Variability of Deep Flow in the Drake Passage from Year-Long Current Measurements

Abstract

To investigate the reasons for the wide variation in previous estimates of transport of the Antarctic Circumpolar Current through the Drake Passage, an analysis of the spatial and temporal variability of currents at 2700 m depth is made from year-long current measurements on six moorings in the Drake Passage. The currents are found to vary over time scales of about two weeks and over spatial scales shorter than 80 km. An average of the six down-channel velocity components is used to estimate the spatially averaged down-channel velocity, or mean flow, at 2700 m. This mean flow varies from 7.6 to?2.9 cm s?1 and has a root-mean-square (rms) amplitude of 2.0 cm s?1 about its time-averaged value. Provided the geostrophic transport relative to 2700 m depth remains constant in time, these variations may be interpreted as temporal variations of 2 60?106 m3 s?1 in total transport with an rms amplitude of 50?106 m3 s?1. The wide variation in previous estimates of transport from short-term measurements can be understood in terms of this observed variation in mean flow. The time-averaged mean flow at 2700 m depth is estimated to be 1.56±1.44 cm s?1 which implies that a transport of 39±36?106 m3 s?1 should be added to the geostrophic transport of about 100?106 m3 s?1 relative to 2700 m to obtain an estimate of the time-averaged total transport through the Drake Passage.