Monday, 19 May 2014

Summer 2014 Outlook Part 1

The time has come, the time that we wait all year for. The summer season is upon us and many are wondering what is in store for parts of north america this upcoming summer.  This season, I decided to break my outlook up into two parts, with part two coming out sometime before the start of June. The discussion below(part 1) deals with a case study I have been wondering about regarding vastly different H5 pattern's among different East based or basin wide el nino events. The "going rule" that most people usually go by is that East Based nino's involve a North Pacific high that is based further west then WB nino's which brings down the PNA overall and pumps the SE ridge.  In addition, East Based nino's tend to force a stronger South-American hadley cell which curbs any North Atlantic blocking. This assumption is usually correct but significant year to year variability is present so that one cannot just say WB nino's are always cold and EB nino are always warm east of the rockies. In the discussion below I will go into detail on what factors you can look for when differentiating between a warm and cold EB/Basin wide nino event.

First off, I want to get a clear picture on where we are headed in the ENSO department before delving any further. Right now, the state of the pacific is undergoing significant change. Warm water continues to invade the ENSO domain, especially across eastern and central areas. Nino 1+2 has surged to 1.3c above normal as of today's update while nino 3.4 is at 0.4c above normal.

SST change since 4/16 is listed below, via the CPC. Notice significant warming of Eastern ENSO regions with slight warming across nino 3.4 and nino 3. This sudden SST warming off South America is a result of an anomalously strong downwelling Oceanic Kelvin Wave(OKW) that formed in February and is continuing to propagate eastwards in the Tropac. This OKW was so strong that it actually modified the base state of the Pacific, causing sub-sfc currents to favour a mean westerly direction. It also helped weaken the upwelling OKW that is following it so that it only acted as a slow down to the upcoming nino instead of denting the anomalous sub-sfc warm pool. For these reasons, I am favouring a high end moderate or low end strong el nino which is on the higher end of the model guidance currently.

Sub Surface Temperature anomaly loop via NOAA
With that in mind I am expecting an east based event in June which slowly transitions to basin wide as we head towards july/august.   For the purpose of this outlook I will be examining years that are EB/Basin wide as that is our expected ENSO positioning this JJA. Bolded/underlined are the years with above normal 500mb height anomalies in the means east of the Minnesota to Texas corridor. Most years did vary from month to month. For example, 1957 had a cold august while June and July were much above normal temperature wise in the east/lakes. I was pretty pleased with my dataset, as even though the sample size was small the numbers were pretty even (4 above normal, 5 below). Beforehand, I did note that there was no conclusive trend with PDO, AMO and solar when related to both dataset's which means that other factors are at work here.

a) East/Basin wide events listed below, with solar mins and maxes noted

East: 1997(Below Normal)  *Smin
East: 1983(above normal)
East: 1972(below normal)
East: 1957(above normal) 
Basin Wide: 1987(above normal)  *Smin
Basin Wide: 1991(above normal) *Smax
Basin Wide: 1965(below normal)  *Smin
Basin Wide: 2009(below normal) *Smin
Basin Wide: 1982(below normal)  *Smax

200mb height anomalies for the JJA period
Group a) which included the below normal EB/BW Nino years. Notice how circumpolar troughs dominate the 30-80N region with slightly above normal heights over the Tropical Pacific.
Group b) Well above normal heights were present along the equator with a strong Great lakes ridge and -PNA pattern.

500mb Omega Anomalies

Colder then normal EB/BW El nino years

 Notice the very strong forcing between the dateline and ~120W in addition to strong African uplift. Weak negative Omega anomalies were present across the northern and Western IO. 

In the above normal EB El Nino years, IO forcing was significantly stronger and extended further east both South and North of the equator. African forcing was significantly weaker then the other group of year as the AEJ(African Easterly Jet) was displaced south in the means.

So how can we actually tell whether this year is headed towards the above normal years(enhanced IO forcing, -PNA, weaker African forcing and +NAO) or the opposite pattern. One thing we look for is persistence as the atmosphere loves to repeat itself, even with changing indices. Below is the H5 pattern for the above normal years in MAM. Notice a very similar type of pattern as noted above.
A strong upper level trough is present across the southern Bering Sea and Sea of Okhotsk in addition to a strong eastern U.S ridge/SW trough.
In the below normal years note the very strong bering sea high which is exactly opposite of dataset 1. This is a very encouraging signal as it makes it easier to differentiate between years that will take path a) or b) as the correlation is high.

OLR Anomalies
In the below normal years tropical forcing was strong across the Southern IO and the African sector in addition to off equator forcing near hawaii. This favours below normal temps across Eastern NA as it weakens the East Asian Hadley cell, increases East Asian cyclogenesis and strengthens the downstream North Pacific high. Also notice how -OLR anomalies extend west of the dateline.
In the above normal years many aspects of tropical forcing are similar(Positive OLR over the central and northern indian ocean). The main difference is that the forcing over the central/eastern tropical pacific is based right along the Equator while the below normal years had more subtropical uplift. The -OLR anomalies also don't extend as far west once you get beyond the dateline.

So how has this year looked so far? If we take a look at tropical forcing notice how -OLR anomalies are virtually non existent across Indonesia.  Notice how Tropac forcing is weaker(more consistent with the below normal years) and also extends further West of the dateline(consistent with the below normal years).

At H5 a Strong North Pacific high, -WPO and tendency for strong East Asian troughiness is in place. This also mirrors the below normal years. 
The West Pacific jet packet amplified anomalously as well.
To conclude part 1, I am expecting this year to behave similar to group A or the Basin wide/East Based nino years that have below normal temps in the JJA means over the Eastern midwest, great lakes, ohio valley and eastern CONUS while the western CONUS enjoys above normal temperatures. In subsequent updates, I will go into further detail regarding the month to month/regional breakdown with precipitation and temperature analysis. I will also talk about some of the closer analogs that I am looking at, in addition to other factors such as the QBO, PDO and solar activity. Any questions, comments etc are more then welcome and I will try to answer them as soon as possible. You can tweet me @blizzardof96 with questions as well.