British Antarctic Survey, Natural Envil'Ol1l11eni Research Council. Cambridge. United ... I Now at: Scott Polar Research Institute, University of. A database of ...
ANTARCTIC PENINSULA CLIMATE VARIABILITY
ANTARCTIC RESEARCH SERIES VOLUME 79, PAGES 61-68
SPATI AL AND TEMPORAL VARIATION OF SURFACE TEMPERATURE ON THE
ANTARCTIC PENINSULA AND THE LIMIT OF VIABILITY OF ICE SHELVES Eli zabeth M. Morris l and David G. Vaughan British Antarctic Survey, Natural Envil'Ol1l11eni Research Council. Cambridge. United Kingdom
Mapping surface air temperature in the Antarctic Peninsula region is made unusually difficult by: the scarcity of meteorological stations, strong climatic gra dients and recent rapid regional warming. We have compiled a database of 534 mean annual temperatures derived from measurements of snow temperature at around 10-m depth and air temperature measured at meteorological stations and automatic weather stations. These annual temperatures were corrected for inter annual variability using a composite record from six stations across the region. The corrected temperatures were then analysed using multiple linear regression to yield altitudinal and temporal lapse rates. A subset of 508 values were then used to produce a map of temperature reduced to sea level and for a specific epoch (2000 A.D.). The map shows the dramatic climate contrast (3-S°C) between the east and west coast of the Antarctic Peninsula in greater detail than earlier studies and also indicates that the present limit of ice shelves closely follows the -9°C (2000 A.D.) isotherm. Furthermore, the limit of ice shelves known to have retreat ed during the last 100 years is bounded by the -9°C and -S oC (2000 A.D.) isotherms, suggesting that the retreat of ice shelves in the Antarctic Peninsula region is consistent with a walllling of around - 4°C. ice shelves around the Antarctic Penin sula as a thermal condition [Vaughan and Doake, 1996]. However, there are significant limitations to Reynolds' analysis; for example, he noted that " if more temperature data were available it would be better if a temporal regression were to be incorporated into the statistical analysi s". We now ha ve the opportunity to improve Reynolds ' map by including new station data , substantially more borehole ternperatures, data from automatic weather sta tions, and by removing interann ual and trend anomalies from the data. We thus present an updated map of mean annual temperature over the Antarctic Peninsula and dis cuss its sign ificance.
1. lNTROD UCTIO N
It has been long-understood, and widely discussed [e.g. Schwerdtfeger, 1974], that the high topography over the
Antarctic Peninsula (Figure I) forms a significant climat ic barrier between the warm Bellingshausen Sea and the colder Weddell Sea. However, the sparse network of cli matological observing stations in the Antarctic Peninsula region is sufficient to reveal only the broadest detail s of this climatic divide. An earlier attempt to map mean annual air temperature supplemented the station data with mean annual air temperatures inferred from ice cores and boreholes [Reynolds, 1981J, and has helped us to under stand the nature of the climatic boundary [e.g. King and Turner, 1997J and to interpret th e geographical Iilll it of
2. DATA A database of mean annual surface temperat ure for the Antarctic Peninsula region (between 400W and 105 oW,
I Now at : Scott Polar Research Institute, Uni ve rsity of Cambridge, United Kingd om
Copyright 2003 by the American Geoph ysical Union 10.\ 029/079ARS05
6\
62
ANTARCTIC PENINSULA CLfMATE VARIABILITY
2.1 Expedition Data Be Il ing h a US en ~
Fara d a y/, '. ' Verna d sky.......
B e llings hausen Sea
~
Orcad as
'" - Esper a n za
Weddell Sea
Measurements of air temperature in the Antarctic Peninsula were sparse before the mid-20th century, when the first scientific bases were established. However, Jones [1990] Iisted mean annual temperatures from 10 expedi tions since 1898 that wintered on or near the Antarctic Peninsula. We have discarded values based on less than 12 months of observation but have used seven mean val ues based on yearlong measurements at 6, 8 or more observations at fixed times during the day (Table I).
2.2 Station Data
Fig. I. Location map of the Antarctic Peninsula
and between 60 0 S and 83 °S) has been constructed using 239 measurements of mean annual air temperature from expedition s, meteorological stations and automatic weather stations, and 295 measurements of snow tem perature at around 10m depth [Morris et al. , 2002]. We have not used proxy temperature data from ice cores , for example, oxygen isotope measurements, because the mean value of the proxy variable over an annual layer may not be an accurate indication of mean annual. tem perature in this area [Vaughan el aI, in press]. Ice core data are indeed useful for studying temperature varia tions over different timescales , but in this paper we are primarily concerned with producing a map of mean annual temperature based on values derived from direct measurements of temperature.
Since the mid-20th century, surface a ir temperatur\. has been measured at scientific stations in the Antarctic Peninsula region , although not all stations achieve an unbroken record. The SCAR READER project (Reference Antarctic Data for Environmental Research) has created a high quality, long-term data set of mean surface and upper air meteorological measurements from il1 situ observations. The data set, which is updated regu larly as the project progresses, is avai lable from the READER web site hosted by the British Antarctic Survey (www.antarctica.ac.uk). Table 2 lists the meteo rological stations and the period during which some or all years have a percentage of observations high enough to calculate an accurate mean annual temperature. We have used the original READER criteria that, for each of 12 months, the data must be >80% complete and there must be no breaks of more than 5 days, to select data for inclusion in our database . (The first criterion has now been made more stringe nt for READER data by raisin!' the required percentage to >90%). We have selected 21l, separate years of mean annual surface air temperatures from 22 stations. Note that the focu s of this paper is on using all sources of mean annual temperature data; we are not here concerned with other station data, such
TABLE I. Mean annual surface air temperatures meas ured by expeditions [Jones, 1990]. All sires we re on land , except for Be/gico, which was trapped in sea ice to the west of the Antarctic Peni nsula in 189 8 and for which we give a mean position. Site
Lat ituderS
Longitude/oW
Elevation/ m.a .s.1.
Date or Peri od
Number of months
BeJgica Snow Hill Island Port Charcot Water Boat Point Winter Island BaTT)' Island Stonnington Island
70.62 64 .50 65 .07 64.8 6525 68 13 68.18
88.58 56.93 64.03 62 .72 64 .27 67.10 67.03
5 13 9 3 34 15 15
1898 1902 ·03 1904 1921 1935 1936 1940
12 18 12 12 12 12 12
MORRIS AN D VAUGHN ANTARCTI C PENINSULA SURFAC E TEMPERATURE
63
TABLE 2. Detai ls o f records of mean an nual air tempe rature from meteorological o bserving station s included in tIle database. Sta tion
Latitude/o S
Long itud e/o W
E levation /m.a.s .1.
Pe ri od
Siple Orcadas Decep tion Esperanza San Mart in Faraday/Ve rnad sky Arturo Prat O ' Higgins Marsh Ade laide Bell in gs bause n Mara lllbio Rothera G reat Wall Ferraz Juba ny Arctowski AIllli rallle Brow n Hope Bay Mat ie nzo Admi ra lty Bay Petre l
75.93 60.75 63.9 8 63.4 68. 13 65.25 62 .5 63 .32 62.2 6 .77 62.20 64.24 6 757 62.22 62.08 62. 24 62.1 6 64.88
84.25 44.72 60 .57 56 .98 6710 64 26 59. 68 57.90 58.96 68.93 58.96 56.66 68. 12 58 .96 58.39 58.66 58.47 62.88 56.98 60.05 58.42 56.2 2
1054 6 8 13 4 II 5 10 10 26 16 198 16 10 20 4 2 7 10 32
1979-86 1996-2000 1959-66 1997 -99 1996-99 1951-95 1996-2000 1996-2000 1996-2000 1963 -74 1970-2000 1996-2000 1978-2000 1996-2000 1995-99 I 996-99 1978-96 1952-83 1953-59 1962-75 1951 -60 1968-7 5
634 64.97 62. 08 63.47
as su mmer temperatures w hich are, of course , va luab le fo r studies of othe r aspects of the Antarcti c Peninsula cli mate
2.3 A utOlll(llic Weflt/ter Swtio1l. The mean annu a l air tempera tures recorded by an wtomatic weat her station (AW ' ) are unlikely to be as acc urate as those derived from mann ed stations but AW Ss do yield data at remote location s that wo uld oth erwise not be sampled. We have inclu ded 22 measure ments of m ean annual surface air temperature from th e 6 Automatic Weath er Stations included in the READER database at tb e time of writing (Tab le 3). The same crite ria for inclu sion are used as fo r the station records. 2.4 Tel/-il1elre SilO II' Temperatures
Mean annual air te mperature at the snow surface is commonly estim ated by assum ing that it is the sa me as the s now tem perature measured at a depth of 10m [Paterson, 1994]. The acc uracy of this ass umpti on depends on the strati grap hy of the snow, the shape of the annual cyc le of air tempera ture and any long-term tem perature trend. Tn a prev ious paper [Morris and Vaughan,
II 18
Number o f complete yea rs 5 3 8 3 4 45 5 5 4 12 13 5 22 4 4 4 15 16 7
JO 10 6
1992], we described a tec hnique to ['e move the effects of measurement depth and date from borehole tempera tures. We have app lied the same technique to I O-m tem peratures in this study. We assu m e that th e acc umulation , densification and meta m o rphos is of po la r snow are c on trolled by the local climate, so that the snow cover at sites with the same tem pe rature hi sto ry, T,(t) , develops broadly the same physical properties. In partic u lar, we suppose that the thermal diffu sivity in the upper 10 m of snow is similar for sites of s imilar T,(r). In this case , a standard set o f snow temperature curv es, T(d,t) , will appl y for all si tes with the same surface temperature hi s tory. The mean a nn ua l surface temperature, TI11 , is then obtained by subtracting a correctio n, aF (T,A,t), fro m the measured borehole temperature, T(d [,![). The a mpli tude of th e first harmonic of th e annu al temperature wave , a, is known, and ra nges from about 4°e at th e northern end of the Antarctic Penin sula to lODe at the South Pole. On the southe rn part of the Peninsula and on Filchner-Ronne Ice Sh e lf a is about I I dc. Th e function F is known at s ites where snow temperatures have been measured at diffe rent depths throughout the year. For the northern part of the Antarctic Peninsula we estimated F from data recorded at Maudheim S ta ti on [Dalrymple el af. , 1966J a nd for so uthern areas, da ta recorded at Plateau
ANTARCTIC PEN[NSULA CLIMATE VARIAB[L1TY
64
TABLE 3. Automatic Weather Stations Station
Latitude/oS
Longitude/o W
E[evation/m.a.s.l .
Period
No . of complete years
Bonaparte Point Butl er Island Limbert Racer Rock Siple Uranus Glacier
64.8 72.21 7542 64.07 75.92 7143
64.1 60.17 59.95 61.61 84.25 68.93
8 91 40 17 [054 780
[992 1990-98 1996-97 J 990-91 1986-91 1987-95
7 2 2 6 4
Station [Weller and Schwerdtfeger, 1977]. Borehole tem peratures are normally made in the austral summer when the snow at I O-m depth is beginning to warm up after the arrival of the winter cold wave through the snow. Thus the corrected mean annua l surface temperature is of the order of 0 - O.s oC warmer than the borehole temperature [Morris and Vaughan, 1992J. \)"Ie have extended the database of borehole measure ments used in Morris and Vaughan [1992] by add ing published and unpublished data corrected for depth and time of year. Thi s yields a total of 285 estimates of mean annual surface air temperature. These are supplemented by a further II sites for which the date of the borehole measurement is unknown but which can be used for spa tial anal ys is.
3. ANALYSIS Broadl y speaki ng, the 10-m temperature data give most infoffilation on spatial variabil ity and the station data give most information on temporal variability. The key to our approach is, how ever, that both types of data are included in a unified analysis, aimed at producing a map of mean annual temperature for a specific epoch.
3.1 Assumptions We assume that the mean annual surface air tempera tures , T,, can be represented by
where for each measurement, qJ is latitude, A, longitude, elevation above mean sea level and r is date of meas urement. The constants, CP, A Z and 00 denote spatial and temporal lapse rates. The assumption of constant rate of ch ange in mean annual temperature over the period cov ered by the database (102 years) is a simplification, but we do not consider that the amount of data available, especially from the earlier yea rs, justifies using a more
Z,
[
complex expression in equation (I). Use of a s ingle func tion, al(l), to describe the interannual variability implies that this is constant over the domain. This assumption is suppol1ed by recent findings [King and Comiso, in press] that there is a good correlation (mostly> 0 .5) betweel. winter temperatures as expressed by passive microwave emissivity across most of the Antarctic Peninsula and the meteorological temperatures at Faraday (now re-named Vernadsky). To estimate al(r), the inter-annual variability, we first estimated mean annual temperature for all years (not only those which have data that satisfy the criteria we set for inclusion in the mean annual temperature database) for each of the stations with long records (Orcadas, FaradaylYernadsky, Bell i ngshausen, Rothera, San Martin and Esperanza). We then calculated the deviation from the trend, for each of these stations. The mean of the surface temperature anomalies is used as an estimate,
