1) The document discusses general circulation patterns like the Hadley cell and polar cell. It also covers jet streams like the polar jet and subtropical jet.
2) El Nino and La Nina events are described as being related to changes in ocean temperatures and atmospheric circulation in the Pacific. During El Nino, warm water shifts eastward.
3) The impacts of El Nino include changes to weather worldwide through teleconnections. The Oceanic Nino Index (ONI) is used to define El Nino and La Nina thresholds.
3. Hadley cell
ïLight, variable winds at the equator are known as Doldrums
ïSimilarly, little wind at 30o N and S is known as the Horse Latitudes
3
6. Scales of Atmospheric Motions
Time and space scale of atmospheric motions
Typical size
Global scale 5000 km
Synoptic scale 2000 km
Mesoscale 20 km
Microscale 2m
Typical life span
6
12. ï at the center of a surface low, the air converges, and then must rise
H
L
ï at the center of a surface high, the air diverges, and must come
from aloft due to sinking motion 12
13. Zonal distribution of precipitation:
0oN---low pressureï cloudy
30oN---high pressureï sunny
45-60oN---low pressureï cloudy
Polar latitudes---high pressure ï clear
13
14. Weather associated with
The Pacific and Bermuda Highs
Pacific High (1) moves northward during
summer ï produced strong subsidence
inversion on eastern side (2) during
winter, it moves south allowing polar fronts
to bring precipitation to SW US
14
Bermuda High transports
moist, warm subtropical air to US
and southern Canada ï This air
can be unstable
17. Polar Jet ï Situated at about 10 km AGL over the polar front (30-70oN)
Subtropical Jet ï Situated above the subtropical highs at about 13 km AGL(20-50oN)
17
19. ïŒ
ïŒ
ïŒ
often have troughs and ridges
generally have a maximum (jet streak) in the base of the trough
transport heat pole ward (cold air south and warm air north)
19
20. Jet stream formation
Polar Jet
10
8
200 mb
500 mb
PGF
700 mb
6
5 km
km
4
2
Warm
Cold
0
EQ
45oN
Large temperature
Gradient at surface
20
NP
25. âą Coastal divergence results in upwelling as cold
water rises to replace surface water
ï§
ï§
brings cold water from deep ocean to the
surface
cold, nutrient-rich water rises to replace the
surface water (good for fishing)
26. Walker Circulation
ï East-west circulations caused by continent and topography
PGF
Typical Walker Circulation
ï rising air is over the western Pacific, and sinking
air is over the eastern Pacific
26
27. Abnormal Walker Circulation
ï El Nino year occurs with an abnormal Walker circulation
Circulation during an El Nino
PGF
ï El Nino refers to eastern movement of
warm water from the western equatorial
Pacific to the eastern equatorial Pacific
27
28. The animation of sea surface temperature (SST)
anomalies to the right shows the unusual warming
that occurred during the 97-98 event.
Note that an anomaly is a departure from some
"normal" value. An anomaly can be either positive
(warm) or negative (cold)
29. El Nino ï related to the abnormal Walker circulation
Spanish name for a little boy
ïduring certain years, the coastal waters near Peru were
abnormally warm, causing unfavorable fishing conditions. This
would occur during the Christmas period
ïoccur every 3-7 years, lasting about one year
La Nina ï related to the strong Walker circulation
Spanish name for a little girl
29
31. Sea surface temperature (SST) in a normal year
Low pressure dominates the western Pacific over warm water, and
high pressure dominates the eastern Pacific over cold water.
31
Fig. 7-32, p. 196
32. SST in an El Nino year
ïŒduring an El Nino year, the SSTs in the eastern Pacific become more warmer
than in a normal year
ïŒhigh pressure shifts from the eastern Pacific to the western Pacific
ïŒlow pressure shifts from the western Pacific to the eastern Pacific
ïŒthis shift in surface pressure is called El Nino Southern Oscillation (ENSO)
32
Fig. 7-32, p. 196
33. El Nino can have a dramatic effect on weather around the
world, creates both precipitation and temperature anomalies.
These changes are referred to as the teleconnections
33
36. Oceanic Niño Index (ONI)
âą The ONI is based on SST departures from average in the
Niño 3.4 region, and is a principal measure for
monitoring, assessing, and predicting ENSO.
âą Defined as the three-month running-mean SST departures
in the Niño 3.4 region. Departures are based on a set of
improved homogeneous historical SST analyses
(Extended Reconstructed SST â ERSST.v3b). The SST
reconstruction methodology is described in Smith et
al., 2008, J. Climate, vol. 21, 2283-2296.)
âą NOAAâs operational definitions of El Niño and La Niña are
keyed to the ONI index.
37. NOAA Operational Definitions for
El Niño and La Niña
El Niño: characterized by a positive ONI greater than or
equal to +0.5 C.
La Niña: characterized by a negative ONI less than or
equal to -0.5 C.
By historical standards, to be classified as a full-fledged
El Niño or La Niña episode, these thresholds must be
exceeded for a period of at least 5 consecutive
overlapping 3-month seasons.
CPC considers El Niño or La Niña conditions to occur when the
monthly Niño3.4 OISST departures meet or exceed +/- 0.5°C along
with consistent atmospheric features. These anomalies must also
be forecasted to persist for 3 consecutive months.
38. ONI (oC): Evolution since 1950
The most recent
ONI value (August
â October 2011) is
-0.4oC.
El Niño
neutral
La Niña