SUMMER OUTLOOK 2010

The autumn to winter of 2009 to 2010 broke multiple records from the tropics to midlatitudes to the arctic.

Starting with the tropics:

Warm waters in the western Pacific steadily collected near the dateline by three significant westerly wind bursts through the autumn and winter months. However, these westerly winds never proceded far into the eastern Pacific where easterly trade winds maintained strength. The Peru current fed colder waters into the extreme eastern Pacific, forcing a west-based El Nino through the winter.

A strong west-based El Nino is rare because normally the SST anomalies force a circulation with positive feedback into the eastern Pacific. 2009-10 was different. The ONI hit 1.8C for NDJ, and 1.7C for DJF, indicating a strong warm episode, but the highest SSTAs were locked in the Nino4 region (near the dateline). This season featured the highest monthly Nino4 SSTs ever recorded for October through February. The strong El Nino powered an active subtropical jet that contributed to the multiple historic snow storms in the Mid Atlantic and Southeast US.

How El Nino is currently progressing:


The anomalies in the central Pacific have begun to diminish in magnitude, as is usual entering boreal spring. Meanwhile the Peru current continues to transport cold waters north, and a negative anomaly has recently developed into the eastern Pacific. Based on recent trends in SSTs, zonal atmospheric momentum and current ENSO models, ENSO should fall back to near neutral this summer, while maintaining an east to west gradient.

Effects on tropical forcing:

This winter has featured persistent tropical forcing near the dateline, with a strong negative OLR signal on the DJF composite, and a nice vertical velocity couplet indicating the meridional overturning cell. This maximum in forcing also coincides nicely with the low level convergence where the WWBs died in their eastward progression. The MJO has consistently lingered in the 7-8-1 territory of phase space. Since no significant reversal in the structure of SST/wind anomalies are expected, there is no reason for this forcing pattern not to continue (a version of Newton's First Law), except for a weaker magnitude through the summer.

Atlantic:

SST anomaly distribution matches very well with the persistent -NAO this winter. The vast majority of the tropical and subtropical Atlantic is above normal right now. In fact, the magnitude of the anomalies competes with even 2005. Negative SST anomalies in the Gulf of Mexico right now, stretching across the middle latitude Atlantic. This is also inline with normal results of a west based El Nino. Should see the Atlantic maintain these warm anomalies, and a general recovery of the Gulf SSTs into the summer.

Atmospheric Angular Momentum:

The last month has featured a significant spike in AAM, with the GWO oscillating around the El Nino attractor. This is a big reversal from the last four years: even the moderate El Nino in 2006-7 saw just a neutral AAM; The last significant +AAM was in 2005. This winter's spike in AAM came largely from strong positive North America mountain torque during the month of January, which was responsible for the pattern change into the February. Currently, the atmosphere is in the process of coming back to equilibrium, with friction torque due to equatorial westerlies, and developing negative mountain torque. With an expected decrease in westerly anomalies in the Pacific, AAM this summer should stabilize near normals, coinciding also with the current long term period of -AAM bias.

Stratosphere:

The QBO dipped below zero in June 2009, and easterly winds continue to steadily lower through the stratosphere. These easterly anomalies assisted with the incredible low index annular mode this winter. January QBO came in at -16.02 and February QBO at -16.98. There has been a deep layer of <-20m/s anomalies above 30mb, that will likely dip down through the summer, and continue affecting the troposphere.

High latitude blocking:

This of course was another major record breaker of the winter. The winter of 2009-10 began with a rapidly falling arctic oscillation, reaching -5.668SD by December 21. Through the rest of the meteorological winter, the AO rose above zero for only two days, and spent an incredible total of 35 days below -4SD! The DJF AO was -3.422, demolishing the previous record low of -2.617 set in 1976-77. The intense ridging over the arctic contributed to the suppression of practically every storm system this winter, with warm temperature anomalies across the subpolar regions and cold anomalies through the middle latitudes.

The height anomaly composite for DJF matches the AO correlation very well with one interesting exception. The below normal heights remain consolidated over the Atlantic Ocean, rather than splitting into two lows over Europe and the eastern US. Trying to match this unique feature to analogs was difficult as the analog composites would always be slanted to either the US or Europe.

The AO has already recovered so far this spring, and will likely avoid the extremeties seen this winter, but with upper level easterly anomalies continuing through the summer, a low index annular mode should redevelop into the summer. Because the summer features shorter atmospheric waves, the way the annular mode affects surface conditions can be variable, and hard to pin down, but factoring tropical forcing into the equation can help.

Soil moisture:

Soil moisture has been shown to have a significant effect on boundary layer temperatures in the summer, with wetter areas having a bias toward cooler temperatures. The Mid Atlantic in particular is entering the spring with wet departures from normal. Eastern Texas up through Missouri and into the Dakotas also have high soil moisture anomalies. The immediate Great Lakes region and portions of the west have drier than normal conditions going into the spring.

Putting it all together:

Using composites of analogs based on the various parameters discussed above, as well as some simple extrapolation of winter trends, here's the outlook for this summer:

June -- Mountain torque forces a +PNA pattern. Below normal heights extending into southeast Canada during this period will keep New England on the cooler side. Ridging will dominate western Canada into the Pacific northwest, with below normal heights influencing California, also consistent with a dying El Nino. The warmest temperatures will be centered in the Ohio Valley, extending into the southeast. There is good potential for severe weather episodes from the Ohio Valley into Pennsylvania, New York and Southern New England as disturbances swing underneath the upper low.


July -- A deep trough in the North Pacific will edge east toward Alaska pushing another trough ashore over California, influencing much of the southwest with below normal temperatures. Ridging will amplify over the south, into the Ohio Valley. Severe weather activity will focus over much of the central plains into the Ohio Valley, with weaker remnant activity affecting the northeast.


August -- North Pacific low slides east, with indications that much of Canada will be affected by below normal heights. The California trough shifts into the central Plains, with strong ridging in the east, although suppressed by hints of a -NAO. Above normal temperatures will dominate much of the east, into the Ohio Valley. The upper Plains will see cooler temperatures, extending down toward Texas. Very good signal for severe weather existing from the Ohio Valley into New England. This could be a very active month for the northeast.


Hurricane Season -- A hyperactive season is likely. Many things are coming together to produce one of the more active hurricane seasons on record. It would obviously be very difficult to beat 2005, but something analogous to 1969, 1995, and 2004 is looking good. The QBO is solidly in negative territory and ready to fall some more with easterly wind anomalies dropping into the upper troposphere, reducing vertical wind shear. A dying El Nino in the spring is frequently a precursor to an active hurrican season in the summer and autumn. First, there is a correlation to above normal Atlantic SSTs, which we are already seeing; and second, if the Eastern Pacific continues to cool, then shear is further reduced.

Preliminary numbers: 19 named storms, 10 of which become hurricanes, 5 of which are major hurricanes.

As for highest risk for landfalls, the season will begin with the Gulf coast and Florida peninsula at highest risk, with the East Coast openning up for business into September.