Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 1:16 PM GMT on July 15, 2010
The equatorial Eastern Pacific Ocean continues to cool, and we have now crossed the threshold into La Niña conditions. 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", fell to 0.8°C below average by July 12, according to NOAA.. The Australian Bureau of Meteorology put this number at 0.7°C below average (as of July 11.) Since La Niña conditions are defined as occurring when this number reaches 0.5°C below average, we are well into the territory of a weak La Niña event. La Niña conditions must be present for several months before this will be officially classified as a La Niña event, but it is highly likely that a full-fledged La Niña event lasting at least eight months has arrived. We started out the year with a strong El Niño, so it may seem surprising that we have transitioned La Niña so quickly, However, historically, about 35 - 40% of El Niño events are followed by a La Niña within the same year. Given current trends, I expect the current La Niña to cross the threshold needed to be defined as "moderate" strength--temperatures at least 1.0°C below average in the equatorial Eastern Pacific--by September.
Figure 1. Progression of El Niño to La Niña over the past year, as measured by SSTs in the the tropical Eastern Pacific in the area 5°N - 5°S, 120°W - 170°W, also called the "Niña 3.4 region". Image credit: NOAA Climate Prediction Center.
It is well-known that both the number and intensity of hurricanes in the Atlantic tend to increase during La Niña events. However, as I discussed in a post last month, since 1995, neutral years (when neither an El Niño or La Niña are present) have had Atlantic hurricane activity equal to La Niña years. The last time we had a strong El Niño event followed by a La Niña event in the same year, in 1998, we had a Atlantic hurricane season 40% above average in activity, with 14 named storms, 10 hurricanes, and 3 intense hurricanes. The season was relatively late-starting, with only one named storm occurring before August 20. I'm thinking this year's season may be similar, though four or more intense hurricanes are a good bet due to the record warm SSTs.
Both El Niño and La Niña events have major impacts on regional and global weather patterns. For the remainder of July and August, we can expect La Niña to bring cloudier and wetter than average conditions to the Caribbean, but weather patterns over North America should not see much impact (Figure 2.) Globally, La Niña conditions tend to cause a net cooling of surface temperatures. Thus, while the past twelve month period has been the warmest globally since record keeping began in 1880, it is unlikely that the calendar year of 2010 will set the record for warmest year ever.
Figure 2. Typical regional weather anomalies observed during June - August when La Niña conditions are present. The Caribbean tends to be cloudier and wetter than average, but there is typically little change to temperature and precipitation patterns over North America. Image credit: NOAA Climate Prediction Center.
The tropics are quiet
There are no threat areas to discuss in the Tropical Atlantic today, and none of the reliable computer models is calling for tropical cyclone development over the next seven days.
I'll have a new post on Friday.
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