Wind-driven upwelling occurs when winds push water offshore and cool nutrient rich water rises from below to replace the surface water that has moved away. This is due to conservation of mass, and Ekman pumping, but you don’t have to worry about those oceanographic details. The nutrients brought into the well-lit surface layer provide food for phytoplankton (or tiny plant cells) which grow quickly, creating areas of higher chlorophyll. These phytoplankton patches feed zooplankton (the insects of the sea), which form patches that attract the baitfish that eat them. The wind basically pumps the food web.
One indicator of where the plankton patches will form is the boundary between the cool upwelled water and the warmer offshore water. At this boundary, temperature gradients form, and these are fronts or temperature breaks that we can map from SST data. In the Bay of Islands, New Zealand, northwest winds blowing over several days will move the surface water offshore by exerting friction on the surface of the ocean. The wind in the southern hemisphere moves water to the left of the prevailing wind direction due to the effect of the rotation of the earth (this is called the Coriolis force by oceanographers). Persistent northwest winds often result in coastal upwelling and the formation of upwelling fronts along the coast. Because these upwelling fronts are areas of rapid plankton growth, as well as being areas that plankton accumulate, they are good areas to target for gamefishing. If the wind blows hard, the front’s may wash out and be less visible at the surface of the ocean, but the plankton patches will still have the available nutrients to grow.
These four days of SST maps show the coastal upwelling developing from the 18th to the 21st December. The red arrows point to the cool upwelled water along the coast. Note how the upwelling intensifies and spreads over the four days.
This map shows the northwest wind blowing along the coast of the Bay of Islands on the 19th December. While the wind speed was variable over the four-day period, the predominant direction was northwest which is upwelling favourable on this coast, driving the surface water offshore.
The temperature gradients were strong enough for clear temperature breaks to develop at the start of the upwelling, but breaks faded as the wind persisted. The red arrows point to the breaks.
There were a lot of clouds during this period, so the chlorophyll map was only clear on the 19th December. The red arrows show the higher chlorophyll along the coast where phytoplankton are growing in the nutrient enriched coastal upwelling.
The intensity of the coastal upwelling can be clearly seen in this SST map. The cool upwelled water stands out clearly, and the close white lines of the temperature contours indicate a temperature front on the seaward side of the upwelling. Compare the temperatures in the two blue circles. There is about a 2 degree C temperature difference, which is quite large on this coast.
The projected wind for the 24th of December shows northwest winds intensifying. Since coast upwelling gets stronger when upwelling favourable winds blow for longer periods, it is likely that the Bay of Islands coastal upwelling will persist and strengthen in the coming days.