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Tidal and Wind-Driven Currents from Oscr FEATURE TIDAL AND WIND-DRIVEN CURRENTS FROM OSCR By David Prandle TWO IMPORTANTASPECTS of tidal currents are (1) be seen by comparing calculations of M, tidal vor- their temporal coherence and (2) their constancy ticity distributions <OV/OX- c?U/OY> from OSCR (over centuries). The first rigorous evaluation of an measurements with corresponding model calcula- Ocean Surface Current Radar (OSCR) system ex- tions (Prandle 1987). ploited these characteristics using sequential de- Tidal Residuals ployments of the one available unit with subse- The propagation of tidal energy from the quent combination of radial components to ocean into shelf seas produces an attendant net construct tidal ellipses (Prandle and Ryder 1985). residual current Uo of 0.5(OUD)cos 0 (0, oscil- Specifications for Tidal Mapping lating current amplitude; c, elevation amplitude: The Rayleigh criterion for separation of closely D, water depth: O, phase difference between lJ spaced constituents in tidal analysis suggests ob- and {). In U.K. waters Uo is typically 0-3 cm s ' servational periods exceeding the related beat fre- compared with 0 of 40-100 cm s ', thus conven- • . enhanced reso- quency, this dictates 15 d of observations to sepa- tional current meters often fail to resolve U~,. lution of the instru- rate the two largest constituents M~ and St. For Moreover, numerical models that accurately sim- tidal elevations this criterion is often relaxed: ulate M z may not resolve U,, with the same accu- mentation reveals however, while elevations show a noise:tidal sig- racy. Year-long deployments of OSCR, in the finer scale dynamical nal ratio of 0(0.1-0.2), the same ratio for currents Dover Straits (Prandle et al., 1993) and the North is 0(0.5). Moreover, wind and wave current com- Channel of the Irish Sea (Howarth et al., 1995), processes. ponents are generally largest at the surface, and enabled, for the first time, these net tidal residual hence for OSCR observations the ratio may be currents to be accurately resolved by direct mea- even higher. Consequently 30-d observational pe- surement. But, as so often occurs in science, en- riods are recommended. Prandle et al. (1993) hanced resolution of the instrumentation reveals show that, from analyses of a sequence of data finer scale dynamical processes. Figure 2 shows a sets of this length, 7 constituents could be deter- residual surface current gyre differentiated from mined with standard deviations of -0.1 of ampli- these long-term measurements in the Dover tude for M e, S 2, and N, and 0.2 for O1, K> M 4, Strait. Such gyres are not exceptional but rather a and MS 4. An associated problem arises with se- characteristic of most coastlines, although rarely lecting specified relationships for closely spaced identified with conventional instrument or air- constituents not explicitly determined; using adja- borne sensors. cent elevation data may be suspect. A further dif- Wind-Driven Currents ficulty arises in shallow water in comparing OSCR results with values from either moored in- Nontidal Currents struments or models based on different vertical Removal of the tidal component from High reference frames. Lane et ell., (in press) indicate Frequency (HF) Radar current observations some possible solutions to this problem. leaves residuals that may include contributions Figure 1 (Prandle 1991) shows typically close from: direct (localized) wind forcing, indirect agreement between tidal ellipse distributions de- (larger scale) wind forcing, surface waves, and rived by combining OSCR measurements from a horizontal and vertical density gradients• Interac- total of 10 sites with values from a fine grid nu- tion of any of these components with the tidal merical model. The extent of this agreement can component can generate significant modulation of the latter contributing an additional nontidal residual. Selective filtering of the nontidal cur- David Prandle, Proudman Oceanographic Laboratory, Bid- rents into frequency bands can be used to sepa- ston Observatory. Birkenhead L43 7RA. UK. rate many of these components. OCEANOGRAPHY'Vo].10, No. 2.1997 57 ~" O0'N- t (m) Fig. 2: Residual (30 d mean) gyre from OSCR Mk II. variability. In shallower water, wind forcing may be partially balanced by surface slopes. In straits these slopes can subsequently generate currents orders of magnitude greater than indicated by lo- calized forcing (Prandle 1993). However, statis- tically significant relationships between wind and residual surface currents have been derived from all OSCR deployments. Both the steady- state Ekman spiral response and rotating inertial currents have been identified (Fig. 3). The steady-state response is typically 1 or 2% of Fig. 1: M 2 tidal ellipses, OSCR (black) versus wind speed, increasing in magnitude in deep model. water and veering increasingly toward the theo- retical deep water values of 45 ° to the right of Persistent residual surface currents of typi- the wind. cally 10-20 cm s -t are regularly observed with Determining Current Profiles From Surface Persistent residual deployments of the OSCR system around the Currents U.K. coast. These can often be dynamically re- surface currents of Construction of tidal current profiles from sur- lated to horizontal density gradients (Prandle face values can be completed, qualitatively, using typically 10-20 cm and Matthews 1990). Likewise jet-like currents simple theory that requires the specification of a of up to 14 cm s -~ have been observed linked s -1 are regularly ob- vertical eddy viscosity coefficient, E, and a bottom with frontal dynamics (Matthews et al., 1993). friction coefficient k (Prandle 1982). Estimates of served with . the Souza and Simpson (1996) show how vertical both E and k may be made from the variation in density gradients modulate surface tidal cur- OSCR system wind-forced response in water of varying depths. rents. Vertical profiles of the wind-forced component in around the U.K. In deep water the oscillatory orbital velocities open seas can similarly be calculated from Ekman of surface waves do not show any obvious aliasing spiral theory. coast. on the Bragg peaks, but in shallow water any asso- By substitution of observed surface currents ciated net drift component may influence "current" into the horizontal momentum equation, local- measurements. ized surface gradients can be constructed. By in- Wind-Forced Currents tegrating these gradients over the region of radar Relating observed wind-driven currents to coverage, both an M 2 co-tidal chart (using a ref- wind forcing is notoriously difficult. Both the erence value) and a mean sea level distribution wind itself and the associated currents exhibit were calculated for the Dover Strait (Prandle et appreciable fine-scale (temporal and spatial) al., 1993). 58 OCEANOGRAPHY'VoL10, No. 2-1997 into major ports, management of coastal dis- charges/withdrawal, tracking of accidental spill- ages, assimilation (updating) of storm surge pre- diction models, etc. In principle, horizontal dispersion coefficients ~. ~ ~ ~ -....~.,,"-~ ~ . can be determined from the spatial variability in velocities observed by HF Radar. In practice, such estimates depend on the spatial resolution, sam- pling period, and instrumental errors of the sys- tem. Experiments to explore such sensitivities in- volving concurrent dye dispersion and drogue ~0.002 m s-' ~-~/bbey tracking would be useful. An exciting prospect is to link such extended ap- Perhaps the great- plications to related development in the radar tech- 15% nology. Thus, for example, a "Grand Challenge" est potential of such for coastal researchers is to predict coastal bathy- synergy is in real- 10% metric evolution, involving long-term continuous measurement of currents, waves, winds, and ba- time operational de- thymetry. An HF Radar system operating at vari- ployments. ........ ,,2,.5 .... able frequency (simultaneous or consecutively) -180 -120 -60 60 120 180 A 0/hour might be optimally tuned for determining each of / i " anticlockwise clockwise these parameters. Fig. 3: Wind-forced currents (shown for 1 m s -j References W; top panel). Histogram of rotation rate of Howarth, M.J., A.J. Harrison, P.J. Knight and R.J. Player, 1995: Measurement of net flow through a Channel. In: residual currents (bottom panel). Proceedings of the IEEE Fifth Working Conference on Current Measurement. IEEE. St. Petersburg, Florida, February 7-9, 121-126. Future Applications Lane, A.. D. Prandle, A.J. Harrison, P.D. Jones and C.J. Jarvis. The OSCR results described here have been Measuring fluxes in tidal estuaries: sensitivity to instru- restricted to the 26-MHz system, the potential of mentation and associated data analyses. Estuarine, Coastal Shelf Sci. In press. the finer scale resolution afforded by operation Matthews, J.P., A.D. Fox and D. Prandle, 1993: Radar obser- at 50 MHz is demonstrated elsewhere. The po- vation of an along-front jet and transverse flow conver- tential for longer range HF Radar current mea- gence associated with a North Sea front. Cont. Shelf surements has yet to be fully exploited. Use of Res., 13, 109-130. HF Radar for permanent monitoring of strategic Prandle, D., 1982: The vertical structure of tidal currents and other oscillatory flows. Cont. ShelfRes., 1, 191-207. straits, such as for the North Channel and Dover __, 1987: The fine-structure of nearshore tidal and resid- Strait described here, could form part of the ual circulations revealed by HF radar surface current Global Ocean Observing System to detect net measurements. J. Phys. Oceanogr., 17, 231-245. trends in oceanic circulation. With the perfor- __~ 1991: A new view of near-shore dynamics based on ob- mance of HF Radar now widely established, the servations from I-IF radar. Progr. Oceanogr., 27, 403-438. __., 1993: Year long measurements of flow through the greatest obstacle to wider use is capital cost: Dover Strait by H.F. Radar and Acoustic Doppler Cur- however, HF Radar systems are not vulnerable rent Profilers (ADCP).
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