Sunday, February 26, 2017

Teleconnections what the heck are they part two


Here is part two of the teleconnection post. This post will cover four major teleconnections. This won't make you an expert. But it will give you an idea of what they are and how they work.
 
The Arctic Oscillation (AO)

Before I can talk about the AO. I have to talk about the Polar Vortex. Because the AO and PV are forever linked.

One other thing to remember, is the phrase geopotential deals with how much work it takes raise a parcel of air from sea level to a certain altitude against the pull of earth's gravity.  Something I show a lot in my weather outlines and outlooks is the 500mb geopotential height charts, to show the state of the atmosphere.

The PV is an area of low pressure circulation over the arctic. The PV is a normal part of the atmosphere and is around the entire year, but most of us only hear about it in the winter. The PV is stronger in the Summer and weaker in the winter.     The strength of the low pressure circulation is the process we call the AO.

The AO is a climate index on the state of the atmospheric circulation over the Arctic. Like all teleconnections it has a positive and a negative state.
 

Positive phase

During the positive phase the AO features below average geopotential heights, which means the PV is strong. The lower area of low pressure at the mid latitudes is higher (weaker), due to the air travels from areas of high pressure to low pressure.  This means the air flow is toward the arctic. Because of this the westerlies and northeasterly trade winds are flowing much stronger.  The stronger winds keep the cold corralled in the arctic.  So the air over the lower 48 is warmer.

Negative phase

In this phase there are above average geopotential heights, which means the PV is weaker. The higher area of low pressure at the mid latitudes is higher (weaker), due to the air travels from areas of high pressure to low pressure.  This means the air flow is away from the arctic.  The winds circling the arctic are weaker, allowing for the PV meander north and south, allowing colder arctic air to invade the mid and low latitudes.  So cold air overruns North America invading the lower 48.  

Impacts on the Eastern U.S.

  • AO+ is associated with above-average temperatures
  • AO- contributes to colder winters and an increase in nor’easters (coastal storms) for New England states

 

North Atlantic Oscillation (NAO)

The NAO is defined by the difference in surface pressure between two atmospheric centers of action, the Icelandic Low and the Azores High.

The NAO is one of the major players in the climate variability in the Northeast and North Atlantic. It's these east west oscillation motions, that represent a north/south swing in pressure across the North Atlantic, that control the strength and direction of the westerly winds and storm tracks.

When the NAO is in its positive phase, we see upper level ridging over the Eastern U.S. The ridge causes a predominant south-westerly flow which  brings warmer temperatures into the Eastern CONUS.

Impacts on the Eastern U.S.

  • NAO index is high (NAO+) it's associated with above-average temperatures.

The NAO is also believed to have an impact on the weather over much of eastern North America. During the winter, when the index is high (NAO+), the Icelandic low draws a stronger south-westerly circulation over the eastern half of the North American continent which prevents Arctic air from plunging southward. This effect can produce significantly warmer winters over the northeastern United States and southeastern Canada.

  • NAO index is low (NAO-) contributes to colder winters and an increase in nor’easters (coastal storms) for New England states

When the NAO index is low (NAO-), the northeastern United States and  even the southeastern United States can incur winter cold arctic outbreaks . A strong NAO- contributes to a weakened jet stream that normally pulls zonal systems into the Atlantic Basin, thus contributing to heat waves.

 


The Pacific North American (PNA)

Is one of the major teleconnections that controls weather for the entire northern Pacific to the Eastern Seaboard of the U.S.

The PNA effect the circulation pattern over the Pacific for every month, except June and July. This circulation moves east over North America. It consists of anomalies in the geopotential height field (normally at the 500 and 700 mb level) over North America.  The disruptions in the Pacific Jet Stream affect weather downstream here in North America.

The positive phase

The positive phase involves above normal geopotential heights of the western US. This typically causes ridging over the western U.S. and troughing over the eastern U.S. This allows Canadian cold air to drop southeastward into the eastern U.S. When the PNA is positive we typically see an increase in southeast and Mid Atlantic winter storminess.

The negative phase

During the negative phase we see the trough over the western U.S. and the ridge over the eastern U.S. This allows the warm moist air from the Gulf of Mexico to lift north and east into the Mid Atlantic and Northeast; so these areas see above normal temperatures and humid conditions. The PNA has a much bigger impact on temperatures during the winter than during the summer. 

Positive phase
 
Negative phase


The Madden-Julian Oscillation (MJO)

The MJO is a pattern of suppressed and enhanced rainfall that shifts eastward in the tropics.  Anomalous rainfall becomes evident at the start over the western Indian Ocean, moves eastward into the equatorial Pacific Ocean, and then into the western hemisphere where the anomalous rainfall pattern becomes less apparent. It takes about 30-60 days to make it completely make it around the equator. During the winter, the MJO can be correlated to enhanced precipitation along the east coast of the United States. 

In the enhanced convective phase, winds at the surface converge, and air is pushed up throughout the atmosphere. At the top of the atmosphere, the winds reverse (i.e., diverge). Such rising air motion in the atmosphere tends to increase condensation and rainfall.

The MJO is quite complicated. But, depending on the time of year each phase can have a different impact on temperature and precipitation across the U.S. The MJO consist of 8 phases. In general the MJO phase response is 12 days.  During the winter months , the MJO can be correlated to enhanced precipitation and cooler temperatures along the east coast of the United States. 

Here are a couple of charts that show on the MJO typically impacts the U.S. during January, February, and March, as well as for June, July, and August.


 
That's enough for part two. Part three will cover the El Nino Southern Oscillation, The Southern Oscillation and briefly cover the other major teleconnections.
 
 

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