The difference in depth of the low between the two states of ENSO is greatest in winter. There is no statistically significant difference in the zonal location of the ASL between the different phases of ENSO. Over 1979–2008 the low has deepened in January by 1.7 hPa dec−1 as the SAM has become more positive. It has also deepened in spring and autumn as the semi-annual oscillation has increase in amplitude over the last 30 years. An increase in central pressure and eastward shift in March has occurred as a result of a cooling of tropical Pacific SSTs that altered the strength of the polar front jet. Copyright © 2012 Royal Meteorological Society

The UK Met Office's Unified Model is used at a climate resolution (N216, ~0.83°×~0.56°, ~60 km) to assess the impact of deep tropical convection on the structure of the tropical tropopause layer (TTL). We focus on the potential for rapid transport of short-lived ozone depleting species to the stratosphere by rapid convective uplift. The modelled horizontal structure of organised convection is shown to match closely with signatures found in the OLR satellite data. In the model, deep convective elevators rapidly lift air from 4–5 km up to 12–14 km. The influx of tropospheric air entering the TTL (11–12 km) is similar for all tropical regions with most convection stopping below ~14 km. The tropical tropopause is coldest and driest between November and February, coinciding with the greatest upwelling over the tropical warm pool. As this deep convection is co-located with bromine-rich biogenic coastal emissions, this period and location could potentially be the preferential gateway for stratospheric bromine