1.
Lecture  13
General Circulation
Jet stream
El Nino and La Nina
1

2.
Hadley cell
Thermal cell

3.
Hadley cell
Light, variable winds at the equator are known as Doldrums
Similarly, little wind at 30o N and S is known as the Horse Latitudes
3

4.
Polar cell
Thermal cell
What is difference between two convergent Lows: ITCZ and subpolar Low?
4

5.
Ferrel cell
Dynamic cell
Ferrel cell is not successful in
explaining the westerlies aloft.
5

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

7.
Semi-permanent pressure areas and seasonal pressure
areas
7
Fig. 7-26, p. 190

8.
Semi-permanent
Pacific High
Semi-permanent
Bermuda High
8

9.
Semi-permanent
Icelandic Low
Semi-permanent
Aleutian Low
9

10.
Seasonal pressure areas:
Canadian High
Seasonal pressure areas:
Siberian High
10

11.
Semi-permanent pressure areas and seasonal pressure
areas
11
Fig. 7-26, p. 190

12.
at the center of a surface low, the air converges, and then must rise
H
L
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 pressure cloudy
30oN---high pressure sunny
45-60oN---low pressure cloudy
Polar latitudes---high pressure clear
13

14.
Weather associated with
The Pacific and Bermuda Highs
Pacific High (1) moves northward during
summer 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 This air
can be unstable

15.
Weather associated with
The Pacific and Bermuda Highs
15

16.
Jet Steams

17.
Polar Jet Situated at about 10 km AGL over the polar front (30-70oN)
Subtropical Jet Situated above the subtropical highs at about 13 km AGL(20-50oN)
17

18.
18
1 knot = 1.151 miles per hour

19.



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

21.
Jet stream formation
Polar Jet
Q: How is the
subtropical jet
formed?
21

22.
Which position is fast?
B
A
22

23.
Jet stream formation
Subtropical Jet
-formed on pole ward side of Hadley cell
-created largely through the conservation of angular momentum
-angular momentum = mVr
m=mass
V=velocity
r=radius
V1
-conservation of angular momentum
m1V1 r1
m 2V 2 r2
constant
r1
m1
V2
r2
m2
23

24.
El Nino and La Nina

25.
• Coastal divergence results in upwelling as cold
water rises to replace surface water


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
 East-west circulations caused by continent and topography
PGF
Typical Walker Circulation
rising air is over the western Pacific, and sinking
air is over the eastern Pacific
26

27.
Abnormal Walker Circulation
 El Nino year occurs with an abnormal Walker circulation
Circulation during an El Nino
PGF
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 related to the abnormal Walker circulation
Spanish name for a little boy
during certain years, the coastal waters near Peru were
abnormally warm, causing unfavorable fishing conditions. This
would occur during the Christmas period
occur every 3-7 years, lasting about one year
La Nina related to the strong Walker circulation
Spanish name for a little girl
29

30.
Previous El Niño Years
1902-1903
1905-1906
1911-1912
1914-1915
1918-1919
1923-1924
1925-1926
1930-1931
1932-1933
1939-1940
1941-1942
1951-1952
1953-1954
1957-1958
1965-1966
1969-1970
1972-1973
1976-1977
1982-1983
1986-1987
1991-1992
1994-1995
1997-1998
2002-2003
2009-
30

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
during an El Nino year, the SSTs in the eastern Pacific become more warmer
than in a normal year
high pressure shifts from the eastern Pacific to the western Pacific
low pressure shifts from the western Pacific to the eastern Pacific
this 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

34.
ONI
Oceanic Nino Index

35.
Nino Region 3.4

36.
Oceanic Niño Index (ONI)
• 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.
• 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 – ERSST.v3b). The SST
reconstruction methodology is described in Smith et
al., 2008, J. Climate, vol. 21, 2283-2296.)
• NOAA’s 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
– October 2011) is
-0.4oC.
El Niño
neutral
La Niña