THE MEAN AND ANNUAL CYCLE OF UPPER LAYER TEMPERATURE-FIELDS IN RELATION TO SVERDRUP DYNAMICS WITHIN THE GYRES OF THE ATLANTIC-OCEAN

Using 28 years of expendable bathythermograph data (1967-1994), we des cribe the mean and annual cycle of the upper ocean temperature fields in the Atlantic from 30 degrees S to 50 degrees N in the context of th e basin-scale wind-driven gyres (Sverdrup stream function field), whic h provide a framework for describing the oceanographic measurements. W e examine the circulation field implied by the temperature distributio ns, which are used as a proxy for the field of mass. Similarities betw een the temperature and stream function fields increase with depth. In the lower to subthermocline depths of the tropical and equatorial gyr es the zonal currents form a closed circulation. A southeastward bound ary current is suggested near and below 150 m that provides closure fo r the tropical gyre, and the equatorial gyre axis is southward of that suggested by the stream function field. Higher in the water column, t he North Equatorial Countercurrent (NECC) may be a surface manifestati on of the North Equatorial Undercurrent (NEUC), where the latter can b e interpreted as the southern limb of the tropical gyre. Because there are large vertical shears in the tropics, the equatorial gyre is not clearly indicated in the vertically integrated temperature field but a ppears below about 200 m. Here, the South Equatorial Undercurrent (SEU C) can be interpreted as the eastward flowing northern limb of the equ atorial gyre and is opposite in direction to the westward flowing Sout h Equatorial Current above. Both the NEUC and SEUC are analagous to cu rrents in the Pacific that are governed by non-Sverdrup dynamics. Desp ite the shortcomings of the data, the mean annual cycle appears to be relatively stable, and we have discounted the possibility that in regi ons where it represents a significant percentage of the total variance , it is changing slowly over the 28 years of record. The wind-forcing fields, which undergo large meridional movements (5 degrees-6 degrees of latitude) during their annual cycle, with some exceptions, have ess entially no counterpart in gyre movements between their seasonal extre mes. Most of the variability associated with the annual cycle is confi ned to the upper 300 m. Greatest variability, where ranges exceed 6 de grees C, occurs in the northwestern Atlantic in late winter and early spring. During this time of year south of the Gulf Stream and below ab out 100 m, water temperatures exhibit a systematic phase lag with dept h. The next largest area of variability, where ranges can also exceed 6 degrees C, resides in the tropical western basin between the equator and 10 degrees N just below 100 m. In the eastern basin, ranges decre ase and shoal. Additionally, the phase fields are consistent with the intensification and relaxation of the tropical ridge-trough system whe re the NECC disappears in March in the west, but the NECC/NEUC complex is strongest in September.