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El Nino Southern Oscillation (ENSO) Forecast

Issued: 9/26/2010
Updated as Warranted

See ENSO page for Links and Current Data
ENSO Archives

Moderate La Nina In-Control
Cold Pattern in-Play Over Entire Equatorial Pacific

Overview: The first few months of 2010 saw a moderate El Nino controlling the equatorial Pacific with a steady flurry of Active MJO Phases reinforcing  westerly wind anomalies at the oceans surface, a warm subsurface flow of water pushing from the mid-Pacific east into Central America, and a solid return flow of warmer than normal surface waters propagating west from there back to the dateline.  This pattern was fueling an active jetstream pattern aloft which in-turn was serving to enhance the development of winter low pressure systems tracking from Japan east towards the US West Coast , pushing the Southern Oscillation Index well into negative territory with those storms pushing further south than normal resulting in a preponderance of westerly swells pushing into the California. 26 significant class swells were documented. Then in mid-March right when the pattern was reaching it's peak, the Pacific equatorial current, which had been flowing west to east, suddenly changed direction and started flowing east to west. Within days the SOI started rising, Active Phases of the MJO ceased and the storm track rapidly broke down. Quite unexpectedly in April the winter surf season was over and a steadily  rising SOI suggested that La Nina was taking control. 

As we went through the later Spring and Summer of 2010, a complete lack of Active MJO phases continued, with the Inactive Phase taking precedence.   The SOI continue to climb and trades over the equator continued solid, though not any stronger than one would expect.  Regardless the tell-tail signal of La Nina developed on the equator in the form of cooler than normal surface waters temperatures building first off the coast of Ecuador and then migrating east under Hawaii and eventually reaching the dateline, at first just slightly below normal but steadily gaining traction with temperature dipping many degrees below normal as of this date. While all this was occurring, momentum in the upper atmosphere from the previous winters El Nino continued to hold some form of control through the spring, with precipitation well above normal over the Southwest US and up to 200% of annual snowfall reported in the Central Sierras (Tahoe area).  And even into early Summer a respectable number of Southern hemisphere storms occurred though originating more in the Southeast Pacific than Southwest, focused swell on South America and up into California. 

It is with that background that we look to the coming Fall and Winter of 2010/2011 with below normal expectations for surf for Hawaii, Canada and the US West Coast.  The paragraphs below describe the state of various indicators used to assess long-term global weather conditions (be it either El Nino or La Nina), followed by a revised set of criteria for assessing it's impact on surf generation potential for the future.

MJO:  The Madden Julian Oscillation (MJO) is a pattern of wind and weather anomalies that run along and over the equator circumnavigating the globe  from west to east in roughly 40-60 day cycles, roughly 20-30 days of the Active Phase followed by 20-30 days of the Inactive Phase at any one point on it's path.  The cycles alternate continuously year after year and are stronger during the northern hemisphere Fall, Winter and Spring season and lessen during the summer.  The classic pulse of the Active Phase of the MJO results in a slackening of trade winds and an increase in rainfall over the area it is present. The Active Phase is directly followed by the Inactive Phase which manifests itself though increased trades winds and reduced rainfall. The Active Phase of the MJO has been implicated in fueling the development of Northern Hemisphere storms during winter months and tropical storms in the Fall over the Pacific , while the Inactive Phase has been implicated in fueling the formation of tropical storm in the Atlantic during summer months.  When the strength of Active Phases of MJO dominate, El Nino indicators strengthen. When the Inactive Phase dominate, La Nina moves to the forefront. That is not to say that the MJO causes either El Nino or La Nina, only that it appear to support the development of either extreme of the ENSO cycle. In other words, when the balance of energy favors the Active Phase, the odds of El Nino developing increases.  When the balance shifts towards the Inactive Phase, La Nina becomes more likely.  More details of the MJO as it relates to the current forecast is presented below.     

     

SOI: 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 for an extended period of time, that indicates average surface pressure is lower over Tahiti and higher over Darwin, symptomatic of El Nino (or over short durations, ~ 20 days, the Active Phase of the MJO). Positive values over longer times indicates La Nina (or the Inactive Phase of the MJO for shorter durations). The greater the negative or positive value over time, the stronger the ENSO (El Nino Southern Oscillation) configuration (be it El Nino or La Nina). During El Nino episodes wind flows from generally high pressure over Darwin towards generally lower pressure over Tahiti, which is a reverse of what a normal state is. When it's positive, the reverse it true, with higher pressure over Tahiti and lower pressure over Darwin, typical of La Nina, with wind flowing east to west, typical of the trade wind pattern for this region, only more so. 

On March 1 2010 the 30 day average for the SOI started moving meteorically into positive territory, starting at -25 and reaching +15 by early May and then heading up to +25 by mid-September.  That's a 50 point change in 7 months with only modest pauses along the way. This is typical of a moderate or strong La Nina. 


Anomalous Sea Surface Temperature: Looking at current seasonally adjusted equatorial Pacific Sea Surface Temperatures (SST), the pattern clearly delineates La Nina. A well defined stream of cooler than normal water extends over the width of the equatorial Pacific starting just off Ecuador and  continuing west over the equator to the dateline and almost to New Guinea.  Temperatures were dipping 1.5-2.0 degrees below normal over that expanse  with perhaps pockets a little cooler. This is the classic signal of La Nina and is the exact opposite of what was occurring even in the early Spring of this year when warmer than normal water was over this exact same stretch of real-estate. Also feeder bands of cooler than normal water, the likely result of upwelling, were occurring, streaming from off the entire US West Coast heading southwest and joining the main flow south of Hawaii to the dateline and another flow pushing from off Chile and Peru tracking northwest to the same junction just east of the dateline.  This was generating the hallmark horseshoe pattern of colder than normal water over the Pacific Ocean driven by stronger than usual northeast winds pushing off the US west Coast and southeast winds tracking off South America. These winds are presumably being driving by stronger than normal high pressure in both the north and southern hemispheres of the Pacific which in turn cause the upwelling and colder than normal water temps. 

Interesting, but in the Atlantic during the previous years El Nino event there was a clearly defined flow of cooler than normal water pulsing off equatorial Africa in the equatorial Atlantic, the exact opposite of what is occurring in the Pacific. Looking back in the historical record a similar pattern occurred during the record breaking El Nino of 1997/98.  And during lesser El Nino years a similar but weaker pattern is evidenced, but not during La Nina years. Now that we're presumably in La Nina, the exact opposite is occurring, namely a massive buildup of warmer than normal water over the equatorial Atlantic and reaching well up into northern Atlantic waters.  This is presumably feeding tropical storm production there, through momentum in the upper atmosphere is still biased towards an El Nino circulation, generating some shear and is keeping a cap on hurricane production.  But with the likelihood of La Nina holding through the winter into summer of 2011, the jetstream will be fully reconfigured in favor of La Nina by then and upper level shear will be drastically reduced, allowing the hurricane season to proceed without restriction. 

During El Nino it is theorized that the Walker Circulation forms over the Pacific warm pool near Ecuador, drawing in surface air from Africa across the Atlantic causing upwelling. This flow of cooler than normal water hinders tropical development in the Cape Verde Storm Corridor. A reverse teleconnection between the Pacific and the Atlantic is known to exist. That is, when the Pacific Ocean is active the Atlantic is inactive from a storm perspective, and visa versa.  And now that were in the beginning of a La Nina phase, it seems the Walker circulations core moves west leaving the Atlantic unaffected and allowing the buildup of warmer than normal waters there. 

Regardless, expect the Atlantic storm season to remain more active than normal in 2010 and even more so in 2011 as La Nina matures. 

Wind Anomalies: Trade winds over the equatorial Pacific typically blow east to west, towards the Philippines and New Guinea, stronger during the spring and summer and less so in the Fall and Winter as low pressure starts building in the upper latitudes with cold front sweeping south towards the equator.     Wind anomaly analysis for Spring and Summer 2010 indicate that on average trades over the equatorial Pacific have been blowing at about a normal velocity, though there have been occasional spurts of slightly stronger than normal trades, through nothing inordinate.  This is in sharp contrast to last year (2009) when El Nino was getting it's footing, where clearly defined incidences of slack trades if not full-on westerly winds were evidenced in the Spring and Fall months.  Rather than blowing east to west they were blowing west to east, or other times blowing less strong than normal, which has the same effect as a reversal of trades in that it supports the generation of a Kelvin Wave (more below). When trades are suppressed, this reduces the amount of upwelling along the coast of South America, allowing the collection of warmer water there (El Nino).  Conversely stronger than average trades there increase upwelling producing cooler than average waters (La Nina). Interesting that though trades have not been blowing noticeably harder than normal, there is clear evidence of upwelling (colder waters over the equator and flowing off the US and South America). Though the velocity may not be harder, we suspect the duration of those trades has been relentless since early Spring, resulting in the cold water pattern discussed above.  It is that 'duration' that would be a trademark of stronger than normal high pressure in both hemispheres of the Pacific. That is, high pressure tends to produce not strong winds, but steady winds.  Conversly low pressure produces gusty-uneven winds.  To generate massive upwelling, one would required the presence of solid unrelenting high pressure. That appears to be the case for the Pacific.  

From an educational perspective, instances of reversed trades/westerly winds that exceed the seasonal norm are called Westerly Wind Bursts (WWB).  These have been implicated in not only transporting warm water east, but also help to fuel tropical development the the Central and West Pacific. Of note: The MJO still has Inactive Phases even during El Nino years, just not as prolonged or strong. 

A Westerly Wind Burst (WWB) is an stronger than normal extended duration of wind that blows from west to east along the equator in the West and or Central Pacific contrary to normal trade winds, forcing warm surface waters to start moving in the same direction as the wind (details below), a hallmark of a strong Active Phase of the MJO and a precursor to El Nino. 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. Clearly we are in a La Nina regime with absolutely no evidence of trade reversal or WWB's since March of 2010.  

Pacific Isotherm: 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 equatorial West Pacific while cold surface and subsurface waters dominate the East Pacific, resulting in a steep angle from east to west, or from a shallow pool of warm waters in the east to a deeper pool of warm water in the west. In El Nino events, warm subsurface water (i.e. Kelvin waves - more below) migrate from the West Pacific to the East and the angle flattens with the depth of warm water becoming more uniform across the width of the equatorial Pacific. Latest data indicates a small pocket of warmer than normal water in the far West Pacific down at 100 meters (2 deg C above normal) and holding steady. And a huge pool of cooler than normal water (-7 deg C) is present south of Hawaii at a depth of 100 meters.  And this cooler water has been locked in-place since near May, only getting cooler while growing in size and not moving. This is in stark contrast to the wave-after-wave pattern of warmer than normal water that tracked east through this region a year ago during El Nino.  

During El Nino events though the MJO signal is weak, it appears that the Active Phases of the MJO are more productive than the Inactive ones.  That is  warm water that successfully makes the multi-thousand mile subsurface journey east to Ecuador erupts to the surface just off the coast there, and weaker than usual trades then blow the warm surface water off to the east, also radiating north up the Mexican coast reaching into California during significant El Nino events. This results in a pool of warm water forming off Central America rather than it's usual place in the West Pacific, flattening the angle of the 20 degree C isotherm across the equatorial Pacific. At this time a La Nina pattern is clearly in control with cool water dominating in the east and no sign of warmer than usual waters pushing east towards Central America to flatten the isotherm angle. 

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 and is typically associated with the Active Phase of the MJO. As the warm surface water gains eastward momentum by forcing of the WWB, 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 Active Phase 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. 

Currently there is no data to indicate that any Kelvin Wave activity has occurred since about Feb of 2010 and if anything, even if one were to try and form, it's eastward progress would be blocked by the massive pocket of cooler than normal water south of Hawaii on the equator.

Pacific Equatorial Counter-Current: There are three currents that run along the Pacific equator. Two run just abreast (a few degrees north or south) of the equator flowing east to west driven by the semi-permanent high pressure centers located in the center of the North and South Pacific.  These high pressure system are what drives equatorial trade winds too. But sandwiched between them is the Pacific Equatorial Counter Current, which flows against the two, running west to east.  Satellite based sensors are used to measure the strength/speed of the counter current. That data can be analyzed to determine if the current is flowing stronger or weaker than in years previous.  In essence,  anomalies in the current can be detected.  A curious fact becomes apparent when looking at long-term trends:  During El Nino years the counter current runs harder than usual to the east, and during La Nina years it runs harder to the west. This is what one would expect, especially since the exact same pattern appears when analyzing tradewinds. 

Looking at recent data the current is running somewhat stronger than normal towards the west, a signal of La Nina. A month previous it was raging to the west and had actually been doing that since April.  Interesting, but starting about March 10-15 (2010) the current changed direction, from flowing anomalously west to east switching to east to west, and has not stopped since. Looking back one year to the summer of 2009, the current was flowing steadily west to east and biased in favor of El Nino. Then in March of 2010, it suddenly reversed direction, the SOI started it's race to positive territory, and the storm machine shut down in the North Pacific like someone flipped a switch.  And there has been no change since. 

Looking at the time of year the current changed for El Ninos in the past (the record starts in '93), most started at least 3-4 months into the year. In these previous events (including the record year of '97) it was not really till the Fall months that the current bloomed to it's full potential.  And interestingly most had some sort of a pause or significant decline in the flow in the July/August timeframe, before the final push into Fall (except 2006).  In many cases, the current changed before the SOI dropped for the first time that year into it's first run of  sustained negative numbers (i.e. the first Active Phase of the year): 2002 current change was Feb 1/Negative SOI starting mid-March, 2006 current change mid-Feb/SOI early May, 09 current mid-Jan/SOI negative late April/though dropping since Feb. 1994 was excluded from the review because it appeared that a multi-year ENSO event was occurring, carried over from '92/93.   This is relatively new data and the record is very short, so no solid conclusions can be drawn, but it provides some foresight for future research. We have not researched when the current changes during La Nina years, but will look into that shortly. But we suspect the mid-Spring timeframe is the determining window, regardless of El Nino or La Nina.         

OLR: When El Nino events unfold at the oceans surface, increased cloudiness/precipitation will develop in the atmosphere above the warmer surface waters, since warmer water supports higher condensation rates above it. The presence of consistent cloudiness or precipitation where it historically shouldn't be is a hallmark of El Nino. Satellite based Outgoing Longwave Radiation (OLR) measurements track reflectivity from clouds over time.  The greater the reflectivity, the less sunlight is being absorbed by clouds. Low reflectivity values represent greater cloud absorption. Current satellite data indicates an almost normal OLR pattern for this time of year.  But since this La Nina is just starting to get organized, and the effects will be more obvious in the winter, we suspect this is more of a lagging indicator that an early indicator.  

Analysis: Reviewing all the data over the past several months, the evidence clearly suggest some form of moderate or stronger La Nina event is developing and that it is already pretty well evolved. It started in the Spring of 2010 with the Pacific equatorial counter current changing direction, followed by a meteoric rise in the SOI with a quick decay of the storm pattern and the Active Phase of the MJO which had been so dominant totally dying. Shortly thereafter a pool of cooler than normal water started to develop over the equator off Ecuador and started building to the west. Then a pocket of cooler than normal water started building 100 meters down under the equator south of Hawaii. In short, this pattern has continued to persist if not build for the past 6 months with virtually no evidence to suggest any sort of an El Nino pattern is to return anytime soon.  

Typically some form of La Nina always develops after a run of El Nino.  There have been multiyear El Nino events, typically associated with the Midoki variety. A Modoki El Nino (Modoki is Japanese for 'the same but different') forms more in the center of the equatorial Pacific (south of Hawaii), in the Nino 4 region, and evolves there, rarely making much eastward headway through it's life and not typically reaching the Ecuador coast. These El Nino events tend to be more weak-to-moderate in strength too, with lesser impact on the northern hemisphere Fall, Winter and Spring storm pattern. They still have an enhancing impact, just not as strong. In contrast the Classic flavor of El Nino starts forming it's warm pool directly adjacent to Ecuador and expands westward as it matures. The environmental impact tend to be more severe.  The most recent El Ninos event (of 2009/2010) was of the classic variety, only one of 5 that have occurred since 1980 (82/83, 87/88, 91/92 [debatable], 97/98 and now 09).  

Regardless, the strength of a La Nina event often is in direct proportion to the strength of the preceding El Nino.  That is, El Nino and La Nina work in a pair.  It's  almost as if the atmosphere, in trying to establish some form of equilibrium, compensates in a strong a fashion as the event which caused it to go out of equilibrium in the first place.  If one considers El Nino a warm anomaly, then La Nina is the cold anomaly that follows to set things 'right'. Most notable is the massive La Nina which followed the huge El Nino of '97/98.  In fact, that La Nina response was so strong it took till 2009 to mount a respectable El Nino event again (though the 2005/2006 El Nino was not too bad). Given the 'rubber band' postulation above, we are expecting a pretty firm multiyear La Nina response as the atmosphere tried to rebalanced after the 2009/2010 El Nino. 

Always of interest is the effects of El Nino and La Nina on the relative level of the Atlantic hurricane season activity. Hurricane activity in the Atlantic is inversely tied to the strength and duration of El Nino and La Nina in the Pacific (inverse teleconnection noted above). A classic El Nino produces strong shearing winds over the Atlantic (during the sumer it forms) that tear the tops off developing tropical storms, rendering them weak and ineffective at evacuating warm moist surface air up high into the atmosphere through the storms eye. In effect, a hurricane is the atmosphere's attempt to create equilibrium, or to restore balance to a system that is too warm, by creating a chimney to vent off the hot air to the upper atmosphere. So if an an inordinate number of hurricanes occur in the Atlantic, or if they are unusually strong, one could conclude that there is a build up of latent heat energy in the ocean and the shearing effects of El Nino are not in-play, which suggests at least a moderate La Nina might be in effect. 

Looking at the Atlantic hurricane activity to date (summer 2010), there has been an active pattern, though most storms have taken a track to the north before reaching the US Eastern Seaboard. A persistent low pressure trough has remained locked over the Great Lakes out to Maine and beyond helping to steer these system to the north and protecting US interests. Such a trough is a direct result of the lingering effects of the El Nino of 2009/2010. Though El Nino was effectively dead by June of 2010, we postulate that is takes at least 6-9 months after that time before it's effects on the upper reaches of the atmosphere fully dissolve.  That is, once An El Nino pattern starts to become established, it generates momentum that continues to live on even once the source of that a pattern is declared dead.   The stronger the event, the longer it's momentum affects the closed system it lives within. There has been limited data that suggest this lingering effect could last up to a year past the cycles demise. Regardless, we believe the momentum of the 2009/2010 El Nino will not be completely absorbed until perhaps late November of 2010. That should sufficiently set up enough upper level shear to protect the Eastern Seaboard of the US through the 2010 hurricane season. But with La Nina holding all through the 2010/2011 winter season, and assuming the same warm water pool is produced in the tropical Atlantic in the summer of 2011, there will be no upper level shear to stop advancing storms encroaching into US territory.  

Conversely, if this were a Modoki El Nino, the impact on tropical development in the Atlantic is theorized to be much reduced, if not actually enhancing the odds for development (we have our doubts about that, especially after reviewing historical data on number of storm days during those years). The presumed theory is that since the Modoki  El Nino forms in the Central Pacific, in-flow to the associated Walker circulation east of the core of the warm pool which normally would be over the Tropical Atlantic is shifted west, over South America. This results in in less shearing if not actually supporting a consistent east to west flow both at the surface and at upper levels. But again we have our doubts.  Also of note, the presence of a classic El Nino in the tropical Pacific supports the development of tropical storms both in the East Pacific (off Mexico) and during WWBs and the Active Phase of the MJO, in the far West Pacific.  There has been no evidence of that this year in the Pacific, with way below normal activity levels recorded.        

The latest El Nino discussion from the Climate Prediction Center/NCEP (September 9, 2010) states that La Nina is in effect and could evolve somewhere between a moderate to strong classification. 

Models: Looking at the MJO models, there is virtually no sign of any significant pulse of the Active Phase of the MJO scheduled for the next month. The Active Phase of the MJO has been virtually absent since April of 2010, and the Inactive Phase has dominated.  There is little hope than much of a change will develop either.  That said, normally during La Nina events there are big swings between the Inactive and Active Phases of the MJO, but with the Inactive Phase dominating the Active Phase has been all but absent. All this suggest is that a La Nina dominated weather pattern is likely to  persist. 

Even further out, of 23 ENSO dynamic and statistical models run in August 2010, all indicate some form of La Nina event persisting through the Winter of 2010/2011 with waters temperatures averaging 1.5 deg C below normal. Most predict this event to peak in Nov/Dec with a gradual slackening of effects into April 2011, though there is much spread in the details from one model to the next. 

LONG-RANGE NORTH PACIFIC STORM AND SWELL GENERATION POTENTIAL FORECAST

Fall/Winter 2010-20101Swell Generation Potential (for California & Hawaii) = 4.0
Rating based on a 1-10 scale: 1 being the lowest (small and infrequent winter surf conditions), 5 being normal/average, and 10 being extraordinary (frequent events of large long period swells)

Methodology (2010) : We have upgraded our methodology again for making long term predictions. In the past we looked solely upon the presence El Nino using the approach that El Nino typically enhances the size, strength, frequency and duration 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 that La Nina typically decrease the size strength, frequency and duration of such systems. After reviewing data from many such years, we still believe that to be true in an absolute sense over the course of an entire season, but also now recognize that the lingering effects of either a strong El Nino or La Nina event last far longer than previous suspected (up to 1 year). So in the Fall and early Winter season directly following a strong El Nino winter, even though La Nina may be in effect, the lingering effects of El Nino on the upper atmosphere could have an enhancing effect on net storm activity. We've are also turning towards the opinion that strong La Nina events can potentially have enhancing effects on net North Pacific storm formation during the Fall season. The net effect is the more extreme the divergence away from a neutral state, the greater the propensity for weather systems to try and return the system to a state of equilibrium in the form of storms (which create winds and therefore waves). In the El Nino state, the focus of the storm is centered more upon the dateline and the Western Gulf of Alaska and the storm track falls south and lasts longer into the winter season, where in a La Nina scenario the focus is more on the Eastern Gulf of Alaska with the track being displaced to the north and timed more on the Fall and early Winter. Therefore, as the swell source moves east, Hawaii becomes a less likely target.

Also the relative strength of MJO phases have an impact on the transport of tropical moisture from equatorial regions of the West Pacific northward to ultimately fall under the influence of the jetstream, increasing the probability for storm formation moving over the dateline and into the Gulf of Alaska. So MJO patterns are now considered in the forecast.

In addition we are become more convinced there is a teleconnection between storm activity in the southern hemisphere and that in the north 6 months later. That is, in years where the net storm activity is up in the southern hemisphere winter (summer in the N Hemi), a corresponding increase in activity could also be noticed in winter in the northern hemisphere (6 months later). The only exception is when there is a strong El Nino or La Nina up north in the winter, then that translates into a net increase in winter activity in the southern hemi 6 months later. Most of this focuses on the strength of the MJO, and seeing how the area it directly impacts is the equatorial Pacific which straddles both the north and south hemisphere's, it would seem reasonable to have an impacts at both poles.

And yet one more possible early indicator is the configuration of the jetstream over the North Pacific starting late July into August over the North Pacific. There is some evidence to suggests a healthy consolidated flow over the NPac early in the Fall season might lead to a continuation of that pattern through the Winter season, and that if an early season split pattern develops, it will continue in that mode through Winter and Spring. A split Northern hemi jetstream does nothing to support surface level gale development.

Forecast Conclusion: All data suggest that a moderate strength La Nina is already in-play and if anything, it will only get stronger as we move into the Northern Hemisphere winter. The presence of colder than normal waters over the balance of the equatorial Pacific, a strongly positive SOI, and massive pool of cold water below the equatorial Eastern Pacific and a complete lack of any Active Phases of the MJO for months now is just the start of the evidence. The buildup of warm water in the Atlantic and a reasonably active hurricane season there only adds to the conclusion. And there is no evidence to suggest that any reversal of this trend is at hand. Historically La Nina events continue developing through the Fall months, peaking out in December with effects on the environment holding well through Spring, and if strong enough, affecting Summer and Fall of the following year (or potentially longer if recent insights in to the effects of ENSO events over a global scale are correct. 

Down south, over the southern hemisphere net winter storm activity was slightly above normal though focused more on the Southeastern Pacific rather than the preferred window in the southwest.  Regardless, this was more a reflection of the lingering effects of the 2009/2010 El Nino rather than a precursor of what's to come.  5 significant class storms/swells occurring with a series of smaller utility class swell mixed in, focused mainly on the US mainland down into Peru and Chile and mostly bypassing Hawaii. 

The North Pacific jetstream pattern has remained looking reasonably good in August and September, and late in September was actively supporting transport of a steady stream of small and weak tropical low pressure centers off Japan tracking northeast over the dateline and then dropping into the Gulf of Alaska. This is almost what one would expect to see if El Nino was in-place. But, there was no early evidence of it in August, typical when one would see such a pattern develop.  In fact, August was barren of any northern hemisphere low pressure development, and did not start to activate until mid- September, about a month behind schedule.  But we have no illusions that this pattern will build any more than currently levels (9/24) and if anything, will wane over the next 30 days. Historically during decent El Nino years tropical systems in the West Pacific turn hard east, transitioning to extratropical status and build while moving over the dateline towards the Gulf.  In the super El Nino of '97, the first Significant class storm of the year occurred in mid-Sept, an extratropical cyclone resulting in 65 kt winds over a large area aimed east producing swell of 10 ft @ 25 sec solid hitting the California coast on Sept 27.    And lesser northwest swell producing systems had occurred as early as mid-August. And even in 2009, the first significant class swell of the season arrived on Sept 12th. Clearly that is not the expectation this year, but it provides some historical context and a model of what can occur in the perfect El Nino scenario. 

What is of interest is that as of this writing, the Northeast Pacific has become very active, as if overnight, and a series of moderate strength gales are forecast or already developing. This is directly attributable to a favorable consolidated south flowing jetstream flowing from the dateline while sinking southeast into the Central Gulf of Alaska.  We believe this a solely the work of leftover energy associated with the previous years El Nino, and is not the direct effects of the building La Nina.  If anything, we suspect La Nina has yet to fully manifest itself in the upper atmosphere and that it will take maybe 2 more months for that transition to occur. So for now a generally positive impact should continue to affect the development of gale in the Northeast Pacific.  

We have previously postulated that stronger La Nina events have the potential to positively impact the production of Fall and Winter storms, but then went on to state we didn't clearly understand if that was the case or whether since strong La Nina events tend to follow stronger El Nino event, that it wasn't just the hangover from El Nino doing the influencing.  For the Fall of 2010, that again is the case.  Regardless, we expect a reasonable start to the 2010/2011 Fla and Winter Season, with the storm track and number of storm developing about on par with a typical season. But we believe the jetstream will start to move into a split mode and the storm track will rapidly falter starting sometime in late December into early January, as Winter becomes firmly entrenched and the last lingering effects of El Nino are washed out of the upper atmosphere and a firmer La Nina signature pattern takes hold.         

We have assigned a swell potential rating of 4.0 for the coming Fall and Winter season, suggesting slightly less than average odds of a historically 'normal' Winter swell production season. But diving down to the details, we expect it to be a hybrid year, with more production on the front end and a rapid decrease as the season moves on, with next to no activity once we reach mid February 2011, with colder than normal air and water temps and an early and strong Spring signature setting in (i.e. brisk cold north winds). 

During El Nino events the standard swell profile is for not only more storm frequency, but stronger and longer lasting ones producing larger and longer lasting swells. And with the jetstream shifting south,. the swell angle tends to favor a more westerly swell angle.  Such a pattern was in abundance during the glory days of the 90's and early 2000's.  Conversely the abysmal surf pattern of 3 year period between 2006/2007-2008/2009 where we would get one day, or even 12 hour swell events, was marked by the presence of La Nina. For this year we believe that will be the case as well, with short lived-swell durations with long breaks in between, perhaps not so much that way early in the season, but increasingly moving toward that pattern as the season progresses.  So if last seasons surf strategy was to plan for for the long run, where endurance and stamina month after month outweight short feasts and 'go-for-broke' assaults, this seasons strategy should be to get all you can while you can cause the odds of it occurring anytime soon again will be low. 

The weather will also play a factor in CA. During El Nino years one expects more moisture than normal with increased snow pack levels in higher elevations of the Sierra on into Nevada and Colorado.  El Nino tends to shift the jetstream southward and flat over the continental US with the California high pressure  system retreating south and west more than normal if not evaporating completely, favoring precipitation for the more southerly positioned ski resorts in the Southwest. But it too causes surface temperatures to rise with a larger amounts of warmer tropical moisture in the mix, meaning the freeze line rises (in elevation) with increased odds for rain at lower lying resorts.   But for La Nina years, the California High pressure tends to take over the waters off the coast there with the jetstream focused on the Pacific Northwest (Central Oregon northward). It is not unusual to have much larger volumes of moisture up there if not outright flooding while a virtual lack of rainfall occurs down into California, especially from Monterey Bay southward.  From a ski and boarding standpoint, going north is a recommended long term strategy.  Of note, during La Nina years, when moisture does move into the California region, and assuming there is sufficient snow base, there is a higher than normal tendency for it to be driven by backdoor fronts, those tracking directly down the Pacific Northwest coast.  They tend to be colder and drier, and when they do arrive, the snow quality is exceptional with bone dry power the result. They are not common, and like the surf profile, come only occasionally. So the strategy is to get it when it comes and not expect more to follow directly.       

This is a preliminary assessment, based on what is known at this time.

And there remains the nagging question of whether we have moved into the Inactive Phase of the Pacific Decadal Oscillation (PDO). The PDO is in-effect is a 20-30 year cycle of weather than slightly mirrors El Nino in the active phase and La Nina symptoms in it's inactive phase. Our thinking is that if we are in-fact in the inactive phase of the PDO (probably since 2000, and will be for the next 20 years), then the odds for favorable winter storm generation conditions are stacked more in favor of La Nina than El Nina, since the inactive phase of the PDO mimics a weak La Nina. But this is mostly just pure speculation.  And there is other data that suggests that we have only been in a 'corrective pattern' since the big 97/98 El Nino. Since that ENSO event was so large and strong, the atmosphere had been trying to re-establish some form of equilibrium for nearly a decade since, and the 2009/2010 season was the first chance since then for a normal pattern to manifest itself. Our thoughts are the historical record is too short and it too soon to know with any certainty whether we are in a down phase of the PDO.  

So assuming a moderate plus strength La Nina in-play and expected to continue if not build through the Fall and Winter of 10/11, we calculate net storm activity will be slightly less than normal, with the worst of it later in the season.  Storms will be shorter in duration and less intense than normal with the potential to cover less surface area, resulting in generally a smaller fetch of decreased duration and less intensity. This should result in less consistent, smaller and shorter period surf, similar to the 3 year period from 2006-2009, and certainly not anything like the 09/10 season.  Of course, we'd be happily  prove wrong.   

(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 OSDPD
Notice the broad area of cooler than normal water temperatures (blue shades) extending along the equator from the Ecuador all the way to the dateline. Also notice the concentration of the coolest waters south of Hawaii to the dateline, at -2.0+ degs C below normal with another pocket building just off Ecuador.  The production of these cooler than normal surface water temperature is driven by stronger than normal trades winds blowing east to west (or if not stronger, at least with more duration than normal). Also note cooler than normal waters flowing from off the US West coast over Hawaii to the dateline and a mirror image flow in the southern hemi from Chile to the dateline. Both these are the product of stronger than normal high pressure in the North and South Pacific causing  enhanced trades, upwelling and hence the colder water temperatures. Also notice the warmer than normal waters temperatures in the Central Atlantic, the result of less than normal trades there and less upwelling and more stagnation, allowing heating to occur. 

SST Anomoly

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Sea Surface Temperature and Surface Wind Anomalies on the Equatorial Pacific
Courtesy: NOAA PMEL
In the top image notice trades wind blowing firmly east to west over the width of the equatorial Pacific. This pattern has been unrelenting since April. There has been no evidence of reversed trades associated with the active phase of the MJO or a Westerly Wind Burst (WWB). In the lower panel notice that surface water temps are above below normal over the equator from off the South American coast west to the dateline and even a bit west if there.  The bulk of the temperature departure is focused from south of Hawaii to the dateline with temperatures -2.0 deg C below normal. The arrows indicate the strength and direction of wind anomalies, which are trending towards and enhanced pattern.  There is no evidence of any west to east blowing winds. 

SST Wind Anomoly

 

20 Degree Thermocline Depth and Position Time Series
Courtesy: CPC NCEP NOAA
(Top Image) The core of warm subsurface water is centered well west of the dateline and making no indication of moving east, symptomatic of La Nina. The thickness of the depth of warm waters in the east is almost non-existent with all warm water off to the west, another indicator of La Nina.
(Lower Image) Notice the pronounced pocket of cooler than normal water 
(-7 deg C below normal) at 140W and 100 meters deep. This pocket is stationary and has been in place for months now, only getting cooler with time. There is not evidence of any Kelvin Waves traveling from west to east 

SST Depth

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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 generally more cooling in water temperatures are forecast off Ecuador by Jan 2011, consistent with development of a moderate to strong La Nina.

SST Forecast

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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 towards El Nino. And when it is positive the trend is towards La Nina. Notice that since March 1 2010 there has been rock solid upward trend, from -25 to +25, highly symptomatic of La Nina. Pauses or fluxes in the upward trend occurred as remnant Active Phase of the MJO tried to get a foothold, but ultimately lost to the stronger and more dominate Inactive Phases. Notice that over the past 3 year record there are broad trends (El Nino and La Nina) and then distinct smaller up and down cycles at 30-45 days each. These are pulses of the MJO. Dips are the Active Phase of the MJO and rises are the Inactive Phase. 

SOI



Pacific Countercurrent Anomalies
Courtesy: OSCAR/NOAA
This image depicts the zonal flow of the Pacific Counter current, which runs roughly on the equator between the West and East Pacific. When the flow is east to west as it normally is (blue), this means nothing, unless is is strong, then that suggest La Nina.  When the flow reverses and moves west to east (red) that suggest El Nino.  
The top panel depicts the absolute flow and speed of the current. Notice the '97 El Nino event is obvious, but all others are more just hints or fragments of something occurring.
The bottom panel depicts anomalies in the current as compared to historical and seasonal averages. In this image departures from normal are clearly obvious, with strong red instances reflective of El Nino and strong blue of La Nina.  Again notice the clear signal of the '97 El Nino event. And the 2009 event provides a decent signature too. But not all red signals necessarily result in an El Nino (i.e 2008) where it tried to start, but failed.  The change in current must be sustained over duration to have a prolonged effect on the atmosphere. Notice that currently we are in a Blue/La Nina phase.  

Pac Current


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