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Chapter 4. Secondary Currents ' Chapter 4. Secondary Currents ’ Tidal currents dominate the flow patterns in most First, the falling rain drops gather horizontal momentum coastal areas. Nevertheless, there are secondary currents because they are carried along at some fraction ofthe wind set into motion through a variety of forces that lead to speed. When they strike the surfaceof the sea this momen- variations in circulation quite distinct from the cyclic tidal tum is then imparted to the water which, like the wind oscillations. The estuarine-type circulation discussed in drag itself, causes the water to be driven forward in the Chapter 2, for example, is a secondary flow pattern main- direction of the wind. In addition, rain drops splashing tained by horizontal differences in water temperature and onto the sea produce an increase in the roughness of the salinity, for which discernible fluctuations in speed occur surface, which effectively enhances the wind drag. Lastly, over periods of weeks to seasons. Wind-generated cur- the natural tendency of the wind speed to diminish to zero rents are secondary flow that may alter speed and direc- very close to the water surface due to friction is partly tion over time spans as short as a few hours. offset by rain drops. Because they lose only 10-20% of Although at any particular instant the presence of their horizontal speed as they fall through the lower few secondary currents may be completely hidden by the metres of the atmosphere, rain drops may actually be stronger tidal flow, their influence can be quite moving faster than the air and, therefore, transfer to it pronounced over long periods of time and can be of some of their momentum. This in turn strengthens the considerable importance in determining the oceanogra- wind close to the sea surface and increases its force on the phy of a particular coastal area. water. Wind-generated surface waves produce a weak trans- port of water in the direction of the waves called the Wind Drift Stokes drift. This is not a wind-current, but is associated with the fact that orbital motions under a wave are not The direct effect of the wind’s drag on a nearly completely closed, and allow the water to advance slightly smooth water surface is felt only in the top few cen- forward with the passage of each wave. The speed of this timetres.This thin layer is then made to move down-wind drift is less than 1/10 the wind-drift and is usually unimpor- at about 3% of the wind speed. (Smooth is used here in tant. the aerodynamic sense in that the wind conforms to any Winds may generate more subtle circulationswithin bumps on the sea surface and does not break up into the surface waters. For instance, the streaks of foam and turbulent patches.) In a 5 m/s wind a thin “skin” of water surface debris that align as windrows along the direction would move at approximately 15 cm/s (3% of 5 m/s), but of the wind are associated with cell-like, circling patterns would have no effect on a boat with a discernibledraught. in the water at right angles to the wind direction (Fig. The shallow penetration of the wind drag can be readily 4.1). Looking downwind, the water to the right of the observed in a pond or sheltered bay where small sub- windrow is circulating anticlockwise while that to the left merged particles, such as pollen, can be seen drifting just beneath the water surface with the wind, while suspended particles a metre or more below the surface remain almost L WINDROINS motionless. The ability of the wind to produce currents to greater wind depths is significantlyenhanced if it is putting energy into -drift surface gravity waves (wind waves). Waves effectively in- crease the wind drag by increasing the roughness of the surface, and make it more difficult for air to flow smoothly over the water. Under these conditions, approximately 40% of the wind energy goes into the waves, of which 5% is lost to breaking crests in the form of whitecaps. The increased wind drag, together with the momentum trans- ferred to the water by whitecapping, leads to substantially deeper wind drift. As the amount of energy in the waves themselves depends on the wind duration and speed as well as its fetch (the unrestricted length of water surface over which the wind blows), the speed and extent of the wind current will depend on these factors also. Therefore, the state of the sea, and not the wind directly, determines the speed and depth of penetration of these currents. Rain, especially heavy rain, associated with storm FIG.4.1. Cellular circulation patterns associated with windrows. Foam and surface debris gather in streaks where currents oftwo Langrnuir cells windsmay further augment the wind‘s ability to drive a converge. Combined effect of separate wind drift (+) and Langmuir surface arrent. This can happen in a number of ways. cells produccs corkscrewlike flow pattern aligned in direction ofwind. - 71 - is circulating clockwise; surface debris gathers or con- the normal tidal streams for a few days following the verges where these two circulations meet to produce sink- passage of a storm front. Current speeds commonly reach ing water. These so-called Langmuir circulations combine 25 cmls off the west coast of Vancouver Island and at the with the stronger currents generated in the wind direction entrance to Queen Charlotte Sound, with a high degree of to produce an overall downwind water motion that some- coherence between the current motions over tens to hun- what resembles a corkscrew. The spacing between adja- dreds of kilometres. cent windrows is roughly equal to twice the length of the dominant surface gravity waves. Although the exact reason for the formation of Lang- Relaxation Currents muir circulations is not completely understood, evidence As well as driving surface drift currents, winds can now suggests they result from a complicated interaction indirectly affect other types of flow. Where the wind drift between the Stokes drift assocated with the waves, and the is restricted by a lee shore, persistent onshore winds will wind-driven current directly formed by the wind drag, or cause the water to pile up against the coast and tilt the sea alternatively, through the action of breaking waves and the surface, an effect that can be simulated by blowing on a wind-driven current. Whatever the cause, a number of bowl of water. the same token, offshore winds will observed features of these circulations can be of practical By produce a sea-level tilt by moving water away from the use. For example, the tendency ofoil slicks to collect along coast. On a large enough scale this can lead to the forma- the convergence bands has been successfully exploited in tion of storm surges on low-lying coasts mentioned earlier cleaning up oil spills at sea. And, as lines of greatest wind- in connection with tides. When such winds weaken or directed surface drift are found along windrows, whereas reverse direction, the raised (or depressed) water surface lines of weakest wind-drift lie midway between adjacent at the shore will seek its equilibrium level. The relaxation windrows, yachtsmen have a natural indicator to set their currents associated with this readjustment of water level course. Clearly, best advantage on a downwind run is may persist for hours or days, depending on the area riding along a windrow. When beating to windward it is affected, and result in perceivable deviations of the surface best to stay midway between the dominant windrows as currents from those expected on the basis of tides and much as possible. local winds alone. Wickett (1973) suggested that the un- Perhaps some of the most important types of wind- usually strong southerly currents of nearly 1.5 mls (3 kn) induced secondary flows are those associated with near observed at a drilling rig at the southern end of Hecate vertically propagating disturbances called inertial (or gy- Strait on Sept. 25,1968, were due to such currents after a roscopic) waves. Generated within the upper ocean by period ofstrong onshore winds. In this particular case, the abrupt changes in wind direction, these inertial currents outtlow was apparently augmented by 7 cm of rain on the are rotary flows whose direction constantly changes over a eastern shore of the Strait and by runoff from adjacent specified period oftime, somewhat akin to the rotary tidal inlets. streams discussed in the previous Chapter. Unlike tidal In partially enclosed basins like harbors and certain streams inertial currents are invariably circularly polarized mlets, disturbance of the surface level by passing storms in that the current vector always rotates clockwise (north- can set up oscillations that cause currents to slosh back and ern hemisphere) and maintains a uniform speed over a forth several times before the system returns to equi- single rotation. Put another way, the tip of the current librium. Sea level and current oscillations of this kind, vector, in the absence of other types of current, traces out called seiches, have periods of minutes to hours and ampli- a circle (see Fig. 3.30b). (In the southern hemisphere, the tudes of 5-10 cm, depending on the depth and geometry sense of rotation is counterclockwise.) Set in motion by a of the basin and the nature of the disturbing mechanism. pulse of momentum from the winds, the currents are Generally speaking, the seiche-currents attain maximum maintained by a balance between the rightward turning speeds midway between the two opposite directions of tilt effect of the Coriolis force (northern hemisphere) and the (see Fig.
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