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
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). Oceanologica Acta, 16, No. 5-6, to loss and damage inherent with oceanographic 457-468. instrumentation. __ and K.D. Ryder, 1985: Measurement of surface cur- The range of synergistic modes of linking HF rents in Liverpool Bay by high-frequency radar. Nature, 315, 128-131. Radar surface current measurements with in situ __ and J. Matthews, 1990: The dynamics of nearshore and shipborne Acoustic Doppler Current Profiler surface currents generated by tides, wind and horizontal (ADCP) (Matthews et al., 1993), remote sensing, density gradients. Cont. Shelf Res., 10, 665-681. numerical modeling, etc., has yet to be fully de- __., S.G. Loch and R. Player, 1993: Tidal flow through the Straits of Dover. J. Phys. Oceanogr., 23, 23-27. veloped. Perhaps the greatest potential of such Souza, A.J. and J.H. Simpson, 1996: The modification of tidal synergy is in real-time operational deployments. ellipses by stratification in the Rhine ROFI. Cont. Shelf Examples include use of HF Radar for navigation Res., 16, 997-1008. [~
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