Neutral to Weak La Nina Conditions In Effect
Signals Still Mixed
Signs of La Nina are evident, with no suggestion of a El Nino anytime soon. The trend over the past two years has been characterized by the persistence of subtle El Nino-like symptoms that we thought were slowly but steadily taking their toll on the atmosphere while building momentum. This pattern started during the winter of 2003/2004 with a generally neutral trend in-place punctuated by occasional signals of a developing El Nino. That trend continued through 2004 with the strength and frequency of the El Nino-like periods increasing, enough to pass over the threshold to be considered a minimal but official El Nino by late August. By early 2005, things looked to be settling back down, but then a major eruption of the Madden Julian Oscillation (MJO) occurred the first week of February, spawning a 40 day run of a very negative Southern Oscillation Index, the strongest for that time of year since the big El Nino of 97/98. A significant increase in Northern Hemisphere storm activity ensued producing a string of significant class swells and producing a strong eastward moving Kelvin Wave. As the MJO and SOI settled back down, the winter storm season faded and we entered the Spring time 'Predictability Barrier. Then in mid-April another negative run of the SOI supported by a renewed active phase of the MJO started, and the Northern Hemisphere storm track tried to respond. But by June and again in September two runs of strongly positive Southern Oscillation Index values were experienced that pretty much squashed any hope for a developing El Nino and it's been looking more like a La Nina pattern ever since.
Looking at current seasonally adjusted equatorial Pacific Sea Surface Temperatures (SST), the affects of the past 6 months of atmospheric changes are apparent. Consistently cooler than average water temperatures have replaced the warmer than normal waters that were present in the East Pacific. Cool water now persists over the entire length of the equator from the dateline eastward, averaging -1.0 to -1.5° C below normal. This is a hallmark of La Nina and has been building since the summer. Otherwise warmer than normal waters persist over much of the South Pacific while neutral temperatures persist in the North Pacific. Latent heat energy that has been present in the tropical Atlantic remains in-place, with water temperatures 2° C above normal across the length of the Cape Verde storm corridor (from West Africa tracking due west into the Caribbean Sea).
Wind anomaly analysis indicates that near normal winds were occurring over the entire length of the equator, blowing briskly from east to west, driven by a strong offshore gradient off Columbia and Peru. These are the trade winds for this area. If either El Nino or a strong burst of the Madden-Julian Oscillation (more details below) was in effect, winds would be averaging from west to east. That is, when the MJO is in an active phase, the trades reverse themselves, and when the MJO is not active, trades return. During La Nina, trades blow much stronger than normal. Since the trades are nether stronger than normal nor have they reversed direction, the wind pattern is neutral. But the cooler than normal waters occurring in the same area are a indicator of the previous months strong trade wind bursts.
Another indicator of El Nino or La Nina is a change in sea surface height. Sea surface height is the height of the oceans surface relative to 'average'. Satellite analysis indicates a thin band of 'below' normal normal height running along the equator from 160W to the coast of Panama and in pockets up to -20 cm below normal, roughly paralleling the wind anomalies and negative sea temperatures occurring in the same area. This is expected and is in sharp contrast to the 'above' normal conditions that persisted in this region during the winter of 2004/2005, averaging 15 cm above normal.
Continuing down the list of ENSO indicators, the 30 day average Southern Oscillation Index (SOI) was negative but just barely so at -1.5, while the 90 day average was positive at 2.9. suggesting ENSO neutral conditions. The last big run of negative numbers was in May and then again in July of 2005, with a big positive streak in September. More recently a bit of a negative run occurred in October and November, but nothing that was breaking the current trend pushing towards more and more positive territory. We have been investigating whether there is any connection between the MJO and the current storm cycle occurring over the North Pacific in December 2005, but the most we could find was suggestions of a weak MJO pulse in the Indian Ocean, and no data suggesting a teleconnection into the Pacific.
Another key indicator in the evolution of either an El Nino or La Nina event is the depth and profile of the 20 degree isotherm (thermocline). During La Nina events, warm subsurface water remains pooled up in the far West Pacific near the equator. Cold surface and subsurface waters dominate the East Pacific, resulting in a steep angle from east to west, going from shallow in the east to deep in the west. In El Nino events, as warm subsurface water (i.e. Kelvin waves) migrate towards the eastern Pacific, the angle flattens and becomes more consistent across the equatorial Pacific. Latest data suggests that a significant thermocline angle was present with warm sub-surface water holed up in the West Pacific at 155E rising sharply and getting most shallow as one traveled east along the equator. This is consistent with La Nina.
And unlike the year before, there is virtually no indication of any 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 of strong westerly winds blowing over the equator (a.k.a. Westerly Wind Burst (WWB) in the West Pacific associated with the MJO. As the warm surface water gains eastward momentum, it sinks and travels well under the oceans surface, only to reappear at the surface when it impacts the South America Coast. This results in the sudden appearance of warm waters along the coast of Peru and Ecuador. Occasional eruptions are normal. Large and consistent eruptions are the hallmark of solid El Nino events. The source of Kelvin Waves, a negative SOI and reversed trades is directly related to the strength and frequency of the Madden Julian Oscillation (MJO). This weather pattern is responsible for the periodic strengthening of the anomalous westerly winds in the West Pacific which drive production of subsurface Kelvin waves, and also drive the SOI negative. When the MJO enters an active phase, El Nino indicators strengthen, and as it fades, so does El Nino. As mentioned earlier, there is no indication of any Kelvin Wave activity nor has there been, with neutral subsurface water temperatures being recorded over the past 5 months and continuing at this time. Subsurface waters 1 degree above normal are all isolated to an area west of the dateline and 100 meters down. Most unremarkable and symptomatic of neutral to mild La Nina conditions.
Reviewing all the data, there emerges a picture consistent with a mild La Nina that is expected to continue through the winter of 2005/2006. Trades are near normal over the Tropical Pacific and sea surface temperatures and heights are below normal in the same area. There is no Kelvin Wave activity and the SOI continues generally positive and cycling more in that direction over time.
Always of interest and a good leading indicator is the relative activity level of the Atlantic hurricane season. This past years activity was record breaking, and there is nothing that can be said here that hasn't been said a thousand times somewhere else. For 2006 Dr. William Gray and the team at the University of Colorado have again predicted an active season (as of the Dec 6, 2005 edition of their forecast).
The latest El Nino discussion from the Climate Prediction Center/NCEP (December 8, 2005) suggests that ENSO-neutral or weak La Nina conditions are to continue during the next 6-9 months which certainly seems reasonable given the overwhelming volume of data indicates that such conditions are in-fact occurring now.
Of 15 ENSO models run in Nov 2005, 8 indicate neutral to mild El Nino and 7 suggests neutral to mild La Nina by August 2006. In other words, the models suggest no major change one way or the other.
LONG-RANGE NORTH PACIFIC STORM AND SWELL GENERATION POTENTIAL FORECAST
Winter/Spring 2005-2006 Swell Generation Potential (for California & Hawaii) = 4.5
There is no data supporting development of either a significant El Nino or La Nina event over the next 9 months, but a fair amount of data suggesting no change at all. We had been previously predicting a slow but steady tendency towards El Nino, but that course has reversed somewhat, though still holding in the neutral zone. El Nino typically enhances the size, strength and frequency of winter North Pacific storms in and around the Gulf of Alaska, thereby improving the likelihood for large winter surf in California and Hawaii. Current data suggests no enhancing effect expected.
So for now, plan on 'normal' Pacific Winter and Spring with the usual periods of enhanced storm cycles followed by periods of inactivity.
(This forecast is highly speculative and based on historical analysis of past La Nina/El Nino events and the latest long-range forecast models)
Sea Surface Temperature Anomalies
Courtesy: NOAA NESDIS
Notice a thin elongated tongue of cooler than normal water (blue) extending west from Ecuador covering most of the eastern Tropical Pacific. This is created by upwelling and slightly stronger than normal trade winds blowing east to west off South America, indicative of mild La Nina conditions.
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. Notice in the lower image the anomalously cool pool of water off South America caused by these same winds that were blowing stronger than normal over the past several months. As a result water temperatures are 1 degree C below normal.
Sea Surface Height Deviation
Consistent with the image above, notice that sea surface heights are slightly below normal right over the equator and were the waters are cooler, as one would expect.
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, increasing the angle between the West and East. This is consistent with a mild La Nina.
(Lower Image) Notice no pockets of anomalously warm water pushing east. This indicates no Kelvin waves are present, consistent with La Nina and ENSO neutral conditions.
Equatorial Pacific Sea Surface Temperature Forecast
Notice that the average of many separate runs of the NCEP model suggest a continuation of cooler waters in the tropical East Pacific into summer of 2006.
Southern Oscillation Index (SOI)
The SOI depicts the difference in pressure between Tahiti and Darwin Australia. When it is consistently negative, that is the pressure is lower in Tahiti than Darwin, the trend is then towards El Nino. And when it is positive the trend is towards La Nina. Notice the recent spikes of upward pressure in July and October, typical of a weak La Nina.