t - Comisión Interamericana del Atún Tropical

CURRENTS IN PANAMA BAY417330° - 150 0 T. Arbitrarily one might assume that the tide-producing forcealternates in that direction. Then when the y-axis is taken along 330° ­150 0 T the conditionV 1 (a - f) = V 2 (a + f)leads to these equations for the alternating tidal force:T x= OT y = [V1 (a - f) + V 2 (a + f)] COS atThis means that the two components of the tide-producing force are circularand have the same magnitude and phase, but have opposite senseof rotation. Because of the Coriolis force this equality of magnitude ofcomponents does not carry over to those of the current. The currentcomponents of the clockwise figure becomeV 1V 2a + fa _ f COS ata + f .= V 2 --f SIn ata­while the counterclockwise current figure has the same components asgiven aboye. Thus when the tidal force is alternating, the magnitude ofthe clockwise current figure exceeds that of the counterclockwise figure.Consequently the components of the sum of the two currents are2fu = ---f V 2 COS ata ­V2a V .a-2= ---f SIn atwhich are the equations for an elliptical current figure in which the currentvector rotates in time elliptically to the right, and for which the ratioof the semi-axes is a/f.Defant (loe. cit.) gives a summary of aH possible cases for the frictionlesstides and indicates that rotations cum sole (clockwise in the northernhemisphere) predominate on the rotating earth. The observations showedthat both right- and left-rotating tidal currents exist in Panama Bay (seeResults and Figure 4). Moreover, the only evident clockwise figure wasthat observed near the sea surface in the deepest water (E6) while thecounterclockwise figures were observed in shaHow water or near the bottom(A6 and C40). Therefore although it is possible to have counterclockwisetidal current figures in the absence of friction, the observationssuggest that bottom friction plays an important role in determining thesense of rotation in Panama Bay. In order to investigate this possibilitythe equations (13), rewritten to include the frictional force so that