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

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. D12, PAGES 13,999-14,007, JUNE 27, 1997 Variability of precipitation over the coastal western Antarctic Peninsula from synoptic observations J. Turner, S.R. Colwell, and S. Harangozo British Antarctic Survey, Cambridge, United Kingdom Abstract. 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. 1. Introduction Numerical models of the atmosphere suggesthat if there is warming across the high southern latitudes, there could be a decrease in the extent of the sea ice and more precipitation in the Antarctic coastal zone [Budd and Sirnrnonds, 1991 ]. Determining the past distribution and variability of precipitation across the Antarctic and its causes is therefore a high priority if we are to be able to understand the changes in years to come and to determine whether they are natural or man induced. However, direct measurement of precipitation on the continent presents many problems because of the strong winds that are a feature of large parts of the coastal region. When the wind speed at a height of 10 m is greater than 5-10 m/s, the snow can be lifted from the ground and moved considerable distances before being deposited again. Meteorologists have had little success with the use of snow gauges, since they rapidly fill with blowing snow, and it is impossible to determine whether the accumulated snow falls as precipitation or is blown into the gauge. Most Antarctic observing stations therefore do not use snow gauges but estimate accumulation from a stake array at a location that is well removed from obstacles. This method is reliable on the relatively flat surface of the ice sheet but not where local and regional topography are a factor. For example, Turner et al. [1995] showed that data from the stake array at Rothera Station on the Antarctic Peninsula (see Figure 1 for places referred to in the text) were strongly influenced by blowing snow episodes and that during the year examined, most of the large increases snow depth occurred as a result of blowing snow. Copyright 1997 by the American Geophysical Union Paper number 96JD /97/96JD ,999 Other techniques used to estimate snow accumulation in the Antarctic include assessment of snow drift density and analysis of snow and ice retrieved from pits and ice cores. These latter methods are particularly powerful; stratigraphy and radioisotope analysis can provide information on annual snow accumulation, although data from deeper cores are difficult to ascribe to a particular year. Nevertheless, data from analysis of short ice cores have allowed the production of a number of maps of annual accumulation [e.g., Bull, 1971; Giovinetto and Bentley, 1985; the latter are reviewed by Giovinetto and Bull, 1987], which show the broad-scale distribution of snowfall across the continent. These maps indicate that the greatest accumulation is in the coastal region with a rapid decrease toward the interior, suggesting the importance of synoptic-scale weather systems that do not generally penetrate far onto the continent [Bromwich, 1988]. It is also possible to use the output of numerical models to compute the net precipitation (precipitation minus sublimation/evaporation) for the Antarctic, which allows the investigation of precipitation variability across a large area [Brornwich et al., 1995]. In the extrapolar regions the synoptic observations from meteorological stations have been used for a number of years to examine the climatological occurrence of different types of precipitation [Tucker, 1961; Petty, 1995]. However, such data have received little attention in the Antarctic. At the coastal stations on the western side of the Antarctic Peninsula these reports have been made for almost 50 years, providing long series of in situ meteorological observations. These data provide subjective observations of rain or snow occurrence, along with an estimation of precipitation intensity, but cannot be converted directly into a snow or rain accumulation because of the lack of in situ "truth" data with which to carry out a calibration. However, they can provide information on precipitation frequency and the ratio of solid to liquid precipitation. Provided that a

2 14,000 TURNER ET AL.' ANTARCTIC PRECIPITATION VARIABILITY 60 ø 75 ø 90 ø 105 ø 45 ø 30 ø 15øW 120"W 60øS 65 ø 70 ø 75 ø 80 ø 85øS Figure 1. Map of the Antarctic Peninsula region showing places referred to in the text. sufficiently long series of observations are available, they should be able to provide information on the variability of precipitation and thus be of relevance in the analysis of accumulation series obtained from ice cores. 2. Data British research stations have been in continuous operation on the western side of the Antarctic Peninsula since the mid-1940s. Argentine Islands (later Faraday Station) was established at 65.4 ø S, 64.4 ø W (altitude 11 m) during 1947 and Adelaide Island (later replaced by the nearby Rothera Station) was established at ø S, ø W (altitude 16 m) in Both stations have operated full surface meteorological observing programs, making three and six hourly reports, respectively. These have always been made by a trained meteorological observer, although at Rothera, the nighttime report (taken at 0600 UTC) is made by a nightwatchman with limited training in observing. In this paper we use only six hourly reports (0000, 0600, 1200 and 1800 UTC). The more recent data from these sites have been entered into a single database; the Argentine Islands/Faraday observations from 1956 to date and the Adelaide Island/Rothera observations from 1962 to date. In the following sections we will simply refer to the two stations as Faraday and Rothera. The synoptic reports in the database have been carefully screened for inconsistencies, such as rain falling when the temperature is well below freezing or any precipitation occurring when there is little cloud. We therefore believe that the data are free of significant errors. The World Meteorological Organization (WMO) SYNOP code for reporting surface meteorological conditions over land provides a comprehensive means of reporting precipitation occurrence and the type of hydrometeors observed. The codes are summarized in the Observer's Handbook [Meteorological Office, 1982]. If precipitation is falling at the observing time, it can be reported as drizzle, rain, snow, or a shower, with the intensity being given as slight, moderate, heavy, or moderate/heavy. The criteria for estimating intensity and type of precipitation are also given in the Observer's Handbook. The code also allows the reporting of precipitation in the last hour or in the last six hours, but we have not used these particula reports because during the night there is no observer keeping a near-continuous watch on the conditions, that is, many precipitation events are missed. The division of falling precipitation into snow or rain/drizzle can be used with reasonable confidence. However, the estimates of precipitation intensity are subjective and not used. There are other problems facing observers, such as differentiating between showers and intermittent precipitation, so we have not carried out a detailed analysis of shower activity, which might have been able to provide information on the nature of the air masses affecting the research stations. Nevertheless, In this paper we examine the variability of precipitation occurrence throughouthe year and on the interannual timescale using the series of synoptic meteorological reports from Faraday and Rothera Stations. We also relate these data to synoptic-scale we examined the Faraday data to determine the sensitivity of the activity via the other meteorological variables and the series of Australian surface and upper air analyses that have been produced since 1972 [Le Marshall et al., 1985]. The quality of these analyses in the early years of the series is questionable because of the lack of observations available [Karoly and Oort, 1987]. However, because this is the longest series of analyses available 1,400 1,300 1,200 1,100 we have used them in this study. First, we consider the nature 1,000 (c) of the precipitation reports and the problems facing observers in 900 (b) the Antarctic. The various types of precipitation observation are 800 discussed, and the reasons for using only certain reports are 700 given. We then presenthe time series of the total number of (a) precipitation reports and examine the trends for different seasons and how they relate to atmosphericirculation and temperature in the Antarctic coastal region. Finally, we consider the phase of 4OO the precipitation and how this has changed since the 1950s Year Figure 2. Number of precipitation reports each year at Faraday. (a) Continuous precipitation, (b) continuous and intermittent precipitation, and (c) continuous, intermittent, and precipitation in the last hour.

3 , TURNER ET AL.' ANTARCTIC PRECIPITATION VARIABILITY 14,001 variations observed in the precipitation record to the particular year at Faraday. Here the number of precipitation reports and the types of reports using three different criteria: (1) only reports of PMSL have an out of phase relationship with the greater number continuous precipitation (precipitation falling at the observing of precipitation reports and the lowest PMSL being found in the time and over the last hour); (2) reports of continuous austral spring and autumn, with higher pressure and fewer precipitation (as defined above) plus intermittent precipitation precipitation reports in the summer and winter. The annual cycle (precipitation falling at the observing time but not for the whole of PMSL is typical of many Antarctic coastal stations and shows of the last hour); and (3) continuous, intermittent, and the well-known "semiannual cycle" discussed by van Loon precipitation in the last hour (but not at the reporting time). [1967]. This cycle is observed because of changes in the The time series of the total number of precipitation events for circumpolar trough, which is the belt of low pressure located each year since 1956 (Figure 2) are very similar, using these between 60 ø and 70øS apparent in the PMSL charts. The trough three criteria, showing the relative insensitivity to the types of is found on the PMSL fields because of the large number of precipitation reports used. However, in all the subsequent synoptic-scale depressions that occur in the zone. These consist sections we will use criterion 2 for the reasons discussed earlier. of large systems that move south from midlatitudes and stagnate All reports of snow, rain, and drizzle were included, but we in the Antarctic coastal zone [Streten and Troup, 1973] and those have not used reports of diamond dust since the amounts of that develop within the circumpolar trough and follow a zonal, precipitation are very small indeed and do not contribute i.e., eastward track [Jones and Simmonds, 1993]. The semiannual significantly to the surface accumulation. cycle in PMSL reflects a deepening and southward migration of the circumpolar trough during the intermediate months and a 3. The Seasonal Cycle of Precipitation filling and northward movement in the summer and winter. The relationship between PMSL and the number of precipitation Reports reports at Faraday thus clearly indicates that the presence and Both Faraday and Rothera have sufficiently long series of depth of synoptic-scale depressions are very important factors in synoptic reports so that the variation in the number of determining precipitation at the coastal sites on the western side precipitation reports over the year can be considered. In Figure of the Antarctic Peninsula. 3 we show the monthly mean number of precipitation reports and At Rothera, some 300 km to the south of Faraday, month-tothe pressure at mean seal level (PMSL), for each month of the month pressure variations are in phase with those at Faraday but are amplified (Figure 3B), indicating that the semiannual cycle is most pronounced at this location. However, the annual cycle in the number of precipitation reports is less clearly linked with the 48[ /. x x' \ // ]993 surface pressure than at Faraday, probably indicating that the synoptic-scale activity is more variable on a year-to-year basis 46 I / //f, / 1992 here than at the northern site. This is also indicated by the fact that the standard deviations of the monthly mean number of "0 precipitation events and surface pressure are slightly larger at the 988 more southerly site o 44 E,.. 42 a. 40 I: 38 ) 36 ß ß - 32 o 30 a Months Pressure -- Precipitation 26 I i I I Months Pressure -- Precipitation Figure 3. Mean number of precipitation events and the mean sea level pressure for each month of the year at Faraday ( ) and Rothera ( ). 4. Temporal Variability of Precipitation Observations Interannual variations The interannual variability of precipitation at Faraday and Rothera is examined through the total number of precipitation reports for each year. Figure 4 shows this quantity for Faraday, 993 along with the mean annual temperature for the station and trend 992 annual total of precipitation reports is clearly a very variable t 991 lines quantity for both and over quantities the period determined 1956 from to 1993 a least varied squares from fit. 365 The to 610 in This is consistent with the large 589 -o interannual variability in the atmosphericirculation around the 988 Antarctic Peninsula which results from changes 987 tracks. Although the annual variability in the number of the / 986 precipitation events is large, there are periods of several years of / consistently more or fewer events than the mean. One of these / 985. lasted from the late 1970s to the 1980s, when an above average- -,,/ 984 number of events was recorded between 1975 and in I I addition, over the period Faraday had only one year with below-normal precipitation. Ice cores collected at various locations across the Antarctic Peninsula also show a greater in depression amount of prec;pitation in the late 1970s/early 1980s [D.A. Peel, personal communication, 1996]. Figure 4 shows the comparable series of annual totals of precipitation reports and the mean annual temperatures for

4 14,002 TURNER ET AL.: ANTARCTIC PRECIPITATION VARIABILITY , > oo. 450 a / ' \ \ /, '\/' \ /, /' ' '\ /, /'\ /' \, Years Precipitation... Temperature than the 5% level). The analysis took into account the autocorrelation in the series, so that only every other year of data was used in assessing statistical significance (Student T test). The lag 1 correlation was less than 0.2, using data from alternative years only. Data for Rothera (Table 1) shows a negative trend in the autumn, but positive values for the rest of the year. The actual rate of increase in the number of precipitation reports for the winter to summer period is actually greater than that found at Faraday, although the shortness of the time series precludes any reliable statistical inference regarding these trends. In the following sections we will examine other meteorological measurements from the Antarctic Peninsula to try and determine the reasons for the variability in the number of precipitation events and the changes observed since the start of meteorological records Table 1. Annual and Seasonal Precipitation Report Variability Trend Trend (Events per Mean Standard Year or Season) Deviation Faraday, Annual Years... Temperature Precipitation Figure 4. Annual total number of precipitation reports and mean annual temperature for Faraday and Rothera. Winter 1.01 *! Spring Summer Autumn Rothera. Both Faraday and Rothera are located within the latitude band of the circumpolar trough, and so high interannual variability in the number of precipitation reports is not unexpected. However, the standar deviation for Rothera (86.1) is higher than that for Faraday (47.8), again indicating that depression activity is more variable at the more southerly site. For the period 1963 to 1993 the annual total of precipitation reports at Rothera ranged from 295 to 680 which is over 150% of the range at Faraday. Long-Term Trends The annual and seasonal mean number of precipitation reports and trends for Faraday for the period are shown in Table 1. These data indicate a year-round increase in the number of precipitation reports, although there is a large variation in the trend throughout the year. The trend is very small in the spring but reaches a maximum in the winter. During the winter season of June to August the number of reports has been increasing at a rate of about one report per season since This may seem a very small increase, but over the 38 years of the record, it has resulted in almost a 50% increase in the number of reports each season from around 95 to 132. Figure 5 shows the number of precipitation reports at Faraday for each winter season since 1956 along with the mean winter temperature and the trend lines. A statistical analysis shows that only the winter trend can be regarded as increasing at a statistically significant level (at less Rothera, Annual Winter Spring Summer Autumn Faraday, Annual Winter Spring !6 Summer Autumn Positive trend values indicate an increasing number of precipitation reports. The trend value marked with an asterisk statistically significant at less than the 5% level. The seasons are taken as summer (December to February), autumn (March to May), winter (June to August) and spring (September to November).

5 ... TURNER ET AL.: ANTARCTIC PRECIPITATION VARIABILITY 14, >e 120.m 110 ' O 7O -2 days (Figure 6). Winds on precipitation days have a strong -4 northwesterly component at this upper level, where the winds are -6 not affected by the local topography. We could therefore expect that in years of greater cyclonic activity and therefore a greater -8 number of reports of precipitation thathere would also be higher mean temperatures ß However, this is nothe case at either -10 g station, and in a number of years the precipitation and -12 g temperature are out of phase. Both the temperature and the -14 precipitation data were therefore compared with other meteorological data from the Antarctic Peninsul and related to the broader-scale circulation via the Australian mean sea level -18 analyses. The strongest relationships found were as follows: The Faraday annual mean temperature is strongly Years influenced by the amount of sea ice in the Bellingshausen Sea,... Temperature -- Precipitation especially during the winter months. This is in line with the Figure 5. Total number of precipitation reports and the mean work of King [ 1994]. Because the sea ice acts as a very efficient temperature for the winter season (June to August) at Faraday insulating layer on top of the relatively warm ocean, the surface over temperatures were lower than average during winters with aboveaverage sea ice extent. The winter air temperatures at the low-lying stations on the western side of the Antarctic Peninsula 5. Links Between Precipitation and Atmospheric Circulation and Sea Ice Extent are therefore influenced directly by the oceanic conditions. 2. The annual number of precipitation reports at Faraday was found to be related in most years to the surface pressure in the Bellingshausen Sea. Turner et al. [1995] showed that most Turner et al. [ 1995] showed that the number of precipitation significant precipitation events at Rothera Station were caused by reports recorded at stations on the coastal western side of the depressions in the Bellingshausen Sea suggesting that on a Antarctic Peninsula was largely determined by the synoptic-scale day-to-day basis, pressure was correlated strongly with the circulation and, in particular, by the location and track of the precipitation over the mid-antarctic Peninsula region. However, major depressions. Time series of the number of precipitation as can be seen in Figure 7, the number of precipitation reports reports, such as that for the winter season at Faraday shown in and mean PMSL at 65øS, 80øW in July indicate that a similar Figure 5, should therefore reflect depression activity in that sector relationship also holds on a monthly basis. In most years the of the Antarctic. One of the winters with a very large number of correlation between the two quantities is very good. However, precipitation reports was 1979, when around 50% more reports in some years the relationship breaks down, such as in 1973, wet,', recorded than the long-ter mean. At this time, the major when pressure was high, yet there were still many precipitation meteorological project called the First GARP Global Experiment reports. This may be explained by lower PMSL in the Weddell (FGGE) was being carried out by the international community. Sea and higher pressure to the west of the Antarctic Peninsula. This project involved the deployment of many additional Such a synoptic situation would place the western side of the observing systems, especially over the Southern Ocean, and the detailed investigation of the atmospheric circulation and its Antarctic Peninsula in a cold, unstable airstream with many showers affecting the area. Normally, such a situation rarely representation in operational numerical weather prediction persists for long periods, but it did so in July 1973, giving the systems. Physick [1981] undertook a detailed analysis of the unusual combination of higher than normal surface pressure and synoptic-scale activity and depression tracks across the southern a large number of precipitation reports. hemisphere using data from the project and noted the anomalously cyclonic nature of the circulation during the year. He found that the circumpolar trough was particularly deep and that around the coast of East Antarctica the large number of depressions resulted in the circulation appearing to consist of a single, dense depression track to the north of the coastline. The The long-term increase in annual mean temperature at Faraday has been investigated by King [1994] who examined the circulation around the Antarctic Peninsula using a number of zonal and meridional flow indices. These were computed using the synoptic meteorological observations from stations on the Antarctic Peninsula and the nearby islands using records starting large number of precipitation reports at Faraday during the winter in the 1940s. These indices suggested that there had been no of 1979 confirms the close association between the cyclonic activity and the number of precipitation events at the stations the western side of the Antarctic Peninsuland also indicates that major circulation changes across that sector of the Antarctic are reflected in the surface reports. At both Faraday and Rothera the annual mean temperature and total number of precipitation reportshow little correlation in particular years (Figure 4). This is somewhat surprising since we systematichange in the atmosphericirculation over this period, and the study therefore concluded that the observed warming was likely a result of oceanographic and/or sea ice extent changes. The increase in the number of precipitation reports at Faraday since the 1950s also implies that a significant change has taken place in some aspect of the atmospheric or oceanic conditions in the region. Oceanographic data in this area were very sparse prior to the 1970s, although since that time, we have had know that most precipitation the western side of the Antarctic reasonable coverage from the polar orbiting satellites. In Peninsula comes from depressions in the Bellingshausen Sea, particular, the passive microwave imagers on the Nimbus 7 and which bring mild conditions to the area as warm fronts move DMSP satellites have provided data that allow the sea ice down from the northwest. This can be seen in the 700 mbar wind roses for Faraday for days with precipitation and also for all conditions to be monitored on almost a daily basis [Zwally et al., 1983]. However, inspection of the seasonal sea ice extent record

6 14,004 TURNER ET AL.: ANTARCTIC PRECIPITATION VARIABILITY a) 10% t 1-10 knots I Ill - 20 knots [ knots knots >= 41 knots 10% t,1-10 knots 0.2% [ Ill - 20 knots [ ]21-30 knots I [31-40 knots >= 41 knots Figure 6. The 700 mbar wind roses for Faraday for precipitation days and all days. and Faraday precipitation data suggests that there is no link between these quantities. If higher sea surface temperatures and reduced sea ice were found in the ocean area to the west of the Antarctic Peninsula, there would be greater fluxes of heat and moisture into the atmosphere and possibly more cloud, particularly at low levels. With the predominantly northwesterly airflow in the area the airstreams would be forced to ascend the steep western side of the Antarctic Peninsula with the formation of additional cloud, possibly giving more precipitation. However, this topographically induced precipitation would tend to fall well above the immediate coastal area where the stations are located. From the data available it would appear that on a year-to-year basis the oceanic conditions have little effect on the number of precipitation events recorded at the stations and that atmospheric conditions and particularly depression activity are having the greatest influence. There are three possible factors that may account for the longterm increase in the number of precipitation events recorded at the low-level stations on the western side of the Antarctic Peninsula: Higher Air Temperatures Robin [1977] examined the link between the free air temperature and the snow accumulation across the Antarctic and found it to be consistent with the Clausius-Clapeyron relationship. This can be explained as the saturation vapor pressure (SVP) setting an upper limit on the amount of water vapor that is available for precipitation. Fortuin and Oerlemans [ 1990] also considered factors that were important in determining the amount of precipitation thai fell across the Antarctic and found that the SVP was by far the most important parameter, particularly in the interior of the continent. Here, above 1500 m, 72% of the variance in precipitation could be explained in terms of the SVP and the convexity of the surface. To date, the relationship between SVP and precipitation low-level sites has not been investigated, but it seems unlikely that the fact the air can hold more moisture would alone result in more precipitation events, since other factors, such as stronger vertical velocities, would be required to give precipitation. It is therefore not possible to say that the increase in the number of precipitation

7 TURNER ET AL.' ANTARCTIC PRECIPITATION VARIABILITY 14, and at Faraday is generally the same, although the ratio of precipitation reports at the two sites is not constanthroughout the year (Figure 3). These differences, expressed as the ratio of the number of precipitation reports at Rothera to Faraday for each month of the year, are shown in Figure 8. In the extended winter period, from April to September, there are from 5 to 10% more 990 c: precipitation events at Rothera than at Faraday and up to 15% 988 less for the remainder of the year. This winter maximum of 986 precipitation at Rothera is probably a result of the much colder 984 air temperatures at the more southerly site, which limit the 982 amount of water vapor that the air can hold, coupled with the Years greater coverage by sea ice. As relatively warm air masses are advected down the Antarctic Peninsula in the northwesterly air flow, they will be cooled, with the development of extensive -- Precipitation Pressure cloud. A greater amount of cloud could then be expected to result in a greater number of precipitation events. The greater Figure 7. Number of precipitation reports at Faraday and the amount of precipitation at Faraday during the remainder of the mean surface pressure at 65 ø S, 80 ø W for the month of July over year, when there is less sea ice present and the two stations have the period a more similar climate, probably reflects the fact that the fronts passing down the Antarctic Peninsula are more active in the more northerly part of the region. At this time of the year the reports on the western side of the Antarctic Peninsula has occurred because of rising air temperatures. temperatures the two stations are more similar than during the winter [King, 1994], so that the moisture holding capacity of the air is less of a factor than the synoptic-scale activity. Reduced Sea Ice in Bellingshausen Sea Throughout the year, Rothera has a higher standard deviation Less winter-season sea ice in the Bellingshausen Sea would (SD) in the number of precipitation reports than Faraday (Table result in greater fluxes of heat and moisture into the atmosphere 1). It is interesting to note that at Rothera both surface pressure and also provide more cloud condensationuclei. An analysis of and temperature also have a higher SD than at Faraday. This is the mean northern limit of the ice at 70øW during the winter does consistent with greater interannual variability in the atmospheric indeed show a downward trend, although because of the large circulation at the more southerly site. The difference between the interannual variability, it is not statistically significant at the 95% SDs in the number of precipitation events at the two sites is level. greatest during the spring and smallest during the autumn. With its more southerly location on the poleward edge of the Changes in Atmospheric Circulation circumpolar trough we could expect observations from Rothera to show a greater interannual variability in the cyclonic activity. More intense and/or a greater number of depressions in the This is borne out in all but two months of the year, with Rothera Bellingshausen Sea would result in more moist, northwesterly air observations having higher SDs in the PMSLs than Faraday. masses, possibly giving a greater number of precipitation events. However, the relatively short observational record for Rothera Such a flow would also have an impact on the sea ice extent, does not at the moment allow us to interpret straightforwardly the which would affect temperatures. An analysis of the Australian differences in the annual cycle of the SD in precipitation reports. surface pressure charts for the winter months does show a This will require further investigation of the synoptic climatology decrease in the surface pressure in the Bellingshausen/Amundsen of the region. Seas, which would imply stronger northwesterly flow. Perhaps more significant, an analysis of depression tracks suggests that since 1973, more depressions have arrived in the Bellingshausen Sea from north of 60øS, compared to systems that developed in 11o the Antarctic. These lows from outside the Antarctic would bring humid air masses toward the Antarctic Peninsula and would increase the probability of precipitation. From the analysis carried out so far, we therefore feel that the increase in the number of precipitation events observed at Faraday is most likely a result of changes in the atmospheric circulation. However, because of the close coupling between the atmospheric and the oceanic environments, more research is required to confirm these preliminary results. loo Spatial Variability The synoptic records for Faraday and Rothera allow some aspects of the spatial variability of precipitation down the western side of the Antarctic Peninsula to be investigated, although clearly, it is not possible to determine fine-scale structure. The number of precipitation events recorded over the year at Rothera lo Months Figure 8. Ratio of the number of precipitation reports at Rothera and Faraday for each month of the year. Figures were computed for the period 1977 to 1993.

8 14,006 TURNER ET AL.: ANTARCTIC PRECIPITATION VARIABILITY ioo $6 I Year... Liquid --Solid Figure 9. The percentage of summer season (December to February) precipitation reports at Faraday which consist of liquid (dashed line) and solid (solid line) precipitation. When we consider only the period (the period for which we have data from both Faraday and Rothera), it is interesting to note that the Faraday precipitation record is characterized by a negative trend at all times of the year. Table 1 shows that for this period the Rothera precipitation trend is large and positive, again highlighting the different climatic conditions at the two sites for this period. 7. The Characteristics of Precipitation The warming trend at Faraday noted by King [1994], with a magnitude of about 2øC over the period since 1956, has had an effect on the characteristics of the hydrometeors recorded at the station. During the winter months, when the warming trend is greatest, the temperatures are very low, and there has been negligible effect on the form of the precipitation. However, during the summer, when the temperatures are close to freezing point (the mean temperatures Faraday for December, January, and February are -0.3 ø, 0.6 ø, and 0.3øC, respectively), the effect is pronounced. Figure 9 shows the percentage of summer season precipitation reports that consist of liquid and solid precipitation for each summer since This shows that at the start of the period there were twice as many reports of snow compared to rain and drizzle. However, by the early 1990s, Faraday was receiving about an equal number of solid and liquid precipitation events. This change in the relative frequency of the two forms of precipitation is statistically significant at less than the 8% level, and the proportion of liquid to solid precipitation is increasing by 2.1% per summer season. Such a change in the nature of the precipitation will only be found in areas where the summertime mean temperatures are close to freezing, which effectively limits this to the western side of the Antarctic Peninsula. However, this change does illustrate the sensitivity of the precipitation type to changes in temperature in certain areas. 8. Discussion and Conclusions Obtaining an understanding of spatial and temporal variations of precipitation across the Antarctic Peninsula is important for many areas of study, including investigation of mass balance, climate change studies, and ice core interpretation. As an aid to I // the interpretation of data from ice cores, it would be of value to understand the types of weather system which are giving the precipitation, e.g., fronts, mesocyclones, etc., and the relative contributions of riming, clear sky precipitation, and snowfall. While the performance of atmospheric models for the high latitudes is rapidly improving [Turner et al., 1996], and it is clear that these models will soon be able to produce accurate predictions of precipitation over most of the Antarctic [Genthon and Braun, 1995], the Antarctic Peninsula is one of the most challenging areas for these models because the topography is so complex. We therefore need to make as much use of the in situ precipitation reports as possible in order to assess how the models are currently performing and to help understand the output from climate change runs made with relatively lowresolution versions of the models. While it is difficult to see how the routine precipitation reports can be converted to accumulation measurements, they do provide insight into precipitation variability and allow precipitation events and temporal variability thereof to be linked to the synoptic-scale activity and climate fluctuations. The work presented here is in general agreement with that of Turner et al. [1995] in showing the importance of synoptic systems in giving precipitation in the Antarctic coastal region. The fact that the semiannual cycle in depression activity was apparent in the precipitation reports suggests that other phenomena, such as E1 Nino-Southern Oscillation (ENSO), may also be searched for in these data. The search for ENSO signals in Antarctic meteorological data has been receiving increasing attention during recent years [Smith and Stearns, 1993] and such a study would be of value to those examining Antarctic ice cores. Ideally, ice cores from close to Faraday and Rothera could be calibrated with station precipitation measurements, but the summer melt at these low-lying stations makes this impossible, and ice core data from nearby sites at a higher elevation will have to be used. The 37% increase the number of winter-season precipitation reports from 1956 to 1993 noted at Faraday suggests that precipitation frequency is increasing in the northern part of the western Antarctic Peninsula. The reasons for the trend cannot, however, be determined unequivocally at this time, although examination of the long series of Australian analyses would suggesthat there has been a change in the origins of lowpressure systems approaching the Antarctic Peninsula over the period studied. However, more studies are needed to ascertain the physical basis for these changes. References Bromwich, D.H., Snowfall in high southern latitudes, Rev. Geophys., 26, , Bromwich, D.H., F.M. Robasky, R.I. Cullather, and M.L. Vanwoert, Atmospheric hydrologicycle over the Southern Ocean and Antarctica from operational numerical analyses, Mon. Weather Rev., 123, , Budd, W.F., and I. Simmonds, The impact of global warming on the Antarctic mass balance and global sea level, in Proceedings of the International Conference the Role of the Polar Regions in Global Change, edited by G. Weller, C.L. Wilson, and B.A.B. Severin, pp , Geophy. Inst., Univ. of Alaska, Fairbanks, Bull, C., Snow accumulation in Antarctica, in Research in the Antarctic, edited by L.O. Quam, pp , Am. Assoc. for the Adv. of Sci., Washington, D.C., Fortuin, J.P.F., and J. Ocrlemans, Parameterisation of the annual surface temperature and mass balance of Antarctica, Ann. Glaciol., 14, 78-84, 1990.

9 TURNER ET AL.: ANTARCTIC PRECIPITATION VARIABILITY 14,007 Genthon, C., and A. Braun, ECMWF analyses and predictions of the Robin, G. de Q., Ice cores and climatic change, Phil. Trans. R. Soc. surface climate of Greenland and Antarctica, J. of Clint., 8, London, Ser. B 280, , , Smith, S.R., and C.R. Steams, Antarctic pressure and temperature Giovinetto, M.B., and C.R. Bentley, Surface balance in ice drainage anomalies surrounding the minumum in the Southern Oscillation systems of Antarctica, Antarct. J. U.S., 20, 6-13, index, J. Geophys. Res., 98, 13,071-13,083, Giovinetto, M.B., and C. Bull, Summary and Analysis of Surface Mass Streten, N.A., and A.J. Troup, 1973: A synopticlimatology of satellite Balance Compilations for Anta.rcti.ca, , Rep. 1, Byrd Polar observed cloud vortices over the Southern Hemisphere, Q. J. R. Res. Cent., Columbus, Ohio, Meteorol. Soc., 99, 56-72, Jones, D.I., and I. Simmonds, A climatology of Southern Hemisphere Tucker, G.B., Precipitation over the North Atlantic Ocean, Q. J. R. extratropical cyclones, Clim. Dyn. 9, , Meteorol. Soc., 7, , Karoly, D.J., and A.H. Oort, A comparison of Southern Hemisphere Turner, J., T.A. Lachlan-Cope, J.P. Thomas, and S. Colwell, The synoptic circulation statistics based on GFDL and Austa!ian analyses, Mon. origins of precipitation over the Antarctic Peninsula, Antarct. Sci., 7, Weather. Rev., 115, , , King, J.C., Recent climate variability in the vicinity of the Antarctic Turner, J., et al., The Antarctic First Regional Observing Study of the Peninsula, Int. J. Climatol., 14, , Troposphere (FROST) project, Bull Ant. Meteorol. Soc., Le Marshall, J.F., G.A.M. Kelly, and D.J. Karoly, An atmospheric van Loon, H., The half-yearly oscillations in middle and high southern latitudes and the coreless winter. J. Atmos. Sci., 24, , climatology of the Southern Hemisphere based on ten years of daily numerical analyses ( ); I, Overview, Aust. Meteoro!. Mag., Zwally, H.J., J C. Comiso, C L. Parkinson, W.J. Campbell, F.D. Carsey, 33, 65-85, and P. Gloersen, Antarctic sea ice, NASA Spec. Pap., Meteorological Office, Observer's Handbook, HMSO, London, NASA SP-459, Petty, G.W., Frequencies and characteristics of global oceanic precipi.tation from shipboard present-weather reports, Bull. Am. S. Colwell, S. Harangozo, and J. Turner, British Antarctic Survey, Meteorol. Soc., 76, , High Cross, Madingley Road, Cambridge, CB3 0ET, England. Physick, W.L., Winter depression tracks and climatological jet streams in the southern hemisphere during the FGGE year, Q. J. R. Meteorol. (Received December 18, 1995; revised June 19, 1996; Soc., 107, , accepted September 17, 1996.)