ALBERT

Description: Geosat altimetric sea level and derived surface geostrophic velocities, shallow current meter velocities, and dynamic height in the low current velocity regime in the southeastern North Atlantic are compared. An attempt is made to determine whether seasonal and interannual variations of geostrophic ocean current in such a low-energy regime can be determined from Geosat altimetry, and whether Geosat altimetry can provide surface currents on a monthly scale that are consistent with what current meter moorings observe. If the latter is true, the possible combination of altimetry and observations together with hydrography to generate believable monthly maps is investigated.

Description: The term Cape Verde Frontal Zone is introduced to characterize the southeastern corner of the subtropical gyre circulation in the North Atlantic Ocean far west of the upwelling area off the Mauretanean shelf. Two water mass fronts, one overlying the other, are identified with a quasi-synoptic set of CTD-OZ and nutrient data from November 1986. In the warm water sphere we encounter North and South Atlantic Central Water (NACWISACW) superimposed on extensions of Mediterranean outflow and Antarctic Intermediate Water. The Central Water Boundary, as the separator of NACW from SACW, represents the southeastern side of the Canary/North Equatorial Current system. It acts as a barrier between the well-ventilated, nutrient-poor inner part of the basin-wide circulation of the North Atlantic and the shadow zone with its lowly oxygenated and nutrified cross-equatorial influx. Year-long current meter records, having fluctuations over typical time scales of 5(1`90 days, attest to the highly variable nature of the Cape Verde Frontal Zone. Incidentally, we observe in the data an intrathermocline eddy, called Meddy BIRGIT, which has a double maximum in the vertical salinity structure. Simultaneous Lagrangian observations by RiCHAttDSON et al. (1989, Journal of Physical Oceanography, 19, 371-383) confirm the expected anticyclonic motion of this salt lens, which must have travelled without significant mixing for at least 2500 km from its likely generation region in the Gulf of Cadiz.

Description: Three data types are compared in the low-current-velocity regime in the southeastern North Atlantic, between 12-degrees-N and 30-degrees-N, 29-degrees-W and 18-degrees-W: Geosat altimetric sea level and derived surface geostrophic velocities, shallow current meter velocities, and dynamic heights derived from hydrographic data from cruises 4, 6, and 9 of the research vessel Meteor. The four current meter daily time series, at depths around 200 m, were smoothed over 1 month; the altimetric geostrophic velocities were computed from sea surface slopes over 142 km every 17 days. The correlation coefficients between the current meter and altimetric geostrophic velocities range between 0.64 and 0.90 for the moorings near 29-degrees-N but between 0.32 and 0.71 for the two around 21-degrees-N; the associated rms discrepancies between the two measurement types range between 1.5 and 4.4 cm/s, which is 49% to 127% of the rms of the respective current meter time series. Dynamic heights relative to 1950 dbar for the months of November 1986 (d(M4)), November 1987 (d(M6)), and February 1989 (d(M9)) were computed from Meteor cruises 4, 6, and 9. Both dynamic heights and altimetric heights (h(M4), h(M6), h(M9)) were averaged over 1-degrees boxes for the duration of each cruise. Differences d(M4) - d(M6) and d(M9) - d(M6) were computed only at bins where at least one station from both cruises existed, Assuming that dynamic heights d in dynamic centimeters are equivalent to sea level h in centimeters, the standard deviation sigma of the differences ((h(M4) - h(M6)) - (d(M4) - d(M6))) and corresponding M9 - M6 values was 2.1 cm. This value (squared) is only 13% of the (5.8 cm)2 variance of the dynamic height differences and is indistinguishable from the 2.7- to 5.6-cm natural variability of sea level in the area expected between the times when the ship and the satellite sampled the ocean. The areally averaged discrepancy for M9 - M6 was only 0.7 cm, but the corresponding value for M4 - M6 was 5.2 cm. A systematic difference between the water vapor corrections used before and after July 1987 is responsible for the M4 - M6 difference. The average M4 - M6 discrepancy is only 0.1 cm using the Fleet Numerical Oceanography Center correction, with a standard deviation of 3.1 cm. In spite of the underlying differences in sampling and physics, including unknown barotropic components not included in our hydrographic dynamic heights, and in data errors, including water vapor, ionospheric, and orbital effects on the altimetry, consistent interannual changes of the mean sea level from the independently obtained altimetric and hydrographic data sets are obtained, and correlated seasonal changes in surface currents are observed with both altimetry and current meters.

Description: Optimum multiparameter (OMP) analysis is used to analyze mixing in the central water boundary of the tropical North Atlantic Ocean. Diapycnal mixing is found to be prevalent in the frontal region. OMP analysis shows that the mixing is unidirectional (South Atlantic Central Water is always mixed upward into North Atlantic Central Water) but cannot identify the process responsible for the observed diapycnal mixing. A histogram of stability ratios Rρ for all mixing lines shows Rρ values between unity and the value found in the parent water masses. It is suggested that this may indicate competition between isopycnal mixing and double diffusion. Double diffusive fluxes are likely to make a recognizable and significant contribution to diapycnal mixing between the Central Waters.