Water Mass Structure during the POLYMODE Local Dynamics Experiment

Abstract

The seven intensive hydrographic surveys of temperature, salinity and oxygen of the POLYMODE local Dynamics Experiment (LDE) (15 May?15 July 1978) were carded out in a 200 km-wide octagonal region centered at 31°05?N, 69°30?W; this location was within the southern portion of the Gulf Stream Recirculation. The vertical structure of the mean and standard deviations of water properties and physical parameters for the experiment are presented. Comparisons of LDE and climatological statistics at 31°N, 70°W showed that the profiles of mean quantities were not markedly different; however, LDE eddy potential energy in the water column above 2000 m was roughly one-half to one-third the long-term climatological values. There were peaks in variability of salinity on potential density surfaces in the 18°C-water, the midthermocline (850 db) and the lower thermocline (1400 db). The 18°C-peak was associated with a marked bimodal salinity distribution, the midthermocline peak with extreme outliers (positive and negative) on the frequency distribution and the lower thermocline with a positively skewed frequency distribution indicating Mediterranean Sea Water influence. Peaks in oxygen variability were observed at the same levels as for salinity. Examples of small anomalous water masses (outliers) are shown in vertical and horizontal sections. The dynamical fields (pressure of density surfaces, dynamic height) were highly anisotropic with the preferred orientation being in the NE/SW direction. There was evidence of northwest translation of dynamical features at a speed of 4 km d?1; the long horizontal scale of the dynamical features (ridges and troughs or elongated eddies) were not resolved by the experiment. Measurements adjacent to the LDE region indicate that the scale in the NE/SW direction was in excess of 400 km. The last four surveys showed the development of a very strong baroclinic feature (velocity shear > 50 cm s?1 between 700 and 1500 db). This jetlike structure, which extended across the survey region, had a gradient (NW/SE direction) with a horizontal able of 100 km. Because of the development of the Strong flow during the period of hydrographic surveys, the results are not typical of the whole period of the experiment; mooring measurements showed that two such events occurred in a 15-month period. There are three major differences between MODE and LDE results: the eddy potential and eddy kinetic energy was 3 to 4 times higher in the LDE; the LDE dynamic height fields were more anisotropic; and them were a number of intense small-scale (horizontal and vertical) property anomalies observed in the LDE that did not have a counterpart in MODE.