Transcript elnino.intro.winterschool.2016x

El Niño / Southern Oscillation
13.10.1997
Overview El Nino
1. History
2. Phenomenon
3. Dynamics
4. Research
History
 El Niño: was always known to fishers in South America.
Coastal waters of the Pacific coast of South America had dramatic warming
events every few years around Christmas time (that why it is call El Nino)
causing large-scale fish dying or immigration to other regions.
 Southern Oscillation: Sir Gilbert Thomas Walker ~1910.
Walker was analyzing the variability of Indian Monsoon variability and found it to
be related to very large-scale atmospheric sea level pressure variability to the
south of India, which he called the Southern Oscillation, as it is south of India. But
it actually is more or less right on the equator.
 ENSO coupling: Bjerknes 1969.
Bjerknes was one of the first researchers that understood that El Nino and the
Southern Oscillation (ENSO) are not only related to each other, but that the
interaction between the two may actually be the causes for the variability in both.
History
 ENSO numerical model: Cane and Zebiak, Science, 1985.
Cane and Zebiak were the first to demonstrate that a numerical model of
the ocean and atmospheric dynamics in the tropical Pacific could
reproduce the ENSO mode. Modern numerical climate models are used to
predict the ENSO evolution for the next few month to one year. It is
basically the only process that allows for seasonal weather forecast in the
tropical regions.
 ENSO research today:
The ENSO is still a subject of ongoing research. Many aspects of the ENSO
mode and how the interaction/feedbacks work are still unclear. It is also
currently researched how ENSO may change in the changing climate and
how ENSO relates to interactions with the rest of the world.
Overview El Nino
1. History
2. Phenomenon
3. Dynamics
4. Research
SST standard deviation
[K]
El Niño event 1997
El Niño event 1997
El Niño event 1997
EOF-1 (44%)
EOF-2 (10%)
EOF-3 (5%)
EOF-4 (5%)
Temperature [oC]
El Niño time series
El Niño power spectrum
Log-log scaling
Log-linear scaling
Subsurface dynamics
Evolution of temperature anomalies, January 1997
Subsurface dynamics
Evolution of temperature anomalies, April 1997
Subsurface dynamics
Evolution of temperature anomalies, September 1997
Subsurface dynamics
Evolution of temperature anomalies, Januar 1998
Southern Oscillation
Correlation SLP vs. NINO3 SST
Overview El Nino
1. History
2. Phenomenon
3. Dynamics
4. Research
SST standard deviation
[K]
ENSO Dynamics
Mean state
Bjerknes Feedbacks
Recharge Oscillator
ENSO Dynamics
Mean state
Bjerknes Feedbacks
Recharge Oscillator
The General Circulation
Momentum :
Coriolis forcing
Pressure gradient
gravity
force
friction
Dynamics at the Equator
Ocean at rest (not responding to winds):
winds
height
warm
cold
Dynamics at the Equator
Ocean Surface:
winds
Eq.
Ocean surface currents
winds
Dynamics at the Equator
Ocean in equilibrium:
winds
height
warm
cold
Dynamics at the Equator
Atmosphere at equator (no SST gradient):
height
Convection (air lifting)
warm
warm
warm
Dynamics at the Equator
Atmosphere in equilibrium (with SST gradient):
height
Walker Circulation
warm
cold
Mean Winds and SST
Mean State
Mean State: Sea Level Pressure (SLP)
Surface pressure
[hPa]
Mean State: Precipitation
Mean State
Variability: SST for different states
La Nina
Normal
El Nino
Variability: Southern Oscillation
Variability: El Nino / Southern Oscillation
ENSO Dynamics
Mean state
Bjerknes Feedbacks
Recharge Oscillator
Bjerknes Coupled Feedbacks
1. SST forced Wind Anomaly
Layer Thickness
Sea Level
SST Anomaly
3. Heat Content forces SST
Heat Content
Anomaly
2. Wind forced Heat Content change
[Co]
Clouds
+
(-)
SST
+
Subsurface
(+)
Winds ()
SST vs. zonal winds
Relation between the zonal wind field and the SST in the box. For the
three tropical oceans separately
zonal winds vs. heat content
Relation between the 20oC isotherm depth field and the zonal wind in
the box. For the three tropical oceans separately
Heat content vs. SST
The local relation between the 20oC isotherm depth field and the SST field.
Mean Sea Surface Temperature
Mean SST
[oC]
ENSO Dynamics
Mean state
Bjerknes Feedbacks
Recharge Oscillator
Recharge Oscillator model of ENSO
Recharge Oscillator model of ENSO
Recharge Oscillator model
dT
= a11T + a12 h + x T
dt
dh
= a21T + a22 h + x h
dt
a11 = T growth rate (damping)
a12 = coupling T to h
a21 = coupling h to T
a22 = h growth rate (damping)
x T = noise forcing T
x h = noise forcing h
a11 = a11A + a11O
a11 = [c1Ct T +c 2 C fT ] + a11O
Ct T = wind response
C fT = net heat response
a11O = T damping (ocean)
Recharge Oscillator model
dT
dT
=-aa11
h
=
T+
+a
a12
12h
11T
dt
dt
dh
dh
=-aa21
=
T+
-a
a22
h
21T
22 h
dt
dt
1
a11 = -0.076 month
1
a12 = +0.021 month
1
a21 = -1.4 month
1
a22 = -0.008 month
Recharge Oscillator model
dT
= - a11 T + a12 h
dt
dh
= - a21 T - a22 h
dt
Recharge Oscillator model of ENSO
El Nino Forecast
Current conditions
El Nino Forecast
El Nino Forecast