Saturday, July 17, 2010

A preliminary look at Winter 2010-11

The past 9 months has featured a plethora of broken records from the Pacific to the Atlantic to the Arctic, stratosphere, and the Sun. Because of this culmination of meteorological phenomena, a set of analog years could not possibly come close to representing the current state of the atmosphere. A more qualitative approach is better this year.

Reviewing what got us where we are now: 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 pushing warm waters into the eastern Pacific. In the winter of 2009-10, 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.

After the last westerly wind burst in February, the thermocline began to rise in the central Pacific, with a layer of cool subsurface anomalies developing. The persistent tropical forcing near the dateline subsided in late March, and the MJO became active through the beginning of May. Trade winds reintensified and by the start of May the central Pacific had cooled substantially. Subsurface anomalies began to affect the far eastern Pacific in the middle of May and these surface anomalies subsequently expanded west into the Nino 3 region. By this time, tropical forcing had made a complete 180, with persistent positive OLR and 200mb convergence anomalies near the dateline and negative anomalies in the Indian Ocean and Atlantic.

During the last four weeks, the equatorial Pacific has continued to cool east of 150E. Along and to the west of 150E SSTs have warmed with notable postive anomalies developing. After heat content in the equatorial Pacific steadily decreased between March and early June, it leveled off through early July, though there is new cooling near 140W. What is notable is that higher OHC is slowly expanding eastward in the far western Pacific, near Indonesia. Overall, OHC anomalies are significantly different from 1998 (a proposed analog). The strong west based El Nino last winter compared to the huge east based El Nino of 1997-8 makes the subsequent states of the Pacific two entirely different beasts. In '98 the EPac maintained above normal heat content through October and there was no push of warmer waters in the WPac and consequently that La Nina hung around for a couple more winters.

This year, there is a very clear west push of +OHC near 5N to 10N which is already being reflected in the far WPac at the equator. We should see warming continue in the western Pacific and extend into the central Pacific by late autumn. La Nina will stick around this winter, but it will be EAST based (Nino3 anomaly < Nino4 anomaly) and the DJF ONI will be around -0.8C to -1C. This is also supported by various forecast model ensembles and roll forward methods. Warmest waters in the world right now are near Indonesia. The only change through autumn is perhaps expand east somewhat. This will enhance low level convergence near 150E sustaining MJO waves likely into phase 6 before collapsing. The few times this winter that the MJO survives into the western and central Pacific will be when blocking is most favorable. This is most likely in late December (also supported by La Nina climatology) and February as the La Nina begins to wane.

In the stratosphere, the QBO has set lowest monthly records for the last few months. Westerlies are now descending pretty quickly and will probably reach 30mb by late December. Ozone took a huge hit from high levels of water vapor associated with the strong El Nino, plus supporting the record breaking negative AO. This can be seen this summer manifested through a persistent strongly positive AAO due to the lack of ozone being transported in the BDC. Again the rapid developing La Nina may come to the rescue as water vapor diminishes significantly near the equator and ozone levels recover, which lends support to a possible stratospheric warming. The positive AAO this summer also correlates with above normal arctic heights / -AO next winter. The -QBO easterlies may hang on just long enough to support a SSW in mid to late November, that will significantly disturb the polar vortex. In the last few months, a -PDO SST signature has become better defined in the north Pacific with warm waters stretching across the basin near 40N. The La Nina pattern will certainly help maintain this general feature. The warm waters in the northwest Pacific will not be an easy road for the PV to take, which will more likely end up in the western hemisphere. I like the chances of colder air making it into the CONUS by early December; with moderation toward the middle of December.

In association with the decaying El Nino, global AAM took a plunge in early March. In early June, the AAM bottomed out again, with the 4th strongest GWO in phases 0.5 - 2.5 for the April-August period on record. This boils down to strengthening trade winds in the Pacific lending support to La Nina through positive feedback. Expect the GWO to continue to be displaced toward the La Nina attractor through the autumn and early winter. With this in mind and the prospect of ridging across the north Pacific, there will be multiple strong frictional torque events this winter. Three to five days prior to the event, there will be a significant storm threat from the midwest into the Ohio Valley and northeast as east based -NAO / Atlantic ridge retrogrades west, and SE ridge breaks down. Could see this happen around December 25th, with -NAO pattern and arctic air settling into the Great Lakes and New England region in the end of December / beginning of January, with continued storm threats through the first half of the month. The pattern will lean back toward a La Nina state by the beginning of February, with overrunning snow/ice threats through the first few weeks. Another torque event by the end of the month will bring winter back into the region into March, though with more La Nina influence.

The issue with looking for arctic outbreaks is the enormous heat release from the El Nino last winter. It's been an issue with strong El Ninos in the past, and especially last winter because the anomaly was further west with SST totals the highest of any El Nino. At least the quick transition to La Nina this summer will likely help shift the pattern to a colder arctic through the autumn, with on average stronger mid latitude ridging (as we've already seen this summer) and +AO/+NAO developing for a few months. This will allow colder air and hopefully a recovery in arctic sea ice and snow cover in preparation for mid latitude winter.

In the Atlantic, record high SSTs have been in place since January. The anomaly distribution matched very well with the record breaking -NAO last winter. The vast majority of the tropical and subtropical Atlantic has been well above normal. Every month so far this year has set a new record TNA, and in fact the May TNA set the all time record. The April and May AMO were the highest since 1878 (keep in mind the trend is removed). The June AMO dropped slightly with 1998 taking the lead (oh no! that year again). Also in the same camp is 2005 and 1995. And like those two years, it is no secret that a very active hurricane season is expected this year.

A weak -NAO has continued into the start of the summer, and the signature SSTA tripole in the Atlantic is expected to continue into the Autumn months. The cold midlatitude anomaly is fairly limited in area and amplitude, and is displaced east of the standard location. Warm anomalies in the Gulf Stream are much more robust than other tripole years. One thought to this point is enhanced baroclinicity helping intensify and guide coastal storms in the late autumn and winter. The tripole will help assist blocking patterns to some extent, though the main theme of the 2010-11 winter will likely be an Atlantic ridge pattern. This will be broken up leading into frictional torque events, which will help set up periods of -NAO blocking.

Most of all, I expect this winter to be remembered for abnormal storminess. I expect huge precipitation anomalies from the Ohio Valley into the northern Mid Atlantic and New England. Whether cold air is present will dictate snow, and it shouldn't be too difficult to tap into some semi-arctic air for these systems. General southeast ridge pattern will dominate otherwise, extending into the northeast in early February. Other warm areas will be southern and central plains into the Rockies, with below normal temperatures in the Pacific northwest.

1 comment:

Anonymous said...

Can we have a summary of how well you think this is turning out now that we are just at the turn of the year ?