24 
Sy, A. et al. (2002): Upper Ocean Climate Ship-of-Opportunlty Programme of BSH - A Status Report 
Research into the variability of the North Atlantic current system and its steering mechanisms 
has been carried on for several years now. Some kind of ocean-atmosphere interaction has 
been considered as a possible explanation and, since WOCE, interest has focused increasingly 
on the North Atlantic Oscillation (NAO). 
Owing to the long-term character of the BSH's upper ocean climate monitoring programme it 
has provided a unique data set of 16 hydrographic sections (CTD and XCTD, Table 3) covering 
at least the upper 600 m of the ocean. That is the layer of strongest current activities. The time 
series of the estimated upper layer volume transports together with the NAO winter (DJF) index 
after Lowe and Koslowski (1998) is displayed in Fig. 16 (top). For the volume transport 
estimates systematic differences apparently do not exist between CTD and XCTD sections and 
between sections located close to the northern or southern boundaries of the GOOS A-2 
corridor. Consequently, all sections can be treated in the same way for this purpose. 
No correlation of any kind can be detected in Fig. 16 (top). However, after a 5-year backward 
phase shift of transports (- 5 years) a functional relationship can be identified. We received the 
best results (explained variance of 62 %) with a second-order polynomial fit (Fig. 17). This 
shows that the ocean and atmosphere interact with a time lag of 5 years. Our results from AX-3 
XBT data, where we found an upper ocean heat content periodicity of about 5 years (Fig. 18), 
supports this result. 
Correlation attempts with the corresponding heat transport time series (Fig. 16 bottom), 
however, provided less clear results. The cause is that the heat content and hence the heat 
transport of the upper ocean is prone to both seasonal and regional heat flux variability. The 
volume transport difference between the northern and southern transects determined for the 
four Gauss cruises 276, 316, 350 and 384 is small and varies between 4 % and 11 %, whereas 
the corresponding differences of heat content and heat transport were found to be up to three 
times larger. This shows that the northern and southern transects cannot be treated in the same 
way as in the case of volume transports, even if the seasonal signal of the surface layer has 
been removed. 
However, after deduction of the 5 northern sections, the correlation of NAO with the heat 
transport time series of the remaining 11 southern sections again provided the best results with 
a second-order polynomial fit and after a time shift of - 5 years (explained variance of 44 %). 
The reduced significance is due to the principle of propagation of errors. 
Our finding does not confirm Lorbacher’s (2000) result of a linear correlation between NAO and 
MOC with a phase lag of only one year, which she interpreted as the effect of a short-term 
change in atmospheric forcing on the overturning. However, her data base differed in some 
significant points from that used here. Including two historical cruises made in 1957 and 1982, 
she used only 5 WOCE hydrographic sections A2/AR19 from 1993, 1994, 1996, 1997 and 1998 
for her analysis. Her data cover a time period of 40 years but have an extremely 
inhomogeneous distribution in time. 
Because a positive time shift cannot be made with the presently existing data, it is not yet 
possible to distinguish between cause and effect, i.e. between active forcing and passive 
reaction of this coupled ocean-atmosphere system. The question who is at the helm, the ocean 
or the atmosphere, as brought to the point by McCartney (1997), must remain open, at least 
until a positive phase shift is applicable to the data. Finally, in the present phase of the 
discussion, the assumption that the NAO dominates oceanic variability leads to another 
question, independent of questions regarding statistical significance: Is an extremely rapid and 
linear response in large-scale circulation reasonable or is a slower, non-linear response more 
plausible? Further results might be available at www.bsh.de.