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El Nino Southern Oscillation (ENSO) Forecast

Issued: 8/16/2006
Updated as Warranted

See ENSO page for Links and Current Data
ENSO Archives

Neutral to Weak El Nino Conditions In Effect
Slight Warming Pattern Taking Hold

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.

Signs of a weak El Nino are present, with momentum trying to build in that direction but not there yet. The trend last fall/winter (2005/2006) was neutral with tendencies towards La Nina, but that was an anomaly looking over the longer arch of the past few years which were characterized by the persistence of subtle El Nino-like symptoms slowly but steadily taking their toll on the atmosphere and building momentum. This pattern started during the winter of 2002/2003 and again in the winter of 2003/2004 with a generally neutral trend in-place punctuated by occasional signals of a developing El Nino. The trend continued through 2004 with the strength and frequency of the El Nino-like periods increasing, enough to pass over the threshold to be considered a minimal but official El Nino by late August. In early 2005, things started to settle back down, but with exception as a major eruption of the Madden Julian Oscillation (MJO) occurred the first week of February, spawning a 40 day run of a very negative Southern Oscillation Index, the strongest for that time of year since the big El Nino of 97/98. A significant increase in Northern Hemisphere storm activity ensued producing a string of significant class swells and producing a strong eastward moving Kelvin Wave. The MJO and SOI settled back down only to again erupt mid-April 2005 with another negative run of the SOI supported by a renewed active phase of the MJO, and the Northern Hemisphere storm track tried to respond. But by June and again in September 2005 two runs of strongly positive Southern Oscillation Index values were experienced that pretty much squashed any hope for a developing El Nino, with a weak La Nina pattern setting up and holding through the winter into the spring of 2006. That trend changed in May with signs of El Nino starting to materialize again.

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 creeped 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 have continued that way to present. A rather deep spurt was noticed early August with daily values down to the -50 to -60 range. The 30 day running average right now is -15, neither extraordinarily low but certainly not high, likely on the border of what would be considered mild El Nino. Occasional downward bursts in the SOI are attributable to the active phase of the Madden Julian Oscillation, which can be traced to periods of enhanced storm activity in regions where the MJO passes as well as fueling the eventual development of full blown El Nino events (more details below).

Looking at current seasonally adjusted equatorial Pacific Sea Surface Temperatures (SST), the affects of the past few months of negative SOI values are apparent. Consistently cooler than average water temperatures cover the Western Pacific where normally warm water resides. A steady stream of water about 0.5° warmer than normal stretches from the west to the east over the equator with tiny pockets up to 1.0° above normal. These waters have migrated east over the equator, and are now pooled up off Ecuador, Central America, and up the Mexican coast into Central California and south into Peru. These waters are not exceptionally warm as one would see during a real El Nino, but the signature pattern is unmistakable. The South Pacific is near normal, but up north a rather warm patch is sitting south of the Aleutian Islands near the dateline, but is unrelated to ENSO. Latent heat energy remains present over the entirety of the North Atlantic as it has for several years, but not as strong over the Cape Verde storm corridor (from West Africa due west into the Caribbean Sea), perhaps signaling unfavorable changes in surface winds and a weaker than forecast tropical storm season there.

Wind anomaly analysis indicates that near normal winds have been occurring over the entire length of the equator, blowing east to west, with occasional weak to moderate bursts of reversed trades in the far Western Pacific (west of the dateline). But none of these have been strong enough or long enough in duration to qualify as a certified Westerly Wind Bust (WWB), a hallmark of the active phase of the MJO and a precursor to El Nino. Most are associated with the temporary passage of tropical storms pushing towards the Orient. Still the fact that warm waters are accumulating off Columbia and Peru suggest a weaker than normal offshore gradient normally present off Northern Peru. Historically if either El Nino or a strong burst of the Madden-Julian Oscillation was in effect, winds would be averaging from 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. Since the trades are not stronger than normal nor have they reversed direction for any length of time, the wind pattern is near neutral. But the warmer than normal waters occurring in the East Pacific remain an indicator of a developing pattern towards El Nino.

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'. Satellite analysis indicates a moderate pocket of 'above' normal height running along the equator from 180W to the Galapagos Islands but not touching the South/Central American coast, at 5-8 cm above normal, roughly paralleling the wind anomalies and positive sea temperatures occurring in the same area. This is not remarkable, but it is a far cry from being negative or indicative of La Nina or to the 'below' normal conditions that were experienced during the fall/winter of 2005/2006.

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) migrate towards the eastern Pacific, the angle flattens and becomes more consistent across the equatorial Pacific. Latest data suggests that a moderate pocket of warm water is transposed to the East, pulled away from the tropical West Pacific and currently sitting under the equator between the dateline and the Galapagos Islands, much as is indicated by the satellite reports above. Theses waters have not reached the Central America Coast and are not expected to unless some forcing event occurs to provide momentum pushing them east. A significant 'shelf' is noticeable at about 120W where the eastern edge of these waters stop with a steep thermocline angle present there into the Central America coast. This is consistent with the push towards El Nino, but we're not there yet. Much warmer waters need to be reaching the Americas first.

The forcing event required would be a series of Kelvin Waves. 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 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 no indication of any solid Kelvin Wave activity, but a consistent channel of water 1° warmer than normal subsurface waters is present extending from the dateline to 120W (near the Galapagos Islands) with 2 imbedded pockets up to 2° above normal. There have been occasional short spurts of westerly winds over the West Pacific, and it is assumed these mini-Kelvin waves originate from them. But they are not particularly impressive, and fall into the statistically normal range for such activity.

Reviewing all the data, there emerges a picture consistent with a neutral pattern pushing towards mild El Nino, but no 'smoking gun' or forcing event appears imminent to push it over the edge. The SOI is negative in bursts, but not markedly and consistently so, with warmer water pushing to the Mid-Pacific but not making rapid progress. There is no significant or consistent Kelvin Wave activity and nothing appears imminent to start that machine in motion. So these occasional signals of El Nino fall within the statistical boundaries consistent with normal conditions. This suggests neutral conditions biased slightly towards El Nino should continue through the Fall/Winter of 2006/2007. Historically if some major change towards El Nino were to occur in the coming months, significant evidence leading up to it's peak would already be accumulating, which it is not.

Always of interest and a good leading indicator is the relative activity level of the Atlantic hurricane season. This past few years activity has been record breaking. For 2006 Dr. William Gray and the team at the University of Colorado have again predicted an active season, but with the latest update (8/3/06) have revised their forecast downward slightly to 15 storm and 7 hurricanes, 3 of which are to be intense. Given the current state of the Tropical Pacific biased towards weak El Nino, the relative cooling of tropical Atlantic waters, the relative lack of anything occurring in the Atlantic and what has formed is on the periphery of the Cape Verde Storm Corridor, and being less than a month away from the historical peak of activity, we suspect their forecast still might be on the high side.

The latest El Nino discussion from the Climate Prediction Center/NCEP (August 10, 2006) suggests that ENSO-neutral conditions to continue over the next 1-3 months with a 50% chance of weak El Nino conditions by year end. This seems reasonable given the current state of the environment.

Of 12 ENSO models run in Aug 2006, 10 indicate neutral conditions by year end and 2 suggest mild El Nino.

LONG-RANGE NORTH PACIFIC STORM AND SWELL GENERATION POTENTIAL FORECAST

Fall/Winter 2006-2007 Swell Generation Potential (for California & Hawaii) = 6.0

There is no data supporting development of either a significant El Nino event or degradation into a La Nina event over the next 6 months. But a steady state seems likely with a slight bias towards El Nino but not growing in the direction strongly. 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. Current data suggests little if any enhancing effect expected.

So for now, plan on 'normal' Pacific Fall/Winter periods of enhanced storm cycles supported by the active phase of the Madden Julian Oscillation followed by periods of relative inactivity.

(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) tucked along Central America, Mexico up into California and southward to Peru. These waters also sit over the equator tracking from the dateline eastward into the East Pacific, typical of weak El Nino conditions. This is created by surface winds blowing west to east over the equator,reverse of the normal trade wind flow there.

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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. One exception, the wind is blowing west to east in the far West Pacific, a hallmark of a Westerly Wind Burst and a precursor to El Nino. Notice in the lower image that slightly anomalously warmer waters are tracking from the West Pacific towards South America caused by these same winds that were blowing tot he east over the West Pacific over the past several months. As a result water temperatures are 0.5-1.0 degree C above normal.

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Sea Surface Height Deviation
Courtesy: NLOM
Consistent with the image above, notice that sea surface heights are slightly above normal right over the equator east of the dateline pushing towards South Americas. 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 El Nino
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(Lower Image) Notice 2 pockets of anomalously warm water pushing east. Each pocket is a weak Kelvin Wave, consistent with weak El Nino
.

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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 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 the recent spikes of downward pressure in June and again August, typical of a weak El Nino.

 

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