Scientists say the waters along the West Coast are warmer than usual because of a lack of upwelling -- the process by which cold, nutrient-rich water rises from the depths to the surface. The colder water supports plankton populations that are an important food source. Here's how upwelling typically occurs: Surface water is dragged westward from the shore by the combined effects of wind and frictional stresses between ocean layers. Colder and saltier water from depths of 164 to 328 feet is drawn upward to replace displaced surface water. Sources: NOAA, the UNiversity of Illinois, Urbana-Champaign