Blue Water Hunting and Freediving - Digital Version 1 - Page 243

appendix- currents 4. Tide reversal—a small tide change between larger changes. 5. Prevailing oceanic currents, in this case the California current. 6. The wind. In most cases, the currents are weakest when the net tidal change is from high to low (Figure A, Examples 1 and 2). When the net change is from low to high, expect strong currents (Figure A, Examples 4and 5). We feel that the prevailing oceanic current opposes and cancels an outgoing tide resulting in less local current, and augments an incoming tide resulting in more local current. In general, expect more current on days when the magnitude of the tide change is large. This is especially true for incoming tides. Estimate the magnitude by dividing the number of feet of tide change by the number of hours during which the change takes place. While the net trend of tidal change is important, a significant reversal in the tide’s direction, can offset the trend (Figure A, Example 4). To get an idea of how these local currents work, imagine a jar of molasses superimposed over the graphical representation of currents available on tide calendars (Figure A). When you tip the jar of molasses, the fluid takes some time to start moving; once started however, it gains momentum. Tipping the jar in the opposite direction does not immediately change the direction of flow; it takes a while to overcome the initial momentum. The steeper the graph, the faster the molasses moves. The flatter the graph, the slower the molasses moves. This seems obvious for simple tide changes with a strong trend (Figure A, Example 2). However, the molasses analogy helps most when we consider the effects of tide reversals on tide trends. A trend is a tidal flow, over several periods, which peaks to high or low. It might contain a small intervening opposite tide—a tide reversal (Figure A, Examples 1, 3, 4 and 5). A trend allows the molasses to build momentum, and a small reversal will do little to overcome its initial momentum. It works the same with the currents; with a large tide trend and a small reversal, expect a strong current (Figure A, Example 5). A strong reversal, on the other hand, will tip the jar of molasses in the opposite direction enough to slow the trend; in this case, expect a weaker current (Figure A, Example 4). At the end of a trend, expect a change in both the direction and force of the local current. Your position on the island is very important. Some locations close to the island are shielded from the current, whereas others are affected by eddy currents (described later). Other locations, far from shore, are less affected by local geography and more affected by open-ocean trends. It’s not uncommon to dive one area in relative calm when at the same time, currents a mile away make diving from an anchored boat impossible. Two factors that are unrelated to the tides are wind-generated currents and large oceanic currents. Several days of strong, steady winds can create a significant but transient surface current. Such currents promote up-wellings of cold, sub-surface, nutrient-laden water that encourages visibility-destroying plankton blooms. The California Current, flowing down the coast, changes in direction and force. It is unpredictable and is probably the wild card responsible for our missed predictions. Figure A summarizes our observations for Santa Barbara Island. While these observations are specific to this island, I have found them to be accurate for other Mexican Pacific Islands, far to the south. You probably will not be able to predict your local currents by using the specifics we use for Santa Barbara Island, but using the same factors we identify, and your own careful observations, you should be able to develop an adequate current-prediction model. Currents from one direction might bring in the fish, while currents from another might kill your spot. Predicting your local currents accurately is a great time saver and allows you to plan vacations with confidence. Local geographic structures have an important impact on currents, sometimes allowing the clever bluewater diver to remain in conditions that cause others to throw in the towel. Look at the bait fish in strong currents. They know where the least current is. The water above and down-current from a subsurface pinnacle will flow faster than the bait fish prefer. Instead, you will find them bunched at the up-current side of the rock. Water driven by the currents acts exactly like 237