|What is La Niña?|
La Niña conditions are currently in place across the equatorial Pacific. La Niña is the opposite of El Niño. These two events are the negative (La Niña) and the positive (El Niño) extremes to a naturally occurring phenomena known as the El Niño Southern Oscillation (ENSO). An oscillation is a motion that repeats itself over a period of time. For ENSO that period is between about 3 to 7 years. The defining characteristics are Sea Surface Temperature (SSTs) anomalies (departures from average) across the central and eastern equatorial Pacific. During La Niña conditions, cooler than average SST’s are found along the equator in the central and eastern Pacific region (right hand figure above). Just the opposite occurs during El Niño events (left hand figure).
The importance of these SST anomalies lies in the fact that they largely dictate where tropical thunderstorms will develop and be the most persistent. Thunderstorms thrive over warm ocean waters in the same way tropical storms and hurricanes do in the Atlantic. During La Niña events, the warmest ocean waters are confined to the western equatorial Pacific region. Therefore, this is the preferred placement for tropical thunderstorms during the Northern Hemisphere cold season. These thunderstorms can be considered as a “bridge” between the ocean and the atmosphere. As these thunderstorms develop, they induce low pressure within the western Pacific region, while high pressure sets up across the eastern equatorial Pacific where thunderstorms are less favorable (Right hand figure above). This leads to stronger easterly trade winds (flow from high to low pressure). In return, these stronger trade winds help reinforce the SST pattern by pushing the warm water west and enhancing the strength of the cool eastern Pacific water. It is this process that produces significant changes to the atmospheric circulation in the tropics and also throughout much of the Northern Hemisphere.
The figure above shows several corridors across the central and eastern equatorial Pacific. SST’s are monitored within these regions. The area labeled as Niño 3.4, which runs from 170° W to 120° W, is used to determine the presence and strength of a La Niña event. If the running monthly average SST anomalies are cooler than -0.5°C within this area, La Niña conditions are present. The strength of LaNiña is determined by the extent that the anomalies fall below the -0.5°C threshold. For example a moderate La Niña is in place if the anomalies exceed -1.0°C. Similarly, a strong La Niña is defined as anomalies below -1.5°C.
The stronger an event is, usually means it will have a greater impact on the placement and strength of the cold season storm track across the Northern Hemisphere. The figure below displays the typical winter season storm track during La Niña events. The main characteristic of the flow pattern is the presence of a large area of high pressure across the North Pacific. This area of high pressure acts as a “block” to the upper level flow, which causes the storm track to buckle northward around the high, and then southward across western North America. Strong and more persistent arctic outbreaks tend to be more common during this type of pattern across the Northern Plains, with above average precipitation possible during the winter and spring seasons.
Current conditions, shown in the figure below, suggest that La Niña conditions are already in place across the equatorial Pacific. The top half of the figure shows observed temperatures across the Pacific, while the bottom half of the figure displays the departure from average. Notice that the warmest water is confined to the far west. However, farther east, a tongue of cool water is emerging along the equator in the central and eastern Pacific. The departures from average suggest that SSTs are already -1.0 to -2.0 C east of the International Date Line (longitude 180°).
The figure below shows that this La Niña event is expected to maintain itself through the winter season. This figure represents the forecast for the average SST anomalies within the Niño 3.4 region. The black line tracks the past conditions while the red lines represent a group of forecasts for the evolution of SST anomalies. Finally, the blue line represents the average of all the red forecasts. Overall, this figure shows that the models are forecasting the possibility of a strong La Niña event for the 2010/2011 winter and spring seasons.
The Pacific Decadal Oscillation
The figure below displays the characteristics of the two phases of the PDO. The positive phase is characterized by warmer than average eastern Pacific water and cooler western and north central Pacific water. However, during the negative phase the warmest SST anomalies are confined to the western and north central Pacific, with relatively cool SSTs across the eastern Pacific.
Considering that the PDO and ENSO are in similar phases, (e.g., La Niña is the cool phase of ENSO and the negative PDO is the cool phase of the PDO) research suggests that this could enhance the typical La Nina winter and spring season impacts. This means that this La Niña event could have a stronger influence on the strength and location of the mean winter and spring season storm track.
The following information is based on local research at the Bismarck, ND office. This information is meant to supplement the official CPC outlook on the local scale. Results from Bismarck are shown. However, the results were similar across much of western and central North Dakota.
The table below displays the average winter monthly and seasonal temperatures and their departures from average (tan cells). Notice that monthly temperatures during -PDO La Niña events (which is forecast this year) are in excess of 2°F below average, and snowfall 1 to 2 inches above average. This may not seem like a lot, but it is significant as these represent the average temperature/precipitation anomalies over 17 similar events. The magnitude of the departures from normal are dampened by averaging over 17 events. The actual departure from normal for a -PDO La Nina event in some cases has been shown to be much higher than the 17 event average.
|Bismarck, ND||-PDO La Niña
Remember that the winter season storm track during these -PDO La Niña events supports the potential for stronger and more persistent arctic outbreaks. To further prove this point, consider the graphs below. These graphs display the probability of exceedance for: the number of winter season days with low temperatures below 0°F (left hand side), the number of winter season days with high temperatures above 45°F (right hand side) and the average winter season temperature (bottom). The green lines represent the probabilities of exceedance for an average year (labeled all years), while the blue lines represent those during -PDO La Niña events. In order to interpret these graphs, consider the 50% and 30% percentiles on the x-axis (horizontal axis). For the number of winter season days with low temperatures below 0°F, an average year would have a 50% chance of having 13 or more days below zero. Similarly, you can gather that there is a 30% chance of exceeding 15 days and a 10% chance of exceeding 18 days. During La Niña events there are a higher number of days below zero for a given probability. For example, the 50th percentile increases from 13 days to around 16 days. Likewise the 30% probabilities of exceedance increases from 15 days to 18 days. Additionally, there are fewer days with temperatures exceeding 45°F for a given probability. This suggests that more persistent arctic outbreaks are leading to higher chances for very cold temperatures and a lower chance of experiencing warm temperatures. This is further supported by the fact that the average winter season temperature during these events is lower given the same probability as a normal year.
Winter season precipitation (snow) tends to be above average during -PDO La Niña events. This is primarily due to the fact that an active storm track is in the vicinity of the Northern Plains. The figures below display the probabilities of exceedance for liquid equivalent precipitation (left figure) and for snowfall amounts (right figure). The overall result is that there are higher probabilities for experiencing higher amounts of winter season precipitation/snowfall during these La Niña events. Eleven of seventeen of these -PDO La Niña events had winter precipitation that ranked in the top 50 wettest/snowiest on record. Of these eleven, seven were in the top 25 and two ranked in the top 10.
During the spring season (March-May) temperatures also show a cooler than average signal. This is due to the fact that the winter season storm track tends to remain in place during the spring. The table below shows that average monthly temperatures remain below average during April and May. This could potentially lead to a late spring season melt. Additionally, there is a strong signal for above median precipitation during April and May. The probabilities of exceedance for spring season precipitation also suggest higher chances for receiving more precipitation than during a typical year. For example, the figure shows that during -PDO La Niña events there is a 70% chance for having more than 4.25” of spring season precipitation and a 50% chance of having more than 5.00”. These same probabilities suggest that during a normal spring season there is a 70% chance of having more than 3.00” and a 50% chance of having more than 4.00” of precipitation.
This strong precipitation signal during April and May, combined with the potential for a later melt, could lead to an enhanced threat for above average flows on area streams during the spring of 2011. Does this mean another year of significant flooding? Not necessarily. The threat for spring flooding is highly uncertain at this time. There are several variables that will determine this threat aside from precipitation and temperatures. This outlook is just meant to act as a guide showing an increased potential for more precipitation and cooler temperatures during the winter and spring season of 2010-2011. This does not mean every day will be extremely cold. It simply means that temperatures averaged over the entire winter and spring season could be colder than the climatological average. Expect the typical significant day to day variability in temperatures commonly experienced during North Dakota winters.
|Bismarck, ND||-PDO La Niña events||17 events|