Stephen A. Harangozo

West Antarctic Peninsula sea ice in 2005: Extreme ice compaction and ice edge retreat due to strong anomaly with respect to climate

In September-October 2005, the juxtaposition of low-and high-pressure anomalies at 130 degrees W and 60 degrees W, respectively, created strong and persistent northerly airflow across the West Antarctic Peninsula (WAP). This had a major impact on regional sea ice conditions, with extreme ice compaction in the Bellingshausen and East Amundsen seas (60 degrees W-130 degrees W) but divergence in the West Amundsen and East Ross seas. This resulted in the former in a highly compact marginal ice zone and ice cover, mean modeled ice thicknesses of >5 m, and an earlier-than-average maximum extent (mid-August). While rapid ice retreat in late winter-spring created a major negative ice extent anomaly, compact ice persisted in the subsequent summer. Other effects were anomalies in air temperature (of +1 degrees C to +5 degrees C) and precipitation rates (to >2.5 mm/d). The patterns in late 2005 are consistent with the occurrence of a weak La Nina and a near-neutral Southern Annular Mode, with a quasi-stationary zonal wave three pattern dominating hemispheric atmospheric circulation. Once a compact ice edge was created, it took only one additional week of strong winds to solidify"" the pack in place. Conditions in 2005 are analyzed in the context of 1979-2005 and compared with the springs of 1993, 1997, 1999, 2001, and 2004. A statistically significant increase of the northerly 10-m wind component between 110 degrees W and 125 degrees W occurred in the Septembers of 1979-2005. No clear trends occur in other spring months. This work underlines the key importance of ice dynamics in recent changes in the WAP sea ice regime.

Variability of precipitation over the coastal western Antarctic Peninsula from synoptic observations

Observations of precipitation events at Faraday and Rothera Stations are analyzed to investigate the spatial and temporal variability of precipitation along the western coastal (Pacific) side of the Antarctic Peninsula. The record of observations made at Faraday since 1956 show a statistically significant increase in the number of winter-season precipitation events. During this season, there are now, on the average, almost 50% more reports of precipitation than during the 1950s. On a year-to-year basis the number of precipitation events is not correlated with the mean surface temperature on the western side of the Antarctic Peninsula but is dependent on synoptic weather system activity. The annual total number of precipitation events at Rothera is also increasing, but because the length of the record is relatively short, this is not statistically significant. The semiannual cycle in the latitudinal location and depth and position of the circumpolar trough is reproduced in the record of precipitation events at both Faraday and Rothera. It is argued that the systematic increase in the number of precipitation events at Faraday since the 1950s is associated with changes in the depression tracks across the Bellingshausen Sea, with an increase in the number of depressions approaching from outside the Antarctic rather than from the west.

An appraisal of NCEP/NCAR reanalysis MSLP data viability for climate studies in the South Pacific

Fifty years of monthly mean sea level pressure (MSLP) data from the NCEP/NCAR reanalysis (hereinafter NNR) are validated against station observations in the extratropical South Pacific and West Antarctica. The period for which the NNR may be considered reliable differs markedly between regions. It is only realistic from ∼1970 across West Antarctica. A lack of surface observations entering the model causes the NNR to be poorly constrained prior to the availability of GTS‐based data in 1967: the latter coincides with a significant improvement in the NNR MSLP across the eastern South Pacific. Substantial spurious negative trends occur within the NNR MSLP at high latitudes; although the decrease is greatest prior to the advent of satellite sounder data it continues into the 1990s south of 60°S. In addition, widely used tropical and extra‐tropical circulation indices are poorly represented in the NNR data prior to the 1960s.

Anomalous atmospheric circulation over the Weddell Sea, Antarctica during the Austral summer of 2001/02 resulting in extreme sea ice conditions

During the Austral summer of 2001/02 exceptionally heavy sea ice conditions were experienced over the eastern Weddell Sea. Satellite microwave imagery showed that large negative (positive) ice anomalies were present from October 2001 to January 2002 over the north-western Weddell Sea (off the coast of Dronning Maud Land). These were a result of anomalously high (low) atmospheric pressure over the South Atlantic (southern Weddell Sea and Bellingshausen Sea), which gave strong north to north-westerly cyclonic flow over the northern and eastern Weddell Sea. This resulted in convergence of sea ice into the southern Weddell Sea and inhibited ice advection along the coast of Dronning Maud land. The atmospheric anomalies around the Weddell Sea were part of an Antarctic-wide amplification of the mean wavenumber 3 pattern resulting in more intrusions of mid-latitude air into the interior of the continent, giving rise to near-record warm temperatures at several locations.

Atmospheric circulation impacts on winter maximum sea ice extent in the west Antarctic Peninsula region (1979-2001)

Increasing evidence exists that the strong warming of the West Antarctic Peninsula (WAP) region since the 1950s is related to reduced sea ice that is likely to be due to changes in the atmospheric circulation. Over twenty years of sea ice extent, ice motion and reanalysed near-surface wind data are used to establish that winter ice extent in the West Antarctic Peninsula (WAP) region is largely determined by the meridional (north-south) atmospheric circulation. A remarkably strong ice extent-wind relationship is found in the WAP. No other Antarctic or comparable sub-Arctic sea ice region shows this. Ice motion data confirm wind-induced drift is crucial for extensive winter ice to occur. Reasons for winter ice extent and winds being more strongly correlated in the WAP than in other parts of the Antarctic are discussed along with implications for understanding the observed warming.

Biodiversity of air-borne microorganisms at Halley station Antarctica

A study of air-borne microbial biodiversity over an isolated scientific research station on an ice-shelf in continental Antarctica was undertaken to establish the potential source of microbial colonists. The study aimed to assess: (1) whether microorganisms were likely to have a local (research station) or distant (marine or terrestrial) origin, (2) the effect of changes in sea ice extent on microbial biodiversity and (3) the potential human impact on the environment. Air samples were taken above Halley Research Station during the austral summer and austral winter over a 2-week period. Overall, a low microbial biodiversity was detected, which included many sequence replicates. No significant patterns were detected in the aerial biodiversity between the austral summer and the austral winter. In common with other environmental studies, particularly in the polar regions, many of the sequences obtained were from as yet uncultivated organisms. Very few marine sequences were detected irrespective of the distance to open water, and around one-third of sequences detected were similar to those identified in human studies, though both of these might reflect prevailing wind conditions. The detected aerial microorganisms were markedly different from those obtained in earlier studies over the Antarctic Peninsula in the maritime Antarctic.

A comparison of five numerical weather prediction analysis climatologies in southern high latitudes

Abstract In this paper, numerical weather prediction analyses from four major centers are compared—the Australian Bureau of Meteorology (ABM), the European Centre for Medium-Range Weather Forecasts (ECMWF), the U.S. National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR), and The Met. Office (UKMO). Two of the series—ECMWF reanalysis (ERA) and NCEP–NCAR reanalysis (NNR)—are “reanalyses”; that is, the data have recently been processed through a consistent, modern analysis system. The other three—ABM, ECMWF operational (EOP), and UKMO—are archived from operational analyses. The primary focus in this paper is on the period of 1979–93, the period used for the reanalyses, and on climatology. However, ABM and NNR are also compared for the period before 1979, for which the evidence tends to favor NNR. The authors are concerned with basic variables—mean sea level pressure, height of the 500-hPa surface, and near-surface temperature—that are available from the basic analy...

An analysis of a 34-year air temperature record from Fossil Bluff (71°S, 68°W), Antarctica

An analysis of a long-term surface air temperature record for Fossil Bluff in the George VI Sound, West Antarctic Peninsula (WAP) documents in detail some important aspects of the climate of this area for the first time. The analysis identifies the close dependency of air temperatures on latitude in the WAP but reveals that the strength of this dependency is greatest in winter. This result along with others leads to the Fossil Bluff climate regime being characterized as ‘continental’ rather than ‘maritime’ as found further north. The WAP as a whole displays large interannual temperature variability but this is greatest in Marguerite Bay rather than the Fossil Bluff area. Evidence is also provided for secular climatic change appearing in summer throughout the WAP over the last few decades. The representativeness of existing Antarctic Peninsula annual air temperature climatologies, based mainly on snow temperature measurements, for the winter and summer periods is also noted.

The relationship of Pacific deep tropical convection to the winter and springtime extratropical atmospheric circulation of the South Pacific in El Niño events

During some El Nino events Rossby wave trains (RWT) are observed to strongly modulate the seasonal atmospheric circulation of the South Pacific extratropics in the austral winter and spring. Here it is shown that seasonally intensified deep tropical convection is confined to the Pacific Intertropical Convergence Zone (ITCZ) close to the dateline in those events with strong RWT modulation but not in other cases. In other cases deep convection either withdraws from the ITCZ or the ITCZ strongly interacts with the South Pacific Convergence Zone (SPCZ) lying at 5-10degreesS. In both cases this points to a weakening of the local tropical Hadley circulation that may be crucial for Rossby wave generation. It is also found that the expected RWT response in the high latitudes can fail to occur even when SST in the central tropical Pacific are very high.

An exceptional winter sea-ice retreat/advance in the Bellingshausen Sea, Antarctica

Abstract The exceptional sea‐ice retreat and advance that occurred in the Bellingshausen Sea, Antarctica during August 1993 was the largest such winter event in this sector of the Antarctic during the satellite era. The reasons for this fluctuation of ice are investigated using passive microwave satellite imagery, ice motion vectors derived from the satellite data, in‐situ meteorological reports and near‐surface winds and temperatures from the European Centre for Medium‐range Weather Forecasts (ECMWF) numerical weather prediction model. The ice edge retreat of more than 400 km took place near 80°W from approximately 1–15 August, although the southward migration of the ice edge was not continuous and short periods of advance were also recorded. Between 16 August and 2 September there was almost continuous sea‐ice recovery. The rate of change of the ice edge location during both the retreat and advance phases significantly exceeded the southward and northward velocity components of ice within the pack, pointing to the importance of ice production and melting during this event. During the month, markedly different air masses affected the area, resulting in temperature changes from +2°C to ‐21°C at the nearby Rothera station. ‘Bulk’ movement of the pack, and compaction and divergence of the sea ice, made a secondary, but still significant, contribution to the observed advance and retreat. The ice extent fluctuations were so extreme because strong meridional atmospheric flow was experienced in a sector of the Southern Ocean where relatively low ice concentrations were occurring. The very rapid ice retreat/advance was associated with pronounced low‐high surface pressure anomaly couplets on either side of the Antarctic Peninsula.