Reduced Atlantic hurricane activity
Trade winds generally steer hurricanes from east to west across the Atlantic Basin. When an El Niño develops, these steering winds slow and can sometimes even reverse the flow. The slower winds from the east interact with the El Niño-forced winds from the west from the Pacific Basin. These interactions increase wind shear (the variation of winds with height) across the Atlantic’s main hurricane development region, limiting tropical cyclone development.
Stretched out jet stream and storm track
During La Niña, the jet stream tends to exhibit deeper troughs and ridges. This typically leads to more intense short-duration precipitation events over relatively small regions along the meridional-like (north-south) jet stream, with extended breaks between events. Conversely, storm tracks during an El Niño year tend to migrate across the country in more of a zonal (east-west) flow. While the dominant storm track will be determined by the exact strength of any El Niño that develops later this year, a persistent zonal jet stream would promote a steadier flow of light-to-moderate precipitation events stretched out over time and over vast geographic regions.
Historical odds favor above-average precipitation for much of the U.S.
Many of the 48 contiguous states have higher probabilities for above-average precipitation during an El Niño year during both the summer and winter. Winter months in the U.S. tend to have more impacts from an El Niño than the summer months, when the polar and subtropical jet streams interact less. In the maps below, the meteorological winter months of December, January and February show clearer precipitation signals than do the summer months.
The winter season map on the right shows a higher risk for extreme wetness in the Southern and Central U.S. (dark green). Notice how much less colorful the summer map on the left is, which indicates higher probabilities for near-normal precipitation.
Late-season drought relief possible in Southwest
“If just the five strong El Niño events are looked at then the rainfall has been above normal four of the five seasons, and all four were at least 140% of normal. However, if only the weak and moderate El Niños are examined then it is seen that six of the 17 years received below normal rainfall, six near normal (80%-120%) and five above normal.”
Californians will no doubt be hoping for a strong El Nino, which could benefit much of the rest of the country as well with moist, nutrient-rich soils. However, even though odds favor increased precipitation for California and especially the Southwest U.S., abundant rainfall is never guaranteed.