Summer Is Coolest Since 1965; Experts Debate Cause

It’s been the coolest summer in 34 years.

Some say you can blame La Nina, at least in part. Some say it’s more complicated than that.

The average temperature in June, July and August at the downtown recording station on the USC campus was 69.9 degrees, 3.1 degrees cooler than normal. The last summer that cool was in 1965, and the last one cooler than that was in 1948, when the average was 69.7 degrees.

“The numbers for this year are really impressive,” said Bill Mork, the state climatologist. “Whenever you get more than 2 degrees cooler than normal, that’s really unusual.”

The numbers are based on a simple calculation: take the high and low temperature each day, average that, and then average all the days in June, July and August.

“It was consistently cool all summer,” Mork said. “This June was the coldest June since 1982, this July was the coldest July since 1991, and this August was the coldest August since 1953.”

The big question is why.

Stuart Seto, a meteorologist with the National Weather Service in Oxnard, said it could largely result from a persistent La Nina oceanic and meteorological condition, the counterpart in Southern California to 1997-98’s drenching El Nino. The disparate meteorological patterns of the two phenomena are generated by fluctuating ocean surface temperatures in the equatorial Pacific.

La Nina led to less rain than usual in Southern California in the winter of 1998-99, and with the condition apparently likely to linger for an additional eight months or so, the coming winter probably will be dry too.

But La Nina has the opposite effect in the Northwest, bringing more rainfall than usual. The lingering condition kept the weather in Washington and Oregon wetter and cooler than normal this spring and summer, and Seto said that frequent, elongated troughs of low pressure dragged the cool La Nina air down into Southern California.

Mork isn’t so sure.

“I think it’s more complicated than that,” he said. “I think it’s due to upper-level [weather] patterns. The whole western 20% of the country has been cool this summer. Who knows why?”

Steve Pryor, a meteorologist with WeatherData Inc., which provides forecasts for The Times, said he tended to agree with Mork.

“Those upper-level troughs have persisted off the coast this summer, and that’s possibly due to La Nina,” he said. “But summer weather patterns in the West, in general, are not that related to the La Nina / El Nino cycle.

“There are so many things going on in the upper atmosphere,” he said. “What governs long-range weather patterns? I’m not prepared to say.”

The current forecast calls for below-normal temperatures to continue in Southern California’s coastal valleys for at least a week to 10 days.

But that’s not to say that we couldn’t have some hot weather ahead of us.

Meteorologists agree that warm, dry Santa Ana winds could start kicking up toward the end of the month, with a continuing possibility of high temperatures well into October.

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Playing It Cool

This year Los Angeles experienced the coolest summer in 34 years. Following are average downtown temperatures for each month this summer, compared with the normal average.

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Normal 1999 Difference Coolest since June 69.7 66.8 -2.9 1982 (65.3) July 74.3 71.8 -2.5 1991 (71.0) August 75.1 71.2 -3.9 1953 (70.7) Summer 73.0 69.9 -3.1 1965 (69.9)

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Source: Office of the California State Climatologist