Post from the climate.gov ENSO Blog – Author: Emily Becker
After several months of flirting, the tropical Pacific ocean and atmosphere appear to have coupled just in time for Valentineās Day and now meet the criteria for El NiƱo conditions. Is it true love? Time will tell, but forecasters expect weak El NiƱo conditions to persist through the spring.
Say yes
For a few months now, the tropical Pacific has met the first two criteria of our āIs It El NiƱo Conditions?ā decision tree (see above).
That is, the sea surface temperature in the NiƱo3.4 region of the tropical Pacific Ocean has been more than 0.5Ā°C above the long-term average, and models were predicting it would stay that way for the next several seasons.
Whatās new over the past month is that weāre seeing signs of El NiƱo-related changes in the atmosphere, with increased clouds and rain in the central Pacific indicating a weaker Walker circulation. One measurement of the strength of the Walker circulation, the Equatorial Southern Oscillation Index, was -0.6 during January, indicating more rising air than average over the eastern Pacific, and less than average over the western Pacific. These changes are enough evidence that the atmosphere is responding to the warmer ocean, leading us to conclude we have El NiƱo conditions!
Be Mine
The Walker circulationāthe atmospheric circulation over the whole tropical Pacific Oceanāis usually driven by intense rising air over the very warm waters of the far western Pacific and Indonesia. This air rises to the upper atmosphere, travels eastward, sinks over the eastern Pacific, and travels back westward near the surface, forming the trade winds.
When El NiƱoās warmer-than-average surface waters develop in the central-eastern Pacific, the Walker circulation is weakened. The extra heat in the ocean surface warms the air above it, leading to more rising motion, and hence more clouds and rain than average over the central-eastern tropical Pacific. Adding a source of rising air over the central-eastern Pacific, and reducing the rising air over the far western Pacific, means the steady upper-level and near-surface winds are disrupted. Those weaker-than-average near-surface winds, in turn, help to keep the surface warmer, in a feedback process critical to the coupled system of El NiƱo.
Call me
The surface warming in the NiƱo3.4 region is currently just a few tenths of a degree above the El NiƱo threshold (0.5Ā°C warmer than average) and actually decreased through January. Most of the climate models predict that the surface temperature anomaly (the difference from the long-term mean) will increase slightly in the near future, and remain above the El NiƱo threshold through the spring.
Bolstering forecastersā confidence that short-term surface warming will rebound and then persist for the next few months is the presence of warmer-than-average water below the surface of the Pacific. This blob, a downwelling Kelvin wave, will travel eastward over the next few weeks, gradually rising and providing a source of warmer waters to the surface.
U R Cute
But what does it all mean? Is this relationship built to last? The future remains to be seen, but forecasters give weak El NiƱo conditions the edge through the spring. After that, chances of a continued El NiƱo drop below 50%. Itās tough to make a successful forecast for later in the year, due in large part to the āspring predictability barrier,ā a notoriously tricky obstacle for computer models. Spring is a time of year when ENSO (El NiƱo/Southern Oscillation, the entire El NiƱo and La NiƱa system) is often transitioning, making it especially difficult to predict what comes next.
Weak El NiƱo conditions mean that El NiƱo isnāt dominating the global circulation, and there is a lower probability of El NiƱo-related global temperature and precipitation effects through the next few months. Other players, including the Madden-Julian Oscillation, may continue to affect weather patterns. For a prediction of the U.S. climate this spring, check out the Climate Prediction Centerās outlook. For all the ENSO information your heart desires, stick with us!