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It's that time of year to attempt to peer past the spring barrier, to glimpse at what ENSO may hold and some possible general weather patterns for the rest of the year. As of March 29th La Nina was present with Region 3.4 at -0.6C. NOAA currently gives a transition this spring to neutral ENSO conditions, a 60% change of developing. In recent weeks the negative temperature anomalies in the Pacific Ocean have warmed some and then cooled again slightly. Generally looking at potential and existing Kelvin Waves, this sort of equatorial wave could change the winds and let the warm water in the West Pacific slosh back to the east. This time of year cyclones from north of Australia to Africa, especially when two or more start pairing up and straddling the equator can cause a Kelvin Wave to get started. Figure 1 shows that is what the tropics are currently attempting, but the storms have been weak. The I's are for invest, areas of interest and have yet to organize. 90B is in the Northern Hemisphere. 90S is below it in the Southern Hemisphere, 99S is to the east, just offshore of Northeast Australia. All three storms have struggled for about 2 1/2days. These storms were proceeded by another cluster of storm a little farther west that never developed. No Tropical Cyclones have managed to form and pull it together enough to be named in weeks. Figure 2. Potential Velocity thru time (vertical axis by date), across the Earth (horizontal axis by longitude). Storms best form when Potential Voracity is positive (brown). Kelvin Waves (blue), Equatorial Rossby Waves (ER marked in red), Madden–Julian Oscillation (MJO in black) and general areas of Low Pressure (purple) all help develop storms. Looking at the CFS forecast, strong storms don't look as likely to form in April, particularly in the area needed to kick up Kelvin waves. Image 3 is the Equatorial Temperature Anomaly of the Pacific Ocean by depth. The top represents the surface where as the bottom is 450m deep in the ocean. This shows some sloshing taking place and heat moving below the surface weakening the current La Nina state. The heat started with quite high temperature anomalies. Those too were weakened. This is probably where some of the forecasts for Modoki La Nina summer are coming from. If that heat around 120W comes to the surface, a cool pool will likely be left out in the Central Region 3.4 while the eastern half warms. Image 4 shows the source of the La Nina, the cold water coming up along South America, being swept out along the equator from there forming well established Tropical Instability Waves . It was cut off a few weeks ago. Region 1.2 (the farthest East Pacific) had an abrupt warming to 0.8C and then quickly dropped to -0.8C over the month of March. If this current wind current flow continues La Nina would continue and most likely strengthen. If it stops this could all warm fairly fast. Image 5 reflects the various outcomes with a wide spread in the models. Statistical models lean a little cooler earlier on. Both Dynamical and statistical models averages point to a cool neutral by summer carrying on into fall. Image 6... SOI reflects the current cluster of invests, current MJO and some stronger storms near a month ago. It has certainly moved away from La Nina. As of 5 days ago the Australian Government Bureau of Meteorology declared La Nina done and conditions neutral based on SOI and the models. NOAA waits til the atmospheric effects take hold on the ocean Sea Surface Temperature in Region 3.4 and then uses the 3 month average there to determine the current ENSO situation.
Overall this year seems to have a thicker spring barrier than some. Neutral looks most likely of the three looking at models and SOI, but extreme temperatures on both ends could easily end up in Region 3.4 over the summer depending on how the Pacific Ocean sloshes and the winds that determine that. Though neither can be ruled out, of the two La Nina looks more likely. If we continue to see a dearth in storms, La Nina especially by the three month ONI standards, may very well remain.
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