Posted by: JeffMasters, 1:48 PM GMT on March 22, 2011

The equatorial waters of the Eastern Pacific off the coast of South America have steadily warmed during March, and it appears increasingly likely that the current La Niña event will be over by June. This week, sea surface temperatures (SSTs) over the tropical Eastern Pacific in the area 5°N - 5°S, 120°W - 170°W, also called the "Niña 3.4 region", warmed to 0.8°C below average, according to NOAA. This puts the current La Niña in the "weak" category for the first time since the event began in July 2010. If these SSTs continue to warm such that they are no more than 0.5°C below average, the La Niña event will be over, and we will be in "neutral" conditions. An animation of SSTs since late November shows this developing warmth nicely. Springtime is the most common time for a La Niña event to end; since 1950, half of all La Niñas ended in March, April, or May. The weakness displayed by the current La Niña event has prompted NOAA's Climate Prediction Center to predict that La Niña will be gone by June. As La Niña continues to wane, we can expect that rainfall over the drought regions of the southern U.S. will gradually return to normal levels by mid-summer.

Figure 1. Latest runs of the long-range El Niño models have 5 predictions for La Niña conditions during hurricane season, 7 for neutral conditions, and 5 for El Niño. Image credit: IRI.

Impact on hurricane season
It is well-known that when an El Niño event is in place, a significant reduction in Atlantic hurricane activity results due to an increase in wind shear. With La Niña likely gone by June, what are the chances of having El Niño in place by the August-September-October peak of hurricane season? Well, our long-range El Niño models do a poor job of making accurate predictions in the spring, a phenomena known as the "spring predictability barrier." True to form, the March predictions by these models are all over the place (Figure 1.) There are 5 predictions for La Niña conditions being present during the upcoming hurricane season, 7 predictions for neutral conditions, and 5 predictions for El Niño. If we look at past history, since 1950, there have been six La Niña events that ended in the spring. During the subsequent hurricane season, two of those years experienced El Niño conditions: 1951 (10 named storms, 8 hurricanes, and 5 intense hurricanes) and 1976 (10 named storms, 6 hurricanes, and 2 intense hurricanes.) The other four years had neutral conditions during hurricane season. These years were 1968 (8 named storms, 5 hurricanes, 0 intense hurricanes), 1989 (11 named storms, 7 hurricanes, 2 intense hurricanes); 1996 (13 named storms, 9 hurricanes, 6 intense hurricanes); and 2008 (16 named storms, 9 hurricanes, and 5 intense hurricanes.) An average hurricane season has 10 named storms, 6 hurricanes, and 2 intense hurricanes. So, three of these six analogue years had five or more intense hurricanes (including one of the El Niño years). Looking at sea surface temperature in the hurricane main development region (MDR), the stretch of ocean between the coast of Africa and Central America, including the Caribbean, February temperatures this year were 0.62°C above average, the 7th highest February anomaly since the late 1800s. Of the six analogue years since 1950 when La Niña ended in spring, only 1996 had a much above average February SST anomaly in the MDR (0.61°C.) Thus, I believe it is a reasonable speculation at this point to predict this year's hurricane season will be similar to 1996, with its 13 named storms, 9 hurricanes, and 6 intense hurricanes--assuming we end up with neutral and not El Niño conditions this fall.

Figure 2. Departure of temperature from average for February 2011. Image credit: National Climatic Data Center (NCDC).

February 2011: 14th - 17th warmest on record for the globe
February 2011 was the globe's 17th warmest February on record, according to the National Oceanic and Atmospheric Administration (NOAA) National Climatic Data Center (NCDC). NASA's Goddard Institute for Space Studies rated February the 14th warmest on record. February 2011 global ocean temperatures were the 10th warmest on record, and land temperatures were the 28th warmest on record. Global satellite-measured temperatures for the lowest 8 km of the atmosphere were average, the 14th or 17th coolest in the 34-year record, according to Remote Sensing Systems and the University of Alabama Huntsville (UAH). The global cool-down from November, which was the warmest November on record for the globe, was due in large part to the moderate strength La Niña episode in the Eastern Pacific. The large amount of cold water that upwells to the surface during a La Niña typically causes a substantial cool-down in global temperatures. The coldest places on the globe in February, relative to average, were Eastern Europe and northeastern Siberia. Central Africa, central China, and western Greenland were exceptionally warm. For the contiguous U.S., February temperatures were near average, ranking the 51st coldest in the 117-year record, according to the National Climatic Data Center. Precipitation was also near average, ranking as the 41st driest February since 1895. February 2011 Northern Hemisphere sea ice extent was tied for the lowest on record in February, according to the National Snow and Ice Data Center. Satellite records extend back to 1979. This is the third consecutive month of record low Arctic sea ice cover.

Mostly offshore winds expected over Japan through Thursday
Radioactive plumes emitted from Japan's troubled Fukushima nuclear power plant will mostly head to the south today, passing just east of Tokyo. Northerly winds wrapping around the back side of an area of low pressure moving out to sea to the east are responsible for this pattern. As high pressure builds in over the next few days, mostly offshore winds will carry radiation from the Fukushima plant out to sea. This should change on Friday, when an approaching low pressure system will once again bring northeasterly onshore winds to Japan, possibly blowing heightened levels of radioactivity into Tokyo.

Figure 3. One-day forecast movement of plumes of radioactive air emitted at 10 meters altitude (red line) and 100 meters (blue line) at 18 UTC (2pm EDT) Tuesday, March 22, 2011 from the Fukushima Daiichi nuclear plant. Mostly offshore winds are predicted to keep the plumes east of Tokyo. Image created using NOAA's HYSPLIT trajectory model.

Figure 4. One-day forecast movement of plumes of radioactive air emitted at 10 meters altitude (red line) and 100 meters (blue line) at 18 UTC (2pm EDT) Wednesday, March 23, 2011 from the Fukushima Daiichi nuclear plant. Offshore winds are predicted to carry radioactivity away from Japan. Image created using NOAA's HYSPLIT trajectory model.

The Miami Herald has an interesting article discussing how Japan's earthquake caused a 3-inch jump in ground water levels in South Florida 34 minutes after the quake struck on March 11.

Seven-day weather forecast for Sendai near the Fukushima nuclear plant

The Austrian Weather Service is running trajectory models for Japan.