On Valentine’s Day 2019, the National Oceanic and Atmospheric Administration (NOAA) announced that El Niño officially arrived. The climate agency stated in a press release:
El Niño conditions across the equatorial Pacific have come together, and we can now announce its arrival,” said Mike Halpert, deputy director, NOAA’s Climate Prediction Center, and ENSO forecaster. “While sea surface temperatures are above average, current observations and climate models indicate that this El Nino will be weak, meaning we do not expect significant global impacts through the remainder of winter and into the spring.
NOAA projected a 55% chance of El Niño persisting through spring. Climate expert David Zierden, the State Climatologist of Florida and a scientist in the Florida Climate Center and Center for Ocean-Atmospheric Prediction Studies at Florida State University, tweeted six indicators that El Niño is quite healthy right now. Here is what it all means.
Sea Surface Temperatures. One of the signature indicators of El Niño is warm sea surface temperatures (SST) in the equatorial Pacific Ocean. Current SSTs are 1-2 degrees C warmer than normal throughout from eastern to central regions of the basin (graphic above). The American Meteorological Society (AMS) Glossary of Meteorology defines El Nino as “A significant increase in sea surface temperature over the eastern and central equatorial Pacific that occurs at irregular intervals, generally ranging between two and seven years.”
Sub-Surface Temperatures. According to NOAA’s Climate Prediction Center website,
During the developing phase of the El Niño, the subsurface ocean structure is characterized by an abnormally deep layer of warm water and an increased depth of the thermocline across the eastern tropical Pacific. Thus, the slope of the thermocline is reduced across the basin. In very strong El Niño episodes, the thermocline can actually become flat across the entire tropical Pacific for periods of several months.
Zierden said that a strong downwelling Kelvin wave is present in the eastern Pacific Ocean (graphic above). During an El Niño, anomalously warm water temperatures are supported by large-scale waves moving across the Pacific Ocean basin toward South America. The NASA Visible Earth website points out:
Under normal conditions, the tropics’ prevailing easterly winds push Sun-warmed surface waters across the Pacific from the Americas toward Indonesia, creating a deep pool of warm water in the western Pacific. During an El Niño, the trade winds falter, and sometimes even reverse, for months. When the winds that maintain the warm pool falter, a large pulse of warm water from the western Pacific slides back toward the east.
Westerly Wind Burst. Zierden also documented the “strongest, most widespread westerly wind anomalies this year over the western and central Pacific (graphic above).” Why is that important? Scholarly research has shown that westerly wind bursts are often associated with El Niño. Research suggests that such wind bursts may be responsible for the downwelling Kelvin wave, eastward equatorial surface currents and the warm pool.
Outgoing Longwave Radiaion. Outgoing Longwave Radiation (OLR) is measured by satellites. Negative anomalies of OLR indicate the presence of convective clouds. Colder clouds that penetrate to higher altitudes emit less infrared energy detectable by the satellite. Deep, cold convection in this part of the Pacific basin is a typically associated with the warm phase of the Southern Oscillation (graphic below).
Southern Oscillation Index. The Southern Oscillation (SOI), according to Dr. Zierden, has reached its lowest level since 2016 (graphic below). SOI is defined by NOAA as ”a standardized index based on the observed sea level pressure differences between Tahiti and Darwin, Australia.” Negative SOI reveals lower than normal air pressure in Tahiti and above normal at Darwin. The pressure values at these locations oscillate between warming (El Niño) and cooling (La Niña) conditions.
The Jet Stream. This factor connects U.S. and global weather patterns to El Niño. The El Niño in the Pacific Ocean modifies global weather patterns through teleconnections. Teleconnections are apparent changes or correlations in meteorological conditions across long distances. According to Dr. Zierden’s tweet, “…the classic enhanced subtropical jet over North America” has been present during the past month. The Climas website at the University of Arizona provides the needed context:
The influence of ENSO on weather…is tied to its ability to change the position of the jet stream—the winds aloft that steer storm systems and dictate where areas of high and low pressure are positioned. During El Niño events, the jet stream over the Pacific Ocean becomes less wavy and splits into a strengthening subtropical jet stream near the equator and a weaker polar jet stream
This pattern change affects global storm tracks, precipitation distributions, and even hurricane activity in the Atlantic basin. Zierden told me by email,
Here in North Florida and the Southeast, the rainy and stormy pattern since October has been consistent with El Nino. Key indices used by climatologist such as Nino 3.4 index has been mostly above the 0.5C threshold since mid-September. Now atmospheric indicators are aligning with warm ocean temperatures, signaling that El Nino conditions could very well last late into spring.
He also expects NOAA to deliver a similar message to his Twitter thread very soon.