ON THE TRANSPORT OF VOLUME AND HEAT THROUGH SECTIONS ACROSS THE NORTH-ATLANTIC - CLIMATOLOGY AND THE PENTADS 1955-1959, 1970-1974

A two-dimensional, vertical-section model is described. The model take s density data along a section as input and calculates the velocity th rough the section relative to the bottom. We have used it to analyze c hanges in the circulation of the North Atlantic by taking as input the objectively analyzed density data of Levitus for the climatological a nnual mean and for the pentads 1955-1959 and 1970-1974. Sections along 55.5-degrees-W, 54.5-degrees-N, and 23.5-degrees-N have been consider ed, and estimates of the poleward heat transport through 54.5-degrees- N and 23.5-degrees-N have been made. Ekman transports are calculated u sing the Hellerman and Rosenstein wind stress field and wind stress an alysed by da Silva et al. for the period 1945-1989. At 54.5-degrees-N, mass balance is achieved by combining with the absolute transport cal culations of Greatbatch et al., giving values for the poleward beat tr ansport of 0.6 PW, 0.7 PW and 0.5 PW for the climatological annual mea n and the pentads 1955-1959 and 1970-1974, respectively. These values compare well with previous estimates obtained using surface heat flux calculations but are not distinguishable from each other. We have perf ormed a similar calculation for 23.5-degrees-N, this time by requiring mass balance through the section assuming that the northward transpor t and flow temperature of the Florida Current was the same in each pen tad and equal to the climatological annual mean value, an assumption w e believe to be justified. The calculated heat transports are 1.2 PW, 1.0 PW, and 0.8 PW for the climatological annual mean and the pentads 1955-1959 and 1970-1974, respectively, with an estimated error of at l east +/-0.3 PW. The climatological value agrees with previous estimate s at this latitude. The corresponding heat transports calculated using potential temperature and geostrophic velocity zonally averaged over that part of the section to the east of the Bahamas are also 1.2 PW, 1 .0 PW, and 0.8 PW. The error in these calculations is likely to be sma ller, indicating that the 1970-1974 value may be less than that for cl imatology. We show that in the 1970-1974 case, the zonally averaged ve locity is more southward above 1000 m and that this accounts for the c hange in the calculated heat transport. We note the sensitivity of thi s change to rather uncertain differences between the climatology and 1 970-1974 data sets in bottom density along the topographic slope. Othe r results concern the vertical distribution of the absolute transport changes diagnosed by Greatbatch et al. Along 55.5-degrees-W, the eastw ard transport of the Gulf Stream relative to the bottom is some 30 Sv less in the 1970-1974 case than in that for 1955-1959, a change compar able in magnitude to that found by Greatbatch et al. However, the maxi mum change found by Greatbatch et al. is displaced to the south of tha t diagnosed relative to the bottom, indicating the importance of chang es in bottom velocity. Greatbatch et al. also considered a case in whi ch the density below 1500 m is assumed to have remained unchanged betw een the pentads, and diagnosed an eastward transport change for the Gu lf Stream of over 20 Sv. On the other hand, the change in eastward tra nsport relative to and above 1500 m is only 7 Sv, indicating that even in this case, changes in bottom velocity play an important role.