Westward moving waves or eddies (Storms) on the Subtropical/Azores Front near 32.5 degrees N? Interpretation of the Eulerian currents and temperaturerecords at moorings 155 (35.5 degrees W) and 156 (34.4 degrees W)

Three Argos buoy-years of Lagrangian data in westward-moving cyclonic eddie s, or Storms, near 32.5 degreesN, together with hydrographic measurements, have shown that Storms move westward at nearly 3 km day(-1). Water in eddie s can be trapped and moved westward by advection within the eddy or by phas e propagation of the eddy pattern, so we cannot say that the flow field (or Eulerian mean) is 3 km day(-1) westward. Two moorings (155 and 156) deploy ed in the Storm Corridor have provided a further 8 instrument-years of Eule rian data. The temperature and current records confirmed that two Storms a year passed each mooring over the 2-year measurement period. As expected, t here is a lag of similar to 1.3 month at mooring 155 (which is 102 km to th e west of mooring 156) with respect to conditions at mooring 156. The progr essive vector diagrams (PVDs) derived from the current meter records exhibi t fairly regular X (east or zonal) and Y (north or meridional) displacement scales that repeat with semi-annual periodicity (SAP). The dominant curren t signal is the north component of the SAP, which reaches an amplitude of 1 8 cm s(-1) for the upper layer or near surface current record (similar to 2 42-m depth). The geostrophic north component values derived from altimetry were in good agreement with the upper layer current meter measurements. The large north component amplitude was not interpreted as evidence for Rossby Waves but rather due to the passage of nine eddies (eight complete) of alt ernate sign (cyclonic, anticyclonic) passing the mooring rigs during the 2- year deployment period. The Y scale shows that the near surface characteris tic or mean maximum azimuthal speed is about 35 cm s(-1) for cyclonic eddie s or Storms, and that this value is reduced to 4 cm s(-1) at 1400-m depth. The residual or mean Eulerian currents range from 8 cm s(-1) for the upper layer currents to 1 cm s(-1) for the deeper currents at similar to 1400-m d epth and are predominantly westward. Simple theoretical considerations and idealised numerical simulations show that the mean westward Eulerian curren t depends markedly on whether the eddy centres pass to the north or south o f the rigs. This raises the question as to what do we mean by Eulerian resi dual currents, even for relatively long records (similar to 2 years). It is shown that the strong near surface westward current (similar to 6 km day(- 1)) measured at mooring 155 is largely due to a westward-moving eddy field with variable centre offsets. The magnitude of the near surface east-west c omponent of flow was estimated as eastward at 2 cm s(-1), The north-south c omponent of mean flow was southward at 2 cm s(-1). The deeper records gave a weak westward flow of similar to 1 cm s(-1) but did not show a significan t southward component for the mean Eulerian flow field. 7.4 float-years of Lagrangian ALACE data in the Subtropical Front region near 740 dbar gave me an east-west flows that were < 0.5 cm s(-1). Overall. it is shown that the eddy structures propagate westward mainly by phase propagation (i.e. a west ward-moving pattern with no westward advection for the current meter to mea sure), though plane Rossby Wave dynamics appeared inappropriate. Theoretica l and modeling considerations show that a speed of 3-km day(-1) westward is too large a value for the self-advection of eddies due to the beta effect. (C) 2001 Elsevier Science B.V. All rights reserved.