|Gulls in stormy sky. Photo by Graham Maranda.|
Weather is a product of planetary heat exchange, driven by solar warming that is much greater at Earth's equator than at the poles. Air and moisture move both vertically (warm air rising and cool air falling) and horizontally (winds, jet streams). Due to Earth's rotation, particles moving across its surface follow curving pathways (the Coriolis effect), clockwise in the Northern Hemisphere.
In the North Pacific, a vast oceanic gyre revolves clockwise around the basin. Its major arc flowing south off our coast is the California Current. Here, very strong winds from the north accelerate the surface current and spur upwelling, crucial to productivity in the marine ecosystem.
Winds are rivers of air that rush outward from high-pressure "ridges" toward low-pressure "depressions" (the steeper the gradient between the two, the greater the wind velocity). Friction exerted by moving air upon the sea surface drives major ocean currents.
Spring northerly winds blow along the California coast due to the high-pressure atmospheric ridge that usually sets up in spring-summer to the west, over the ocean. In most years a steep gradient forms in May-June between this North Pacific High and a deep low-pressure center over the Southwest, where warmed air rapidly rises.
By about November, the Pacific High weakens and migrates south, and the Aleutian Low deepens, spinning winter storms in our direction.
In some years, the Pacific High is weak or positioned too far south in spring; then north winds may be too weak to generate much upwelling in the California Current. Also, in this fluid system a rogue "blocking high" can persist, resulting in ocean-atmosphere anomalies.
In El Niño years, a much larger-scale atmospheric fluctuation occurs in the equatorial Pacific—the Southern Oscillation. Scientists call this phenomenon by both names, shortened to ENSO. During El Niño, great ocean gyres in both hemispheres slow down or reverse near the equator, letting warm water from the western Pacific swamp the normally cold-water realms off the Americas.
Consequences include poor productivity (suppressed upwelling) in the California and Peru currents and widespread weather anomalies, such as greater-than-average rainfall along the West Coast. When ENSO flips again, very cold water can return to the eastern Pacific, with weather effects that often include drought in the U.S. Southwest.