The NAOAO framework merits and shortcomings 1141 SLP and temperature

SLP fields can be used to infer surface winds. An analysis of SLP can hence help us understand how the NAO/AO relates to high-latitude SAT variability and recent trends. Figure 11.11 shows SLP patterns, composite anomalies and composite differences for the region north of 50° N based on extremes of Hurrell's NAO index. Data are drawn for the years 1947-96. Immediately evident is that the SLP signature, while strongest in the vicinity of the Icelandic Low (a maximum difference of 22 hPa between extreme positive and negative states), extends well into the Arctic Basin (see panel e). Over the Pole, there is a composite difference of about 8 hPa. Building on previous discussion, the Icelandic Low, while weaker under the negative NAO/AO, is also shifted to the southwest. The low composite also shows a separate weak closed low over the Barents Sea (compare panels a and b).

Recall that one of the earliest-noted manifestations of the NAO/AO is the "seesaw" in SAT over Europe and northeastern North America. Under the positive mode, the SLP pattern associated with the strong Icelandic Low will tend to advect cold high-latitude air over Greenland and eastern Canada, consistent with the tendency for negative

High NAO index composite SLP (mb) Low NAO index composite SLP (mb)

High NAO index composite SLP (mb) Low NAO index composite SLP (mb)

High NAO index composite SLP anomaEies (mb) Low NAO index composite SLP anomalies (mb)
(High NAO index composite) minus (low NAO index composite)
Figure 11.11 Distributions of (a), (b) winter SLP and (c), (d) SLP anomalies for composites of winter months representing high-index and low-index states of the NAO, (e) the change in SLP from the high-index composite to the low-index composite (from Dickson et al., 2000, by permission of AMS).

temperature anomalies in these areas. By comparison, an opposing strong advection of warm lower-latitude air over northern Europe and Scandinavia is consistent with positive temperature anomalies in this area. Under the negative mode with a weak and southward-shifted Icelandic Low, winds over Greenland and eastern Canada are weaker (positive temperature anomalies). Winds over northern Europe and Scandinavia, while still southerly, are also much weaker than under the positive NAO mode, and have opposing temperature anomaly signals.

Hurrell (1995; 1996) can be credited with pointing out that winter NAO temperature signals are much more widespread than the classic "seesaw". Based on regression analysis between the NAO index and gridded temperature fields (1935-94), Hurrell (1996) calculated the change in temperature corresponding to a unit positive deviation of the NAO index. Figure 11.12 provides an updated analysis over the period 19002002. Strong positive changes associated with a one standard deviation positive change in the NAO index extend across northern Eurasia. Smaller positive responses are found over the eastern United States, while negative responses are found over eastern Canada and southern Greenland. This pattern clearly resembles the pattern of winter

Figure 11.12 Changes in surface temperature (x 10-1 °C) corresponding to a unit deviation of the NAO index, computed over the winters (December-March) of 1900-2002. The Arctic Ocean and other regions with insufficient data are not contoured. The NAO index is based on PC1 of the sea level pressure field for the Atlantic sector (from Hurrell et al., 2003, by permission of AGU).

Figure 11.12 Changes in surface temperature (x 10-1 °C) corresponding to a unit deviation of the NAO index, computed over the winters (December-March) of 1900-2002. The Arctic Ocean and other regions with insufficient data are not contoured. The NAO index is based on PC1 of the sea level pressure field for the Atlantic sector (from Hurrell et al., 2003, by permission of AGU).

temperature anomalies for 1981-2002 given in Figure 11.2, indicating that much of the observed recent change can be related to the change toward a more positive NAO. Using a multivariate linear regression analysis, Hurrell (1996) concluded that the NAO accounts for 31% of the interannual variance of Northern Hemisphere extratropical temperatures over the past 60 years (through 1994) while ENSO accounts for 16%. The latter reflects the ENSO link with the strength of the Aleutian Low and amplitude of the downstream Pacific North America (PNA) mid-tropospheric wavetrain over North America.

Similar results are obtained with the AO. Thompson et al. (2000) calculated the spatial distribution of linear trends in winter (January-March) SLP and SAT over the period 1989-97 (For which the AO went from a strongly negative to a positive state), and used regression techniques to assess the part of the trends linearly related to the AO index. The remainder (residual) is the part of the trends not linearly related to the AO (Figure 11.13). Following earlier discussion, the AO trend is associated with a negative trend in SLP (note that they actually use the height of the 1000 hPa surface, which is closely related to SLP) largest near the Pole, locally exceeding 70 m (see also Walsh et al., 1996). The SAT trend pattern is similar to that shown in Figure 11.2. Nearly all of the SLP decreases over the Arctic Basin, roughly half of the warming over Siberia, and all of the cooling over eastern Canada and Greenland, are linearly related to the AO trend. The residual SAT trends indicate warming over most of the hemisphere. The most obvious feature of the residual SLP trend is the pressure falls over the North Pacific, which occur in conjunction with warming over eastern Canada and Alaska. These are interpreted in terms of a control by "ENSO-like" interdecadal variability discussed in Section 11.1.2. These residual warmings, as well as those over Eurasia, are rather large, illustrating the limitations of the NAO/AO framework. In particular, the warming over Alaska seen in Figure 11.13 seems to have strong links with the PDO. Hartmann and Wendler (2005) show that warming over Alaska from 1951 to 2001 (for both mean annual and seasonal conditions) can be associated with the abrupt shift in the PDO from its negative phase (1951-76) to its primarily positive phase from 1977 to 2001. The deeper Aleutian Low in the latter period advected warm and moist air into the region. Precipitation hence also increased. Trends, however, are sensitive to the period chosen for analysis, illustrating the importance of taking into account the influence of abrupt regime shifts.

Along similar lines, the winter trend pattern over land areas shown in Plate 8 is different from that in Figure 11.13, with the satellite retrievals showing areas of cooling over central and eastern Eurasia and less pronounced warming over Alaska. Although limitations of the satellite retrievals cannot be discounted, we must consider the influence of the different analysis periods. The 1981-2003 period evaluated in Plate 8 straddles the period when the NAO/AO moved from negative to strongly positive, and then back toward a more neutral state. This will dampen the expression of the NAO/AO. Interesting in this regard is that, despite regional winter cooling, warming still dominates. Furthermore, the NAO/AO cannot give a firm accounting of the summer warming seen in Plate 8.

Figure 11.13 The 30-year (1968-97) linear trends (January through March) in (left) SLP expressed as 1000 hPa geopotential height and (right) SAT. Total trends are shown in the top panels. The middle panels show the part of the trends linearly congruent with the monthly AO index. The bottom panels show the components of the trends that are not linearly congruent with the AO. The contour intervals are 15 m (30 yr)-1 (-22.5, -7.5, +7.5 . . . ) for SLP and 1 K (30 yr)-1 (-1.5, -0.5, +0.5 . . . ) for SAT (from Thompson et al., 2000, by permission of AMS).

Figure 11.13 The 30-year (1968-97) linear trends (January through March) in (left) SLP expressed as 1000 hPa geopotential height and (right) SAT. Total trends are shown in the top panels. The middle panels show the part of the trends linearly congruent with the monthly AO index. The bottom panels show the components of the trends that are not linearly congruent with the AO. The contour intervals are 15 m (30 yr)-1 (-22.5, -7.5, +7.5 . . . ) for SLP and 1 K (30 yr)-1 (-1.5, -0.5, +0.5 . . . ) for SAT (from Thompson et al., 2000, by permission of AMS).

While the studies just discussed do not address the Arctic Ocean, this area has been examined by Rigor et al. (2000, 2002). For the period 1979-97, over most of the Eurasian side of the Arctic Ocean, at least half of the observed trends in winter SAT are linearly related to the corresponding AO trend (Rigor et al., 2000). Rigor et al. (2002) also find that spring and autumn SAT anomalies over the Arctic Ocean are correlated with the AO index during the previous winter. As discussed in Chapter 7 and earlier in this chapter, significant reductions have been observed in the summer ice cover, dominated by reductions along the shores of Siberia and Alaska. Rigor et al. (2002) showed that sea ice anomalies in different years could be understood on the basis of atmospheric forcing as far back as the previous winter, placing the observed sea ice trend in a more general AO context. The argument is that changes in the winter wind field associated with the positive AO promote an anomalous production of young thin ice in leads and polynyas along the Eurasian and Alaskan coasts. The thinner ice results in larger heat fluxes to the atmosphere in spring and hence higher SATs. Thinner ice and earlier spring melt, fostered by the higher SATs, provide preconditioning mechanisms to favor large summer ice losses. With less ice at summer's end, there will be larger heat losses to the atmosphere in autumn as the large open-water areas cool and re-freeze. Hence, the regional patterns of SAT rise over the Arctic Ocean are viewed by Rigor et al. (2000) as not so much a direct effect of temperature advection associated with the AO, but an indirect effect related to changes in the ice cover. Lending weight to this view, Plate 8 shows large temperature trends for spring and autumn in areas of sea ice retreat. We will return to this issue shortly when we elaborate on the sea ice record.

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