As we move into each season, many ask the question “What is the long range seasonal weather forecast outlook? While initially that may seem like a basic question that should have an “easy answer,” given the advances in computers, as you start to unfold all of the components that influence our longer term patterns, you quickly realize it is not. Then as you include the finer details of the day-to-day weather changes, you quickly realize that answering that question is still complex, simply because the atmosphere is COMPLEX!

Even so, there are some drivers that provide insight of the overall medium to long range trends. You just have to recognize that we will still have day-to-day and weekly fluctuations.

What are the Main Drivers of Multi-Month Seasonal Forecasts?

One of the main drivers of seasonal forecasts is the El Niño-Southern Oscillation (ENSO) state. The ENSO state is an evaluation of the sea surface temperatures in a specific region of the Pacific Ocean along the equator. Most people are probably familiar with the terms El Niño or La Niña which describe the ENSO state. The primary indicator for the ENSO state is the Oceanic Niño Index (ONI). The ONI value is the Pacific Sea Surface temperatures over a 3-month running average period. While there are several sections that are monitored, the key region is the Nino 3.4 area (shown in the graphic below.)

ENOS Nino regions

 

What are La Nina and El Nino?

La Niña conditions exist when the Pacific Ocean sea surface temperatures in the Niño 3.4 area are COOLER than normal and the index is -0.5 or lower.

El Niño conditions exist when the Pacific Ocean sea surface temperatures in the Niño 3.4 region are WARMER than normal resulting in an ONI index value of 0.5 and higher. 

Neutral is when the ONI value ranges between +0.5ºC and -0.5ºC.

The Pacific Ocean tends to go through cycles. According to the International Research Institute for Climate and Society at Columbia University 

Historically speaking….El Niño and La Niña events tend to develop during the period of Apr-Jun and they: 

  • Tend to reach their maximum strength during October – February.
  • Typically persist for 9-12 months, though occasionally persisting for up to 2 years
  • Typically recur every 2 to 7 years”

The graphic below shows the ONI index since 2008. The areas in blue represent periods of La Niña and the regions in yellow representing times of El Niño.

 

For the latest Sea Surface Temperatures (SST) see the loop below

In addition, the graphic below shows the most recent ONI observations and forecasts. 

 

 

US Seasonal Impacts of La Niña and El Niño 

For the US, La Niña and El Niño tend to influence mainly winter regimes. However, when you look globally, the ENSO state can have impacts with any season. Below shows the impacts of a La Niña pattern.

While the following shows the main impacts with an El Niño pattern. 

For more information about La Niña and El Niño, Climate.gov has additional insight. North Carolina State University also has additional details about ENSO.

How Does ENSO Influence Hurricane Development?

La Niña and El Niño can also impact hurricane activity in both the Atlantic and Pacific basins.  In a La Niña scenario, the Atlantic Basin tends to be more active which increases the chances for the continental U.S. and the Caribbean Islands to experience hurricanes. However, even with an active hurricane season, if the storms do not move across land, then the land impacts tend to be less with main effects to offshore operations. In contrast, during a La Niña, the Pacific tends to be less active.

The graphic below shows the typical hurricane activity in compared to a normal year.

With an El Niño the reverse occurs and the Atlantic/Caribbean tends to have less activity, while the Pacific is more active.

For more information about ENSO and hurricanes see the Climate.gov article focusing on Hurricane development.  

ENSO and Droughts in the US

Since the ENSO state tends to shift the weather pattern, a longer term La Niña event can promote a long dry period which can lead to a drought, especially across the southern part of the US. However, the winter of 2016-2017 is an excellent example of how ENSO is not the only driver and other atmospheric influences do play a role. In the fall/winter of 2016-2017, instead of it being a dry period which is what was anticipated with a La Niña winterwe received significant precipitation across the parts of the west.

For more information about ENSO and Droughts see National Drought Mitigation Centers article on ENSO and Droughts.

 

ENSO and Severe Weather?

According to an NWS Climate Prediction Center Article, since the ENSO state influences the jet stream patterns, there are some connections between ENSO and severe weather outbreaks. With El Niño,

the jet stream is oriented from west to east across the southern portion of the United States. Thus, this region becomes more susceptible to severe weather outbreaks. During La Niña the jet stream and severe weather are likely to be farther north. 

So while we are not able to forecasts for specific events, ENSO does influence which regions of the country will be more prone to severe weather events.

Other Factors that Influence the Weather Pattern

While ENSO pattern influences the general weather pattern, there are other factors that play a role. Some of the more known factors include the Madden-Julian Oscillation (MJO), North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO). Note: in recent years many in the media have referred to the AO as the “Polar Vortex.” While the term “Polar Vortex” is a new term, the understanding of the Arctic Oscillation by meteorologists is NOT new.

What is MJO?

MJO is an atmospheric wave disturbance that tends to move eastward in the tropics. It generally rotates around the globe on a 30 to 60-day cycle. MJO is different from ENSO as it is an atmospheric disturbance vs a stationary sea temperature shift. In addition, MJO is a wave pattern that moves around the globe, thus we can see multiple MJO events in a given season. Since it has a higher variability or intraseasonal tropical climate variability which changes from week-to-week, it is more difficult for the longer range models to identify the details of this feature.

For more information about the MJO see Climate.gov

What is NAO?

The North Atlantic Oscillation (NAO) evaluates two atmospheric pressure areas in the North Atlantic. Changes in the pressure levels in these regions, influence the path of the jet stream, especially for the eastern US. These shifts in jet stream impact the temperature and precipitation patterns.  When we are in a positive NAO situation, the eastern U.S. typically sees stronger winter storms and overall is in a wetter period. When we have negative NAO, the eastern U.S. is generally colder and drier. (Note: The MJO interacts with the NAO, but they are separate atmospheric features.)

What is AO?

The Arctic Oscillation Index evaluates what is occurring with the wind and pressure patterns in the Arctic (This feature tends to be more of a winter impact). When the index is in a negative phase, the upper-level winds are weaker, which allow cold Arctic air to push farther south than normal. So we tend to get cold outbreaks in the US. In addition, storm tracks tend to be farther south. When the AO is in a positive phase, the polar winds are stronger which keeps the cold air “locked” farther north and allows the storm track to remain north.

For more information about the NAO and AO, see the North Carolina State Climate office’s article on NAO and AO.  

Note: while the AO it is a different feature than the NAO and the MJO, they can all interact which is a part of what makes seasonal outlooks challenging.

How does MJO interface with ENSO?

Since the MJO is an atmospheric wave in the tropics, it is influenced by the ENSO state. The graphic below shows how the two features can phase or oppose each other since they move on different timescales.

 

When ENSO and MJO are in phase and they both are supporting a wetter pattern, their phasing can lead to an extremely wet pattern. But when they are in phase with a dry pattern, it can cause a very dry pattern.

Final Note about Seasonal Outlooks

A key point about the outlooks, they are multi-month averages and generally are heavily weighted by the ENSO state. The outlooks will not provide details for the day-to-day swings in the weather or details with individual storms. Interactions with features such as the MJO, AO and NAO will impact the path of low-pressure systems that can cause different conditions than what the broader outlooks may indicate. Finally, the complexities of ice, snow, heavy rain and thunderstorms are finer scale and more detailed than what the climate models are able to represent.  So you could experience “extremes” in a given period but when the multi-month information is averaged together, the longer range forecasts still verify.

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