The terms of the steady-state, homogeneous turbulent kinetic energy budgets are obtained from measurements of turbulence and fine structure from the small autonomous underwater vehicle (AUV) Remote Environmental Measuring Units (REMUS). The transverse component of Reynolds stress and the vertical flux of heat are obtained from the correlation of vertical and transverse horizontal velocity, and the correlation of vertical velocity and temperature fluctuations, respectively. The data were obtained using a turbulence package, with two shear probes, a fast-response thermistor, and three accelerometers. To obtain the vector horizontal Reynolds stress, a generalized eddy viscosity formulation is invoked. This allows the downstream component of the Reynolds stress to be related to the transverse component by the direction of the finescale vector vertical shear. The Reynolds stress and the vector vertical shear then allow an estimate of the rate of production of turbulent kinetic energy (TKE). Heat flux is obtained by correlating the vertical velocity with temperature fluctuations obtained from the FP-07 thermistor. The buoyancy flux term is estimated from the vertical flux of heat with the assumption of a constant temperature-salinity (T-S) relationship. Turbulent dissipation is obtained directly from the usage of shear probes.A multivariate correction procedure is developed to remove vehicle motion and vibration contamination from the estimates of the TKE terms. A technique is also developed to estimate the statistical uncertainty of using this estimation technique for the TKE budget terms. Within the statistical uncertainty of the estimates herein, the TKE budget on average closes for measurements taken in the weakly stratified waters at the entrance to Long Island Sound. In the strongly stratified waters of Narragansett Bay, the TKE budget closes when the buoyancy Reynolds number exceeds 20, an indicator and threshold for the initiation of turbulence in stratified conditions. A discussion is made regarding the role of the turbulent kinetic energy length scale relative to the length of the AUV in obtaining these estimates, and in the TKE budget closure.

[1] We report observations of the structure of the front that surrounds the plume of the Connecticut River in Long Island Sound (LIS). Salinity, temperature, and velocity in the near-surface waters were measured by both towed and ship-mounted sensors and an autonomous underwater vehicle. We find that the plume front extends south from the mouth of the river, normal to the direction of the tidal flow in LIS and then curves to the east to parallel the tidal current. The layer depth at the front and the cross-front jumps in salinity and near-surface velocity all tend to decrease as distance from the source increases. This is qualitatively consistent with the prediction of layer models. In the across-front direction, the plume layer depth increases from zero to the asymptotic value within a few times the plume depth ($5 m). Vertical motion is generated in this zone, and there is evidence of overturning. Farther from the front, the high-frequency salinity standard deviation decays exponentially with a length scale of 30 m. Assuming that the salinity fluctuations are a consequence of turbulence, we find that the rate of turbulent kinetic energy dissipation decreases exponentially in the across-front direction with a decay scale L G % 15 m. Estimates based on AUV-mounted shear probes are consistent with this estimate. We present an explanation of the physics that determines L G and provide a simple formula to guide the choice of resolution in models that are designed to resolve the frontal structure.

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Thermal fronts in the North Pacific from June 1976 to May 1980 have been mapped at the 300-m level by using temperature data obtained in the region 30ø-45øN, 160øE-160øW in the TRANSPAC XBT Volunteer Observing Ship Program. The origins of the paths of the major frontal bands in the study region (i.e., the Kuroshio Extension, the Subarctic Fronts, and the Subtropical Front) appear to be related to the principal bathymetric features of the western mid-latitude North Pacific (i.e., the Shatsky Rise, the Emperor Seamount Chain, and the Hess Rise). In the long-term mean, the Kuroshio Extension Front at 35øN was observed to have bifurcated at the Shatsky Rise near 160øE into two bands. One band continued northeast along the Shatsky Rise to 42øN to become the North Subarctic Front. The other band continued zonally eastward where it encountered the Emperor Seamounts, near 36øN, 170øE, and trifurcated into bands which tracked through passages between the Seamounts near 37øN, 34øN, and 32øN, respectively. Downstream, the most northern of these three bands became the South Subarctic Front near 39øN, the middle band continued as the Kuroshio Extension Front at 35øN, and the most southern band became the Subtropical Front near 32øN. The middle band tracked north of the Hess Rise, while the most southern band tracked south of the Hess Rise. A near-meridional frontal band linked the North and South Subarctic fronts in the Emperor Seamount region near 170øE. In the annual mean and individual seasonal mean maps, the essential character of topographic influence of the long-term mean was observed. However, the individual frontal bands varied seasonally about the long-term mean by _+2 ø of latitude. Also, all four major frontal bands were not always observed, and intermittent linkages and additional filaments also occurred.

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Temperature data obtained from June 1976 to May 1978 in the TRANSPAC ships‐of‐opportunity XBT program have been used to examine the large‐scale evolution of synoptic thermal fronts from 165°E to 135°W, from 30° to 50°N in the mid‐latitude North Pacific on a 2° latitude by 5° longitude grid. These results were referenced to a climatology computed from all XBT data collected in the region from 1968–1974. The primary indicator of frontal strength and position was the zonal continuity of the modulus of horizontal temperature gradient in the upper 300 m. Over the 2‐year period, the Subarctic Front was observed as a quasi‐zonal coherent feature, ranging in latitude between 38° and 46°N, strongest at the sea surface, decreasing in strength toward the east. This front was found to have had two realizations, a northern one dominant in winter‐spring between 42° and 46°N and a southern one dominant in summer‐fall between 38° and 42°N. Generally, these frontal realizations were vertically coherent with depth. The transition from the southern front to the northern front occurred in either November or December of both years, while the transition from the northern to the southern front took place in either March or April. In winter‐spring, there existed considerable overlap of the existence of these two fronts; however, in summer‐fall, the northern front was not observed. The northern front had a tendency to be displaced southward 2°–4° through the winter‐spring period, while the southern front had no consistent tendency over the 2 years. At depth, the strength of these fronts shows no significant variability over the 2‐year period, and the two fronts are comparable in strength. Two other thermal fronts were observed intermittently, i.e., the Subtropical Front in the eastern part of the region between 30° and 36°N and the Kuroshio Extension Front in the western part of the region along approximately 36°N. These fronts were observed to form transient links or filaments with both realizations of the Subarctic Front.