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
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.
