THE CALIFORNIA DELUGE : Who Left the Door Open? : System Moves Aside, Letting Rain Pour In

In the folklore of California’s water experts, it is called the “storm door”--a puzzling but powerful weather system that effectively controls how much winter rain falls from Eureka to San Diego.

The storm door--a high-pressure system that in most years dominates the coast of California--blocks any storms forming over the Gulf of Alaska from moving south. But when it unexpectedly swings wide open, as occurred this week, the result is visible from any window in Southern California--torrential, unceasing, unstoppable rain.

Weather experts outside California talk knowingly about the effects of El Nino, Southern Oscillations, global warming and sunspot cycles when they contemplate the current downpour. Even chaos theory gets a speculative mention.

State water experts such as chief hydrologist Maurice Roos, however, are content just to say the storm door is jammed open.

The door is a massive seasonal high pressure system that in most years squats like a sumo wrestler off the California coast from spring to fall, effectively preventing any rainstorms brewing to the far north from drenching the state.

As the season cools into winter, the ridge of high pressure normally drifts south slightly and the storm door opens just a crack. That allows seasonal rains to replenish the state’s water supplies while deflecting most serious storms into Canada, where the jet stream pushes them down into the central United States. If the door jams shut, the state experiences drought.

“The high-pressure ridge sits (along) the West Coast and tends to control the storm tracks,” state climatologist William Mork said.

But a high-altitude river of air--60 miles wide and a mile thick--crossing the Pacific from Japan at more than 190 m.p.h. has blown the high-pressure system away and allowed a wet low-pressure system to take its place. That system is spinning storm after storm into the state.

Several climate researchers say the unusual storm systems are small symptoms of a remarkable, prolonged warming of the western tropical Pacific Ocean. This has increased the surface water temperature by one or two degrees Fahrenheit--enough to significantly affect weather patterns and atmospheric pressure in unpredictable ways.

Climate experts have paid considerable attention to El Nino--a recurring Pacific Ocean current that typically begins around Christmas--and its effects on weather throughout the world. Some researchers suggested that California’s rain this week may be caused by a mild El Nino.

James W. Hurrell, a scientist in the climate analysis section of the National Center for Atmospheric Research, said since 1990 the region has experienced four El Ninos--so many they effectively run together. The long-term effects of that prolonged El Nino on California’s weather are unfathomable.

“We are in an event that we have never seen before,” Hurrell said.

The extended El Nino comes on the heels of an equally substantial, decade-long change in the north Pacific that dramatically altered circulation patterns in the atmosphere, shifting the moist air of low-pressure systems over the Aleutians along the west coast of North America.