William M. Connolley

Recent rapid regional climate warming on the Antarctic Peninsula

The Intergovernmental Panel on Climate Change (IPCC) confirmed that mean global warming was 0.6 ± 0.2 °C during the 20th century and cited anthropogenic increases in greenhouse gases as the likely cause of temperature rise in the last 50 years. But this mean value conceals the substantial complexity of observed climate change, which is seasonally- and diurnally-biased, decadally-variable and geographically patchy. In particular, over the last 50 years three high-latitude areas have undergone recent rapid regional (RRR) warming, which was substantially more rapid than the global mean. However, each RRR warming occupies a different climatic regime and may have an entirely different underlying cause. We discuss the significance of RRR warming in one area, the Antarctic Peninsula. Here warming was much more rapid than in the rest of Antarctica where it was not significantly different to the global mean. We highlight climate proxies that appear to show that RRR warming on the Antarctic Peninsula is unprecedented over the last two millennia, and so unlikely to be a natural mode of variability. So while the station records do not indicate a ubiquitous polar amplification of global warming, the RRR warming on the Antarctic Peninsula might be a regional amplification of such warming. This, however, remains unproven since we cannot yet be sure what mechanism leads to such an amplification. We discuss several possible candidate mechanisms: changing oceanographic or changing atmospheric circulation, or a regional air-sea-ice feedback amplifying greenhouse warming. We can show that atmospheric warming and reduction in sea-ice duration coincide in a small area on the west of the Antarctic Peninsula, but here we cannot yet distinguish cause and effect. Thus for the present we cannot determine which process is the probable cause of RRR warming on the Antarctic Peninsula and until the mechanism initiating and sustaining the RRR warming is understood, and is convincingly reproduced in climate models, we lack a sound basis for predicting climate change in this region over the coming century.

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.

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.

Evaluation of the sea ice simulation in a new coupled atmosphere‐ocean climate model (HadGEM1)

A rapid increase in the variety, quality, and quantity of observations in polar regions is leading to a significant improvement in the understanding of sea ice dynamic and thermodynamic processes and their representation in global climate models. We assess the simulation of sea ice in the new Hadley Centre Global Environmental Model (HadGEM1) against the latest available observations. The HadGEM1 sea ice component uses elastic-viscous-plastic dynamics, multiple ice thickness categories, and zero-layer thermodynamics. The model evaluation is focused on the mean state of the key variables of ice concentration, thickness, velocity, and albedo. The model shows good agreement with observational data sets. The variability of the ice forced by the North Atlantic Oscillation is also found to agree with observations.

Spatial and temporal variability of net snow accumulation over the Antarctic from ECMWF re-analysis project data

Forecasts from the ECMWF re-analysis project (ERA) covering the period 1979–1993 are used to examine the spatial and temporal distribution of net snow accumulation (precipitation minus evaporation) over the Antarctic continent. There is generally good agreement between the spatial distribution of net accumulation in the model data, when the 15 year mean annual accumulation is considered, and the equivalent maps produced in earlier studies from in situ data. One of the major differences is the westerly displacement of the accumulation maximum on the western side of the Antarctic Peninsula as a result of the model orography having a high degree of smoothing in the east–west direction. The mean annual net accumulation in the ERA data for the whole of the continent is 151 mm year−1, equivalent to a total accumulation of 2106×1012 kg year−1. The mean accumulation value is in reasonable agreement with the best estimates from glaciological data, which recent studies have suggested is in the range 150–170 mm year−1. The lower value from ERA is partly a result of overestimation of evaporation/sublimation from the large ice shelves during the summer and spring, and an underestimation of the precipitation in the interior of the continent. The accumulation from the ERA data is the same as that computed from the operational ECMWF forecasts. During the 15 year data period, the mean accumulation varied from 129.1 mm (1987) to 171.8 mm (1984). Considering the continent as a whole, net accumulation was at a minimum in the summer season, although in coastal parts of West Antarctica, the minimum occurs in spring. The ERA accumulation data for the interior of the continent show no annual cycle and are significantly smaller than the available in situ measurements. At certain coastal sites in West Antarctica there is a clear relationship between annual precipitation and cyclone activity, although in East Antarctica such a relationship is only apparent in monthly data.

Validation of the Surface Energy Balance over the Antarctic Ice Sheets in the U.K. Meteorological Office Unified Climate Model

Abstract Surface radiation measurements and other climatological data were used to validate the representation of the surface energy balance over the East Antarctic Ice Sheet in the U.K. Meteorological Office Unified Climate Model. Model calculations of incident and reflected shortwave radiation are in good agreement with observations, but the downward component of longwave radiation at the surface appears to be underestimated by up to 20 W m−2 in the model. Over much of the interior of Antarctica this error appears to be compensated for by an overestimate in turbulent transport of heat to the surface, while over the steep coastal slopes the heat flux is in good agreement with observations but the surface temperature is too low. The excessive heat flux over the interior results largely from the use of an inappropriately large bulk transfer coefficient under very stable conditions, suggesting that the surface heat flux scheme in the model is not ideally formulated for the conditions that prevail in the Ant...

The role of the non-axisymmetric antarctic orography in forcing the observed pattern of variability of the Antarctic climate

The pattern of inter-annual variability in the atmospheric circulation around Antarctica has a maximum over the Amundsen-Bellinghausen Sea (ABS), which is particularly strong during the winter (June, July and August). By using an atmosphere-only general circulation model the causes of this maximum have been investigated. In particular we study the effect of the non-axisymmetric nature of the local surface forcing (sea surface temperatures, sea ice and orography) by imposing axi-symmetric forcing fields at high southern latitudes. The results of these experiments show that the non-axisymmetric nature of the Antarctic orography is sufficient to explain the variability maximum in the ABS.

THE MYTH OF THE 1970s GLOBAL COOLING SCIENTIFIC CONSENSUS

Climate science as we know it today did not exist in the 1960s and 1970s. The integrated enterprise embodied in the Nobel Prizewinning work of the Intergovernmental Panel on Climate Change existed ...

The Antarctic climate of the UKMO Unified Model

We examine some aspects of the performance of the United Kingdom Meteorological Offices new climate model over Antarctica. Pressure and temperature fields are presented as a basic check on the model climate. The gradient of pressure between mid-latitudes and high southern latitudes is too great, resulting in an Antarctic trough that is too deep by 4–6 hPa. Temperature is well modelled though the interior is slightly too cold in winter. Precipitation is interesting because of its relevance to mass balance and therefore changes in sea level. The simulation of the pattern of accumulation is good despite somewhat high values at places in the coastal areas, with an areally-averaged value of 182 mm y−1. We also look at the phenomena of the coreless winter and the katabatic winds which are a consequence of the intense radiative cooling. These two effects may provide a useful diagnostic of the model performance.

Long‐term variation of the Antarctic Circumpolar Wave

[1] During the period 1968-1999, the character of circum-Antarctic anomalies in sea level pressure, sea ice edge, and sea surface temperature changed substantially. An Antarctic Circumpolar Wave (ACW) is only clearly visible in the period 1985-1994. Before, and perhaps after this, the signal, particularly in sea level pressure, is quite different with no clear sign of precession. Accompanying the change from precessional to nonprecessional modes is a change in the spatial pattern of variability and a change in the predictability of atmospheric anomalies from oceanic forcing. Evidence from general circulation model integration suggests that during the precessional mode of the ACW, there is enhanced predictability, as would be required to support a coupled ocean-atmosphere interaction.