Published: May 14,2015
First declared by NOAA scientists in early March, the current El Nino continues to settle in, with more pronounced sea-surface temperature anomalies becoming evident in the equatorial Pacific Ocean.
Weekly
sea-surface temperature anomalies (degrees Celsius) from Jan. 4 - May
9, 2015. The Nino 3.4 region used as a defining area of El Nino or La
Nina conditions is highlighted by the black rectangle. El Nino is an anomalous, yet periodic, warming of the central and eastern equatorial Pacific Ocean. For reasons still not well understood, every 2-7 years, this patch of ocean warms for a period of 6-18 months.
This week, both NOAA and Australia's Bureau of Meteorology issued outlooks that forecast El Nino to persist into northern hemisphere possbily into the winter months. NOAA's Climate Prediction Center gave an 80 percent chance the El Nino will last through the rest of 2015.
There's also an increasing chance El Nino may become moderate or strong, and, thus, play a stronger role in your weather.
Statistical
and dynamical model forecasts from April 2015 of sea-surface
temperature anomalies (degrees Celsius) until February 2016 in the
region of the equatorial Pacific Ocean in which El Nino is defined. What does warm water have to do with the weather?
Schematic
comparison of sea-surface temperature and most persistent
rain/thunderstorm locations in neutral vs. El Nino conditions in the
equatorial Pacific Ocean. A 'Sea Change' in Atmospheric Circulation
Typically, easterly trade winds near the equator pile warm water into the western Pacific Ocean. Conversely, the resultant upwelling, or upward movement of deep, cold ocean water keeps the eastern and central Pacific Ocean cooler.Thunderstorms require at least some degree of warm, humid air near the surface, so they're more numerous and persistent over the western Pacific warm pool, and much less so in the eastern equatorial Pacific.
During an El Nino, these trade winds weaken, and may at times reverse from west to east. Warmer western Pacific water then slowly sloshes back toward the central, even eastern Pacific Ocean in what's known as an equatorial-trapped Kelvin wave.
Therefore, the most persistent thunderstorms will shift from the western to the eastern and central Pacific Ocean in an El Nino.
This trade wind reversal and the resulting reorientation of thunderstorms changes the atmospheric circulation not just over this swath of the equatorial Pacific Ocean, but can also have far-reaching impacts on the atmospheric circulation.
Weather Impacts
(NOAA/CPC)
(NOAA/CPC)
1) El Nino is not the sole driver of the atmosphere at any time. Day-to-day variability in the weather pattern, including blocking patterns, forcing from climate change and other factors all work together with El Nino to determine the overall weather experienced over the timeframe of a few months.
2) No two El Ninos are exactly alike. The intensity matters for impacts.
El Nino's clearest impact on northern hemisphere weather patterns occurs from late fall through winter.
Looking at past moderate-strong El Ninos, here are the upshots for temperatures and precipitation from late fall through winter in the U.S.:
- Wetter: Southern U.S. from California to the Carolinas then up parts of the East Coast
- Drier: Parts of the Ohio Valley, Great Lakes, Northwest and Northern Rockies
- Cooler: Desert Southwest, Southern Plains, northern Gulf Coast
- Warmer: Northern tier of states from the Pacific Northwest to the Northern Plains, Great Lakes, and Northeast
Note these are impacts that are typically expected, but they aren't always the rule.
Residents of the western states may remember the flooding that struck California during the strong 1997-98 El Nino. In February 1998, a series of storms caused an estimated $550 million in damage and killed 17 people in California. A total of 35 counties were declared federal disaster areas. This fits into the bucket of the wetter-than-average winter you would typically expect in a moderate or strong El Nino.
Interestingly, during the previous winter there was also major flooding in California and it was even more costly with a total price tag of $1.8 billion, according to Jan Null, a consulting meteorologist in California. However, El Nino was not present that winter and rainfall for the season was near average. The flooding was the result of excessive rainfall that fell in a short time period combined with snowmelt from late December to early January.
The weak El Nino in the winter of 2006-07 provided a totally different story than what we saw in the very strong 1997-98 El Nino winter.
California had its 23rd driest winter season on record when looking at the three-month period from December 2006 to February 2007. In Los Angeles, the entire water year from July 2006 to June 2007 was the driest on record with just 3.21 inches of rainfall.
So, those hoping for drought relief next winter in the Golden State shouldn't immediately draw a conclusion that significant rains are ahead in any El Nino year. The strength of the El Nino can play a role in the outcome. In addition, heavy rainfall can occur with or without El Nino present and that was the case in the winter preceding the strong 1997 to 1998 El Nino.
What About Hurricane Season?
There is a body of scientific evidence linking the occurrence of El Nino with increased wind shear in the tropical Atlantic Basin, which is one factor – along with dry air – that limits the development and strengthening of tropical cyclones.Take the 2013 Atlantic hurricane season. Yes, 14 storms formed, but only two reached hurricane strength. Neither of these hurricanes reached major hurricane status, which is defined as Category 3 or stronger on the Saffir-Simpson Hurricane Wind Scale.
When considering overall season activity via the ACE index, 2013 was the least active Atlantic hurricane season since 1994. There was no El Nino in place in 2013.
Now, consider the 2004 Atlantic hurricane season. Four hurricanes - Charley, Frances, Ivan and Jeanne - hammered Florida in less than six weeks. There were 15 storms and nine hurricanes that season, which is an active one by any measure, and it developed despite a weak El Nino.
Taken together, the five El Nino hurricane seasons since 1995 averaged about 11 storms, 5 hurricanes, and 2-3 major hurricanes, a reduction of four storms, 3 hurricanes, and 1-2 major hurricanes from the average since 1995.
Prior to the current active phase of Atlantic hurricane activity (pre-1995), there were several other relatively slow hurricane seasons: 1982, 1986, 1987, 1991 and 1994. The 1982 season was particularly inactive, with only six tropical storms and two hurricanes. The next year, despite one of the strongest El Ninos on record finally fading by early summer, only four storms formed the entire season.
Exactly where the equatorial Pacific Ocean warms in an El Nino matters, as well.
- Warming in the eastern equatorial Pacific: lower numbers of Atlantic tropical cyclones
- Warming in the central equatorial Pacific: higher numbers of Atlantic tropical cyclones
While we have focused on numbers of storms and hurricanes, it's ultimately a storm's path that matters for impact.
"Even if we get a strong El Nino, that doesn't mean no U.S impacts (from hurricanes or tropical storms)," says Dr. Phil Klotzbach, a tropical meteorologist and researcher at Colorado State University.
Klotzbach notes that Hurricane Betsy hit both Louisiana and Florida in 1965 and Agnes flooded out the eastern U.S. in 1972, both during strong El Ninos. All it takes is one intense, landfalling hurricane to make many forget an El Nino was even there.
So, ultimately, if El Nino does form, it may exert some influence on the numbers. However, that is not a sure thing, and all it takes is one hurricane making landfall in a populated area to change perceptions of an active season.
(MORE: 2015 Hurricane Season Outlook)
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