California will remain in the stranglehold of drought at least until September, even as a climate system in the tropical Pacific Ocean that would have brought rainfall to the parched state appears to be weakening, according to the National Oceanic and Atmospheric Administration's monthly climate update released yesterday.

Weather watchers had been hoping that an El Niño, which occurs when an area of the tropical Pacific Ocean warms by at least 0.5 degrees Celsius above normal, would bring moisture to the West Coast (ClimateWire, May 16).

An El Niño is still 60 percent probable by the middle of the summer and 82 percent likely to develop by the end of the year, said NOAA and the International Research Institute for Climate and Society (IRI). But a strong El Niño, caused by hotter (more than 1.5 degrees C warmer than average) temperatures in the Pacific Ocean, now seems unlikely.

Instead, meteorologists are watching for a moderate to weak event.

"We are nicely on track for a weak-to-moderate, but still potentially impactful, El Niño event as we go towards the fall or winter," said Stephen Baxter, meteorologist and seasonal forecaster at NOAA's Climate Prediction Center, during a media briefing.

The downgrade is due to weather patterns playing out in the equatorial reaches of the Pacific Ocean. An El Niño forms when the ocean becomes abnormally hot, as it did earlier this year. The magnitude of the warming led some to predict a "Super Niño".

Since then, temperatures have cooled somewhat in the equatorial Pacific waters. And the warmer ocean has not efficiently transferred much of its heat to the atmosphere in a process known as convection. Without the transfer of heat, the atmosphere will not warm. Hot air tends to rise, creating low pressure pockets that can trigger local weather events such as increased precipitation.

"The atmospheric indicators aren't as reflective of developing El Niño conditions [as previously]," Baxter said.

NOAA and IRI will continue to watch the Pacific keenly as both the El Niño and its cooler sibling, La Niña, develop between April and June. Conditions this month will dictate how strong or weak the event will be later this year.

The probability that an El Niño will not occur is 18 percent, Baxter said.

California relives the dry '70s

Translating a moderate-to-weak El Niño into rainfall predictions for California can be tricky.

Muddying the predictions is a second climate system in the Pacific Ocean called the Pacific Decadal Oscillation (PDO), which alternates from hot to cold every few years. The PDO has been in the "cool" phase since the late 1990s due to an influx of cool water from the Arctic.

When an El Niño is strong, it can switch the PDO to the warm phase. A warmer ocean would create low pressure anomalies much closer to California, triggering rainfall.

Conversely, a weak El Niño would not have much of a California connection, Baxter said.

"We've seen this in late '70s—that was the period when we had a couple of weak El Niños back to back. It didn't do much for California rainfall," he said.

In fact, the severity of the current drought in California is comparable to one that afflicted the state back in the late '70s, according to NOAA.

Alaska breaks climate records

Farther north, parts of Alaska witnessed unusual weather this year with quite a few climate records being shattered, said Richard Thoman, climate science and services manager at NOAA's National Weather Service.

The northernmost human settlement of Barrow saw the warmest January to May on record with temperatures at 2.5 degrees Celsius above zero, Thoman said.

In Yakutat, a community that receives the most rainfall (150 inches a year) in the United States, precipitation was abnormally low. Less rain and snow fell between February and May than in any year since climate observations began 70 years ago, Thoman said.

In Denali National Park and Preserve, the road leading into the wilderness was completely clear of snow and ice before May, the first time in history that this has happened, Thoman said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500