Mild El Nino Continues
One Kelvin Wave Hits, A Second On the Way
Signals of a weak El Nino continue to be evidenced statistically, though the atmosphere does not show any overt signs of responding directly to it's presence yet. Starting with the winter of 2003/2004 a generally neutral trend was in place with occasional signals of a developing El Nino. That trend continued through the summer of 2004 with the strength and frequency of the El Nino-like periods increasing, enough to pass over the threshold to be considered a minimal El Nino by some in late August. Whether this evolves into a 'real' El Nino with the traditional circulatory changes for the winter season is still debatable, but it seems reasonable that the impact of these episodes on the overall North Pacific winter weather pattern should have a mildly positive influence from a storm generation potential.
Looking at current seasonally adjusted Sea Surface Temperatures (SST) across the equatorial Pacific, the affects of the low level atmospheric changes are apparent. Consistently warmer than average water temperatures have persisted on the equator near the dateline since last winter, building more distinctly over the past 2 months. In July, the warming trend became more pronounced and the pool of warm water started moving east towards Ecuador, typical of El Nino. Current water temperatures are about 0.5 ° C higher than in August, totaling 1.0 to 1.5° C above normal extend from just well northeast of New Guinea eastward across the dateline to the mid-Pacific with fragmented anomalies eastward to due south of California. This general trend was consistent over the past several months with a pronounced cool tongue -2° C below normal extended west off Central America and typical of a neutral summer-time pattern. But in early October that tongue dissipated and a warm tongue replaced it, extending westward off Ecuador with water 2-3 ° C above normal now positioned near the Galapagos Islands. This is interesting. Elsewhere in the Pacific warmer than normal waters continue across much of the North Pacific, but not nearly as strong as was indicated this summer. Most of the South Pacific remains in the normal range with only a small pocket of slightly warm water east of New Zealand. Warmer than normal waters are also present in the Tropical Atlantic, typical of the normal 'mirroring' that occurs between the Pacific and Atlantic.
QuikSCAT imagery and wind anomaly analysis indicates that the normal trade winds in the far West Pacific have reversed direction over a small area northeast of New Guinea, but not covering nearly as much area as in previous months. Near neutral/normal winds cover the remainder of the tropical Pacific. But much variability in wind speed and direction has been noted in the past few months, attributable to the strength and phase of the Madden Julian Oscillation (MJO - more details below). In general, the MJO is not in an active phase and so trades are normal, as would be expected. During a typical El Nino this time of year one would expect to see slack to no trades with bursts of westerly winds from the Philippines eastward to nearly 120W. It looked like that was developing in August, with a string of active typhoons developing south of Japan tracking northward in-sync with a very active phase of the MJO. But that phase has since dissipated, and so have the anomalous trades. Conversely, if La Nina was in-effect, the trades would be blowing hard east to west across the entire equatorial Pacific, which is clearly not the case. In all, winds look neutral for the time of year.
Another indicator of El Nino or La Nina is a change in sea surface height. Seas surface height is the height of the oceans surface relative to 'average'. As indicated by satellite analysis, a broad band of waters running about 15 centimeters higher than normal spans the width of the equatorial Pacific from New Guinea eastward to the Galapagos Islands and into south America. This is above normal, but not overly so. Of note though is that these higher waters are more pronounced than last year and higher than those indicated in the last report, presumably indicative of warmer waters under the oceans surface and the potential development of a weak El Nino.
Continuing down the list of ENSO indicators, the 30 and 90 day average Southern Oscillation Index (SOI) are not strongly indicating El Nino. This is the one indicator that has continued to give us some pause/concern. This index compares barometric surface pressure between Darwin, Australia and Tahiti. A negative anomaly (signaling El Nino) indicates that lower than normal surface pressure is present over the Central Pacific while higher than normal pressure is in the West Pacific. A positive SOI (La Nina) means the pressure pattern is reversed. During April 2004 the 30 day trend was very negative, indicating El Nino, but returned to normal in early May and even trending towards La Nina. Then again in June the trend went even stronger to the negative only to return to a near neutral state late in the month. And then again in July the trend dipped well into the negative range, only to rebound some in early August, but not giving up the slight negative tendency. Yet another negative pulse developed in late September and early October consistent with renewed MJO activity and an increase in tropical activity in the Western Pacific. Yet another weak burst of negative anomalies started in early November, but is easing up in recent days. These dips are associated with an increase in the Madden Julian Oscillation (MJO) over the West Pacific during the noted time-frames (more details below). The 90 day trend, which attempts to average out the sporadic peaks and valleys in the curve now suggests a slight tendency towards El Nino, but nothing that stands out, and has not really changed since the last report. It seems unlikely than any large scale shift will develop anytime soon given the trend over the past several months.
Another key indicator in the evolution of either an El Nino or La Nina event is the depth and profile of the 20 degree thermocline. During La Nina events, warm subsurface water remains pooled up in the far West Pacific near the equator. Cold surface and subsurface waters dominate the East Pacific, resulting in a steep angle from east to west, going from shallow in the east to deep in the west. In El Nino events, as warm subsurface water (i.e. Kelvin waves) migrate towards the eastern Pacific, the angle flattens and becomes more consistent across the equatorial Pacific. Latest data suggests that the thermocline is almost identical to last year, which itself had tendencies towards El Nino. If anything, it looks like it has given up a little ground from a few months earlier but nothing dramatic. The core of the central warm pool has moved to 170E, where it was near 180W in our last report. This suggests no real change.
But like the months before, the area of most interest is the clear indication of Kelvin Wave activity. 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 strong westerly winds blowing over the equator in the West Pacific associated with the MJO. As the warm water gains eastward momentum, it sinks and travels well under the oceans surface, only to reappear at the surface when it impacts the South America Coast. This results in the sudden appearance of warm waters along the coast of Peru and Ecuador. Occasional eruptions are normal. Large and consistent eruptions are the hallmark of solid El Nino events. The source of Kelvin Waves, a negative SOI and reversed trades is directly related to the strength and frequency of the Madden Julian Oscillation (MJO). This weather pattern is responsible for the periodic strengthening of the anomalous westerly winds in the West Pacific which drive production of subsurface Kelvin waves, and also drive the SOI negative. When the MJO enters an active phase, El Nino indicators strengthen, and as it fades, so does El Nino. In August there was clearly a Kelvin Wave with temperature 4.0° C above normal positioned 75 meters under the oceans surface on the equator at 120W, approaching the South American coast, impacting it a few weeks later. The warm water tongue streaming west off South America is the result of that impact. And another Kelvin wave is on the way attributable to the renewed active phase of the MJO in late September. It is currently positioned near 160W, 125 meters under the surface and 3° C above normal tracking east. It should help to continue the warm water tongue off South America when it impacts there in early December. If El Nino were developing strongly, we would expect to see more of these events occurring, fueled by renewed MJO activity, a negative SOI and anomalous trades on the oceans surface. This is happening, but at a very leisurely pace, consistent with a mild El Nino.
Reviewing all the evidence, there is solid evidence of a developing weak El Nino, but nothing to indicate anything more. Trade winds are reversed over a small area in the West Pacific, Sea surface temperatures are above normal over a moderate area in the Tropical Pacific typically associated with El Nino as are sea surface heights. There is a solid periodic Kelvin Wave activity with one en route to the South American coast. The SOI continues marginally negative, and periodically well negative when the MJO is active, and is showing no signs of moving into the positive range.
Of interest was the unbelievably active hurricane season in the Atlantic. This is consistent with forecasts produced by Dr. William Gray and the team at the University of Colorado. Their April 2nd forecast suggested that mild La Nina conditions were expected in the Eastern Pacific through the summer based on trends at key locations in all oceans over the previous 6 months. This also contributed to their assessment that, like most years in the recent past, enhanced tropical storm activity was expected for the the Atlantic hurricane season (northern hemisphere summer). In fact, Hurricane Ivan earned the lead with the most intense hurricane days of any storm on record since 1900 (10 days). William Gray had some concern in his last update published August 6 that suggested that strengthening El Nino signals in the Pacific would have a negative impact on the development of tropical systems in the Atlantic, and they tuned their forecast downward. That clearly did not happen, indicating that El Nino signals from the Pacific did not manifest themselves as strong as expected in the Atlantic and therefore did not have the dampening affect on the hurricane season as was expected. This suggests that the signals in the Tropical Pacific have not yet produced any large scale circulation changes in the upper levels of the atmosphere, and are not yet affecting global weather patterns. Also there is no data suggesting the development of more rainfall than normal over the warm pool in the Tropical Pacific, something one would expect to see if the Walker Circulation was be modified by the rising moist air over the Central Pacific.
But the latest El Nino discussion from the Climate Prediction Center/NCEP suggests that a weak El Nino is in place and expected to continue developing though the winter of 2005. Their assessment substantiates that some form of El Nino activity appears to be underway. The big wild card in all the analyses remains to be the MJO. If it becomes more active and consistent, the meteorological signals that are driving El Nino symptoms will likely be more robustly manifest. If the MJO falters, with an on again-off again like pattern, then a more ' normal' year will likely result. It remains too early to tell completely, but data so far seems to indicate a trend is towards a mild and late season El Nino.
Of 11 ENSO models run in October, about half indicate neutral conditions while the other half indicate mild El Nino. this is no different that what has been forecast over the past 6 months. None indicate La Nina. Given that we are now in the prime El Nino development window, and the over-all meteorological pattern suggests a mild El Nino, it seems reasonable that such a trend will persist. we have factored this into our most current swell potential rating.
LONG-RANGE NORTH PACIFIC STORM AND SWELL GENERATION POTENTIAL FORECAST
Winter-Spring 2004/2005 Swell Generation Potential (for California & Hawaii) = 6.5
There continues to be no data supporting development of a significant El Nino event this year, but lot's of data suggesting we are currently in a mild one. Assuming no significant change over the next few months, this would be good. It is not uncommon to have a series of years where mild El Nino conditions persist. They slowly and subtly change the atmospheric weather patterns in favor of producing slightly stronger and more frequent winter storms in the North Pacific, but not so much as to cause full blown storm conditions to fill the North Pacific basin. During strong El Nino events, storm conditions often take over the coastal regions of Hawaii and the US West Coast, making the large waves produced by these strong winter storms unrideable from a surfing perspective and inflicting needless damage to property and civilian livelihood. The spectacle produced by such events is certainly not worth the cost to humans and wildlife. But the subtle guiding influence of a mild El Nino strung out over several years can produce a series of climatic changes that favor the development of storms in both the North and South Pacific without the associated poor local weather and damaging winds and rains. The long-term affects of such a pattern do not dissipate in a season either, but can continue for a year after the main trend reverses. We are hopeful that such a pattern is developing.
All current signals and predictive data continues to suggest we are in a mild El Nino pattern, and that it will last through the Winter and Spring seasons. For the short-term, no significant change in the storm and surf pattern is forecast. But as El Nino develops, a slow but steady enhancement to the normal storm track should be expected, with some increase in the size and consistency of the winter surf pattern occurring by late December, and certainly better than the past 2 years. The El Nino enhancement could also have the ability to extend the winter surf season a month or so longer than normal, but not strongly.
So for now, plan on 'normal' Fall and early Winter seasons with a slow evolution towards a mildly more active pattern late Winter.
(This forecast is highly speculative and based on historical analysis of past La Nina/El Nino events and the latest long-range forecast models)
Sea Surface Temperature Anomalies
Courtesy: NOAA NESDIS
Notice a broad area of warmer than average water (yellow and orange) extending across the entire Pacific Ocean on the equator. A similar pattern is also mirrored in the Atlantic. Of particular interest is the tongue of much warmer than average waters (orange to red) near South America. Last report this area was cooler than normal (dark blue), suggesting that the effects of multiple Kelvin Waves is having the desired warming affect.
Sea Surface Temperature Anomalies and Average Surface Winds on the Equatorial Pacific
Courtesy: NOAA PMEL
In the top image notice winds actually blowing from west to east, against the normal trade wind flow in the far West Pacific. But for the most part trades are in affect (east to west). Notice in the lower image the pool of warmer than normal water in the equatorial Pacific extends from the west to the east, much more so than las report but a little less concentrated.
Sea Surface Height Deviation
Like the images above, notice that sea surface heights are somewhat above normal right over the equator and over the same area where the ocean is warmer. This area is better defined than the last report in August.
20 Degree Thermocline Depth and Position Time Series
Courtesy: CPC NCEP NOAA
(Top Image) A core of warm subsurface water is centered in the West Pacific but has retreated to the west about 10 degrees as compared to the last report. This image is identical to last years scenario.
(Lower Image) Notice a broad pocket of anomalously warm water (3° C) at 165W and at about 125 meters deep. This is a Kelvin Wave produced a month ago by strong westerly winds in the West Pacific by a pulse of the MJO.
Equatorial Pacific Sea Surface Temperature Forecast
Notice that multiple runs of multiple models (started at 2 separate times) suggest a continuation of a mild warm trend through the winter of 2004/2005 into the summer months. This is consistent with 50% of the available ENSO modeling tools/suites.
Equatorial Pacific Sea Surface Temp Forecast
NCEP Model: Notice Sea Surface Temps are to remain mildly warm into the Summer of 2005, typical of a weak El Nino.