† The contribution of R. J. Allan was written in the course of his employment at the Met Office, UK, and is published with the permission of the Controller of HMSO and the Queen's Printer for Scotland. ‡ The contributions of these authors were prepared as part of their official duties as US Federal Government employees.The Twentieth Century Reanalysis (20CR) project is an international effort to produce a comprehensive global atmospheric circulation dataset spanning the twentieth century, assimilating only surface pressure reports and using observed monthly sea-surface temperature and sea-ice distributions as boundary conditions. is similar to that of current three-day operational NWP forecasts. Intercomparisons over the second half-century of these surface-based reanalyses with other reanalyses that also make use of upper-air and satellite data are equally encouraging.It is anticipated that the 20CR dataset will be a valuable resource to the climate research community for both model validations and diagnostic studies. Some surprising results are already evident. For instance, the long-term trends of indices representing the North Atlantic Oscillation, the tropical Pacific Walker Circulation, and the Pacific-North American pattern are weak or non-existent over the full period of record. The long-term trends of zonally averaged precipitation minus evaporation also differ in character from those in climate model simulations of the twentieth century.

Several papers have described a significant trend toward the positive phase of the Southern Hemisphere annular mode (SAM) in recent decades. The SAM is the dominant mode of atmospheric variability in the Southern Hemisphere (SH) so such a change implies a major shift in the broadscale climate of this hemisphere. However, the majority of these studies have used NCEP-NCAR reanalysis (NNR) data, which are known to have spurious negative trends in SH high-latitude pressure. Thus, given that the SAM describes the relative atmospheric anomalies at mid-and high southern latitudes, these errors in the NNR data have the potential to invalidate the published findings on changes in the SAM. Therefore, it is important that a ''true'' benchmark of trends in the SAM is available against which future climate scenarios as revealed through climate models can be examined. In this paper this issue is addressed by employing an empirical definition of the SAM so that station data can be utilized to evaluate true temporal changes: six stations are used to calculate a proxy zonal mean sea level pressure (MSLP) at both 40Њ and 65ЊS during 1958-2000. The observed increase in the difference in zonal MSLP between 40Њ (increasing) and 65ЊS (decreasing) is shown to be statistically significant, with the trend being most pronounced since the mid-1970s. However, it is demonstrated that calculated trends in the MSLP difference between 40Њ and 65ЊS and the SAM itself are exaggerated by a factor of 3 and 2, respectively, in the NNR. The SH high-latitude errors in the early part of this reanalysis are greatest in winter as are subsequent improvements. As a result, the NNR shows the greatest seasonal trend in the SAM to be in the austral winter, in marked contrast to observational data, which reveal the largest real increase to be in summer. Equivalent data from two ECMWF reanalyses, including part of the new ERA-40 reanalysis, are also examined. It is demonstrated that ERA-40 provides an improved representation of SH high-latitude atmospheric circulation variability that can be used with high confidence at least as far back as 1973-and is therefore ideal for examining the recent trend in the SAM-and with more confidence than the NNR right back to 1958.

The Reference Antarctic Data for Environmental Research (READER) project data set of monthly mean Antarctic nearsurface temperature, mean sea-level pressure (MSLP) and wind speed has been used to investigate trends in these quantities over the last 50 years for 19 stations with long records. Eleven of these had warming trends and seven had cooling trends in their annual data (one station had too little data to allow an annual trend to be computed), indicating the spatial complexity of change that has occurred across the Antarctic in recent decades. The Antarctic Peninsula has experienced a major warming over the last 50 years, with temperatures at Faraday/Vernadsky station having increased at a rate of 0.56°C decade −1 over the year and 1.09°C decade −1 during the winter; both figures are statistically significant at less than the 5% level. Overlapping 30 year trends of annual mean temperatures indicate that, at all but two of the 10 coastal stations for which trends could be computed back to 1961, the warming trend was greater (or the cooling trend less) during the 1961-90 period compared with 1971-2000. All the continental stations for which MSLP data were available show negative trends in the annual mean pressures over the full length of their records, which we attribute to the trend in recent decades towards the Southern Hemisphere annular mode (SAM) being in its high-index state. Except for Halley, where the trends are constant, the MSLP trends for all stations on the Antarctic continent for 1971-2000 were more negative than for 1961-90. All but two of the coastal stations have recorded increasing mean wind speeds over recent decades, which is also consistent with the change in the nature of the SAM.

While global mean surface air temperature (SAT) has increased over recent decades, the rate 28 of regional warming has varied markedly 10 , with some of the most rapid SAT increases 29 recorded in the polar regions [11][12][13] . In Antarctica, the largest SAT increases have been 30 observed in the Antarctic Peninsula (AP) and especially on its west coast 1 : in particular, 31Vernadsky (formerly Faraday) station (Fig. 1) experienced an increase in annual mean SAT 32 of 2. 8° C between 1951 and 2000. 33 The AP is a challenging area for the attribution of the causes of climate change 34 because of the shortness of the in-situ records, the large inter-annual circulation variability 14 35 and the sensitivity to local interactions between the atmosphere, ocean and ice. In addition, 36 the atmospheric circulation of the AP and South Pacific are quite different between summer 37 (December -February) and the remainder of the year. 38Since the late 1970s the springtime loss of stratospheric ozone has contributed to the 39 warming of the AP, particularly during summer 7 . However, during the extended winter 40 period of March -September, when teleconnections between the tropics and high southern 41 latitudes are strongest 15 , tropical sea surface temperature (SST) anomalies in the Pacific and 42Atlantic Oceans 16 can strongly modulate the climate of the AP. The teleconnections are 43 further affected by the mid-latitude jet, which influences regional cyclonic activity and AP 44SATs. While the jet is strong for most of the year, during the summer it is weaker, there are 45 fewer cyclones, and tropical forcing plays little part in AP climate variability. 46The annual mean SAT records from six coastal stations located in the northern AP 47 (Fig. 1) show a warming through the second half of the Twentieth Century, followed by little 48 change or a decrease during the first part of the Twenty First Century 17 . We investigate the 49 3 differences in high and low latitude forcing on the climate of the AP during what we 50 henceforth term the 'warming' and 'cooling' periods, focussing particularly on the period 51 since 1979, since this marks the start of the availability of reliable, gridded atmospheric 52 analyses and fields of sea ice concentration (SIC). We use a stacked and normalized SAT 53 anomaly record (Fig. 2a) response to stratospheric ozone depletion and increasing greenhouse gas concentrations 5,18 . 68The trend in the SAM led to a greater flow of mild, north-westerly air onto the AP (Extended 69 Data Fig. 2a), with SAT on the northeastern side increasing most because of amplification 70 through the foehn effect 7 . This atmospheric circulation trend contributed to the large decrease 71 in SIC in summer (Extended Data Fig. 3a) and for the year as a whole (Fig. 3a). However, 72there was no significant trend in annual mean sea level pressure (SLP) across the AP during 73 4 the warming period (Fig. 3b). During the summer, tropical climate variability had little 74 influence on the AP SATs 15 and the trend in the...