Sy, A. et al. (2002): Upper Ocean Climate Ship-of-Opportunity Programme of BSH - A Status Report
21
The depth range of the seasonally mixed layer in the GOOS A-2 corridor covers the upper
40 - 50 m in summer (Fig. 12), deepening to 200 - 300 m west of the Mid-Atlantic Ridge and up
to 500 m in areas east of the ridge during fall and winter due to cooling and mixing, in good
agreement with Robinson et al. (1979) and McCartney and Talley (1982). The largest
mixed-layer depth of 650 m was found close to the European shelf.
Of course, the temperature of the mixed layer (Fig. 13) shows the same seasonal development
as its depth. The warming phase lasts from March to September, followed by cooling (and
deepening of the mixed layer) until February. The variability of the monthly mean temperature,
however, does not exceed 4.5 °C between the warmest month of September (15.5 °C) and the
coldest month of February (11 °C).
Time series of the annual mean heat content anomalies from 1988 to 2000 (Fig. 14) show as a
clear signal a period of relatively low heat content until 1991 followed by a period of relatively
high values in 1992 and 1993. The next cold period in 1994 and 1995 and the warm period
since 1996 are not developed as clearly as the preceding periods. The annual mean heat
content values of the coldest year 1990 (28.7 GJ/m 2 ) and of the warmest year 1993 (33,5
GJ/m 2 ) give a range of variability of only about 15 %.
Since 1998, CTD full-depth sections carried out in the framework of GOOS and CLIVAR have
been designed to form so-called inverse boxes, i.e. volume transport conserving closed areas,
in order to improve geostrophic current estimates by application of the simple concept of
continuity or the more sophisticated inverse method (Wunsch, 1978; Sy, 1988). A first result is
presented in Fig. 15 which compares the main branches of the NAC as observed during the
three box cruises 1998, 2000 and 2002. The optimum reference level for the geostrophic
calculation was chosen at a depth where the baroclinic box transport imbalance vanished. The
ocean above the reference level may be interpreted as an advection path of the NAC, and thus
represents the upper branch of the MOC in the North Atlantic with its advection of warm water of
subtropical origin flowing in northeastern direction into the Nordic Seas. The current branches
through the box boundaries were identified and determined as NAC branches by the same
continuity principle and by water mass (T/S) analysis.
A comparison of the three realizations in Fig. 15 gives a good overall impression of the extreme
variability of the NAC regime. In the southwest, the huge inflow of the NAC as the northeastward
extension of the Gulfstream, and its recirculation eddy are typical. However, the net
inflow into the western box decreased from 20 Sv (10 6 m 3 /s) (1998) and 24 Sv (2000) to 12 Sv
(2002). The transport calculation for the northern boundary of the western box confirms this
result. After following the 4000 m depth contour east of the Grand Banks, the NAC leaves the
western box due north and again the transport is dramatically reduced from 17 Sv (1998) and
18 Sv (2000) to 10Sv (2002).
Along its course the NAC often divides into several branches and meanders (Sy, 1988). The
main branch, the subpolar front, separates the cold subpolar gyre from the warm subtropical
gyre. East of the Mid-Atlantic Ridge the NAC finally turns north. In 1998 this change of direction
took place significantly closer to the ridge than in 2000 and 2002, indicative of the eastward
expansion of the Subpolar Mode Water of the subpolar gyre (Bersch, 1999).
Although the volume transport of the NAC appears to be highly variable in Fig. 15, we found
only small changes in the total volume transport figures through the GOOS A-2 corridor
transect, despite the decrease of the main NAC branch between 2000 and 2002. The
necessary compensation comes from a unique branch observed only in 2002 which enters the
eastern box from the south.
