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
i.cgiicated 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 i.cgiicated 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 i.cgiicated 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.cgiace 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.cgiace
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.cgiace.
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.cgiay, 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
di.cgiaced 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 .cgiit pattern develops, it will continue in that mode through Winter and Spring. A .cgiit
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.cgiay 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.cgiace. 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 .cgiit 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.cgian 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 .cgiay 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.cgius strength La Nina in.cgiay 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.
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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.
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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 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.cgiace for months now, only getting cooler
with time. There is not evidence of any Kelvin
Waves traveling from west to east
.
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.
.
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.
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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.
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