La
Nina Waiting in the Wings
Cooler Waters Take Over Tropical East Pacific
In
early 2007 a marginal El Nino was in control of the tropical
Pacific, influenced by occasional bursts of energy from the Madden
Julian Oscillation (MJO). It was expected that it would have
a net positive impact on the production of winter storms in the
North Pacific, resulting in a moderately higher instance
of large winter surf along the north shores of the Hawaiian Islands
and into California. But given it's late formation in 2006 and
it's rather sporadic nature, cycling between occasional bursts
of MJO influenced storm activity
followed by long spells of complete inactivity and possibly the
overriding push into the inactive phase of the Pacific Decadal
Oscillation (PDO), the result was a winter of far less large
surf than has been experienced in years. The one bright spot
was the extension of the winter surf season into May of 2007.
This is one of the hallmarks of El Nino's effects, as was evidenced
by a series of significant class storms pushing into the Gulf
of Alaska in April and May of 2006. But otherwise it's impacts
were completely unremarkable. Since then we have been slowly
but steadily pushing towards a pattern more consistent with a
very weak La Nina, as evidenced by cooler than usual waters over
the tropical East Pacific and a trend towards positive Southern
Oscillation Index values. The paragraphs below describe the state
of various indicators that are used to asses the state of either
El Nino or La Nina, followed by an assessment on it's impact
on surf generation potential for the future.
First
we look at the Southern Oscillation Index (SOI). This number
compares surface pressure over Darwin Australia with pressure
over Tahiti. If this value is negative that indicates
average surface pressure is lower over Tahiti and higher over
Darwin, symptomatic of El Nino. In this configuration
wind flows from high pressure (Darwin) towards lower pressure
(Tahiti), which is a reverse of what normally is experienced.
When it's positive, the reverse it true with higher pressure
over Tahiti and lower pressure over Darwin, typical of La Nina.
And wind flows east to west, typical of trades for this region,
only more so. The first 5 months of 2007 recorded nearly consistent
30 day average values in the negative range, typical of El Nino.
But the average never dipped more than 15 points below normal,
symptomatic of the weak nature of the El Nino that was in.cgiay.
Then in June the index spiked up into positive range peaking
at near +11, only to subside again quickly back into negative
territory. And as of August it started to rebound again, reaching
+2.4 as of this date. Statistically it's essentially neutral.
The downward bursts in the SOI are attributable to the active
phase of the Madden Julian Oscillation, which are the building
blocks of El Nino and can be traced to periods of enhanced storm
activity in regions where the MJO passes (more details below).
Prolonged neutral and positive values are symptomatic
of the inactive phase of the MJO. As of this time there is no
clear trend one way or the other based on the SOI, with one concluding
we are in a neutral phase.
Looking
at current seasonally adjusted equatorial Pacific Sea Surface
Temperatures (SST), the pattern looks a bit less 'neutral'
than the SOI would have one believe. A thin but clearly
defined tongue of cooler than normal water is extending
along the equatorial Pacific starting at Ecuador pushing
west to a point south of Hawaii. This area is
neither broad nor particularly strong, but waters are clearly
not warmer than normal, and indicate symptoms of La Nina.
Also the clearly defined horseshoe pattern attributable to La
Nina is set up over the Western Pacific, with warmer than normal
waters being di.cgiaced north from the tropical West Pacific
up into the Gulf of Alaska and south towards Tahiti. This suggests
strong than normal easterly winds are in.cgiay over the equator
pushing water to the west and then off to the north and south.
This is marked departure from 6 months earlier during the late
winter when warmer than normal waters covered the tropical East
Pacific and a weak El Nino was trying to get established. During
El Nino events, warm waters from the far West Pacific start migrating
east along the equator in response to a shift in the SOI and
are blown there by a reversal of trade winds (more to follow).
The South Pacific is exhibiting near normal water temperatures
while up north a mixed pattern is in.cgiay under the Aleutians.
Alternating pockets of colder water are off the Kuril Islands,
warmer water over the dateline, cool waters again in the
Central Gulf of Alaska and more warm water over the immediate
Canadian coast. Slightly warmer than average water is evidenced
along the US Atlantic coast, through the Caribbean and into the
Gulf of Mexico. But the documented mirror-image cooler than
normal water is also faintly showing up pushing off Africa moving
towards northern Brazil, just like it is off Ecuador in the Pacific,
another La Nina indicator. So in all the La Nina signature is
evident, though not strong.
Wind
anomaly analysis indicates that on average winds have been blowing
stronger than normal off Ecuador pushing to about the mid-equatorial
Pacific, from east to west, typical of La Nina. These offshore
winds (relative to Ecuador) have caused upwhelling along the
coast and produce the cooler than average waters discussed above.
There has been no evidence of prolonged Westerly Wind Bursts,
except for short-lived durations attributable to tropical systems
passing through the West Pacific. And those instances have been
tied to the active phase of the MJO (yes, the MJO still has active
phases even during La Nina, just not as prolonged or strong).
A Westerly Wind Burst (WWB) is an extended duration of winds
that blow from west to east along the equator in the West
and Central Pacific contrary to normal trade winds, forcing warm
surface waters to start moving in the same direction of the
wind (details below), a hallmark of a strongly active phase of
the MJO and a precursor to El Nino. None of these conditions
are in evidence and if anything, the opposite is true. Historically
if either El Nino or a strong burst of the Madden-Julian Oscillation
is in effect, trade winds that normally blow from east to west
reverse themselves and blow west to east. That is, when the MJO
is in an active phase, the trades reverse themselves in the West
Pacific, and when the MJO is not active, trades return. During
La Nina, trades blow much stronger than normal. We are currently
in an inactive phase of the MJO with normal if not enhanced trades
in.cgiay. And the cooler than normal waters occurring
in the East Pacific remain as evidence of the tendency more
towards La Nina.
Another
indicator of El Nino or La Nina is a change in sea surface height.
Sea surface height is the height of the oceans surface relative
to 'average'. Current data suggest there is no trend one way
or the other. Sea heights over the tropical east Pacific or neutral/normal.
Another
key indicator in the evolution of either an El Nino or La Nina
event is the depth and profile of the 20 degree isotherm (thermocline).
During La Nina events, warm subsurface water remains pooled
up in the far West Pacific near the equator. Cold surface and
subsurface waters dominate the East Pacific, resulting in a steep
angle from east to west, going from shallow warm water in the
east to warm water being deep in the west. In El Nino events,
as warm subsurface water (i.e. Kelvin waves - more below) migrate
towards the eastern Pacific, the angle flattens and the depth
of warm waters becomes more uniform across the equatorial Pacific.
Latest data indicates a solid pocket of cooler than normal water
in the east and is actually being transported to the west, pushing
from Ecuador at the oceans surface and digging in deeper to the
150 meter mark as it pushes west, reaching a temperature almost
3 degrees C below normal at 140W. These cooler waters are
effecting the subsurface thermocline the whole way west to the
dateline. Warmer waters remain pooled up in the extreme Western
Pacific and at no time since June have they made any progress
east, even when being forced by a short lived WWB. This is consistent
with the development of a weak to moderate La Nina.
A
Kelvin Wave is a pocket of warm water that travels under the
oceans surface from west to east at a depth of about 150-200
meters. It is generated by a burst of strong westerly winds blowing
over the equator (a.k.a. Westerly Wind Burst (WWB) in the West
Pacific associated with the MJO. As the warm surface water gains
eastward momentum, it sinks near the dateline and travels well
under the oceans surface, only to reappear at the surface when
it impacts the South America Coast. This results in the sudden
appearance of warm waters along the coast of Peru and Ecuador.
Occasional eruptions are normal. Large and consistent eruptions
are the hallmark of solid El Nino events. The source of Kelvin
Waves, a negative SOI and reversed trades is directly related
to the strength and frequency of the Madden Julian Oscillation
(MJO). This weather pattern is responsible for the periodic strengthening
of the anomalous westerly winds in the West Pacific which drive
production of subsurface Kelvin waves, and also drive the SOI
negative. When the MJO enters an active phase,
and does so frequently, El Nino indicators strengthen. As consistent
active phases of the MJO fade, so does El Nino.
Currently there is no evidence of any Kelvin Wave activity occurring
and none has occurred since at least June, even when short
lived WWB's have occurred. The resulting warm waters pushing
east from these WWB events have not made a dent in the cooler
than normal waters firmly entrenched in the Eastern Tropical
Pacific. This is symptomatic of La Nina.
When
El Nino events unfold at the oceans surface, precipitation
will develop in the atmosphere above the warmer surface waters,
since warmer water supports higher condensation rates above it.
The presence of consistent precipitation where it historically
shouldn't be is a hallmark of El Nino. Current satellite data
indicates no increases in precipitation over the eastern equatorial
Pacific. If anything, abnormal precipitation has been consistently
indicated over the eastern Indian Ocean and has tried to make
headway into the Western Pacific, but has been denied entrance
of any magnitude. The MJO travels against the
grain of the equator from west to east developing initially in
the Indian Ocean. It seems that the MJO has not been strong enough
to do much to transport this moisture eastward. This is consistent
with the development of La Nina.
Reviewing
all the data, it is appears that a La Nina
event is trying to get some footing. Cooler water is
in.cgiace over the Eastern Equatorial Pacific with no
Kelvin Wave activity occurring to dislodge it. And active phases
of the MJO have seemed weaker than normal, not providing any
help in breaking down prevailing easterly winds over the East
Pacific. If anything they have served to hold it at bay, but
even that might be at risk in the near future. The
SOI was negative earlier in the year into the summer, but that
seems more of a hangover from the weak El Nino
phase that occurred earlier this year. Historically it is
normal for the environment to rebound from an El Nino event with
an equally strong La Nina episode. And that appears what is trying
to happen. But since the preceding El Nino of 2006/2007 was
so inordinately weak, perhaps it's co.cgiimentary La
Nina might do the same. Though that is more of a guess than based
in fact at this early date. So the SOI appears to be the lagging
indicator, with the ocean temperature and surface wind anomalies
being the leaders in the transition. So the SOI will be the
one to watch for over the coming months.
Always
of interest is the relative activity
level of the Atlantic hurricane season. We believe this is inversely
tied to the strength and duration of El Nino and La Nina in the
Pacific. El Nino produces
strong shearing winds over the Atlantic 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. So if during
the next 2 months an inordinate number of hurricanes occur
in the Atlantic, or if they are unusually strong, one could conclude
that the shearing effects of El Nino are not in.cgiay, which suggests
at least a moderate La Nina might be in effect. Currently watching
the development of category 4 hurricane Dean in the Caribbean
(forecast to reach Cat 5), this seems like a logical
conclusion. But there needs to be more than one to make a trend.
Dr Grey and associates at the University of Colorado are forecasting
17 tropical storms and 9 hurricanes, 5 of which will be intense
this season resulting in a total of 185% the normal tropical
activity. A normal year produces 9.6 named storms and 5.9 hurricanes.
His research suggests neutral to light La Nina conditions expected
during this years hurricane season.
The
latest El Nino discussion from the Climate Prediction Center/NCEP
(August 9, 2007) states that ENSO neutral conditions to
continue through the month, with a slightly greater than 50%
probability La Nina developing over the next several months.
They also note the typical transition time toward La Nina
occurs during August-September-October. Looking at
the MJO models, the inactive phase of the MJO is currently in
control and peaking out, and expected to have run it's course
by the end of the month. The active phase of the MJO is also
starting to brew in the central Indian Ocean. With a 30-45 day
cycle, but not knowing how strong or weak this phase will become,
it seems reasonable that at worst a mild La Nina will be in
effect for the winter of 2007/2008.
Of
18 ENSO models run in July 2007, all but two indicated neutral
to cool conditions through year end. But of that, only one suggested
radically cool conditions and most suggested very modest temperature
departures (-1 deg C) or less, bordering on neutral.
LONG-RANGE NORTH PACIFIC STORM AND SWELL GENERATION POTENTIAL FORECAST
Fall/Winter
2007-2008 Swell Generation Potential (for California & Hawaii)
= 4.0
Rating
based on a 1-10 scale: 1 being the lowest, 5 being normal/average,
and 10 being extraordinary
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. Conversely La Nina typically decrease the size strength,
frequency and duration of such systems.
There
is no data supporting development of El Nino
over the next 6 months and all data supporting the mild
development of La Nina, or at least holding in ENSO neutral conditions.
From a swell generating perspective this is not good news. On
top of that there is also data suggesting we have moved well
into the inactive phase of the Pacific Decadal Oscillation (PDO),
which 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), and La Nina is occurring over top of that,
then the odds for favorable winter storm generation conditions
are less than normal. In fact we're thinking that since
last years El Nino was such a bust, that the PDO had an overarching
hand in that as well. In addition, during La Nina events, tropical
activity in the Eastern Pacific is normally suppressed, limiting
summer and early Fall tropical induced south swell activity relative
to the US mainland. In addition, the shift towards enhanced trades
tends to suppress the strength of the active phase of the MJO relative
to it's emergence into the Western Pacific from the Indian Ocean,
which in-turn suppresses the transport of tropical energy and
moisture northward. The result is reduced number of tropical systems
in the Western Pacific that curve north eventually turn extratropical
moving towards the Gulf of Alaska. In fact, we suspect
this conditions suppresses the total flow of moisture energy
transported northward feeding developing winter storms, limiting
their strength and duration. So assuming a very weak La Nina, with
the added effects of an inactive PDO, we calculated net storm activity
will be less than normal and when it does occur, the storms will
be shorter in duration and and less intense than normal. If anything
the rating above assumes almost ENSO neutral conditions. A downward
adjustment will be needed if full scale La Nina conditions develop.
It
is very interesting to compare this years forecast with last years
Fall forecast, when El Nino was occurring. Especially compare
the charts to view the marked difference between the two:
Previous Forecast
(This forecast is highly speculative and based on historical analysis of past La Nina/El Nino events and the latest long-range forecast models)
Sea Surface Temperature Anomalies
Courtesy: NOAA NESDIS
Notice a thin stretch of cooler than normal
water (dark blue) extending along the equator from Ecuador to a point south
of Hawaii. This is almost typical for this time of year, caused
by strong easterly winds there creating upwhelling of cooler water from deep
in the Pacific. This is consistent with normal to slightly enhanced trades for
the area. Also note warmer than normal waters extending from New Guinea northeast
to the Gulf of Alaska and southeast to Tahiti. This in more symptomatic of La
Nina, and caused by the enhanced trades blowing from east to west over the equator.
.
Sea Surface Temperature Anomalies and Average Surface Winds on the Equatorial Pacific
Courtesy: NOAA PMEL
In the top image notice winds blowing from east to west over the entire Tropical
Pacific Ocean, the standard trade wind pattern for this time of year (if not
slightly enhanced). There is no evidence of reversed trades which would be associated
with either the active phase of the MJO or El Nino. Notice in the lower pane
that surface temps are lower than normal over the equator in the east and slightly
warmer than normal in the far west. The arrows indicate the strength and direction
of wind anomalies, which aren't too far off of normal. In all there appears to
be a slight tendency towards La Nina.
.
Sea Surface Height Deviation
Courtesy: NLOM
This image depicts Sea Surface Height deviations as indicated by satellite
altimeter. Warmer waters bulge higher and cooler waters push deeper.
The area of most interest relative to El Nino/La Nina is over the equator.
Notice that sea surface heights are essentially normal on the equator. A length
of warmer waters extends almost the width of the Pacific at 10N, but does
not indicate the presence of El Nino.
.
20 Degree Thermocline Depth and Position Time Series
Courtesy: CPC NCEP NOAA
(Top Image) The core of warm subsurface water is centered in the far West Pacific
and is not making any headway to the east, which would be symptomatic of El Nino.
The very shallow depth of warm waters in the east are more an indicator or La
Nina, but only faintly.
(Lower Image) Notice the pronounced pocket of cooler water (2
deg C below normal) pushing west and deeper from 120W. This is
indicative of the trades wind blowing offshore at the oceans surface,
forcing cooler water deeper almost in a reverse Kelvin Wave.
this pattern has been in.cgiace for 2 months now.
.
Equatorial Pacific Sea Surface Temperature Forecast
Courtesy: NOAA/NCEP
Notice that the average of many separate runs of the NCEP model suggest a strengthening
of the cooler trend with waters in the Nino3.4 region reaching their lowest point
in October, about 1 deg K below normal and consistent with a weak 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 July of last year there has been
a steady tendency for the trend to be moving upward even though there are distinct
cycles within that overall trend (MJO pulses). Dips are the active phase
of the MJO and rises are the inactive phase. The current inactive phase of the
MJO is evidenced by the rising pressure regime at the very tail end of the image.
But the data is essentially neutral (neither El Nino or La Nina suggested).
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