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El Nino Southern Oscillation (ENSO) Forecast

Issued: 11/29/2006
Updated as Warranted

See ENSO page for Links and Current Data
ENSO Archives

Moderate El Nino In-Control
Waters Warming - Atmosphere Yet to Respond

Note: We apologize for the long time between updates to this section of the site. There really hasn't been much occurring with regards to ENSO until now so we've focused on coding new content.

Starting in May the first signs of a potential El Nino materialized, then proceeded to build and fade cyclically through our last update mid-August. Since then a major building cycle occurred in the month of October, but has again relented. All this is tied to the cyclical active and inactive phases of the Madden Julian Oscillation (MJO) and has produced a net buildup of warm waters over the equatorial regions of the Eastern Pacific typical of an El Nino event. But the anticipated swell enhancing effects on the atmosphere above have yet to become manifest, though certainly expected over the coming month or so. The paragraphs below describe the evidence that leads to an El Nino conclusion, followed by an assessment on it's impact on surf in the future.

First we look at the Southern Oscillation Index (SOI). This number compares surface pressure over Darwin Australia with pressure over Tahiti. If this value is negative that indicates average surface pressure is lower over Tahiti and higher over Darwin, symptomatic of El Nino (wind flows from high pressure towards lower pressure). When it's positive, the reverse it true with higher pressure over Tahiti and lower pressure over Darwin, typical of La Nina. The first 5 months of 2006 recorded a string of nearly consistent daily positive values. But starting May negative values crept into the record with occasional 8-10 day runs of reasonably deep negative values (-20 or so). By late June almost all the daily values were negative and continued that way into August with a rather deep spurt noticed early August with daily values down to the -50 to -60 range. Then things relaxed late August through September only to dig deep again starting October 1 and held through the end of the month when the 30 day running average dropped to -15, the same as it did the end of August. These values are not extraordinarily low but certainly not normal, indicative of moderate El Nino conditions. November has been a mixed month with some negative and some positive values, all netting out about near 0 or neutral. The downward bursts in the SOI are attributable to the active phase of the Madden Julian Oscillation, which are the building blocks of El Nino and can be traced to periods of enhanced storm activity in regions where the MJO passes (more details below). Prolonged neutral and positive values are symptomatic of the inactive phase of the MJO.

Looking at current seasonally adjusted equatorial Pacific Sea Surface Temperatures (SST), the affects of the past 5 months of overall negative SOI values are apparent. A well defined tongue of warmer than normal water is strung along the equatorial Pacific starting on the dateline and building as it approaches the coast of Ecuador. Temperatures are up to 2° C above normal, which is respectable and worthy of notice. The strongest pocket of these waters is over the intersection of the equator and the dateline with lesser temps tracking towards the South American coast but averaging 1.5° above normal. This is a marked improvement from even 3 months earlier when a classically well defined warm pool was not really in-place yet, but is now. During El Nino events warm waters from the far West Pacific start migrating east along the equator in response to a shift in the SOI and are blown there by a reversal of trade winds (more to follow). The South Pacific is near normal other than a small path of warmer than normal water east of new Zealand, and up north normal waters temps are occurring over the bulk of the North Pacific. Latent heat energy remains present over the entirety of the North Atlantic as it has for several years, including the Cape Verde storm corridor (from West Africa due west into the Caribbean Sea), but did not seem to play a role in the late season tropical storm season for that region.

Wind anomaly analysis indicates near normal winds currently occurring over the entire length of the equator, but that was not the case until recent. An extended reversal of trades started in early October consistent with the fall in the SOI as the active phase of the MJO took hold, blowing west to east on the equator over a multiple week duration in October strong enough to qualify as a certified Westerly Wind Bust (WWB), a hallmark of the active phase of the MJO and a precursor to El Nino. The warmer than normal waters currently running over the equator in the East Pacific and into Ecuador are a direct testament to the October WWB event. Historically if either El Nino or a strong burst of the Madden-Julian Oscillation is in effect, trade winds that normally blow from east to west reverse themselves and blow west to east. That is, when the MJO is in an active phase, the trades reverse themselves in the West Pacific, and when the MJO is not active, trades return. During La Nina, trades blow much stronger than normal. We are currently in an inactive phase of the MJO with normal trades in-play. But the warmer than normal waters occurring in the East Pacific remain as solid evidence of the active phase of the MJO from just a month ago and have pushed up the intensity of the El Nino pattern.

Another indicator of El Nino or La Nina is a change in sea surface height. Sea surface height is the height of the oceans surface relative to 'average'. No current data is currently available.

Another key indicator in the evolution of either an El Nino or La Nina event is the depth and profile of the 20 degree isotherm (thermocline). During La Nina events, warm subsurface water remains pooled up in the far West Pacific near the equator. Cold surface and subsurface waters dominate the East Pacific, resulting in a steep angle from east to west, going from shallow in the east to deep in the west. In El Nino events, as warm subsurface water (i.e. Kelvin waves - more below) migrate towards the eastern Pacific, the angle flattens and becomes more consistent across the equatorial Pacific. Latest data indicates a solid pocket of warmer water is being transposed to the East, pulled away from the tropical West Pacific and currently sitting under the equator due south of California. These waters are 5° C above normal and are tracking towards the South American coast, and direct product of the Westerly Wind bursts of October. Once they hit there, a major warm upwhelling is expected and surface water temperatures will likely head up. Compared to a year ago there's a significant difference in the incursion of warm water eastward into the South American coast. This is consistent with the development of a moderate El Nino approaching the mature phase.

A Kelvin Wave is a pocket of warm water that travels under the oceans surface from west to east at a depth of about 150-200 meters. It is generated by a burst of strong westerly winds blowing over the equator (a.k.a. Westerly Wind Burst (WWB) in the West Pacific associated with the MJO. As the warm surface water gains eastward momentum, it sinks near the dateline and travels well under the oceans surface, only to reappear at the surface when it impacts the South America Coast. This results in the sudden appearance of warm waters along the coast of Peru and Ecuador. Occasional eruptions are normal. Large and consistent eruptions are the hallmark of solid El Nino events. The source of Kelvin Waves, a negative SOI and reversed trades is directly related to the strength and frequency of the Madden Julian Oscillation (MJO). This weather pattern is responsible for the periodic strengthening of the anomalous westerly winds in the West Pacific which drive production of subsurface Kelvin waves, and also drive the SOI negative. When the MJO enters an active phase, El Nino indicators strengthen, and as it fades, so does El Nino. Currently there is a solid Kelvin Wave in flight positioned under the equator south of California en route to Ecuador as mention above. This is definitely the most impressive one so far from this El Nino event.

As El Nino events unfold at the oceans surface, precipitation will develop in the atmosphere above the warmer surface waters, since warmer water supports higher condensation rates above it. The presence of consistent precipitation where it historically shouldn't be is a hallmark of El Nino. Current satellite data indicates minor increases in precipitation just west of the dateline but nothing else of interest in the greater Pacific. Of note, major increases in precipitation are developing in the Western Indian Ocean, possibly consistent with the start of a new active phase of the MJO. (The MJO travels against the grain of the equator from west to east developing initially in the Indian Ocean).

Reviewing all the data, it is clear that an El Nino event is occurring. The SOI has been negative in bursts over the past 6 months and has not ventured at all up into the positive range for more than a few days at a time. Warmer water is in place over the entire Eastern Equatorial Pacific with a significant Kelvin Wave poised to impact the South American coast in the next few weeks. And it is likely another active Phase of the MJO is starting to develop in the Indian Ocean. This suggests a moderate El Nino is occurring and should continue through the Winter/Spring of 2006/2007. What is interesting is that this El Nino event started to develop later in the year than when most historically form. It is unclear whether this means the whole cycle will be offset to the right a few months, or whether it will just result in a shorter overall lifespan for this event. That is yet to be known.

Always of interest is the relative activity level of the Atlantic hurricane season. This past few years activity was record breaking. But contrary to the forecast produced for 2006 by Dr. William Gray and the team at the University of Colorado, 2006 turned out to be a complete dud. The forecast was for an active season in August, with 15 storms and 7 hurricanes, 3 of which are to be intense, then revised downward to 11, 6 and 2 in Oct with 9,5 and 2 actually occurring. The team gave much credit to the downward hurricane activity to the fast onset of El Nino. In fact, they indicated the onset of this El Nino was perhaps the fastest ever recorded. To paraphrase: "The warming of the eastern and central Pacific during August through October 2006 has been truly remarkable. Only (the record breaking event of) 1997 witnessed a larger temperature increase in Nino 3 anomalies from June-July to August-September than did the 2006 season. But, in 1997, June-July Nino 3 anomalies (2.1ºC) were already well above average while 2006 June-July anomalies (0.1ºC) were not. This was by far the largest percentage warming of SST anomalies between June-July and August-September in the tropical Pacific for a year that had El Niño conditions in August-September." In short, the rapid onset of El Nino and the ENSO models inability to forecast it's coming caused the forecast to be skewed to the high side. El Nino produces strong shearing winds over the Atlantic that tear the tops off developing tropical storms rendering them weak and ineffective, which was clearly the case this year. But this causes us some concern (see below).

The latest El Nino discussion from the Climate Prediction Center/NCEP (November 9, 2006) states that El Nino conditions to continue into the spring of 2007. This seems reasonable given the current state of the environment.

Of 18 ENSO models run in Nov 2006, all indicated warm conditions through year end. Compare that with 12 models run in Sept where 10 indicated neutral conditions by year end and 2 suggested mild El Nino. Clearly there is some work that needs to be done with the long range ENSO models.

LONG-RANGE NORTH PACIFIC STORM AND SWELL GENERATION POTENTIAL FORECAST

Winter/Spring 2006-2007 Swell Generation Potential (for California & Hawaii) = 7.0

There is no data supporting a degradation into a La Nina event over the next 6 months and lot's of data supporting the continued development of El Nino. From a swell generating perspective this is good news. The issue is that the atmosphere above the Pacific Ocean as of now has shown absolutely no reaction to the development of El Nino, even with the occurrence of 2 solid active Phases of the MJO . Dr Grays team is attributing the bust of a hurricane season in the Atlantic to the evolution of El Nino. If this were true, one would think the usual Pacific Basin atmospheric enhancements normally attributable to El Nino would also be in evidence. But instead net Pacific storm activity has been well below normal with not one significant class storm occurring so far this season. We suspect that El Nino might have had the impact on the Atlantic summer tropical climate as indicated by Dr Gray and team, mainly because the tropical Atlantic is the first stop in the eastbound global equatorial conveyor belt, but those anomalies have not been established long enough to circumnavigate the entirety of the globe to start feeding the Indian Ocean area moving eastward into the Pacific. That said, our current working theory is that it take 6 months from the time the first solid El Nino indices become established in the Pacific before the usual upper anomalies manifest themselves and start influencing Western Pacific upper level weather.

El Nino typically enhances the size, strength and frequency of winter North Pacific storms in and around the Gulf of Alaska, thereby improving the likelihood for large winter surf in California and Hawaii. And our theory is mainly attributable to the late start of El Nino. Our current thinking is that due to the late start of El Nino the atmosphere is still reconfiguring itself and has yet to get all the pieces in place to support winter storm development in earnest. Net tropical activity in the Pacific was a little better this year, but nothing more than average. And the fall storm season has been a total bust. But there's no denying El Nino is in-play, and it will have it's impact on the atmosphere in time. We suspect the season will begin late and last longer than normal, with significant storms likely occurring well into the Spring. We'll be watching for the next active phase of the MJO to see whether it has a positive impact on surface level storm development. The big active phases of the MJO in August and October produced nothing of interest, with a major split jetstream flow in control of the entire North Pacific. But that appears to be waning and a good indicator will be to monitor what impact the next developing active phase of the MJO has on net storm activity in the North Pacific.

As such we've actually increased the Storm/Swell Potential Rating from 6 to 7 for the Winter/Spring season, betting on El Nino's influence having a moderate but late coming impact.

(This forecast is highly speculative and based on historical analysis of past La Nina/El Nino events and the latest long-range forecast models)

Sea Surface Temperature Anomalies
Courtesy: NOAA NESDIS
Notice a broad area of warmer than normal water (orange/red) extending over the equator from the dateline east into South America typical of moderate El Nino conditions. This is created by surface winds blowing west to east over the equator, a reversal of the normal trade wind flow there (east to west).

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Sea Surface Temperature Anomalies and Average Surface Winds on the Equatorial Pacific
Courtesy: NOAA PMEL
In the top image notice winds blowing from east to west over the entire Tropical Pacific Ocean, the standard trade wind pattern for this time of year. Data during October indicated a semi permanent area of winds blowing west to east (reverse of normal) when the MJO was in it's active phase. . Notice the results of that MJO pulse in the lower image where significantly anomalous warmer waters are tracking from the West Pacific towards South America caused by these same winds in October that were blowing to the east over the West Pacific. As a result water temperatures are 1.5-2.0 degrees C above normal.

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Sea Surface Height Deviation
Courtesy: NLOM
This is an old image before the MJO pulse of October got established. Notice that sea surface heights are slightly above normal right over the equator pushing into South America but not overly so. If we had a more recent image (none available due to the outage of the Jason-1 satellite) it is likely seas surface heights would be far higher. Warmer waters raise the oceans surface slightly.

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20 Degree Thermocline Depth and Position Time Series
Courtesy: CPC NCEP NOAA
(Top Image) The core of warm subsurface water is centered slightly east of normal in the West Pacific and are pushing east further than normal, decreasing the angle between the West and East. This is consistent with a mild to moderate El Nino
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(Lower Image) Notice the very pronounced pocket of anomalously warm water (5 deg C above normal) pushing east from 120W. This is a strong Kelvin Wave produced from the Westerly Wind Burst and active phase of the MJO in October
.

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Equatorial Pacific Sea Surface Temperature Forecast
Courtesy: NOAA/NCEP
Notice that the average of many separate runs of the NCEP model suggest a continuation of warmer waters in the tropical East Pacific well into Spring of 2007.

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Southern Oscillation Index (SOI)
Courtesy: BOM
The SOI depicts the difference in pressure between Tahiti and Darwin Australia. When it is consistently negative (that is surface pressure is lower in Tahiti than Darwin Aust), the trend is then towards El Nino. And when it is positive the trend is towards La Nina. Notice that since June it has been in the negative range with small cycles within that timespan up to current with the dips being the active phase of the MJO. The current inactive phase of the MJO is evidenced by the rising pressure regime at the very tail end of the image (but that is likely the head back down in about 3 weeks with a new pulse of the MJO expected),

 

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