Persistent westward flow in the Azores Current as seen from altimetry and hydrography

Temporal and spatial variability of the eastward flowing, jet-like Azores Current is investigated by applying a new method for combining altimetry with hydrography in order to produce absolute surface geostrophic currents [Challenor et al., 1996]. The altimeter surface measurements contain information about both the geoid and the geostrophic ocean circulation: the geoid signal dominates and is generally not sufficiently well known at the small length scales required to identify the mesoscale ocean signal. In this study, hydrography is used to distinguish the mean ocean signal from the geoid along an ERS 1 satellite track south of the Azores during both 3-day repeat phases and thereby determine cross-stream absolute dynamic topography of the Azores Current at high temporal resolution. Hydrographic data collected near 28 degrees W in March 1992 is combined with ERS 1 altimeter data from January-March 1992 and January to early April 1994. Time series of 3-day repeat profiles of absolute surface geostrophic velocity perpendicular to the altimeter/cruise track are examined. The Azores Current absolute velocity profile along this track is rather coherent: the current variability is less than the mean. Surprisingly, a persistent westward flow of similar to 25 cm s(-1) is observed near 35 degrees N, 28 degrees W. This flow appears to be a ''retroflection'' from the main eastward flowing current. An isopycnal analysis reveals the westward flow to be cooler (by 0.5 degrees C) and fresher (by 0.15 practical salinity units) than the main body of the Azores Current. The now appears to persist throughout each of the above 3-month periods in late winter/early spring, but the question of whether this is a seasonal feature or more permanent remains open. It is suggested that the westward flow is topographically controlled.