An ENSO-Neutral Winter

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Lynne Anthony
6 years ago
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In making a long-range outlook, it is vital to start with an understanding of the key factors that could shape seasonal temperature and precipitation anomalies.

Winter outlooks often are weighted towards the anticipated El-Nino-Southern Oscillation (ENSO) regime.

Short term anomalies, as indicated by specific teleconnections, will more likely shape the ultimate outcome for winter.

1 An ENSO-Neutral Winter This issue of the Blue Water Outlook newsletter is devoted towards my thoughts on the long range outlook for winter. You will see that I take a comprehensive approach to this outlook which includes pre-existing land-based parameters, longer-range teleconnections, and climate modeling. In making a long-range outlook, it is vital to start with an understanding of the key factors that could shape seasonal temperature and precipitation anomalies. While all long range outlooks are difficult, and often contain limited accuracy, this years winter outlook will be even more complex. Winter outlooks often are weighted towards the anticipated El-Nino-Southern Oscillation (ENSO) regime. The high probability of an ENSO-Neutral winter (as described within this report) will lead towards increased seasonal uncertainty. Short term anomalies, as indicated by specific teleconnections, will more likely shape the ultimate outcome for winter. While there is a degree of accuracy in the prediction of these anomalies, predictive skill is limited to the short term. Each season sets the stage for the following one to some degree. Winter is a critical season as it can shape broad-scale water resources well into the new year. Winter precipitation patterns, especially late winter, will play a key role in the recharge of water resources and set the stage for spring agriculture operations. I will also be keeping an eye on the development of critical precipitation excesses or deficits that could lead towards operational disruptions. The following 6 parameters were considered in my outlook for winter: Past trends, ENSO, Land-based interactions, Climate models, Sea ice considerations, and Teleconnections. This outlook is only a start to BWO coverage of this important season. Updates, insight, and possible changes, can be expected on a frequent basis throughout the winter season. I hope that you find the information contained in this report useful for your operational planning. I will be tracking and analyzing the development of winter season trends, variables, and anomalies throughout the winter season. Blue Water Outlook 1

You likely have heard about the important role of feedback within a BWO briefing this past summer.

Feedback is important in the consideration of summertime rainfall, however the role is much less significant during the cold season.

While pre-existing soil moisture impacts on the winter outlook are minimal, there were several anomalies of note. A wet spring-summer anomaly over the Southeast U.S. has been diminishing, if not ended.

Let s keep these trend anomalies in mind in the formulation of the winter forecast. The coverage and extent of the snowpack can impact future precipitation.

Recent studies have shown that observed rapid declines in sea ice in autumn can be associated with an unusual number and intensity of extreme cold events and large snowfalls in mid-latitudes.

2 You likely have heard about the important role of feedback within a BWO briefing this past summer. Anomalies of precipitation can have a feedback role where wet areas are more likely to receive additional precipitation while dry areas tend to remain dry. Feedback is important in the consideration of summertime rainfall, however the role is much less significant during the cold season. Hot summer sunshine and rapid crop growth enhances evapotranspiration (ET). However, during the winter, with cold temperatures, a lower sun angle, and low vegetation growth, ET is much less. While pre-existing soil moisture impacts on the winter outlook are minimal, there were several anomalies of note. A wet spring-summer anomaly over the Southeast U.S. has been diminishing, if not ended. A persistent dry anomaly over the central Midwest persists. A new wet anomaly has developed over the Northern Plains. Let s keep these trend anomalies in mind in the formulation of the winter forecast. The coverage and extent of the snowpack can impact future precipitation. Often, areas of low pressure, and subsequent winter precipitation, will repeatedly move along the edge of a well-developed snow pack. Recent studies have shown that observed rapid declines in sea ice in autumn can be associated with an unusual number and intensity of extreme cold events and large snowfalls in mid-latitudes. The image below plots the extent of arctic sea ice extent, along with a comparison with normal and historical average levels. A cool and stormy arctic summer maintained overall sea ice extent, especially compared with the record low amount last year. Arctic sea ice dropped to a seasonal low on September 13th with very little change for the rest of the month. Arctic sea ice then increased significantly towards the very end of September due to strong seasonal cooling. With sea ice either remaining unchanged or expanding during the early part of fall, there are no clear signs that sea ice patterns will bring colder or snowier weather to the nation this winter. Blue Water Outlook 2

3 Recent-past and longer-term precipitation and temperature trends might be of assistance in the winter outlook. Can one identify either a short-term or longer-term winter trend in temperature or precipitation? The following graphs show key trends for the Contiguous U.S. for the 4-month period consisting of December through March a key water recharge period. Regional plots are contained on the following regional pages. U.S. Winter Long-Term Temperature Trend U.S. Winter Short-Term Temperature Trend Since 1900 Since 2000 Normal Trend Line A clear and significant warming trend has been observed over the long-term period (top/left). The image to the top/right indicates a shorter-term period, since For the Contiguous U.S., note that the vast majority of the past 13 years have experienced above-normal cool-season temperatures. U.S. Winter Long-Term Precipitation Trend U.S. Winter Short-Term Precipitation Trend Since 1900 Since 2000 There has been greater variability in precipitation trends. The longer-term trend indicates slightly greater precipitation (top/ left). However, taking a look at the short-term trend (top/right) you can see that recently years have tended dry. SUMMARY: An analysis of recent trends would tend to point towards warmer-than-normal cool-season temperatures with slightly below-normal precipitation. Keep in mind that these are national trends and that there could be much more variability in regional analysis. Blue Water Outlook 3

4 The El Niño-Southern Oscillation (ENSO) signal is typically used as a major predictor for the winter season. El Niño is a band of unusually warm sea temperatures that develop from time to time while La Niña conditions represent unusually cool sea surface temperatures. Sea surface temperatures are easily and reliability observed using a variety of sensors. Research has provided strong linkages between the ENSO regime and associated precipitation and temperatures patterns for certain regions. ENSO has been in a neutral phase since early in 2013 (left). Key models predict a high probability that neutral conditions will persist through the winter months (left/below). With ENSO being a large component of any winter forecast an ENSO-neutral winter will provide few, if any, clues as to the direction of either temperatures or precipitation. There are signs of warming starting very late in the winter or during the spring with increasing possibilities of El Niño by the middle of It will be important to watch for any sign of early warming of the NINO3.4 SST Index throughout the winter. An early increase in Pacific Ocean temperatures might point towards enhanced spring rainfall. However, for now, this is only a remote possibility. Note the convergence of the two black lines above. This indicates decreasing chances for ENSO-Neutral and increasing chances for El Niño the first half of Blue Water Outlook 4

While the general rule is that an ENSO-Neutral regime will not trend precipitation one way or the other, it might be of interest to look at past similar ENSO years.

Precipitation anomalies were determined for these similar years. While there is no significant anomaly trend for much of the nation, as one might suspect, there are two areas of interest.

It is interesting to see a fall precipitation anomaly over the Midwest that does generally align with the composite (left).

5 While the general rule is that an ENSO-Neutral regime will not trend precipitation one way or the other, it might be of interest to look at past similar ENSO years. The current neutral conditions have been unusually persistent. Composites were identified for similar years with persistent ENSO-neutral conditions heading into winter. Precipitation anomalies were determined for these similar years. While there is no significant anomaly trend for much of the nation, as one might suspect, there are two areas of interest. DRY Dry anomalies are a possibility for parts of the Midwest, centered over Missouri, as well as the Pacific Northwest. It is interesting to see a fall precipitation anomaly over the Midwest that does generally align with the composite (left). While it might be a stretch, is it possible that this is a sign of a developing winter precipitation anomaly? Persistent hemispheric imbalances in atmospheric pressure will likely shape winter temperature and precipitation patterns. Unfortunately, long-lead forecasts of these imbalances, as identified by specific short-duration teleconnections, cannot be forecast much in advance. Perhaps we can look at some recent winter trends in one specific teleconnection, the North Atlantic Oscillation (NAO), for some clues. The following graph plots NAO data starting in More frequent but moderate positive events Intense and deeper negative events You can see that there is both seasonal and annual variability within the NAO (left). It would seem that there have been less but more extreme low or negative events with more frequent but more moderate positive events. However, in an assessment of the winter season outlook, a more recent analysis might be warranted. Taking a look back at the past 5 winter season, negative NAO months have outnumbered positive months by a factor of two to one. Thus, based on this admittedly-narrow perspective, data might hint towards a slight bias towards more frequent cold NAO events. Blue Water Outlook 5

The PDO tends to be a background teleconnections with shorter-term teleconnections tending to have an overriding nature.

6 In additional to short-term anomalies such as the AO and NAO, there are long-duration, decadal oscillations. The Pacific Decadal Oscillation (PDO) can influence temperature and precipitation patterns during the winter. The PDO tends to be a background teleconnections with shorter-term teleconnections tending to have an overriding nature. Decadal oscillation typically work in tandem with the ENSO regime, intensifying its impacts. The PDO is currently in a long-term negative, or cool, phase. Impacts from a coolphased PDO will likely be subtle, and often overshadowed by other factors. However, there are some possible anomalies to consider. Under ENSO-Neutral conditions, winter impacts of the PDO are likely to be modest. However, one of the more likely stronger correlations, especially in light of climate models, might be decreased precipitation over the South and Southwest U.S. Anomalies NW U.S. Temperatures Southeast U.S. Temperatures Southern U.S. Precipitation NW and Great Lakes Precipitation Cool Phase PDO Below Normal Above Average Below Average Above Average The Atlantic Multi Decadal Oscillation (AMO) is another long-duration oscillation of sea surface temperature. It has been in a warm phase since the mid 1990s. During a warm phase AMO, droughts can be more frequent over the Midwest and Southwest. One of the stronger correlations with a warm AMO is an increase in rainfall over Central and South Florida. The warm phase AMO, while a secondary contributor to shorter-duration oscillations, might point towards reduced winter precipitation over the Midwest and Southwest. BWO will cover these two long duration oscillation in much more details in subsequent issuances over the winter months. Blue Water Outlook 6

Climate models are often used as a primary predictor of season temperature and precipitation patterns.

The International Research Institute (IRI) is one of many seasonal models and one widely used in winter predictions.

There are no significant areas of above-normal (green-shaded) precipitation forecast. There are several areas where below-normal precipitation is forecast (yellow-shades).

7 Climate models are often used as a primary predictor of season temperature and precipitation patterns. A pool of multiple models, or an ensemble of models, often is utilized to provide more accuracy and identify a more reliable solution. The International Research Institute (IRI) is one of many seasonal models and one widely used in winter predictions. The next two images show the IRI precipitation model for the months of December, January, and February. There are no significant areas of above-normal (green-shaded) precipitation forecast. There are several areas where below-normal precipitation is forecast (yellow-shades). These areas include parts of Florida, the Southeast and Southwest U.S. Yellow = Below Normal Precip These next two images indicate the IRI temperature forecast with blue shades indicating cooler-than-normal temperatures and red shades indicating above-normal temperatures. The strongest signal appears to be a call for above-normal temperatures over the South and Southwest U.S. Red = Above Normal Temps Blue Water Outlook 7

8 The two images below indicate that recent ( ) winters have trended on the dry side with significant variability in temperature. The wet spell that developed last winter and extended into the summer has been broken. The ENSO regime is a major cool-season regional predictor for the Southeast U.S. A Neutral-ENSO winter will make this season hard to predict. The BWO winter outlook is aligned towards climate models which seem to have a reasonable grasp on the current dry spell. Based on these models, precipitation is likely to average below normal over the 3-month winter period. A cool phase PDO might suggest a bias towards above average winter temperatures, however under an ENSO-Neutral regime, this bias likely will be slight. Winter Temperature Trends Winter Precipitation Trends In consideration of the cool-phased PDO, a temperature outlook of higher chances of above normal temperatures looks reasonable. Remember that short-term teleconnections will likely have a greater role in the seasonal outlook than long duration oscillations. The recent trend in the NAO towards occasional strong negative events could continue this winter. Strong NAO events tend to align semistationary fronts over parts of the Southeast U.S., with mixed precipitation including the possibility of occasional significant snow or ice events during the coldest part of winter. It is possible that the winter will average out with below-normal precipitation but still experience several significant winter events. One variable worth watching is the rate of NINO3.4 sea surface temperature warming as indicated by climate models. A more rapid warming rate than currently expected by climate models could point towards enhanced late winter and early spring precipitation. It could also reinforce the prospects of messy late winter mixed precipitation events. Blue Water Outlook 8

9 While ENSO-Neutral conditions will not provide much clue to the winter outlook, climate models indicate the strongest forecast anomalies for the U.S. extend over the Southwest U.S. These anomalies call for warmer-than-normal temperatures and generally below-normal precipitation. While the above-normal temperature pattern covers almost the entire South, the precipitation pattern is not as consistent. The core of below-normal precipitation is centered over the Southwest U.S., only brushing western sections of Texas and Oklahoma. It is possible that anomalies of above-normal temperature might be more pronounced over western sections of Oklahoma and Texas, with readings averaging closer to normal over the rest of the region. Winter Temperature Trends Winter Precipitation Trends Trend maps of temperature and precipitation for recent winters are shown to the left. These two graphs indicate that there has been significant variability in winter temperature and precipitation over the South since However, a factor that does leans towards below-normal precipitation is the continued cool phase Pacific Decadal Oscillation (PDO). Southern sections of the U.S. trend towards belownormal precipitation during cool PDO events. While an ENSO-Neutral regime, and recent variability in trends, will tend to reduce overall confidence in the winter outlook, other factors point towards and warmer and drier winter. Climate models and the cool-phased PDO hint towards drier and warmer conditions centered over the Southwest, but likely spreading eastward into parts of Oklahoma and Texas. Keep in mind that short-term teleconnections, such as the AO & NOA, will likely have a greater role in the seasonal outlook than long duration oscillations. The recent trend in the NAO, towards occasional strong negative events, could likely continue this winter. Strong NAO events tend to align semi-stationary fronts over parts of the South U.S., with mixed precipitation including the possibility of occasional significant rain or mixed precipitation events during the coldest part of winter. Blue Water Outlook 9

The following two graphs plot recent (2000-2012) winter temperature and precipitation trends for the corn and soybean belts.

Winter Temperature Trends 2000 2012 Winter Precipitation Trends 2000-2012 The winter recharge of both upper and lower soil moisture is important during the winter months to support subsequent summer

10 The following two graphs plot recent ( ) winter temperature and precipitation trends for the corn and soybean belts. Note that winters have been steadily been getting warmer while precipitation has been more variable. Winter Temperature Trends Winter Precipitation Trends The winter recharge of both upper and lower soil moisture is important during the winter months to support subsequent summer agriculture operations. Ample winter/spring moisture last season ( ) helped support, to a degree, agriculture operations and yields, across areas where drought conditions developed. An ENSO-Neutral winter will more likely than not point towards equal chances for either above or below normal temperatures and precipitation. However, as indicated in this report, there are some signs of past ENSO-Neutral years resulting in drier-than-normal winter conditions over parts of the Midwest, especially lower Midwest. Keep in mind that short-term teleconnections, such as the AO & NAO, will likely have a greater role in the seasonal outlook than long duration oscillations. The recent trend in the NAO, towards occasional strong negative events, could likely continue this winter. Areas of low pressure often moved across the Midwest during strong NAO events bringing a wide variety of precipitation. This includes the possibility of significant snow events during the coldest part of winter. Blue Water Outlook 10

Here s what a weak El Nino usually brings to the nation with temperatures:

Here s what a weak El Nino usually brings to the nation with temperatures: Time again for my annual Winter Weather Outlook. Here's just a small part of the items I considered this year and how I think they will play out with our winter of 2018-2019. El Nino / La Nina: When looking