Atmospheric moisture budget over Antarctica and the Southern Ocean based on the ERA-40 reanalysis

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 28: (2008) Published online 5 March 2008 in Wiley InterScience ( Atmospheric moisture budget over Antarctica and the Southern Ocean based on the ERA-40 reanalysis Hanna Tietäväinen a * and Timo Vihma b a Department of Physical Sciences, Division of Atmospheric Sciences, University of Helsinki, Finland b Finnish Meteorological Institute, Helsinki, Finland ABSTRACT: The atmospheric moisture budget over Antarctica and the Southern Ocean was analysed for the period on the basis of the ERA-40 reanalysis of the European Centre for Medium-Range Weather Forecasts. Meridional transport by transient eddies makes the largest contribution to the southward water vapour transport. The mean meridional circulation contributes to the northward transport in the Antarctic coastal areas, but this effect is compensated by the southward transport by stationary eddies. The convergence of meridional water vapour transport is at its largest at S, while the convergence of zonal transport is regionally important in areas of high cyclolysis. Inter-annual variations in water vapour transport are related to the southern annular mode (SAM). The eastward transport has a significant (95% confidence level) positive correlation with the SAM index, while the northward transport has a significant negative correlation with SAM near 60 S. Hydrological balance is well-achieved in the ERA-40 reanalysis: the difference between the water vapour flux convergence (based on analysis) and the net precipitation (precipitation minus evaporation, based on 24-h forecasts) is only 13 mm yr 1 (3%) over the Southern Ocean and 8 mmyr 1 (5%) over the continental ice sheet. Over the open ocean, the analysis methodology favours the accuracy of the flux convergence. For the whole study region, the annual mean flux convergence exceeded net precipitation by 11 mm yr 1 (3%). The ERA-40 result for the mean precipitation over the Antarctic continental ice sheet in is 177 ± 8mmyr 1, while previous estimates range from 173 to 215 mm yr 1. For the period , the ERA-40 data do not show any statistically significant trend in precipitation over the Antarctic grounded ice sheet and ice shelves. From the ERA-40 data, the annual average net evaporation (evaporation minus condensation) is positive over the whole continent. Copyright 2008 Royal Meteorological Society KEY WORDS Antarctica; ERA-40 reanalysis; water vapour; flux convergence; transient eddies; stationary eddies; net precipitation Received 14 May 2007; Revised 16 November 2007; Accepted 24 December Introduction Precipitation is the only significant source term in the mass budget of the Antarctic ice sheet. Over the majority of the ice sheet, evaporation is close to zero and there are no significant sources of water vapour. Hence, precipitation is dependent on the atmospheric moisture transport from lower latitudes. Owing to its effects on the distribution of precipitation and evaporation, atmospheric moisture transport is also important for the freshwater budget of the Southern Ocean. Further, through the cloudradiative forcing the moisture transport strongly affects the heat budget of the snow and ice surfaces. Estimates of the moisture transport depend on the data employed, and more accurate quantitative results are urgently needed to better understand the factors controlling the mass, heat and freshwater budgets and their sensitivity to changes in climate. The mass budget of the Antarctic ice sheet is controlled by precipitation, evaporation, condensation, melt-water * Correspondence to: Hanna Tietäväinen, Finnish Meteorological Institute, P.O. Box 503, FI Helsinki, Finland. hanna.tietavainen@fmi.fi discharge, the basal melt of the underwater parts of the ice sheet, net export of blowing snow, and iceberg calving. On the scale of the whole ice sheet, the important terms are precipitation, iceberg calving, and basal melt, while the others are minor terms, although they may be locally or regionally important in the coastal areas. A nonzero mass budget of the grounded ice sheet changes the global sea level, and recent studies (Thomas et al., 2004) have indicated a rapid reduction of glaciers in coastal West Antarctica. On the other hand, satellite altimetry data show that the East Antarctic ice sheet has grown in the period (Davis et al., 2005; Zwally et al., 2005). Davis et al. (2005) interpreted this as being primarily due to an increase in snowfall accumulation, while Monaghan et al. (2006a) concluded that there has not been any significant change in snowfall since the 1950s. One of the problems in estimating the mass budget of the Antarctic ice sheet is that it is very difficult to measure the accumulation. Precipitation measurements by gauges are liable to errors due to blowing snow, and the gauge network is very sparse and concentrated in coastal regions. Moreover, in the interior of the continent, Copyright 2008 Royal Meteorological Society

2 1978 H. TIETÄVÄINEN AND T. VIHMA precipitation falls mostly in the form of clear-sky diamond dust precipitation (Ekaykin et al., 2004), which cannot be recorded by snow gauges. Snow stake measurements and estimates based on snow-pit stratigraphy are also very liable to errors, and their spatial coverage is poor. Owing to wind-driven sublimation, the spatial variability of the surface mass budget on the kilometre scale is an order of magnitude higher than its temporal variability on the centennial time scale (Frezzotti et al., 2004). The thickness of the ice sheet can be measured by satellite altimetry, but altimetry data do not directly allow one to distinguish between the various terms of the mass budget. To separate the accumulation term (precipitation), relevant estimates can be made on the basis of the atmospheric moisture budget. Analysis of net precipitation and ice-core data can also be combined to support each other (Steig et al., 1994; Monaghan et al., 2006a). In addition to the Antarctic continental ice sheet, the freshwater flux in the form of net precipitation is also important for the Southern Ocean. A positive freshwater flux at the sea surface lowers the surface salinity and strengthens the stratification of the ocean boundary layer, further affecting the formation and melt of sea ice. Experiments applying ocean circulation models have suggested that the formation of Antarctic Bottom Water, an essential component in the general circulation of the global ocean, is sensitive to the surface freshwater flux (Stössel and Markus, 2004). This is also the case for the development of large-scale open ocean polynyas (Marsland and Wolff, 2001), which allow enhanced evaporation and cause a strong feedback to the freshwater flux. In the icefree Southern Ocean, the annual evaporation is more than 50% of the precipitation (Kållberg et al., 2005), while in the sea ice zone evaporation is mostly restricted to leads and polynyas (Vihma et al., 2002). Hence, in the sea ice zone, analogously to the continental ice sheet, the net precipitation strongly depends on the transport of moisture from lower latitudes. Bromwich (1979, 1988) and Connolley and King (1993) utilized rawinsonde sounding data in calculating the moisture transport, while Slonaker and van Woert (1999) applied satellite data, and Giovinetto et al. (1992, 1997) used surface observations. Atmospheric model results have been applied to analyse the modelled precipitation minus evaporation (Ohmura et al., 1996; Cullather et al., 1998; Bromwich et al., 2004; Simmonds et al., 2005; van de Berg et al., 2006; Monaghan et al., 2006a,b) and to calculate the meridional moisture flux (Howarth, 1983) and its convergence (Bromwich et al., 1995; Genthon and Krinner, 1998; van Lipzig and van den Broeke, 2002). The effects of cyclones on the moisture transport have been addressed in several studies, most of them being case studies (Stone et al., 1989; Stone and Kahl, 1991; Naithani et al., 2001). In addition, the origins of weather systems that have precipitated over the Antarctic have been identified by trajectory analysis (Reijmer et al., 2002; Russell et al., 2004). A systematic division between the contributions of the mean meridional circulation (MMC) and disturbances has been made, e.g. by van Lipzig and van den Broeke (2002), and Bromwich et al. (1995, 2000), but they did not separately consider the contribution of stationary eddies (SE). Following the classical approach of Palmén and Vuorela (1963), we will divide the meridional water vapour transport into contributions by the MMC, SE and transient eddies (TE). For the Antarctic, a division into these three components seems to have been made previously only by Genthon and Krinner (1998), who analysed meridional energy transport across 70 S. During recent decades, we have witnessed a dramatic improvement in the accuracy of atmospheric model analysis and forecasts, in particular, for the Southern Hemisphere (Simmons and Hollingsworth, 2002; Bromwich and Fogt, 2004; Uppala et al., 2005). We, therefore, consider it essential that new estimates of the atmospheric moisture budget over the Antarctica and Southern Ocean are calculated on the basis of recent model analysis. The ERA-40 reanalysis (Uppala et al., 2005) of the European Centre for Medium-Range Weather Forecasts (ECMWF) will be applied in this study. Our objectives are the following: a. to provide, on the basis of ERA-40, a comprehensive picture of the horizontal and vertical distribution as well as the meridional and zonal transport of water vapour over the Antarctic and Southern Ocean in different seasons, b. to analyse the relationship between the atmospheric moisture budget and the large-scale atmospheric circulation, as characterized by the indices of the southern annular mode (SAM), semi-annual oscillation (SAO), and southern oscillation (SO), c. to quantify the role of the MMC, SE and TE in the moisture transport in various latitudes, d. to analyse the spatial and seasonal distributions of precipitation and evaporation as well as the trends in precipitation during the modern satellite era (since 1979), and to compare them with previous estimates, and e. to obtain quantitative results for the net precipitation (precipitation minus evaporation) over the Antarctic and Southern Ocean, and to compare them with the water vapour flux convergence, taking into consideration the sources of uncertainty in both results. The reanalysis data set and our methodology is described in Section 2, while the vertical and horizontal distribution of water vapour is presented in Section 3. In Section 4, we address the distribution of precipitation and evaporation, the transport of water vapour being analysed in Section 5. Section 6 divides the meridional water vapour transport between the MMC, SE and TE, and Section 7 compares the water vapour flux convergence with net precipitation. We pay detailed attention to one year (2000), while the vertically integrated properties and the total transport are analysed primarily for the period , also paying attention to the evolution of the accuracy of ERA-40 due to changes in observation methods in the late 1970 s.

3 ERA-40 ATMOSPHERIC MOISTURE BUDGET OVER ANTARCTICA AND THE SOUTHERN OCEAN Data and methodology The study focuses on the period from 1979 to 2001, the period in which satellite data were assimilated into ERA-40. This caused a major improvement in the quality of ERA-40 in the Southern Hemisphere, while before 1979 the reliability of ERA-40 in the Antarctic and Southern ocean was poor (Simmons and Hollingsworth, 2002; Monaghan et al. 2006a, b), at least in non-summer months (Bromwich and Fogt, 2004; Bromwich et al., 2007). To quantify the differences in ERA-40 before and after 1979 and to make comparisons with older studies, we also utilized the data from the whole ERA-40 period from 1958 to Even for the period , the ERA-40 reanalysis is not free of errors: Trenberth et al. (2005) observed errors in total column water vapour (TCWV) over the oceans, and there are specific problems in the Antarctic (Reijmer et al., 2005): the surface temperature is overestimated and the katabatic winds in the coastal regions are underestimated because of the limited horizontal model resolution. Further, Vaughan et al. (1999) noticed that the ERA-40 mean net precipitation values are somewhat lower than results based on in situ data and remote sensing observations. Despite these imperfections, ERA-40 is regarded as the most accurate reanalysis over the Southern Hemisphere (Bromwich and Fogt, 2004; Monaghan et al., 2006b; Bromwich et al., 2007; Dell Aquila et al., 2007), and will therefore be applied in this study. The ERA-40 reanalysis is based on a forecast model at T159 resolution (approximately 125 km in the horizontal). The vertical resolution is 60 levels, of which more than half are in the troposphere. A three-dimensional variational data assimilation (3DVAR) technique is used (Uppala et al., 2005). The TCWV, cloud ice and cloud water, vertically integrated northward and eastward fluxes of water vapour, vertically integrated flux convergences, as well as precipitation and evaporation were collected directly from the ECMWF data archive on a 1 by 1 grid covering the region S. The precipitation and evaporation are from forecast fields, while the other variables are analysis based on 6-h forecasts (as the first-guess field) and assimilated observations. The water vapour data assimilated into ERA-40 are humidity profiles from radiosondes and radiances from a number of instruments, e.g. the vertical temperature profile radiometer (VTPR) and special sensor microwave/imager (SSM/I) on-board NOAA satellites and the high resolution infrared radiation sounder (HIRS/2 and HIRS/3) from both the Television Infrared Observation Satellite (TIROS) operational vertical sounder (TOVS) and advanced TOVS (ATOVS). Short-time precipitation forecasts tend to suffer from the long model spin-up period (Bromwich et al., 2002). We compared ERA-40 net precipitation from 6-h and 24-h forecasts for the 10-year period , and it appeared that the annual net precipitation from 6-h forecasts was approximately 10% smaller than that from 24-h forecasts. In addition, 24-h forecasted annual net precipitation was closer to the annual water vapour flux convergence. For this reason, we have made use of the 24-h forecasts. Additional ERA-40 data were collected from the year 2000 for a more detailed study focusing on the division of the transport into contributions from the MMC, SE, and TE. This particular year was selected for the more detailed studies because (1) it was close to the end of the ERA-40 period with more satellite data on air humidity and wind assimilated into the reanalysis than in the earlier years and (2) in 2000, the SAM was close to zero (0.26), and hence it can be considered as a representative year (SAM has had an increasing trend in the recent decades). SAM is an annular structure with pressure anomalies of opposite sign in the middle and high latitudes (Thompson et al., 2000). In its positive phase, SAM is characterized by a southward shift of the circumpolar jet stream with increased westerly winds near 60 S and decreased westerly winds near 40 S. The mid-latitude pattern of SAM is asymmetric with zonal three-wave structure. We utilized the SAM values from the data base of the British Antarctic survey, BAS (at these were calculated according to Marshall (2003). ERA-40 data on horizontal wind and specific humidity (analysis at 6-h intervals) were collected from 11 pressure levels (1000, 925, 850, 775, 700, 600, 500, 400, 300, 250 and 200 hpa) for the year The surface pressure data were also collected. The integrations through an atmospheric column were made from the surface pressure, p 1, to the 200 hpa pressure level, p 2. Taking the vertical integral of specific humidity, q (in kg kg 1 ), through an atmospheric column, we get the TCWV, Q, inkgm 2. The quantity is the same as precipitable water (or water vapour) in millimetres: p2 Q = 1/g qdp (1) p 1 where g is the acceleration due to gravity. For the vertically integrated water vapour flux ( F q,inkgm 1 s 1 ) we get: p2 F q = 1/g q V dp (2) p 1 where V is the wind vector. F q can be divided into northward (meridional) and eastward (zonal) components. At any given pressure level, the meridional water vapour flux can be divided into the contributions of MMC, SE, and TE (Palmén and Vuorela, 1963): [qv] = [q][v] + [q }{{} v ] + [q }{{} v ] }{{} MMC SE TE (3) where v is the northward wind component, the overbar denotes temporal averaging, square brackets denote zonal averaging, and the prime and the star denote deviations from the temporal and zonal means, respectively. The deviations from the temporal mean were calculated

4 1980 H. TIETÄVÄINEN AND T. VIHMA (1) from the annual time average and (2) from the temporal mean for each month, to separate the contribution of the weather disturbances from seasonal variations. The results based on the two methods were very close to each other, and in Section 6 we show only the results based on (1). The components of the vertically integrated water vapour flux are given by: p2 [ F q ] = 1/g [qv] dp = F q,mmc + F q,se + F q,te p 1 p2 = 1/g [q][v] dp p } 1 {{} MMC p2 1/g [q v ] dp } p 1 {{ } TE p2 1/g p 1 [q v ] dp } {{ } SE (4) The atmospheric moisture budget can be expressed in terms of water vapour convergence and net precipitation: dq = F q P E (5) dt where P is the precipitation and E is the surface net evaporation (evaporation minus condensation), and dq/dt represents a storage term. The angle brackets denote areal averaging. For long-time averages, the storage term can be neglected and thus convergence equals net precipitation. If a model is in hydrological balance this equality is valid. The hydrological balance in ERA-40 and ERA-15 (the 15-year reanalysis of the ECMWF covering the period ) was evaluated by Bromwich et al. (2002) and (for ERA-15) Genthon and Krinner (1998): a significant improvement in ERA- 40 compared to ERA-15 is that an approximate annual hydrological balance is achieved for the 24-h forecasts. 3. Specific humidity and total column water vapour On the basis of ERA-40 data, the sum of total column cloud ice water and cloud liquid water is only around 1% of the TCWV. Hence, this study concentrates solely on the atmospheric water vapour. Vertical cross-sections of specific humidity for S along longitudes 0, 90, 180, and 270 E are shown in Figure 1 (d), and (e) shows the vertical crosssection along longitude 64 W, which goes approximately through the Antarctic Peninsula. The isolines of specific humidity slant towards the south, i.e. the amount of water vapour at a certain height is higher over the ocean than over the continent. In addition to the oceanic origin of the water vapour, the specific humidity gradient is also due to the northward increase in air temperature, which controls the atmospheric capability to hold water vapour: air saturated with respect to ice at 20 C holds only 8% of the water contained in saturated air at 10 C. Hence, the latent heat of condensation plays rather a passive role in the atmospheric dynamics over the Antarctic ice sheet. Over the Southern Ocean, however, the latent heat flux from the sea is a significant energy source for synoptic and mesoscale weather systems (White and Simmonds, 2006). The difference in humid-air penetration into West and East Antarctica can be clearly seen from Figure 1. In East Antarctica (Figure 1 and ), air masses with a humidity exceeding 0.5 g kg 1 are already blocked by the steep slope at S, whereas in West Antarctica humid air climbs a fairly gentle slope and air masses with a specific humidity exceeding 0.5 g kg 1 reach 80 S (Figure 1(c) and (d); the former includes the almost flat Ross Ice Shelf). In the sector of the Antarctic Peninsula (Figure 1(e)), air with a specific humidity of maximum 0.5gkg 1 reaches 84 S. However, air with higher specific humidity values is blocked further north by the mountainous peninsula. A particular feature at 80 S, 270 E is a tongue of dry air generated via katabatic winds and penetrating below the moist air mass (Figure 1(d)). The highest specific humidity values, around 5 g kg 1, are found near the surface at the northern edge of the study area. The seasonal distribution of TCWV with sea ice concentration of 0.5 is presented in Figure 2. Over the sea, TCWV increases rather symmetrically northward, while over the ice sheet the differences between the dry East Antarctica and more humid West Antarctica are evident. Seasonal variations in the atmospheric water vapour content are large, particularly south of 60 S. The highest TCWV values over the continent are found in summertime (DJF) on the northern tip and coastal regions of the Antarctic Peninsula, where TCWV reaches on average 7 8 kg m 2. Over the Southern Ocean at S, summertime (DJF) TCWV is on an average 12 kg m 2. During the austral winter (JJA), corresponding values are 3 4 kg m 2 on the Antarctic Peninsula and 8 kg m 2 over the Southern Ocean at S. Day-to-day variations in TCWV related to weather systems are large, particularly in West Antarctica. Seasonal variations in TCWV are also larger in West Antarctica, while on the high plateau of East Antarctica TCWV remains below 1kgm 2 in all seasons. The massive ice sheet in East Antarctica creates an effective barrier for synoptic-scale weather systems bringing humid air to the continent, whereas the western side of the continent is topographically lower and more indented, allowing weather systems to penetrate further inland. The Antarctic Peninsula acts as a wall for the westerly winds, collecting humid air on its western side and keeping its eastern side drier. Most of the water vapour in the Antarctic atmosphere originates from the open parts of the Southern Ocean (Bromwich and Weaver, 1983). The seasonal distributions of both TCWV and sea ice extent closely follow that of air temperature. Inter-annual variations in TCWV over the Antarctic ice sheet correlated with the SAM index in the period

5 ERA-40 ATMOSPHERIC MOISTURE BUDGET OVER ANTARCTICA AND THE SOUTHERN OCEAN S 85 S 80 S 75 S 70 S 65 S 60 S 55 S 50 S Pressure (hpa) Pressure (hpa) Pressure (hpa) Pressure (hpa) Pressure (hpa) S 85 S 80 S 75 S 70 S 65 S 60 S 55 S 50 S (c) S 85 S 80 S 75 S 70 S 65 S 60 S 55 S 50 S (d) S (e) S 85 S 80 S 75 S 70 S 65 S 60 S 55 S 50 S 85 S 80 S 75 S 70 S 65 S 60 S 55 S 50 S Latitude (degrees South) Figure 1. Annual specific humidity (g kg 1 ) by ERA-40 for along the longitudes 0 E, 90 E, (c) 180 E, (d) 270 E and (e) 64 W. The cross-section (e) goes approximately through the Antarctic Peninsula. Note the non-linearity at the lower end of the contours. The white areas represent the Antarctica land/ice mass (correlation coefficient r = 0.48; significant at 95% confidence level): positive SAM was associated with low water vapour values over the ice sheet. In the Southern Ocean at S, there was, however, no significant correlation. The distribution of water vapour based on the ERA-40 reanalysis fits with the results of Connolley and King (1993) solely based on rawinsonde sounding data: an annual mean of 4 kg m 2 along the coast of East Antarctica and 0.6 kg m 2 on the high plateau. The ERA-40 results can be compared with at least one study based on an entirely independent method: Miao et al. (2001) presented the distribution of TCWV over Antarctica based on data from the Special Sensor Microwave/Temperature 2 (SSM/T2) instrument. Except for some synoptic-scale variations, the daily-averaged TCWV maps from 1997 and 1998 showed similar spatial distributions to the ERA- 40 reanalysis for the corresponding seasons. 4. Precipitation and evaporation The averages of seasonal net evaporation (evaporation minus condensation) and precipitation for the period are presented in Figures 3 and 4, while Table I shows estimates for the annual means on the

6 1982 H. TIETÄVÄINEN AND T. VIHMA (c) (d) Figure 2. Seasonal distribution of total column water vapour (kg m 2 ) by ERA-40 for in DJF, MAM, (c) JJA and (d) SON. The thick black line represents the seasonal distribution of a sea ice concentration of 0.5 by ERA-40. Note the non-linearity of the scale. (c) (d) Figure 3. Seasonal net evaporation (mm) by ERA-40 for in DJF, MAM, (c) JJA and (d) SON. Note the non-linearity of the scale. Antarctic continental ice sheet based on the results of Bromwich et al. (2004), Monaghan et al. (2006b), and this study. Comparisons in Table I demonstrate a large scatter between estimates based on various models and methods; the results of a single regional model are very sensitive to lateral boundary conditions (compare the Polar MM5 results from Monaghan et al. (2006b)) and the results are also very sensitive to the exact study period (compare the ERA-40 results for and ). According to this study, the net evaporation typically increases towards the north, the values in the sea ice zone being between those over the continent and over the open ocean (Figure 3). In winter (JJA), local

7 ERA-40 ATMOSPHERIC MOISTURE BUDGET OVER ANTARCTICA AND THE SOUTHERN OCEAN 1983 (c) (d) Figure 4. Seasonal precipitation (mm) by ERA-40 for in DJF, MAM, (c) JJA and (d) SON. Note the non-linearity at the lower end of the scale. Table I. Estimates of precipitation and evaporation (mm yr 1 ) over the Antarctic continental ice sheet from Bromwich et al. (2004), Monaghan et al. (2006b), and this study. Study Estimate Method Value (mm yr 1 ) Bromwich et al. (2004) E Modelled sublimation from Polar MM5, Jul 1996 Jun ± 1 P Modelled precipitation from Polar MM5, Jul 1996 Jun ± 15 P NCEP-DOE AMIP-2 reanalysis (NCEP-2), ± 12 P ERA-15, ± 7 P ECMWF TOGA, ± 20 P Dynamic retrieval method with ERA-15 data, ± 5 P Dynamic retrieval method with ECMWF TOGA data, ± 16 Monaghan et al. (2006b) P Polar MM5 with boundary conditions from ERA-40, P Polar MM5 with boundary conditions from NCEP2 reanalysis, P ERA-40, P NCEP2 reanalysis, P Japanese 25-year Reanalysis, P Composite of the above results, This study P ERA-40, ± 8 E Net evaporation from ERA-40, ± 1 maxima often occur as a thin band around the coast. This is at least partly due to the presence of coastal polynyas (Stössel and Markus, 2004). A summertime (DJF) local maximum in net evaporation can be found in the Ross Sea located in the region of the southerly barrier winds (62 68 S, E). According to the ERA-40 reanalysis, during the austral autumn and winter, net evaporation is negative over most of the continent because of the deposition of hoar frost. However, during summer evaporation takes place even on the high Antarctic plateau, and as an annual average the net evaporation is positive over the whole continent. The latter is in agreement with King et al. (2001), Bromwich et al. (2004), and van den Broeke et al. (2005). The regional climate model experiments of van de Berg et al. (2005) resulted, however, in negative evaporation over most of the high plateau. The sea ice coverage over the Southern Ocean varies from about km 2 in February to about km 2 in September (Parkinson, 2004). This means significant seasonality in the atmosphere ocean exchange. In the western Weddell Sea (64 73 S, W), because of the year-round larger

8 1984 H. TIETÄVÄINEN AND T. VIHMA sea ice concentrations, the ERA-40-based evaporation is less than elsewhere in the sea ice zone (Figure 3). In summer (DJF), however, net evaporation in the Weddell Sea roughly equals the values over the other sea areas at corresponding latitudes. In summer, the sea ice remaining in the Weddell Sea does not significantly reduce evaporation, as the surface temperatures of the open ocean and (often melting) sea ice are not far from each other. Over the open ocean, evaporation is large in all seasons and largest in winter, when it exceeds 180 mm. The precipitation (Figure 4) generally increases towards the north, but there are anomalous regions: in the Atlantic sector, the precipitation is, in all seasons, lower than the zonal mean, while the maximum values (over 1000 mm per year) south of 60 S are reached over a narrow region of the west coast of the Antarctic Peninsula. On the coast of East Antarctica, annual precipitation is less varying between 300 and 700 mm, but over the ocean the sectors of E, E, and, in autumn (MAM), E also represent positive anomalies. The zonal variations in precipitation are related to quasi-stationary cyclones in the circumpolar trough (Bromwich, 1988), and the precipitation pattern in Figure 4 resembles the pressure pattern with wave number three. Precipitation in the Weddell Sea is less than in other areas at the same latitude because of the Antarctic Peninsula blocking humid westerly winds and depressions on its western side. Air on the west coast of the Peninsula is generally 7 K warmer than at similar latitudes and elevations on the east coast (Vaughan et al., 2003), a fact which has a large effect on the potential water vapour content. Additionally, evaporation values are low, especially in the western Weddell Sea. Over the ocean, the maximum precipitation is received in autumn, when the cyclonic activity is largest. This is related to the SAO, which controls the annual cycle of the mean sea level pressure at middle and high latitudes of the Southern Hemisphere (van Loon, 1967): in autumn the circumpolar pressure trough is at its most southerly location. Autumn maximum in precipitation has been observed in Southern Ocean islands, for example in the Marion Island (Rouault et al., 2005). Over the continental ice sheet, large seasonal differences occur mostly near the coast (Figure 4). The western flank of the Antarctic Peninsula and the coast of West Antarctica are known as areas where many synoptic-scale weather systems are driven by westerly winds and where many mature and declining lows are found. Extra-tropical cyclones are responsible for most of the precipitation falling over the coastal region of Antarctica (Turner et al., 1995; King and Turner, 1997). The precipitation generation in coastal regions is strongly influenced by the fact that the poleward-moving, moist air masses are blocked by the steep marginal ice slopes and experience direct orographic lifting with adiabatic cooling (Bromwich, 1988). The latter mechanism is particularly important on the west coast of the Antarctic Peninsula (van Lipzig et al., 2004). On the East Antarctic high plateau, the annual precipitation is very low, below 50 mm, and clear-sky precipitation comprises a large fraction of the total precipitation. Bromwich et al. (2004) reported a broad peak of snow accumulation in the interior of Antarctica during March October. This peak is believed to result from the increased clear-sky precipitation during the polar night when moisture saturation is enhanced by the strong radiative cooling (Bromwich, 1988). This seems, however, not to be present in the ERA h precipitation forecast. On the contrary, ERA-40 precipitation over the East Antarctic plateau is greatest during summer. The time series of annual precipitation are shown in Figure 5: the results prior to 1979 are only shown to demonstrate the dramatic impact of the start of satellite data assimilation on ERA-40. Owing to this effect, the data from show an artificial upward trend of +1.5 mm yr 2 for the annual precipitation in Antarctica. For the satellite era , we get a weakly negative trend, 0.15 mm yr 2, but according to the Cox Stuart test it is not statistically significant, neither were any of the trends we calculated for 10 -wide latitude bands. Inter-annual variations in precipitation over the Antarctic ice sheet did not have any significant correlation with the SAM index (r = 0.15), but those over the Southern Ocean had at a confidence level just below 95% (r = 0.37, instead of the 0.41 required). Annual mean evaporation in ERA-40 did not depend on the SAM index. Several previous studies have identified positive trends in Antarctic precipitation, net precipitation or snow accumulation. These include analysis of ice-core data by Morgan et al. (1991), observations of snow accumulation at the South Pole from 1955 to 1992 by Mosley-Thompson et al. (1995), and studies based on atmospheric model analysis. Cullather et al. (1998) calculated Antarctic net precipitation from ECMWF operational analysis and precipitation from National Centers for Environmental Prediction (NCEP) reanalysis for , and found a positive trend of +2.0 to +2.5 mm yr 2 (our result for the same period for ERA-40 net precipitation is 0.20 mm yr 2, but the trend is not statistically significant). Bromwich et al. (2004) applied three different methods to estimate the Antarctic precipitation: the dynamic retrieval method (DRM), the Polar MM5 model simulations, and atmospheric reanalysis and analysis. The DRM, ECMWF operational analysis and ERA-15 reanalysis, as well as the NCEP2 reanalysis, showed an upward trendof+1.3to+1.7mmyr 2 for the mean precipitation over all of Antarctica from 1979 to The precipitation trend was, however, weakly downward over much of the continent (Bromwich et al., 2004; their Figures 19 and 20). In the recent study of Monaghan et al. (2006a), the conclusion drawn was that there has been no statistically significant change in Antarctic snowfall since the 1950s. The study was based on a combination of ERA-40 reanalysis and observations, primarily from ice cores. Based on Polar MM5 simulations, Monaghan et al.

9 ERA-40 ATMOSPHERIC MOISTURE BUDGET OVER ANTARCTICA AND THE SOUTHERN OCEAN Precipitation mm Precipitation mm Year Year Figure 5. Time series of precipitation (mm) by ERA-40 in latitude bands S (solid line) and S (dashed line) and in latitude bands S (solid line) and S (dashed line) and in the Antarctic continental ice sheet (dash-dotted line) during (2006b) came to the same conclusion for the period Transport of water vapour The southward branch of the Southern Hemispheric Ferrel cell transports warm and humid air in the lower troposphere towards Antarctica as far as to S, where it encounters the northward branch of the Antarctic circulation cell, and rising motion occurs. The main contributor to the southward heat and moisture transport is, however, TE, i.e. synoptic-scale weather systems (Trenberth and Stepaniak, 2003). Most of the weather systems develop around S at the polar front separating the temperate mid-latitude air and cold polar air masses (King and Turner, 1997). Baroclinic disturbances move eastwards with the westerly winds, but they also move slowly south towards the Antarctic coastline. The ERA-40 results for the vertically integrated meridional water vapour flux (Figure 6) clearly demonstrate that the average meridional water vapour transport is southwards. Northward transport is restricted to the Ross Sea, the Weddell Sea and certain regions in the continent, above all the sector E. It is partly related to the SAM wave number three pressure pattern but also to topographic effects on the wind field. The latter are particularly important near the Lambert Glacier at 70 E, where strong katabatic winds prevail, and in the western Ross Sea at 170 E, where the northward wind component has its maximum because of strong barrier winds. The largest values for northward water vapour transport, over 10 kg m 1 s 1, are found over the latter region being co-located with the summer (DJF) evaporation maximum (Figure 3), but the transport is much smaller than the southward transport over more northerly regions of the Southern Ocean. The latter has its maximum roughly in the same sector ( E) as the major continental area of northward transport. In Table II the meridional water vapour transport through latitudes 50, 60, 70 and 80 S is presented, based on this and previous studies. In addition to presenting the ERA-40 results for the modern satellite era (1979), we present the ERA-40 results for the whole period and for the same periods as addressed in previous studies. Although the accuracy of ERA-40 prior to 1979 is significantly worse than that after 1979 (Monaghan et al., 2006a; Bromwich et al., 2007), we see that the ERA-40 results for and differ from each other less than the ERA-40 results differ from the results of other studies (Table II). The most dramatic differences are found between the early study of Howarth (1983) and ERA-40 results for the same period, especially in the northern latitudes. The results of Giovinetto et al. (1992, 1997), based on surface data, are larger than the ERA-40 results, but the coverage of the surface data was substantially poorer than that of ERA-40. Results from Bromwich et al. (1995) address a period in the modern satellite era, and are close to the ERA-40 results for the same period, although slightly smaller at each latitude. Slonaker and van Woert (1999) used a combination of TOVS and SSM/I satellite data from Their estimation of the water vapour flux across 50 S is the largest one in Table II, but their estimate across 60 S is the smallest. Their result was

10 1986 H. TIETÄVÄINEN AND T. VIHMA Figure 6. Annual meridional water vapour flux (kg m 1 s 1 ) by ERA-40 for Contour interval is 10 kg m 1 s 1. Negative values indicate southward water vapour transport. calculated for only one year, but according to ERA- 40 this year did not differ much from the average for (Table II). The large difference between ERA-40 and the results of Slonaker and van Woert (1999) is interesting, as Bromwich and Fogt (2004) suggested that the assimilation of TOVS data into ERA-40 was the main reason for the increase in the quality of ERA-40 data from 1979 onwards. The differences may have mostly originated from the wind field, which in Slonaker and van Woert (1999) was entirely based on satellite data. Seasonal averages for meridional water vapour transport across certain latitudes are presented in Table II and the seasonal geographical distributions in Figure 7. At the latitudes presented in Table II, the southward flux is largest during the austral autumn. The largest southward water vapour transport values are found over the Southern Ocean between 120 and 180 E. Particularly in summer (DJF), a tongue of large southward transport is present to the west of South America and the Antarctic Peninsula. According to ERA-40, the mean meridional wind is northerly in this region south of 55 S. Further north it is close to zero, but cases with northerly winds are associated with high air moisture. The smallest values of southward water vapour flux occur basically during the austral summer (DJF), which is mostly due to the reduced southward transport off the West Antarctica coastline at E. Further to the north, at 50 S, the southward flux is, however, smallest during winter (Table II). The northward flux over the western Ross Sea is largest during the austral summer and spring. In summer (DJF), the atmospheric water vapour content is at its highest (Figure 2) and strong southerly winds persist in the region throughout the year. At all times of the year, the Antarctic Peninsula acts as a wall separating the maximum southward water vapour transport on its western side from the northward transport and weaker southward transport occurring on its eastern side (Figure 7), as also observed by Russell et al. (2004). The zonal water vapour flux is rather symmetric with respect to the South Pole (Figure 8). The water vapour transport is mostly eastward in the westerly wind belt, but westward in the Antarctic coastal area, in the easterly wind belt. It is generated by the combined effect of the large-scale pressure field (the circumpolar trough lies north of it) and the coriolis-induced turning of the down-slope katabatic winds. In summer, when atmospheric humidity content is at its largest, the westward transport in the coastal area is over 20 kg m 1 s 1.The Transantarctic Mountains, which extend roughly from the Ross Sea to the Weddell Sea, create an area of westward water vapour transport between East and West Antarctica. The zonal water vapour transport is at its minimum during winter. Compared to the meridional transport, the zonal transport is clearly larger, but in the study area its convergence is balanced by its divergence; the meridional transport is more essential for the continental-scale net precipitation and mass balance. Both zonal and meridional water vapour transports are related to the SAM index. The correlation between the annual mean zonal water vapour flux and the SAM

11 ERA-40 ATMOSPHERIC MOISTURE BUDGET OVER ANTARCTICA AND THE SOUTHERN OCEAN 1987 Table II. Results for meridional water vapour flux across latitudinal belts 50, 60, 70 and 80 S. The flux is defined as positive northward. Meridional water vapour flux (kg m 1 s 1 ) Study Period 50 S 60 S 70 S 80 S Howarth (1983) 1 September 1973 to 31 August ERA-40 (this study) as above Bromwich et al. (1995) ECMWF TOGA and NMC ERA-40 (this study) as above Giovinetto et al. (1992, 1997) Surface data ERA-40 (this study) Slonaker and van Woert (1999) ERA-40 (this study) as above This study ERA Standard deviation ERA Standard deviation Summer (DJF) Autumn (MAM) Winter (JJA) Spring (SON) (c) (d) Figure 7. Seasonal meridional water vapour flux (kg m 1 s 1 ) by ERA-40 for in DJF, MAM, (c) JJA and (d) SON. Contour interval is 20 kg m 1 s 1. Negative values indicate southward water vapour transport. index depends on the longitude being significant at the 95% level almost everywhere (Figure 9). The accuracy of the SAM indices calculated on the basis of ERA40 have been criticized by Bromwich et al. (2007), but the dependence between the zonal flux and the SAM index is qualitatively similar with the SAM index based on ERA-40 or the BAS data set (Figures 9 and 10). The meridional water vapour flux has a weaker correlation with the SAM index, reaching significance at 95% level only near 60 S. The correlation is negative over the Southern Ocean (positive SAM yields southward water vapour transport due to the wave number three structure)

12 1988 H. TIETÄVÄINEN AND T. VIHMA (c) (d) Figure 8. Seasonal zonal water vapour flux (kg m 1 s 1 ) by ERA-40 for in DJF, MAM, (c) JJA and (d) SON. Contour interval is 40 kg m 1 s 1 for the positive values and 20 kg m 1 s 1 for the negative values. Negative values indicate westward water vapour transport. Correlation coeff Longitude (degrees East) Correlation coeff Latitude (degrees South) Figure 9. Correlation coefficient between the zonal water vapour flux (positive eastward) and the SAM index based on observations (solid line) and based on ERA-40 (dashed line), and correlation coefficient between the zonal water vapour flux and the SO index (dash-dotted line), same for the meridional water vapour flux (positive northward). The 95% significance level for the correlation coefficient (0.41) is also shown (thin solid line with circles). and significantly positive only at 80 S when the index is calculated from ERA-40 (Figure 9). The SO index has qualitatively similar but weaker (insignificant) effect than SAM on the meridional transport. For the zonal transport, the SO index has its highest correlations in the regions where SAM has its lowest ones (Figure 9).

13 ERA-40 ATMOSPHERIC MOISTURE BUDGET OVER ANTARCTICA AND THE SOUTHERN OCEAN 1989 SAM index Year Figure 10. Time series of the annual SAM index based on observations (solid line) and based on ERA40 (dashed line), and the vertically integrated eastward water vapour flux (kg m 1 s 1 ) across longitude 180 E (dash-dotted line) during Zonal water vapour flux kg/(ms) 6. The components of meridional transport and the flux convergence In Figure 11 the water vapour flux in 2000 is divided into those due to the MMC, SE and TE. It can be seen that TE are responsible for most of the southward water vapour transport in high southern latitudes. The effect of SE is, in general, small except that in the Antarctic coastal area around 70 S when there is significant southward transport compensating the northward transport by the MMC. Stationary eddy transport mostly takes place in West Antarctica, where the strongest negative stationary disturbance in the geopotential height field is located. Around 68 S the influence of the katabatic wind starts to decrease; here the MMC s northward transport of water vapour reaches its maximum, and north of this latitude begins to decrease because of the influence of the mid-latitude Ferrel cell. North of 65 S the MMC transport becomes southerly, but at latitude 50 S it is still small (around 6 kg m 1 s 1 ) compared to the transport due to TE (around 22 kg m 1 s 1 ). The water vapour flux components and the total flux through given latitudes are presented in Table III. In 2000, the magnitude of the total southward meridional water vapour transport is in latitudes 50, 60 S, and 70 S slightly smaller but in latitude 80 S larger than the annual mean for (Table II). The water vapour flux is smallest during the austral summer and largest during autumn and winter. The seasonal absolute variations decrease towards the south (as the flux magnitude decreases) but the seasonal relative variations increase towards the south. For example, in 2000 the transient eddy transport across 80 S ranges from 0.6 (summer, DJF) to 1.1 kg m 1 s 1 (autumn and spring, MAM and SON), while across 50 S the corresponding seasonal range is from 20.0 (spring, SON) to 26.1kgm 1 s 1 (autumn, MAM). The meridional water vapour convergence related to the three flux components, MMC, SE and TE, is presented in Figure 11. As can be concluded from the flux strengths (Figure 11) the water vapour convergence related to transient eddy transport is the largest and always positive. Near S the transient eddy accumulation peaks at over 300 kg m 2 yr 1, while the total water vapour flux accumulation is over 450 kg m 2 yr 1. Table III. ERA-40 meridional water vapour flux (kg m 1 s 1 ) across latitudinal belts 50, 60, 70 and 80 S divided into mean meridional circulation (MMC), stationary eddies (SE) and transient eddies (TE) in The total transport, i.e. the sum of the components, is also shown. The flux is defined as positive northward. 50 S 60 S 70 S 80 S MMC SE TE Total transport On the Antarctic high plateau the average meridional flux convergence varies around 50 kg m 2 yr 1, decreasing towards the south. The flux convergence related to SE is mostly negative but small in magnitude over the ocean, and positive over the continent. At its maximum the flux divergence related to SE removes 140 kg m 2 yr 1 of water vapour at 67 S. South of 69 S the stationary eddy convergence is compensated largely by the divergence of the transport by the MMC. South of 50 S water vapour flux convergence is larger than divergence in the north-south direction, while in the east-west direction convergence and divergence are in balance on the circumpolar scale (Figure 12 and ). The areal average for the zonal water vapour divergence over S is zero, while for the meridional water vapour divergence the average is clearly negative: 405 kg m 2 yr 1. One area of significant meridional water vapour flux convergence is located off the East Antarctic coastline between longitudes of 120 and 180 E, where northward water vapour transport from the continent and southward transport from the ocean encounter each other (Figure 6). Strong divergence in the east-west direction occurs in the same region. Large values of meridional water vapour convergence are also found on the western coast of the Antarctic Peninsula and West Antarctica. In areas of strong cyclolysis, water vapour flux convergence can be expected, while flux divergence can be expected in areas of strong cyclogenesis. In their Figure 3 Simmonds et al. (2003), present the difference between

14 1990 H. TIETÄVÄINEN AND T. VIHMA 5 Flux convergence kg/(m2yr) Meridional water vapour flux kg/(ms) Latitude (degrees South) Latitude (degrees South) Figure 11. Meridional water vapour flux (kg m 1 s 1 ) by ERA-40 for the year 2000 divided into the contributions of mean meridional circulation (solid line), stationary eddies (short dashed line) and transient eddies (dash-dotted line). The total water vapour flux is also shown (long dashed line). As above but convergence of the meridional water vapour flux components (kg m 2 yr 1 ) Figure 12. Divergence of zonal and meridional water vapour flux (kg m 2 yr 1 ) by ERA-40 for the cyclogenesis and cyclolysis rates in the high southern latitudes in the austral winter and summer. Cyclolysis exceeds cyclogenesis almost everywhere south of 50 S: the largest differences can be found along the West Antarctic coast and over the ocean near longitudes of 30 and 120 E. The differences are to some extent collocated with the ERA-40 total water vapour convergence: large water vapour convergence occurs along the West Antarctic coast and near longitudes of E (Figure 13). Simmonds et al. (2003) also concluded that quasi-stationary cyclones appear in the southern parts of the Weddell and Ross Seas, which are locations of high cyclogenesis and cyclolysis frequencies. Comparing the areal distribution of stationary eddy water vapour transport from ERA-40 with cyclone density, it can be noticed that northward transport of water vapour occurs on the western side of both the Weddell and Ross Seas, consistent with the locations of the quasi-stationary features (not shown). A similar division between the dynamic components was presented in Genthon and Krinner (1998), who calculated the ERA-15 meridional energy transport across 70 S. The southward transport of sensible and latent heat across 70 S turned out to be dominated by TE, with a smaller contribution by SE. Energy transport by the MMC was directed northwards. The dominant role of TE in the water vapour transport was also shown by van Lipzig and van den Broeke (2002), but they did not analyse the contribution of SE. They noted that during years with weak mean katabatic winds the moisture convergence over Antarctica is large. In these years, the circumpolar trough is strong, and the