Convergence and Downwelling at a River Plume Front

JULY 1998 O'DONNELL ET AL. 1481 Convergence and Downwelling at a River Plume Front JAMES O'DONNELL University of Connecticut, Groton, Connecticut GEORGE O. MARMORINO AND CLIFFORD L. TRUMP Naval Research Laboratory, Washington, D.C. (Manuscript received 14 July 1997, in ®nal form 23 September 1997) ABSTRACT The small-scale structure of the circulation and hydrography at the frontal boundary of the Connecticut River plume in Long Island Sound has been resolved using a novel combination of instruments: a towed acoustic Doppler current pro®ler (ADCP) and a rigid array of current meters and conductivity±temperature sensors. Observations were made during the latter half of the eastward ebb tide, when the river plume was well established and the front was moving to the west at approximately 0.3 m s21. Two across-front transects revealed a horizontal convergence rate in the across-front velocity components at 0.6 m of 0.05±0.1 s21. This was associated with a salt-induced horizontal density gradient of 1022 kg/m4. Observations obtained during a period in which the towed ADCP was caught in the zone of maximum surface convergence showed signi®cant downwelling with a near-surface maximum of 0.2 m s21. Vertical velocities of this magnitude are consistent with observed mag- nitudes of the convergence rate at 0.6 m assuming volume ¯ux continuity and weak alongfront variations. Quantitative comparison of the velocity observations to the model of Garvine show reasonable agreement, though the vertical distribution of vertical shear and strati®cation could be improved. Within 20 m of the front, observations reveal regions of strong subsurface horizontal gradients in density and velocity that are not well described by the model. This limitation is a consequence of the assumption of similarity in the vertical structure of the ®elds. Both the magnitude and the across-front variation of the vertical velocity component observed agree with the theoretical predictions. The authors conclude, however, that the fundamental dynamics in the model are adequate to describe the general structure of the plume layer thickness. 1. Introduction Imberger 1987) suggested that the horizontal scale of It is well established that freshwater runoff transports variations in the density and current structure was less particulates and dissolved material from land to estuaries than 100 m and comparable to the resolution of the mea- and the coastal ocean and that a quantitative understand- surements. Qualitative estimates of the width of the lines ing of the mechanisms of mixing and dispersion of this of foam and detritus that are often associated with frontal ef¯uent is essential to improving our prediction capability convergences are much smaller, of order 1 m. This small- for estuary and ocean circulation. Though the ¯ow mag- er scale is also consistent with the results of laboratory nitude and the geometry of every river estuary is unique, experiments on gravity currents, for example, Simpson during periods of high discharge, many have been ob- and Britter (1979), which indicate that the horizontal served to form a large plume of brackish water at their scale of the front should be a few times the thickness of mouths (Garvine 1974b; Stronach 1977; Ingram 1981; the buoyant layer. Freeman 1982; Lewis 1984; Luketina and Imberger Recent development of models of the dynamics of the 1987). Undoubtedly, many more smaller, and seldom sur- interior of river plumes (Garvine 1984, 1987; O'Donnell veyed, rivers exhibit this behavior intermittently. 1988, 1990) have suggested that the ¯ow at the frontal Convergent surface fronts are a common feature in boundary in¯uences the dynamics in the interior of the reports of observations of river plumes. Systematic ob- plume. This suggests that advancement in the understand- servations of the morphology of fronts have been few. ing of plumes requires a more detailed appreciation of Early work (e.g., Garvine and Monk 1974; Luketina and the properties of the front. The wide variety of, and the uncertainty in, the spatial scales of variability in river plumes, from 1 to 103 m, together with their transient nature has made detailed observations of their properties Corresponding author address: Dr. James O'Donnell, Department of Marine Sciences, University of Connecticut, 1084 Shennecossett dif®cult. However, technological advances in navigation Road, Groton, CT 06340-6097. and current measurement have provided new tools with E-mail: [email protected] which to observe the nature of river plume fronts. In this q 1998 American Meteorological Society Unauthenticated | Downloaded 09/28/21 03:08 AM UTC 1482 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 28 paper we report some results of an expedition to resolve the structure of the plume of the Connecticut River in Long Island Sound with a rigid array of current and conductivity±temperature sensors and a towed acoustic Doppler current pro®ler (ADCP). We then evaluate the consistency of the different types of observations and compare them to the predictions of the model of Garvine (1974a). In the next section we describe the main features of the Connecticut River plume and review prior observations of the frontal structure. Observations of dynamically related features are also summarized. Based on these observations and the theoretical work of Garvine, we es- timate the scale of the horizontal convergence and con- sequent downwelling velocity with emphasis on its de- pendence on the horizontal scale of variation. In section 3 we describe the methods and equipment that we em- ployed and then present the results in section 4. The observations and the model of Garvine are compared in section 5. The techniques and analyses are discussed and the paper summarized in section 6. 2. Review a. Observations During the spring freshet, the Connecticut River can deliver 2000 m3 s21 of freshwater to Long Island Sound (LIS). The fate of this water was the subject of a series of reports by Garvine (1974b, 1975, 1977) and Garvine and Monk (1974). This work established that a large area FIG. 1. (a) Garvine and Monk's (1974) observations of the density (st) structure of the plume front. Dots show sample locations. (b) of eastern Long Island Sound was covered by a thin (2 Garvine and Monk's (1974) observations of the across-front velocity m) surface layer of brackish water that was separated component (cm s21) measured in the front relative frame. Dots show from the Sound water by a line of foam and detritus, sample locations. referred to as a front. This river plume was observed to be strongly in¯uenced by the tidal ¯ow in the sound and was located to the east of the river mouth on the ebb and sor. The velocity ®eld was obtained (in a frame of ref- to the west on the ¯ood. The motion ®eld in the plume erence moving with the front) using a mechanical current during its formation was observed by Garvine (1977) meter lowered from a vessel that maintained a ®xed dis- using drifters and drogues that were tracked visually by tance from the front. Though these techniques yielded aerial photography. These measurements revealed that the ®rst detailed picture of the frontal structure, dynamic there was little spatial structure in the velocity ®eld in interpretation was limited by the relatively large uncer- the buoyant layer, that there was strong circulation normal tainties resulting from platform positioning, motion and to the tidal ¯ow in the denser water, and that the buoyant rotation, and nonsynoptic sampling. An example of their layer had negligible in¯uence on the deeper motion. Most observations is presented in Fig. 1. In a qualitative sum- importantly, the observations con®rmed the existence of mary of their observations, Garvine and Monk suggested strong horizontal shear and convergence in the surface that the characteristic horizontal scale of variation of the motion at the front. temperature, salinity, and density was between 20 and Garvine and Monk (1974) reported observations of the 50 m; however they noted that qualitative observations density and motion ®eld in the vicinity of the front with of water color showed signi®cant changes over scales of a resolution of approximately 20 m. They circumvented order 0.5 m. Other exploratory measurements of the ver- navigation problems by making observations in a ref- tical component of velocity in the front with dye tracked erence frame that moved with the front and measured by divers suggested a downwelling of several centimeters distance from the front by stretching line between a small per second. boat in the frontal convergence and the instrument plat- Observations of a similar, but much less buoyant plume form. Their hydrographic observations were obtained us- front in Koombana Bay, Australia, were reported by Lu- ing a pump to sample water at a variety of depths and ketina and Imberger (1987, 1989). Measurements of the pass it through an on-deck conductivity±temperature sen- hydrography and current velocity were obtained using a Unauthenticated | Downloaded 09/28/21 03:08 AM UTC JULY 1998 O'DONNELL ET AL. 1483 vertically pro®ling CTD and current meter mounted on Here r` is the density of the denser ambient water, g is a submerged tower and supplemented by a ship-towed one-half the density anomaly of the surface water at the CTD package. Though the quality of the velocity ob- plume-front boundary xb, and r(x) describes the across- servations was superior to those of Garvine and Monk front variation of the surface buoyancy. Note that both (1974), the spatial resolution in the vertical pro®les in g and r(x) must be speci®ed and that the depth of the the vicinity of the front was approximately 50 m and pycnocline, D(x), is the only characteristic of the density insuf®cient to properly resolve the scales ;O(10 m) re- ®eld that is predicted; D(x) must satisfy D(0) 5 0 and vealed by the hydrographic observations.

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