Analysis and assimilation of temperature and altimetry data in the North Atlantic Ocean

Observations of subsurface temperatures in the North Atlantic between 1950 and 1994 have been analysed to investigate interdecadal variability. Previous studies have concentrated on the more plentiful sea surface temperature data or on limited hydrographic data. Here, all subsurface temperature data, principally from bathythermographs, are used to investigate the development and evolution of temperature anomalies. During most of the period, a dipole feature was seen. The western subtropical gyre and the ocean north of 50°N were occupied by anomalies of opposite sign. These regions appear to show a cycle of warming and cooling with a period of approximately 30 years but the data record is too short to determine whether this is a regular oscillation. There is evidence of propagation of temperature anomalies along the paths of the major upper ocean currents, namely along the Gulf Stream and North Atlantic Current and also along the return currents on the southern edge of the subtropical gyre. An intense warm anomaly on the northern edge of the subtropical gyre developed between 1988 and the end of the data in 1994. The Gulf Stream front became steeper and isotherms were depressed by up to 150m. This is consistent with a stronger Gulf Stream and an intensification of the circulation of the subtropical gyre. This is compared with a cold anomaly in 1966-1972 which is opposite in nature, indicating a weaker subtropical gyre circulation. A scheme developed by Cooper and Haines (1996) has been employed to assimilate sea level anomaly data from the TOPEX/POSEIDON altimeter with a climatological hydrography of the North Atlantic.