The Atmosphere Brings us Weather, the Oceans Bring us Climate

A Look at La Niña and the North Pacific

La Niña conditions in the tropical and subtropical Pacific and a few of its climate cousins have been battling it out the last few months.

La Niña, the opposite phase of the Southern Oscillation from El Niño, is a natural, periodic climate oscillation in the tropical and subtropical Pacific Ocean where sea surface temperatures become colder than normal in the central and eastern equatorial Pacific, but warmer in the western Pacific. These altered sea surface temperatures bring with them an atmospheric chain reaction.

The Pacific Ocean is currently in the throes of one of the strongest La Niña events in recorded history. Warm water in the tropics and subtropics has moved westward, producing warm water ocean conditions around Australia, Indonesia, and the Philippines. The central and eastern tropical Pacific Ocean is much colder than average.

“This very strong La Niña is having a significant role on climate worldwide,” said Delta Science Program Lead Scientist Cliff Dahm during the January Delta Stewardship Council meeting. “Eastern Australia is suffering through some of its worst floods in recorded history. Newspapers have said more area in Queensland is flooded than the areas of France and Germany combined. These areas are seeing the opposite end of the spectrum than areas in the southern part of the U.S.”

Dahm went on to say that Southern California has been spared the normal dryness, but the vast majority of the rest of the southern U.S. is “very much in drought, in particular, Texas, Louisiana, Georgia and Florida-a common byproduct of La Niña events.”

Globally, 2010 was the wettest year in the historical record, Dahm said. Massive precipitation events in Pakistan and Australia as well as the snowstorms that blasted across the Korean Peninsula, United Kingdom and central and eastern U.S. are some examples. What’s happening in California where wet conditions have prevailed this past fall? Climate indices (indicators) provide some insight.

Climate indices are now used to better predict temperature, precipitation, river runoff, and biological responses at time scales of months to up to a year in advance throughout the world. Below is a look at some climate indices that have been in the news of late.

El Niño (La Niña) Southern Oscillation (ENSO)

In addition to the changes in ocean surface temperatures, sea surface elevations, tropical winds, the distribution of clouds, direction and strength of ocean currents, and vertical ocean temperature measurements are used to determine the extent and strength of El Niño and La Niña conditions in the tropical Pacific.

While very strong El Niño events such as those in 1982-1983 and 1997-1998 produced large flows for the rivers of California, La Niña conditions are generally well correlated with below-normal precipitation and reduced water from snowmelt in Southern California and the southwestern U.S. The effects of a large La Niña event on precipitation and river flows in Northern California are much less predictable since it falls in the transition zone between the normally wetter conditions in the Pacific Northwest and the drier conditions in the southwestern United States.

Pacific Decadal Oscillation (PDO)

The PDO was derived as an index for the strength of salmon runs in Alaska and the West Coast of the continental United States. The PDO off California is manifested in changes in ocean surface water temperature in the California current. Fluctuations in the phase of the PDO have been shown to link to the long-term strength of runs of salmon on the West Coast of the U.S. including the rivers of California. The PDO generally shifts phases (warm or cool) on decadal time scales. The phase of the PDO shifted in 2009-2010.

The condition of the strong La Niña and a phase change in the PDO combined to bring California some wild fall 2010 weather with greater than expected precipitation for a La Niña year. La Niña should weaken in the coming months, however, with a likely return of average or near-average sea surface temperatures by summer, according to the U.S. Climate Prediction Center in its Feb. 10 website update.