Diagnosing changes in the North Atlantic circulation using a vertical section model

Xu, Jie (1992) Diagnosing changes in the North Atlantic circulation using a vertical section model. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (14MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

A two-dimensional vertical section model is described. The model takes density data along a section as input and calculates the velocity through the section referenced to the bottom. We have used it to analyse changes in the circulation of the North Atlantic by taking as input the objectively analysed density data of Levitus(1982,1989) for the climatological annual mean and for the pentads 1955-59 and 1970-74. -- Sections along 55.5°W, 64.5°W, 54.5°N and 23.5°N have been considered and estimates of the poleward heat transport through 54.5°N and 23.5°N have been made. Ekman transports are calculated using the Hellerman and Rosenstein(1983) wind stress field and wind stress analysed by da Silva and Levitus for the period 1945-1989. At 54.5°N, mass balance is achieved by combining with the absolute transport calculations of Greatbatch et al.(1991), giving values for the poleward heat transport of 0.6PW, 0.7PW and 0.5PW for the climatological annual mean and the pentads 1955-59 and 1970-74, respectively. The estimated error is ±0.2PW. These values compare well with previous estimates obtained using surface heat flux calculations and suggest that the poleward heat transport in 1970-74 may have been marginally less than in 1955-59. We have performed a similar calculation for 23.5°N, this time by requiring mass balance through the section assuming that the northward transport and flow temperature of the Florida Current was the same in each pentad and equal to the climatological annual mean value, an assumption we believe to be justified. The calculated heat transports are 1.2PW, 1.0PW and 0.8PW for the climatological annual mean and the pentads 1955-59 and 1970-74, respectively, with an error of ±0.3PW. The climatological value agrees with previous estimates at this latitude and there is again the suggestion that the 1970-74 value is less than the others, particularly in comparison with the climatological annual mean. Along 54.5°N, the reduced heat transport in 1970-74 is attributed to a deeper North Atlantic Current and a warmer return flow in the Labrador Sea. Along 23.5°N, on the other hand, the southward flow over the interior of the North Atlantic is more surface confined in 1970-74 than in the climatological case. -- Other results concern the vertical distribution of the absolute transport changes diagnosed by Greatbatch et al.(1991). Along 55.5°W, the transport of the Gulf Stream referenced to the bottom was some 30Sv less in 1970-74 than in 1955-59, a change comparable in magnitude to that found by Greatbatch et al.(1991). However, the maximum change found by Greatbatch et al. is displaced to the south of that referenced to the bottom, indicating the importance of changes in bottom velocity. Greatbatch et al. also considered a case in which the density below 1500 m is assumed to have remained unchanged between the pentads, and diagnosed a transport change for the Gulf Stream of over 20Sv. On the other hand, the change in transport referenced to and above 1500m is only 7Sv, indicating that even in this case, changes in bottom velocity play an important role. There is a suggestion in the model results that in the 1970-74 pentad, the northern recirculation gyre of the Gulf Stream was weaker than in both 1955-59 and the climatological case.

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