Background: The Changing Arctic
The Arctic environment is changing, and the key variables are changing in locations and over time scales that are not being monitored with present observing systems. Several recent oceanographic expeditions indicate that the influence of the Atlantic water in the Arctic Ocean has increased. Data collected beginning in 1993 indicate that the boundary between the eastern (Atlantic) and western (Pacific) halocline assemblies has moved (Morison et al., 1998a; Carmack et al., 1995; McLaughlin et al., 1996; Carmack et al., 1997; Steele and Boyd, 1998). Earlier it was approximately aligned with the Lomonosov Ridge, but now it lies nearly parallel with the Alpha and Mendeleyev Ridges. This is illustrated in Figure 1 (pdf) which shows the differences between temperature and salinity measured in 1993 from the USS Pargo (Morison et al., 1998a) and climatological temperature and salinity from the Joint U.S.-Russian Atlas of the Arctic Ocean: Winter Period (EWG, 1997; Gore and Belt, 1997). The EWG Atlas (EWG, 1997) is a compilation of Russian and Western wintertime hydrographic data taken from 1948 to 1987. It has been objectively gridded and separated into decadal and total statistics. For this comparison the temperature and salinity from the Atlas have been interpolated to the cruise track of the Pargo, and the differences between 1993 and the climatology are plotted as color contours along the Pargo track.
Figure 1a (pdf) shows that the salinity in the upper 250 m has increased dramatically in a wedge extending from the Lomonosov Ridge to a front roughly aligned with the Alpha and Mendeleyev Ridges. This represents an advance of about 55° of longitude in the position of the front between the saltier surface waters of the eastern Arctic and the fresher waters of the western Arctic. The temporal salinity increase in the Makarov Basin is comparable to the instantaneous spatial variability over the whole Arctic Ocean. Comparison with statistics in the EWG Atlas also indicates that the change is several times the typical interannual variability in the Makarov Basin.
Figure 1b (pdf) shows that temperature has increased in the warm core of Atlantic water over the Lomonosov Ridge, with the maximum temperature being over 1°C greater than at any time in the observed past. A less intense warm core appears over the Mendeleyev Ridge, and there is a general warming in the Makarov Basin centered near 200 m. The temperature maximum has become both warmer and shallower (Carmack et al., 1997; Swift et al., 1997). The increased temperature over sloping topography suggests that the warm water is a tracer that is carried along by this flow. Swift et al. (1997) use this idea to infer that the warming in the Arctic Basin is due to changes in the temperature of the Atlantic water inflow.
The shoaling of the Atlantic water suggests that the halocline, which isolates the surface from the warm Atlantic water, is growing thinner. The cold halocline layer in the Eurasian Basin has retreated during the 1990s and now covers significantly less area than in previous years (Steele and Boyd, 1998; Ekwurzel, 1998). In 1995, the Makarov Basin was the only region that still had a true cold halocline layer. Since the cold halocline layer insulates the surface and sea ice from the heat of the Atlantic water, halocline changes could have profound effects on the surface energy balance and sea ice in the Arctic.
There have been changes in the Canada Basin as well. McPhee et al. (1998) report that during the fall of 1997, multiyear ice near the center of the Beaufort Gyre was anomalously thin. The upper ocean was also both warmer (relative to freezing) and substantially less saline in 1997 than in the mid 1970s. Macdonald et al. (1999) suggest the change began in 198990. The increased heat content, combined with the thin ice, suggests that during the summer of 1997 the ice concentration was lower and allowed more solar radiation to enter the upper ocean. These effects may be due to reduced ice convergence in the Beaufort Sea (McPhee et al., 1998; Macdonald et al., 1999). In 1998 the summer ice extent in the same Beaufort Sea region was a record minimum.
The observed oceanographic changes appear to have begun in the late 1980s. In 1991 a slight warming was observed near the Pole (Anderson et al., 1994; Rudels et al., 1994), and Quadfasel et al. (1991) report warmer than usual temperatures in the Atlantic water inflow in 1990. The differences from climatology seen during the subsequent cruises are much larger. The large differences between the 1991 and 1993 data suggest that we are seeing an event unique to the 1990s.
The changes in the Arctic Ocean are related to changes in the atmosphere (Swift et al., 1997; Dickson et al., 1997). The observed shift in ocean frontal position is associated with a decadal trend in the atmospheric pressure pattern (Morison et al., 1998; Steele and Boyd, 1998). Figure 2 (pdf) (from Steele and Boyd, 1998) shows the fields of atmospheric pressure and ice drift measured by the International Arctic Buoy Programme (IABP) (Colony and Rigor, 1993 and Rigor and Colony, 1995) averaged for 197987 (a) and 198896 (b). The patterns of pressure and ice drift for 1988-96 are shifted counterclockwise 40°60° from the 197987 pattern, similar to the change in frontal position inferred from Figure 1. The yearly average pressure maps of the IABP indicate that the shift in the atmospheric pressure pattern began around 198889. After 1988, the annual average Beaufort High was weaker and usually confined to western longitudes. This is consistent with the decrease in the annual mean atmospheric surface pressure in the Polar Basin, which has been below the 197995 mean every year since 1988 (Walsh et al., 1996).
The most comprehensive picture of the change in the atmospheric pattern is presented by the leading empirical orthogonal function (EOF) of sea-level pressure for the Northern Hemisphere. Thompson and Wallace (1998) show that this EOF, termed the Arctic Oscillation (AO), is more strongly coupled to surface air temperature variations over the Eurasian Continent than is the North Atlantic Oscillation (NAO). As shown in Figure 3a (pdf), the AO has a dominant negative region centered over the North Pole. It has strong positive lobes over the North Pacific and North Atlantic. As shown in Figure 3b (pdf), the AO index has been rising since the mid-1960s, accompanied by an increase in Northern Hemisphere surface air temperature over the same period. The spatial distribution of the temperature increase confirms the results of Chapman and Walsh (1993) and Martin et al. (1997) that surface air temperatures in the Arctic have been increasing. The largest upward trends are over the Russian Arctic. Rigor (1999) finds the AO accounts for more than half the warming trends over Alaska, Eurasia, and the eastern Arctic. The AO is associated with low pressure over the Arctic Ocean, so that the particularly rapid increase in the AO index shown after the late 1980s, Figure 3b, agrees with the results of Walsh et al. (1996) and with the timing and sense of change in upper Arctic Ocean circulation. The time series of the 50-hPa and surface coefficients are very well correlated, meaning that the change in the atmosphere extends from the stratosphere to the surface.