La Nina Still Fighting

The big climate news this week was the official declaration from NOAA that El Nino has begun. SSTs in the equatorial Pacific Ocean remain slightly above normal (0.5-1 C) with the warmest anomalies along the equator between 150 W and 110 W. The 1 C anomaly has actually shrunk in size the last few weeks as the trade winds have remained slightly stronger than normal (0.5-1 m/s) just south of the equator between the Dateline and 100 W. However, subsurface (50-75 m) anomalies have actually increased to 3-4 C between 130 W and 100 W so any relaxation of the trades in this area will result in the anomalously warm water rising closer to the surface.




An analysis of the equatorial Pacific Ocean waters certainly confirms that a weak El Nino is ongoing, but the global wind and tropical convective signals suggest that La Nina is still valiantly fighting. Recent positive mountain torque has since dropped near climatology with very weak negative contributions coming from East Asia and North America. Frictional torque has increased the last few days, possibly due to anomalously strong westerly winds associated with low pressure centers between 40-45 S. Despite this, relative AAM remains near 1 standard deviation below climatology. Furthermore, the wind signals remain weakly projected in GWO phases 1-3, the La Nina attractor. The SOI has also generally remained positive since June 22 as stronger than normal high pressure has persisted near Tahiti, and in fact, the 30 day average is close to 4.




In addition to the wind signals, the tropical convective signals also favor a La Nina type pattern with strong convection moving out of the Indian Ocean and slowly into Indonesia. However, OLR anomalies suggest another area of convection centered near the Dateline, and this will need to be watched closely over the next couple of weeks for persistence and/or strengthening. If this area indeed persists and/or strengthens and the convection over Indonesia moves further east, the tropical convective signals will finally begin favoring an El Nino pattern.


The 250 mb streamfunction composites for GWO phases 1-2 seem to depict the current global circulation, especially the circulation across the United States. A strong upper anticyclone dominates the Southern Plains while a weak shortwave trough is located over the Pacific Northwest. This upper air pattern will quickly flatten out through the middle of next week before a strong shortwave trough digs into the Eastern U.S. by the end of the week. This will provide a chance of rain and slightly cooler temperatures to the Southeast and continued cool weather across the Great Lakes and Northeast. The upper level ridge will move westward and will bring heat back to the Western U.S.
With the weak GWO projection likely to continue, it won't be of much value in the medium and long range. However, keeping with the idea that the current global circulation pattern persists, the heat ridge will likely slide back to the east by Week 2. This means a return of the warmth to the Southern Plains and the Southeast. The main storm track will be across the northern part of the country where periodic episodes of severe weather will occur. Passing shortwaves should also prevent the Great Lakes and Northeast from seeing prolonged hot weather in the foreseeable future.