Significant changes in our worldwide circulation / air pressure pattern will be occurring over the next few months as we transition from a cold-Nina regime to a warm-Nino ENSO event. Consequently, the ramifications of this developing El Nino will be shown in the occurrent weather patterns — i.e the precipitatio nand temperature anomalies. My summer forecasts for both 2007 and 2008 were met with very good success, so hopefully I have a decent handle on the evolution of this year’s warm season.

Major factors to examine:1) El Nino, 2) PDO / AMO

1) El Nino of 2009

The image below depicts the tropical Pacific, from the NW coast of South America westward across the entire ENSO region. Note the sea surface temp anomalies in particular — locallized areas of +0.7 / +0.9c appearing, with widespread numbers in the +0.5c territory. Keeping in mind the thresholds for nino intensity: A) Weak Nino = +0.5c to +0.9c B) Moderate Nino = +1.0c to +1.4c, C) Strong Nino = +1.5c and greater; we are most definitely weak nino status right now. The weekly ENSO numbers show region 3.4 (the crucial determining region) as +0.5c as of the latest update — which is weak nino.

The onset of this Nino event can also be seen in the pressure pattern we’ve transitioned into over the past couple months — the Southern Oscillation Index (SOI) is essentially a measure of atmospheric pressure differences between two general zones in the tropical pacific — Tahiti and Darwin. Generally speaking — when the SOI is in a predominately positive state, like it’s been the past 2 years, one can expect increased easterly trade winds, which upwell colder waters in the central Pacific ( La Nina ); however, when the SOI is in an overall negative modality, as it’s morphed into the last 6-8 weeks, increased westerly trade winds initiate, resulting in warming of the trop Pacific waters ( El Nino ). I.e. – what we’re seeing now.

The SOI has proven to be a pretty accurate predictor of oncoming cold/warm ENSO events as noted by this data below:

Essentially what this information indicates is that a negative SOI for 1-2 months is almost always a precursor for El Ninos.

The weak El Nino of 1951-52.
— attained nino status in JAS of 1951
— monthly SOI averaged negative beginning March 1951; the March, April, and May numbers were -1.3, -1.1 and -1.7 respectively.

The weak El Nino of 1963-64.
— attained nino status inJJA of 1963
— June SOI number was solidly negative at -1.6, before the onset of the nino. The decline was apparent as MAY 1963 was near 0 SOI.

The weak El Nino of 1968-69.
— attained nino status in OND of 1968
— monthly SOI averaged negative as early as August

The weak El Nino of 1976-77.
— attained nino status in ASO of 1976
— monthly SOI was first negative in June

The weak El Nino of 1977-78.
— attained nino status in ASO of 1977
— monthly SOI began negative streak in March of 1977

The weak El Nino of 1969-70.
— attained nino status in ASO of 1969
— monthly SOI continued negative several months prior to the onset and throughout the winter.

The 1965-66 Nino.
— attained nino status in MJJ of 1965
— monthly SOI turned negative in April

The 1957-58 Nino.
— attained nino status in MAM of 1957
— monthly SOI negative by February of 1957

The 1972-73 Nino.
— attained nino status in AMJ of 1972
— monthly SOI turned negative in April of 1972

The 1982-83 Nino.
— attained nino status in AMJ of 1982
— monthly SOI averaged negative as early as February, neutral March, then persistently negative April and beyond

The 1986-87 Nino.
— attained nino status in JAS of 1986
— monthly SOI negative by August

The 1987-88 Nino.
— continuation of the prior year
— monthly SOI negative streak ended February of 1988 — the last month that was considered nino.

The 1991-92 Nino.
— attained nino status in AMJ of 1991
— monthly SOI persistently negative beginning in February

The 1994-95 Nino.
— attained nino status in AMJ of 1994
— monthly SOI began negative streak in January of 1994

The 1997-98 Nino.
— attained nino status in AMJ of 1997
— monthly SOI began negative streak in March 1997

The 2002-03 Nino.
— attained nino status in AMJ of 2002
— monthly SOI began negative streak in March 2002

The 2006-07 Nino.
— attained nino status in JAS of 2006
— monthly SOI negative streak began in May 2006

Climate forecast models have caught onto the El Nino idea as well:

Most in the weak to moderate range.

The CFS is notorious for it’s extreme bias, and it’s forecasting a strong nino for the upcoming year (which BTW would be a disaster for next winter)

2) PDO / AMO

I’ve grouped these two multidecadal indices together as they will both be undergoing similar changes over the next few months — the Atlantic and Eastern Pacific should transition warmer as we head deeper into the summer due to the upcoming El Nino.

Note how cold the water in the East Pacific was in early April — strong neg PDO indicative of LA NINA signature.

Now here in late May — the developing el nino has already begun to take a toll on those very chilly waters:

I expect this process to continue — gradual erosion of the cold East pacific throughout the summer, and transition into a neutral PDO, probably slightly positive by autumn as the El Nino will be full-fledged by then (at least weak, probably moderate).

The AMO — i.e. the atlantic SSTA regime has taken a hit from the past 2 yrs of Nina conditions — pretty cool water presiding. However, that should also change over the coming months. This is to be expected as we are still in the midst of the long term +AMO (warm) decadal phase which started in 1995. They’re generally 25-30 year cycles, so the transition to a consistently cold Atlantic should not be expected until sometime around 2020.

Summer Outlook 2009

With all these factors in mind, along with a few less important ones, I feel fairly confident for this summer’s temp and pcpn progression.

I’ve come up with three analogs in particular which fit 2009 very well based on the above factors — 1951, 1957, and 2002. The latter two summers preceded moderate el nino’s (which I believe will occur this year).

Below is what the composite of those 3 summers (June – July – August) look like temperature wise. Anomalies generally slightly above normal in our area.

June was the overwhelming warmer than normal summer month and August overwhelming colder than normal in these developing EL NINO seasons…

While July was near normal, however, due to the warm Atlantic, I’ve erred on the warm side for July.

So here his my temp departure forecast for NYC/metro area as per usual:

June: +1 to +2
July: 0 to +1
August: -1 to -2
Summer (JJA) : 0 to +1

In terms of precipitation — oncoming El Nino’s tend to be wetter than normal in the East. In particular I’m expecting increased T-storm activity in June, followed by July, with August potentially being the driest summer month WRT normal.

To sum up: A near to slightly above normal temp summer with wetter than normal conditions should prevail.

As far as 90F heat — I think we have heat wave opportunities in both June and July. Remember – even though June should be warmer compared to normal, July’s averages are naturally higher than June.

Verification

Monthlies:

June – F

July – F

August – C

Monthly score: D-

Overall temp score: C-

Precip score: B

Final grade: C-