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The Weather Classroom
Your Alaskan guide through the murky waters of El Nino and La Nina -- Section 2
John Papineau -- NWS Anchorage
Mid-latitude Impacts
Up until now we have only considered what occurs in the tropics within about 20°N/S of the equator.
There are however important impacts that EN and LN produce outside of the tropics in the middle and at
times in the high latitudes. Keep in mind that the largest impacts typically occurring during the
winter months. Probably the largest impact occurs as changes intropical heating and rainfall
alters the strength and position of the jet stream winds (there are three jet streams: sub-tropical,
polar and arctic). The jet streams are bands of strong winds on the order of 100-200 mph that exists
at an altitude of roughly 6 to 8 miles, depending on latitude and time of year. There is
considerable week-to-week variation in the strength and position of these jets. The sub-tropical
jet for example is not continuous around the globe, but when it is present it is usually found over
the eastern Pacific and Atlantic Oceans between 20° and 30° (in both hemispheres). The polar jet
is the most continuous of the three in space and time, it typically ranges between 30° and 60° latitude.
The arctic jet comes and goes and is much weaker than the other two jets. All of these jets are much
stronger during the winter than at other time of the year.
Jet stream winds do not simply move along stationary tracks from west-to-east around the globe,
rather they frequently follow 'wave-like' patterns. The crests of the waves are called 'ridges'
(areas of high pressure) and the troughs of the wave are called something unique like 'troughs'
(or a 'low' if the pattern is circular in nature). For the most part, large weather systems move
along the northern edge of the polar jet stream. As you might imagine, storms moving along the
western edge of a ridge transport warm and moist air from lower to higher latitudes. On the east
side of a ridge (which maybe the west side of a trough), cooler and often drier air is transported
from higher to lower latitudes. Over the course of a winter season there is a continual sea-saw
between ridges to troughs over Aleutian Islands and Gulf of Alaska. Where a particular ridge or
trough forms is very important to what type of weather Alaska will experience. In addition, due
to the large size of our state, one type of weather pattern may not of course cover the entire state.
Above: A comparison of El Nino's warm strip in Dec. 1997 and Dec. 2002. Sea surface temperature anomalies
in these maps were computed from measurements of sea surface temperature collected by the AVHRR sensor
on the NOAA polar orbiting satellites.
http://science.nasa.gov/headlines/y2003/14mar_elnino2002.htm
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How Events are Measured
There are several different ways to measure ENSO events. The oldest method consists of taking
the sea-level pressure difference between Tahiti and Darwin. When the pressure is considerably
higher in Darwin for at least four months, an El Nino usually occurs. Conversely, when the pressure
in Tahiti is higher, the mode switches to La Nina. The back-and-forth (on seasonal timescale)
nature of these pressure changes has been termed the Southern Oscillation, while the sea-level
pressure values are called the Southern Oscillation Index (SOI). A different index, called Nino3.4
is based on the temperature of the water in the central and eastern equatorial Pacific Ocean
(within 5° of the equator) and how that temperature compares to the long-term average water
temperature in the same region (anomaly). An EN is likely when the water temperature anomaly
is greater than +1oF for a number of consecutive months. Likewise, LN are likely when the water
temperature anomaly is greater than -1°F. A third index is the so called Multi-ENSO Index (MEI),
which examines five different parameters. These parameters include the SOI and Nino3.4 but also
the wind field, air temperature and cloudiness. The important point to remember about these indices-
they do not always agree on the existence of a particular ENSO event. The largest events match
well across all three indices but the moderate events may or may not match up. The National
Weather Service, whether good or bad, has adopted Nino3.4 as it's index of choice.
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