The NCPP “Missouri River Basin/La Nina Pilot” held monthly webinars in collaboration with NOAA's Climate Predictions Center and Central Region climate services director, the South Dakota State Climatologist and Missouri Basin River Forecast Center. These webinars presented a discussion on ongoing climate anomalies and climate outlooks to stakeholders from the Missouri River Basin region. This FAQ provides background information on seasonal outlooks developed at the NOAA Climate Prediction Center and on two of the important climatic influences on the region: La Nina and the Arctic Oscillation.
What is a Seasonal Outlook?
A seasonal outlook uses the current conditions in the oceans, snow and ice distribution, as well as the atmosphere as a starting point, just as a weather forecast would. But there is no attempt for an actual prediction of individual storms. Rather, the outlook tries to put bounds on the atmospheric (and ocean) flow over the course of a few weeks and months, and thus describes the general "flavor" the weather might more likely take in comparison to the “climate normal".
What does Climate Normal mean?
A climate normal does not have to be a weather / climate condition that is recurring year after year, but rather describes the central part of a range. Seasons typically are characterized by conditions falling between any two end points such as warm and cold, wet and dry, or stormy and calm. A seasonal outlook tries to describe how the coming weeks and months likely are going to fall within such a previously experienced range in a region. It provides a guide to the local audiences, such as farmers, businesses, and resource managers, for whom there can be great benefit in getting a "heads up" about the expected "flavor". Of course this is particularly the case if indications are that a season is more towards one of the extreme ends of the experienced range.
How to interpret the seasonal climate outlooks?
The seasonal climate outlooks are issued by the Climate Prediction Center at the National Weather Service. The outlooks contain maps developed based on probabilities. These probabilities show increased chances toward below or above average conditions at various locations or equal chances where there is not enough skill to change the outlook. Generally the deeper the colors you see the more confidence the outlook has. Thus lighter shades tend to have a lower confidence and lower probability. The white areas are pretty much equal chance of above, normal or below. The outlooks are produced regularly and focus on periods of length 1-week, 2-weeks, 1 month, 2, 3, 4 and 5 month in the future.
Temperature Outlook for 2011/11/23-27
In the plot above the outlook indicates a greater than normal probability for warm conditions in the central and eastern parts of the conterminous US and greater than normal probability for colder conditions in Alaska.
Precipitation Outlook for 2011/11/23-27
In terms of precipitation, the outlook indicates greater than normal chance of wet conditions in the north-eastern and western parts of the country while the middle of the country and Alaska are forecasted to be dry with a greater than normal probability.
What are El Nino and La Nina?
Globally, the most powerful driver of climate for the coming months and seasons is located in the tropical Pacific, where the strongest convection slips along the equator between the far western Pacific and more central to eastern Pacific. Handily, not only is this tropical Pacific variability the strongest of the modes of climate variability from year to year, it is also one of the most predictable (see other modes of climate variability - there will be a link to another page here).
Generally, the convection tends to stay in the western section over warm waters of what is called the warm pool in the tropical Pacific. But every few years, 2-7 on average, an El Niño develops when the warm equatorial waters spread eastward and displace the cool upwelling waters below the surface. The convection follows this warm water and with that changes the flow of where tropical moisture enters into the higher latitudes. In reverse, stronger trades can push the warm water far westward, leading to enhanced upwelling along the equator and thus to colder eastern Pacific, a phase called La Niña.
What are Modes of climate variability?
“Natural variability of the climate system, in particular on seasonal and longer time scales, which predominantly occurs with preferred spatial patterns and time scales, through the dynamical characteristics of the atmospheric circulation and through interactions with the land and ocean surfaces. Such patterns are often called regimes, modes or teleconnections. Some examples are the North Atlantic Oscillation (NAO), the El Niño-Southern Oscillation (ENSO), the Northern Annular Mode (NAM; previously called Arctic Oscillation, AO) and the Southern Annular Mode (SAM; previously called the Antarctic Oscillation, AAO).”
How do El Nino and La Nina influence the path of the storm systems entering the Western US during winter?
Important for North America is how the high pressure system off the West Coast is connected to El Nino and La Nina (see Figure below). The high pressure system, commonly called the Hawaiian High, tends to be weakened and displaced under the warm El Niño, or it gets strengthened under the cool La Niña phase. It’s this Hawaiian High that is operating as a guide for the jet stream, and thus to storms that hit the west coast.
(Jet stream is the term for the relatively strong winds concentrated within a narrow stream in the atmosphere – Glossary of Meteorology, AMS).
A weaker subtropical system under El Niño allows for more storms to propagate far southward with increased rain into southern California and the Southwest. The far Northwest tends to get less precipitation than normal. Under the other extreme, La Niña, the subtropical high pressure system tends to be more robust and therefore can generally more effectively block storms from penetrating southward along the West Coast. The Pacific Northwest, in such La Niña years tends to get a much larger amount of precipitation while the Southwest tends to remain dry.
From wrh.noaa.gov through voices.wahsingtonpost.com
How does La Nina typically impact the Missouri River Basin?
For the Missouri River Basin, the difference between El Niño and La Niña is somewhat less direct than for the West Coast because it is located a little further down the "train" of the flow. Despite being further downstream, the figures below show how on average the distribution of precipitation, temperature and snow during winter tends to separate clearly between El Niño and La Niña over the upper Missouri Basin.
The maps below are from http://www.cpc.ncep.noaa.gov/products/precip/CWlink/ENSO/enso.anal.shtml
Most often (with about 60-70% of occurrence) the temperatures are lower in the Missouri river basin compared to neighboring areas during a La Nina year, while they are average in an El Nino year.
The precipitation anomalies are not that focused in one direction indicating that conditions are average during either La Nina or El Nino years.
The snow anomalies indicate that the area with greater snow amounts includes the northern and western parts during DJF and expands to include the entire basin in March during La Nina years, while El Nino years are characterized with much less snow in general.
How were the 2010/2011 and 2011/2012 winter seasons different?
Despite the fact that the two winters of 2010/11 and 2011/12 were characterized by moderately strong La Niña patterns the temperature and snow amounts distributions in the Missouri River basin and around the US differed markedly. The 2011/12 winter was substantially warmer than the 2010/11. Along with the higher temperatures in the 2011/12 winter came the significantly reduced snow cover, both indicators of a storm track that was located further north.
While it is well known that not all El Niño / La Niña events are the same (somewhat different spatial patterns of where the sea surface temperatures peak, how the convection is displaced), it is also important to recognize that the influence from the Pacific is not the only driver of year to year climate variability. The winters 2010/11 and 2011/12 are a perfect example to illustrate that.
From the tropical side, the two winters looked quite similar, but from a polar perspective, where the other side of the jet stream guidance comes from, the early winters could not have been more different! The 2010/11 winter was marked by a circulation around the pole that was diverted into numerous waves leading to extreme exchange between lower latitude warmer air with polar cold air. This led to frontal systems repeatedly dropping southward over the Midwest, bringing cold air and accumulating large amounts of snow. This year the flow around the Northern Hemisphere high-latitudes was almost circular with a strong pressure gradient guiding the winds in a west to east direction, inhibiting exchange between the lower latitude warmer air and the polar cold air.
The metric to describe this flow around the pole is often summarized as the "Northern Annular Mode" or "Arctic Oscillation". It describes how the circulation is oriented around the pole (or within a segment, e.g. the North Atlantic and Europe) – see the figure below.
Effects of the Positive Phase | Effects of the Negative Phase
of the Arctic Oscillation of the Arctic Oscillation
(Figures courtesy of J. Wallace, University of Washington)
A positive phase brings stronger circulation around the North pole, stronger Westerlies, fewer cold outbreaks to the south and a storm track at a more northern location for the US.
A negative phase is related to weaker circulation around the North pole, more cold air outbreaks to the south and a southern displacement of the storm track over the US.
Looking at the differences between 2010/11 and 2011/12 one can see that this high-latitude annular mode was opposite, and not just opposite, but each time rather close to an extreme. Despite the comparable influence from the tropics between these two years, it is clear that on the polar side the steering could hardly be any more different. Indeed, in 2011/12 over North America the storm track was drawn northward and remained extremely zonal with hardly any fronts dropping southward to bring cold air and snow over the Midwest. There was a short disruption of the zonal flow in late January and early February, and immediately cold air reached southward (not only over N. America, but equally so over Europe after having experienced a warm early winter).
This comparison of the jet stream and its orientation as being driven both from the tropics as well as from the high-latitudes illustrates the complexity that is involved in weather and interannual climate variability.