Moderator: S2k Moderators
Synopsis: La Niña is expected to last at least into the Northern Hemisphere spring 2011.
During November 2010, the ongoing La Niña was reflected by below-average sea surface temperatures (SSTs) across the equatorial Pacific Ocean (Fig. 1). For the second straight month, only small changes were evident in the Niño SST indices, which ranged from –1.3oC to –1.7oC at the end of the month (Fig. 2). The subsurface oceanic heat content (average temperatures in the upper 300m of the ocean, Fig. 3) also remained well below-average in association with a shallower-than-average thermocline across the central and eastern equatorial Pacific (Fig. 4). Convection remained enhanced over Indonesia and suppressed over the western and central equatorial Pacific (Fig. 5). Enhanced low-level easterly trade winds and anomalous upper-level westerly winds continued over the equatorial Pacific. Collectively, these oceanic and atmospheric anomalies reflect a moderate-to-strong La Niña.
Consistent with nearly all ENSO forecast models (Fig. 6), La Niña is expected to peak during November-January and to continue into the Northern Hemisphere spring 2011. Thereafter, the fate of La Niña is more uncertain. The majority of forecast models and all of the multi-model combinations (thicker lines) indicate a return to ENSO-neutral conditions during the Northern Hemisphere spring and early summer. However, a smaller number of models, including the NCEP Climate Forecast System, suggest that La Niña could persist into the summer. Historically, there are more multi-year La Niña episodes than El Niño episodes, but other than support from a few model runs, there is no consensus for a multi-year La Niña at this time. Consequently, La Niña is anticipated to continue into the Northern Hemisphere spring, with no particular outcome favored thereafter.
Likely La Niña impacts during December 2010-February 2011 include suppressed convection over the central tropical Pacific Ocean, and enhanced convection over Indonesia. Impacts in the United States include an enhanced chance of above-average precipitation in the Pacific Northwest, Northern Rockies (along with a concomitant increase in snowfall), Great Lakes, and Ohio Valley. Below-average precipitation is most likely across the southern states, extending into the Mid-Atlantic region. An increased chance of below-average temperatures is predicted for the northernmost western and central states, and a higher possibility of above-average temperatures is forecast for much of the southern and central U.S. (see 3-month seasonal outlook released on November 18th, 2010).
Current Conditions
As of mid-December 2010, SSTs continue to indicate moderate to strong La Niña conditions in the central and eastern equatorial Pacific. For November the SST anomaly in the NINO3.4 region was -1.45 C, indicative of moderate to strong La Niña conditions, and for the September-November season the anomaly was -1.53 C. Currently the IRI's definition of El Niño conditions rests on an index of SST anomalies, averaged over the NINO3.4 region (5S-5N; 170W-120W), exceeding the warmest 25%-ile of the historical distribution, and similarly for La Niña relative to the 25%-ile coldest conditions in the historical distribution. The NINO3.4 anomaly necessary to qualify as La Niña or El Niño conditions for the Dec-Jan-Feb and the Jan-Feb-Mar seasons are approximately (-0.65C, 0.65) and (-0.55, 0.50), respectively.
Expected Conditions
The most recent weekly SST anomaly in the NINO3.4 region is -1.5 C indicating moderate to strong La Niña conditions in the tropical Pacific; this is approximately the same as the -1.45 C level observed in November. What is the outlook for the ENSO status going forward? December is a time of the year during which the observed ENSO state usually indicates the ENSO conditions to be expected into the first one to three months of the following year, and, if there is an ENSO event in progress (as is currently the case) that event is typically peaking in strength and perhaps even starting to decrease slightly. One might ask whether the current La Niña condition is therefore guaranteed to persist over the coming several months, and whether it would be expected to begin weakening in the coming one or two months. A clue to the answers lies in the moderate strength and extent of negative subsurface sea temperature anomalies at depth in the central and eastern Pacific, whose upper portion has encompassed the surface waters since approximately mid-June. The subsurface negative heat content anomaly has slightly decreased from one to two months ago, but is still substantial. Sea temperature anomalies below the surface often portend SST anomalies to be expected in the following few months, as they are subject to the climatological upwelling in the eastern part of the basin. Current low-level wind anomalies indicate much enhanced trades, especially in the western and west-central portion of the tropical Pacific, and very strongly positive traditional and equatorial SOI indices. These atmospheric features imply above-average upwelling activity in the eastern tropical Pacific, at least for the near term, and suggest that the negative subsurface sea temperature anomalies will likely continue to find their way to the surface during the coming few months. The continued surfacing of below-average subsurface waters implies a likelihood for continuation of the strength of the currently moderate to strong La Niña conditions, although additional cooling appears unlikely at this point as we are near or even just past the usual time of peak strength. In view of the steady strength of the negative anomalies of the east-central tropical Pacific SSTs, we may have already seen the peak strength in October, but there could just as likely be a continuation of the currently moderate to strong SST anomaly level as there could be a slow decrease in their strength during the coming month. Above-average subsurface waters exist in the western tropical Pacific, and although they have been edging eastward during the last few months they do not appear poised to displace the large volume of below-average water to their east for at least a two or more months.
Presently, the models and observations taken together indicate probabilities of approximately 98% for maintaining La Niña conditions, near 2% for returning to ENSO-neutral conditions, and nearly 0% for developing El Niño conditions during the Dec-Feb 2010/11 season in progress. Going forward, probabilities for La Niña continue at 94% or greater through Jan-Mar 2011. In northern spring 2011 the probabilities for La Niña begin to weaken at a faster rate, declining to about 64% by Mar-May and to 43% for Apr-Jun.
The above assessment was made in part on the basis of an examination of the current predictions of ENSO prediction models as well as the observed conditions. For purposes of this discussion, El Niño SST conditions are defined as SSTs in the NINO3.4 region being in the warmest 25% of their climatological distribution for the 3-month period in question over the 1950-present timeframe. The corresponding cutoff in terms of degrees C of SST anomaly varies seasonally, being close to 0.40 degrees C in boreal late-spring to early-summer season and as high as 0.75 degrees C in late boreal autumn. La Niña conditions are defined as NINO3.4 region SSTs being in the coolest 25% of the climatological distribution. Neutral conditions occupy the remaining 50% of the distribution. These definitions were developed such that the most commonly accepted El Niño and La Niña episodes are reproduced.
The models show unanimous agreement regarding the continuation of La Niña conditions into the earliest months of 2011, but vary somewhat in their predictions of the rate of decreasing strength starting from Feb-Apr season. Most statistical and dynamical models call for at least moderate La Niña strength (stronger than -1C) through the Jan-Mar season, and all models except for one (of 22) indicate a waning of strength during Jan-Mar relative to the initial Dec-Feb season. For the Dec-Feb, Jan-Mar and Feb-Apr seasons, 100% of the models are predicting La Niña conditions, while none predicts ENSO-neutral conditions. From Mar-May onward, some models begin indicating a return to neutral ENSO conditions, but not to the point of being a majority until late northern spring and early summer. By Jun-Aug, a few models call for weak to moderate El Niño conditions. At lead times of 4 or more months into the future, statistical and dynamical models that incorporate information about the ocean's observed subsurface thermal structure generally exhibit higher predictive skill than those that do not. Among models that do use subsurface temperature information, 3 of 12 (25%) predict ENSO-neutral SSTs for the May-Jul seasons, 7 of 12 (58%) predict La Niña conditions, and 2 of 12 (17%) predict El Niño conditions. (Note 1). (Note that La Niña conditions for May-Jul require a NINO3.4 SST anomaly of -0.50 or stronger, and El Niño conditions require 0.45 or stronger.) Caution is advised in interpreting the distribution of model predictions as the actual probabilities. At longer leads, the skill of the models degrades, and skill uncertainty must be convolved with the uncertainties from initial conditions and differing model physics, leading to more climatological probabilities in the long-lead ENSO Outlook than might be suggested by the suite of models. Furthermore, the expected skill of one model versus another has not been established using uniform validation procedures, which may cause a difference in the true probability distribution from that taken verbatim from the raw model predictions.
An alternative way to assess the probabilities of the three possible ENSO conditions is to use the mean of the predictions of all models, and to construct a standard error function centered on that mean. The standard error would be Gaussian in shape, and would have its width determined by an estimate of overall expected model skill for the season of the year and the lead time. Higher skill would result in a relatively narrower error distribution, while low skill would result in an error distribution with width approaching that of the historical observed distribution. This method shows probabilities for La Niña at near 100% for Dec-Feb and Jan-Mar, remaining at 96% or more through Mar-May 2011. (Note that the threshold for La Niña weakens from approximately -0.65C to -0.40C between Dec-Feb and Mar-May, due to the seasonality of the interannual variance). The forecasters believe, however, that the Mar-May model-based probability may be higher than it should be, due to a common model bias of persisting ENSO episodes for too long a duration at the end of their typical seasonal cycle. Model probabilities for La Niña are 51% for May-Jul, and decrease to the climatological probability of 25% for Aug-Oct. The same cautions mentioned above for the distribution of model predictions apply to this alternative method of inferring probabilities, due to differing model biases and skills. In particular, this approach considers only the mean of the predictions, and not the range across the models, nor the ensemble range within individual models.
cycloneye wrote:Which model will nail the future of ENSO in the comming months?
Here are two of the most important ENSO Models and they are not on the same page as CFS is for a two year La Nina duration,while,the Australian POAMA Model is Neutral by the Summer. Which one will be right?
A two year La Nina duration? If the CFS (NCEP) model is right,that is what is in the cards.
http://img832.imageshack.us/img832/3861 ... sstsea.gif
However,the POAMA Model says Neutral ENSO by the Summer.
http://img827.imageshack.us/img827/315/poamanino34.gif
Ptarmigan wrote:cycloneye wrote:Which model will nail the future of ENSO in the comming months?
Here are two of the most important ENSO Models and they are not on the same page as CFS is for a two year La Nina duration,while,the Australian POAMA Model is Neutral by the Summer. Which one will be right?
A two year La Nina duration? If the CFS (NCEP) model is right,that is what is in the cards.
http://img832.imageshack.us/img832/3861 ... sstsea.gif
However,the POAMA Model says Neutral ENSO by the Summer.
http://img827.imageshack.us/img827/315/poamanino34.gif
If we have a prolonged La Nina, it would be a first since Fall 1998 to Winter 2001. Prolonged La Ninas happen more than a prolonged El Nino.
KWT wrote:Really 2007 to winter 2008-2009 was a prolonged La Nina as well, not as long as some other events and it did have weaker gaps but that period was pretty much a La Nina pattern till the El Nino of 2009 kicked in during April.
The reason that I did not mention 2007-2009 was because the NOAA did not consider Winter 2008-2009 as La Nina, even thought it should been. Winter of 2008-2009 was interesting because it snowed and for the most part was mild and dry. April 2009 was very wet. 2009 was one of the years that went from La Nina to El Nino. I noticed years that went from La Nina to El Nino are rare, but when it happens, the winters are colder. Some of the coldest winters occurred when the previous winter was La Nina. 
Current Conditions
As of mid-January 2011, SSTs continue to indicate moderate to strong La Niña conditions in the central and eastern equatorial Pacific. For December the SST anomaly in the NINO3.4 region was -1.53 C, indicative of moderate to strong La Niña conditions, and for the October-December season the anomaly was -1.52 C. Currently the IRI's definition of El Niño conditions rests on an index of SST anomalies, averaged over the NINO3.4 region (5S-5N; 170W-120W), exceeding the warmest 25%-ile of the historical distribution, and similarly for La Niña relative to the 25%-ile coldest conditions in the historical distribution. The NINO3.4 anomaly necessary to qualify as La Niña or El Niño conditions for the Jan-Feb-Mar and the Feb-Mar-Apr seasons are approximately (-0.55C, 0.50) and (-0.45, 0.40), respectively.
Expected Conditions
The most recent weekly SST anomaly in the NINO3.4 region is -1.7 C, indicating moderate to strong La Niña conditions in the tropical Pacific; this is just slightly cooler than the -1.53 C level observed in December. What is the outlook for the ENSO status going forward? January is a time of the year during which the observed ENSO state is often beginning to move toward weaker anomaly values, particularly if an ENSO episode has been occurring. One might ask whether the current La Niña condition should therefore be expected to weaken, and if so, at what rate. In the current case, negative subsurface sea temperature anomalies have continued to occupy the central and eastern equatorial Pacific, and have shown little tendency to weaken even in the last one to two months as the event has been in the process of discharging at the surface. Sea temperature anomalies below the surface often portend SST anomalies to be expected in the following few months, as they are subject to the climatological upwelling in the eastern part of the basin. Current low-level wind anomalies indicate much enhanced trades, especially in the western and west-central portion of the tropical Pacific, and very strongly positive traditional and equatorial SOI indices. These atmospheric features imply above-average upwelling activity in the eastern tropical Pacific, at least for the near term, and suggest that the negative subsurface sea temperature anomalies will likely continue to find their way to the surface during the coming two or more months. The continued surfacing of below-average subsurface waters implies a likelihood for short-term (at least one month) continuation of the strength of the currently moderate to strong La Niña conditions, despite that the seasonal cycle of ENSO suggests a typical decline in strength at this time of year. Above-average subsurface waters exist in the western tropical Pacific, and although they have been edging eastward during the last few months they do not appear poised to displace the large volume of below-average water to their east for at least two or more months, and possibly longer.
Presently, the models and observations taken together indicate probabilities of approximately 98% for maintaining La Niña conditions, near 2% for returning to ENSO-neutral conditions, and nearly 0% for developing El Niño conditions during the Jan-Mar 2011 season in progress. Probabilities for La Niña decrease slightly to 88% for Feb-Apr, and to 67% for Mar-May. In late northern spring the probabilities for La Niña weaken at a faster rate, declining to 46% for Apr-Jun and to 27% for Jun-Aug and for the following several seasons.
The above assessment was made in part on the basis of an examination of the current predictions of ENSO prediction models as well as the observed conditions. For purposes of this discussion, El Niño SST conditions are defined as SSTs in the NINO3.4 region being in the warmest 25% of their climatological distribution for the 3-month period in question over the 1950-present timeframe. The corresponding cutoff in terms of degrees C of SST anomaly varies seasonally, being close to 0.40 degrees C in boreal late-spring to early-summer season and as high as 0.75 degrees C in late boreal autumn. La Niña conditions are defined as NINO3.4 region SSTs being in the coolest 25% of the climatological distribution. Neutral conditions occupy the remaining 50% of the distribution. These definitions were developed such that the most commonly accepted El Niño and La Niña episodes are reproduced.
The models show unanimous agreement regarding the continuation of La Niña conditions into part of the second quarter of 2011, but vary somewhat in their predictions of the rate of decreasing strength starting from the Jan-Mar season. Most statistical and dynamical models call for at least moderate La Niña strength (stronger than -1C) through the Feb-Apr season. For the Jan-Mar, Feb-Apr and Mar-May seasons, 100% of the models are predicting La Niña conditions, while none predicts ENSO-neutral conditions. Following Mar-May, some models begin indicating a return to neutral ENSO conditions, but not to the point of being a majority until the May-Jul season. By Jul-Sep, two of the 23 models call for weak El Niño conditions. At lead times of 4 or more months into the future, statistical and dynamical models that incorporate information about the ocean's observed subsurface thermal structure generally exhibit higher predictive skill than those that do not. Among models that do use subsurface temperature information, 6 of 13 (46%) predict ENSO-neutral SSTs for the Jun-Aug seasons, 6 of 13 (46%) predict La Niña conditions, and 1 of 13 (8%) predict El Niño conditions. (Note 1). (Note that La Niña conditions for Jun-Aug require a NINO3.4 SST anomaly of -0.50 or stronger, and El Niño conditions require 0.45 or stronger.) Caution is advised in interpreting the distribution of model predictions as the actual probabilities. At longer leads, the skill of the models degrades, and skill uncertainty must be convolved with the uncertainties from initial conditions and differing model physics, leading to more climatological probabilities in the long-lead ENSO Outlook than might be suggested by the suite of models. Furthermore, the expected skill of one model versus another has not been established using uniform validation procedures, which may cause a difference in the true probability distribution from that taken verbatim from the raw model predictions.
An alternative way to assess the probabilities of the three possible ENSO conditions is to use the mean of the predictions of all models, and to construct a standard error function centered on that mean. The standard error would be Gaussian in shape, and would have its width determined by an estimate of overall expected model skill for the season of the year and the lead time. Higher skill would result in a relatively narrower error distribution, while low skill would result in an error distribution with width approaching that of the historical observed distribution. This method shows probabilities for La Niña at near 100% for Jan-Mar and Feb-Apr, declining to 95% for Mar-May and 69% for Apr-Jun. (Note that the threshold for La Niña weakens from approximately -0.55C to -0.45C between Jan-Mar and Apr-Jun, due to the seasonality of the interannual variance). The forecasters believe, however, that the northern spring model-based probabilities for La Niña may be higher than it should be, due to a common model bias of persisting ENSO episodes for too long a duration at the end of their typical seasonal cycle. Model probabilities for La Niña are 46% for May-Jul, and decrease to less than 40% for Jul-Sep and beyond. The same cautions mentioned above for the distribution of model predictions apply to this alternative method of inferring probabilities, due to differing model biases and skills. In particular, this approach considers only the mean of the predictions, and not the range across the models, nor the ensemble range within individual models.
Synopsis: ENSO-Neutral or La Niña conditions are equally likely during May-June 2011
La Niña persisted during January 2011 as reflected by well below-average sea surface temperatures (SSTs) across much of the equatorial Pacific Ocean (Fig. 1). However, some weakening was evident in certain atmospheric and oceanic anomalies, in part due to Madden-Julian Oscillation activity. Most Niño indices were between –1oC and –1.5oC at the end of January, with the easternmost Niño-1+2 region returning to near-average (Fig. 2). A lessening of the negative subsurface oceanic heat content anomalies (average temperatures in the upper 300m of the ocean, Fig. 3) was observed mostly in association with an eastward shift in the above-average temperatures at depth in the central equatorial Pacific (Fig. 4). Convection remained enhanced over Indonesia and suppressed over the western and central equatorial Pacific (Fig. 5). Also over the western and central equatorial Pacific, the anomalous low-level easterly and upper-level westerly winds decreased in magnitude. Collectively, these oceanic and atmospheric anomalies reflect an ongoing, mature La Niña that has begun to weaken.
Nearly all of the ENSO model forecasts weaken La Niña in the coming months (Fig. 6). A majority of the models predict a return to ENSO-neutral conditions by May-June-July 2011, although some models persist a weaker La Niña into the Northern Hemisphere summer 2011. Recent trends in the observations and models do not offer many hints on which outcome is more likely. Also, model skill is historically at a minimum during the Northern Hemisphere spring (the “spring barrier”). Therefore La Niña is expected to weaken during the next several months, with ENSO-neutral or La Niña conditions equally likely during May-June 2011.
Expected La Niña impacts during February-April 2011 include suppressed convection over the west-central tropical Pacific Ocean, and enhanced convection over Indonesia. Potential impacts in the United States include an enhanced chance of above-average precipitation in the Northern Rockies and western regions of the Northern Plains (along with a concomitant increase in snowfall), Great Lakes, and Ohio Valley. Below-average precipitation is favored across much of the southern states. An increased chance of below-average temperatures is predicted for much of the West Coast and northern tier of states (excluding New England), and a higher possibility of above-average temperatures is forecast for much of the southern and central U.S. (see 3-month seasonal outlook released on January 20th, 2011).
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