El Nino Update and Long Range Forecast

A weak El Nino episode continues across the equatorial Pacific Ocean with anomalously warm pockets in the central and eastern Pacific. Although most of the anomalies are generally ~1 C above normal, a small anomaly in excess of 2 C is present near 105 W. With the warmer than normal water continuing across the equatorial Pacific, all Nino regions remain above 0.5 C. With an increase in the trade winds in July, we saw the very warm subsurface water cool and shrink. In fact, a cool anomaly is now present between 50 and 100 meters below the surface near 110 W. However, warmer than normal water is present 150 meters below the surface between the 160 W and the Dateline.

In recent weeks, trade winds have lessened and allowed for warming across the equatorial Pacific. Running 5-day averages show weaker than normal trade winds existed across the entire equatorial Pacific. As a result, a large area of greater than 1 C anomalies was present between 135 W and 165 E. The 29 C isotherm is located the farthest east (170 W) it's been in almost six weeks. Furthermore, the 30-day average of the SOI has finally fallen back to around -5. All of this evidence supports a weak El Nino signature, and the large majority of computer models suggest this will strengthen as we head into fall. This episode should peak by the end of the year, followed by a slow decrease in strength as we head into 2010.



Needless to say, this El Nino event has not had the ocean and atmospheric support that many El Nino events have had. Whether the ocean has led the atmosphere or vice versa, the ocean and atmosphere have rarely been working in tandem at the same time for the last few months. Global relative AAM remains near climatology after being negative for the last week. Mountain torque has spiked to near 2 standard deviations above normal, with positive contributions from the Americas and tropics. Frictional torque is near climatology after peaking at around 1 standard deviation above normal in the last week. The tendency of relative AAM remains positive and is contributing to a weak GWO phase space projection in Octant 4. The current upper air pattern generally supports this phase based on 250 mb composites centered on August 26.




An incoherent MJO signal has been observed for the last several weeks, and this appears likely to continue for the next couple of weeks. Notwithstanding, pulses of enhanced uplift will likely continue across the Arabian Sea, Bay of Bengal, eastern Pacific, and the Atlantic basin for the next 10-14 days. This will pose a heightened risk for tropical cyclone activity in the Atlantic and eastern Pacific. After this time period, forecasts show that the Atlantic basin, in particular, may see a lull in tropical activity. Near the Dateline, OLR anomalies have been higher than normal for most of August, but recent indications are that negative anomalies may be returning. If this continues over the next week, it would suppress convection across the Arabian Sea and Bay of Bengal.

With the GWO in octants 8 and 1 a couple of weeks ago, these effects will begin to be felt in the long range forecasts. The north Pacific jet will extend this week and will result in the formation of a trough in the Pacific Northwest during the latter half of this week. For our area, we'll begin the week under the influence of an unseasonably strong trough. This will result in below normal temperatures and precipitation for much of week 1. By the end of week 1, a flat ridge will become more established over our region and result in near normal to slightly below normal temperatures and precipitation. Given the weak GWO projection, I'm slightly less confident about week 2, but we should be dominated by a flat ridge aloft for the first half of week 2. This will result in near to slightly below normal temperatures and precipitation. The flat ridge may once again dominate this period, but the influence of a trough to the west will likely create better chances of rain with slightly below normal temperatures near the end of week 2. Overall, I don't see any signs of oppressive heat in the next couple of weeks.