Altimeter-Derived Loop Current Metrics

Abstract

Altimeter-derived Loop Current metrics are used to describe Loop Current intrusion and eddy separation in the Gulf of Mexico for the time period from 1 January 1993 through I July 2004. Time series of Loop Current extent, boundary length, area, circulation and volume are determined by tracking the 17-cm contour in the daily sea surface height maps. This contour closely tracks the edge of the high-velocity core of the Loop Current. The altimetric Loop Current tracking is qualitatively and quantitatively evaluated by direct comparisons with Loop Current thermal fronts observed in sea surface temperature imagery during times of good thermal contrast and cloud free conditions. Eddy separation events are identified by the changes in the Loop Current length associated with breaking of the tracking contour, with a total of 16 Loop Current eddies causing significant changes in the Loop Current extent during the time period. Statistical and spectral analyses are used to characterize the Loop Current variability and separation frequency. Eddy separation occurs most frequently at 6, 9 and 11.5 months, with little or no power at the annual frequency. Significant low frequency power is found near a period of 17 to 22 months, which is associated with far southern retreat of the Loop Current and separation periods near 18 months. A bimodal distribution of the retreat after eddy separation is identified that influences the duration of the subsequent separation period. When the entire Loop Current retreats below 25N the average separation period following retreat is 16.2 months, much longer than the 5.5 month average for the cases where part of the Loop Current remains north of 25°N. More interesting is a nearly linear relationship between Loop Current retreat and subsequent separation periods. Thus the irregular retreat of the Loop Current after separation may explain in part why the Loop Current exhibits such an irregular eddy separation period.