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How does El Nino get to San Diego?

You've heard about the strong, possibly record breaking El Nino weather event that has been building for months. The El Nino system has the potential to bring heavy rain to Southern California. In San Diego, previous El Ninos have almost doubled our average 10 inch annual rainfall. There are no certainties, though, because it is so difficult to predict storm tracks and rainfall amounts. Ideally, we'd get enough sustained precipitation to fill our reservoirs but not coming so hard and fast that we have flood damage. Also ideally, we'd want temperatures cool enough over the Sierra Nevada and Rockies for the precipitation to fall as snow and replenish our sadly depleted snowpack. Remember, only about 20% of our water supply is local, the rest coming from the Colorado River and Northern California, so weather patterns in those regions really matter.


On average, we know what El Nino events, occurring roughly every 2-7 years, do in different parts of the world. While an El Nino can be good news for Southern California rainfall, it can be bad news on the other side of the Pacific, in Australia for example, where the shift in weather patterns means a greater risk for drought and wildfires. It's hard to be exact in predictions because El Nino is part of the complicated Earth System. Globally, ocean temperature and circulation, high and low atmospheric pressure cells, air masses, ice distribution in glaciers and ice sheets, and topography interact and influence the climate on a large scale. Locally, smaller scale factors tie in with the global system to determine our weather in the coming months.


Getting back to what is El Nino - it's an oscillation in the ocean-atmosphere system in the tropical Pacific. El Nino begins in the equatorial Pacific. Trade winds that usually blow off the coast of Chile weaken, allowing warmer water to move east, causing the central and eastern equatorial Pacific ocean to warm. Current ocean surface temperatures are more than 2 C above normal - that's big! As anybody who's been in the ocean lately knows, there has been unprecedented warmth in the eastern Pacific, stretching from Chile to Alaska. The ocean affects the atmosphere and vice versa. A low pressure system associated with the warmer ocean water migrates to the eastern Pacific. Low pressure systems (a hurricane is an extreme low) bring stormier, wetter weather. Cooler water and a high pressure system move into the western Pacific, bringing drier conditions. Large scale atmospheric circulation cells and ocean currents connect conditions at the Equator all the way to the poles. Typically, during a strong El Nino, the wintertime storm track shifts south and we expect a wetter winter in the Southern U.S., a drier winter in the Northwest, and warmer winter in the Northern states. Will El Nino get us out of our current drought? No, it might help, but we will need more than one wet winter to replenish reservoirs, groundwater, and snowpacks. In any case, drought cycles, worsened by global warming, are part of life here in our SoCal desert/Mediterranean climate. So keep up the water conservation and support water recycling!


By Jennifer Olim