Results from the implementation of the Elastic Viscous Plastic sea ice rheology in HadCM3

We present results of an implementation of the Elastic Viscous Plastic (EVP) sea ice dynamics scheme into the Hadley Centre coupled ocean-atmosphere climate model HadCM3. Although the large-scale simulation of sea ice in HadCM3 is quite good with this model, the lack of a full dynamical model leads to errors in the detailed representation of sea ice and limits our confidence in its future predictions. We find that introducing the EVP scheme results in a worse initial simulation of the sea ice. This paper documents various enhancements made to improve the simulation, resulting in a sea ice simulation that is better than the original HadCM3 scheme overall. Importantly, it is more physically based and provides a more solid foundation for future development. We then consider the interannual variability of the sea ice in the new model and demonstrate improvements over the HadCM3 simulation

Preconditioning of iterative methods for linearised or linear systems

SIGLEAvailable from British Library Document Supply Centre- DSC:D92328 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

The Antarctic temperature inversion

In the interior of the Antarctic ice sheet the surface temperature inversion averages over 25°C in the winter months. The negative buoyancy of the near-surface air drives the katabatic windflow, which has important consequences for the climate of Antarctica. Radiosonde measurements of the inversion are combined with recent GCM results in an attempt to assess the accuracy of proposed connections between the surface temperature and the inversion strength by comparing the limited observational verification data with the much wider coverage that a climate model allows. This indicates that, using multi-annual data, the continent-wide RMS error of deducing the inversion strength from a regression technique is approximately 2ċ9°C, whereas using a method based upon differences between summer and winter temperaures has a RMS error of approximately 2ċ5°C

A modeling and observational study of East Antarctic surface mass balance

We examine simulations of the surface mass balance of the sector of the East Antarctic Ice Sheet between 2.4°W and 110.5°E as produced by the United Kingdom Meteorological Office Unified Climate Model. Estimates of the actual mass balance of this sector can be obtained from glaciological observations of snow accumulation and from studies of the atmospheric water vapor budget using radiosonde observations. The former technique gives an average sector accumulation of 104 mm yr−1, and the latter yields 157 mm yr−1. The modeled accumulation in this sector, 122 mm yr−1, lies between these two estimates, suggesting that the model can accurately represent the processes controlling surface mass balance. However, examination of the atmospheric water vapor budget in the model shows that only 30% of the water vapor precipitated in this sector is carried by resolved-scale transport. Although the model produces the “correct” accumulation in present-day climate simulations, it is not clear that this would change appropriately if the model were used to simulate future climates. By producing synthetic estimates of water vapor transport at radiosonde station locations we have used the model data to investigate the uncertainties in estimating sector accumulation from radiosonde data. The results of this study indicate that the radiosonde technique will tend to overestimate sector accumulation, thus reconciling the two observational estimates

Sensitivity of modelled atmospheric circulation to the representation of stable boundary layer processes

We examine the sensitivity of the modelled climate of the third generation Hadley Centre climate model to changes in the parametrization of surface and boundary-layer fluxes under stable stratification. Replacing the model's standard parametrization with one in which fluxes decrease more rapidly with increasing stability generates statistically-significant changes in modelled 500 hPa height. The largest changes are seen across the North Atlantic and North Pacific and occur during Northern Hemisphere summer, when persistent stable atmospheric boundary layers form over the western sides of these oceans. The atmospheric response in a coupled version of the model is stronger than in the atmosphere-only version, suggesting that feedbacks involving sea surface temperatures, surface fluxes and atmospheric circulation are important in determining the response

The performance of the Hadley Centre climate model (HADCM3) in high southern latitudes

An assessment of mean atmospheric and oceanic data from a 100-year segment of a pre-industrial control run of version 3 of the Hadley Centre climate model is presented. The model output has been verified against in situ measurements from expeditions, data from the research stations and mean fields from the 15 year re-analysis project of the European Centre for Medium-range Weather Forecasts (ECMWF). The wave number 3 pattern of the mean sea-level pressure (MSLP) and 500-hPa height fields are handled reasonably well, but the climatological troughs over the Bellingshausen Sea and at 130°E are too deep in winter by about 9 hPa at the surface. This is a result of positive sea-surface temperature (SST) errors over the tropical, eastern sides of the major ocean basins. These overly deep surface troughs result in the Antarctic coastal easterlies being too strong along the coast of Marie Byrd Land and much of the coast of East Antarctica. The circumpolar trough is too deep in summer by about 1.5 hPa and is located several degrees too far north in winter. Near-surface air temperatures over the interior of the Antarctic are in error by several degrees where the model has incorrect orographic height. The low-level temperature inversion is too strong over the Antarctic plateau. Precipitation minus evaporation over the interior of the Antarctic is slightly too low. The maximum in sea ice extent and the phase of the semi-annual oscillation (SAO) both lag the best available verification data by about 1 month

Significant warming of the Antarctic winter troposphere

We report an undocumented major warming of the Antarctic winter troposphere that is larger than any previously identified regional tropospheric warming on Earth. This result has come to light through an analysis of recently digitized and rigorously quality controlled Antarctic radiosonde observations. The data show that regional midtropospheric temperatures have increased at a statistically significant rate of 0.5° to 0.7°Celsius per decade over the past 30 years. Analysis of the time series of radiosonde temperatures indicates that the data are temporally homogeneous. The available data do not allow us to unambiguously assign a cause to the tropospheric warming at this stage

Antarctic Peninsula climate variability and its causes as revealed by analysis of instrumental records

Climate observations made since the mid twentieth century reveal that the Antarctic Peninsula is a region of extreme climate variability and change. The pattern of change is, however, both seasonally and spatially inhomogeneous. Limited data from the east (Weddell Sea) coast indicate that surface air temperatures here are rising at around 0.03 degreesC per year in all seasons. On the west (Bellingshausen Sea) coast, summer temperature trends are similar to those prevailing on the east coast but, in winter, warming trends of over 0.1 degreesC per year are observed, making this the most rapidly warming part of the Southern Hemisphere. Rapid warming is confined to the very lowest levels of the atmosphere and warming of the free troposphere over the Peninsula is not statistically significantly different from the Southern Hemisphere average. Interannual variations in winter temperatures on the west coast are strongly correlated with variations in atmospheric circulation and sea ice extent, suggesting that both atmospheric and ice/ocean processes may be contributing to the long-term warming. However, there is little observational evidence to support long-term atmospheric circulation changes. Coupled atmosphere-ocean general circulation model (AOGCM) experiments, forced with observed greenhouse gas increase, fail to reproduce the observed pattern of warming around the Peninsula. However, current AOGCMs may not be sophisticated enough or of high enough resolution to represent all of the processes that control climate on a regional scale around the Antarctic Peninsula