Structure and Origin of 18°C Water Observed during the POLYMODE Local Dynamics Experiment

Abstract

Two distinct types of 18°C water (Subtropical Mode Water) were observed during the POLYMODE Local Dynamics Experiment (LDE; May?July 1978; 31.0°N, 69.5°W). These were revealed on isopyncals by salinity histograms which were bimodal. Salinity was highly correlated with oxygen, vortex stretching, and 17.5°?18.5°C thickness. The correlations are positive between salinity and both oxygen and thickness and negative between salinity and vortex stretching. The origins of the two water types are deduced using a variety of measurements in the Sargasso Sea including apparent oxygen utilization, vortex stretching and salinity. It is found that the modes were formed approximately 16 months prior to the LDE during the severe winter of 1976/77. Sharp horizontal salinity gradients between the two LDE water types are comparable to those observed more than a year earlier, and the spatial scale (?100 km) of the regions of saline mode water is smaller in the LDE than immediately after the 1976/77 winter (?200 km). Thew observations suggest that the characteristics of newly formed 18°C water may persist for several years despite strong mesoscale stirring in the Gulf Stream Recirculation Zone.