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Total column (surface to 300 hpa)
V
U
m s -1 dec -1
2.0
- 2
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0
2
Fig. 9.5 23-year trends (1979-2001) in zonal ( left ) and meridional ( right ) wind derived from
TOVS temperature profiles over the entire year for the entire column, surface to 300 hPa (Courtesy
of J. Francis)
An attempt to improve the three-dimensional wind fields for climate reanalyses
was undertaken by Francis et al. ( 2005 ) using satellite-derived temperature profiles
from the TIROS Operational Vertical Sounder (TOVS). The TOVS Polar Path-
finder (TOVS Path-P) product (Francis and Schweiger 1999 ), specifically designed
for polar applications, was used to compute the thermal wind with a mass conser-
vation technique (Zou and Van Woert 2002 ; Francis et al. 2005 ). One drawback of
this approach is that the resultant wind field would be nearly geostrophic and would
not take into account any significant ageostrophic motions in the flow (Zou and Van
Woert 2001 ). The thermal wind product would therefore be less accurate in regions
of significant ageostrophic flow, such as in the entrance and exit regions of jet
streaks, and in strongly curved flows, as the geostrophic balance only occurs when
there is no curvature in the flow.
Nevertheless, an examination of the TOVS-derived thermal winds has proven
useful for Arctic climate studies. Trends and anomalies for the period 1979-2001
were calculated for both meridional and zonal winds at eight levels between the
surface and 300 hPa (Francis et al. 2005 ). It was found that zonal winds are more
westerly over Eurasia and the western Arctic Ocean, while westerlies have weak-
ened over northern Canada (Fig. 9.5 ). Combined with the corresponding pattern in
meridional winds, these results suggest that the polar vortex has, on average, shifted
toward Siberia. Changes in meridional winds are consistent with observed trends in
melt onset date and sea ice concentration in the marginal seas.
Because winds derived from polar-orbiting satellite imagers better capture the
ageostrophic component of the wind and because they have been shown to improve
weather forecasts (Key et al. 2003 ; Velden et al. 2005 ), they could also be used to
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