Transcript Document

El Niño, La Niña and ENSO
La Niña
Time mean
El Niño
Correlation coefficient of annual-mean sea-level pressure with
pressure at Darwin.
Darwin
Tahiti
ENSO time series and spectrum
year
period (years)
El Niño impacts: Global
Surface temperature (ºC)
Precipitation (mm/day)
NCEP/NCAR Reanalysis 1950-2000
ENSO mechanism: the Bjerknes feedback
Bjerknes feedback - equatorial sections
La Niña
longitude
longitude
ocean
atmos
El Niño
El Niño and Southern oscilation
Southern oscillation index (SOI) = p(Tahiti)-p(Darwin)
SOI (hPa)
Nino 3.4 index (N34) = SST averaged over 120°W-170°W and 5°S- 5°N.
N34 (oC)
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Year
Southern Oscillation index (SOI) vs. Nino 3.4 SST
(N34)
N34 (degC)
Regression line
Pearson’s corr. coeff
Variance explained
N34 = 28.5 – 0.4 SOI
c = – 0.83
c2 = 0.70
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SOI (hPa)
EOFs in 2 dimensions
N34 (degC)
e2
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SOI (hPa)
Teleconnection map
correlation of 500 hPa height at base point (20N,160W) with all other points
base point
Teleconnectivity map, 500 hPa height
PNA
NAO
The map is constructed as follows:
-for each point in the grid, build a teleconnection map using that point as base point;
-assign to that point the maximum (absolute value of) anticorrelation found in the teleconnection map
-draw contours of the resulting field, add arrows showing points connected by max anticorrelation
Pacific/North American pattern (PNA)
500mb height, 1-point correlation map, base point 20N,160W
warm
warm
cool, wet
base point
North Atlantic Oscillation (NAO)
The “NAO index” is defined as the difference in surface pressure measured at Stykkisholmur (Iceland) and Lisbon (Portugal) or Ponta
Delgada (Azores): NAO index = pAzores – pIceland. High positive value of the index means pressure is very low over Iceland and very high over
the Azores.
The map shows the regression of the the NAO index onto the northern hemisphere surface pressure field.
NAO impacts
Temperature
Precipitation
dry
wet
A cartoon of the NAO
High
Low
NAO time series
anthropogenic
climate change?
NAO index time series
Index according to Jones et al. 1997: pGibraltar -pStykkisholmur
or natural variability?
AO or NAO?
no correlation!
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Leading EOF of northern hemisphere surface pressure
Regression of NAO index on surface pressure
The leading EOF of surface pressure shows a pattern which is similar to the NAO in the Atlantic, but is more zonal -- there is a second
“center of action” in the Pacific. This pattern has been labeled the “Arctic Oscillation”, to emphasize that it represents a zonally-symmetric
oscillation centered on the North Pole. In this view, the NAO is just a locally-enhanced manifestation of the global, zonally-symmetric “annular
mode”. However, there is no significant correlation between points in the Atlantic and in the Pacific, I.e no zonal teleconnection between
Atlantic and Pacific basins.
Physics of the AO or “annular mode”
Regression of leading EOF’s amplitude onto
zonal-mean zonal wind
Climatological zonal-mean wind
The idea behind the “annular modes” is that:
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1.
The AO is just the surface signature of a mode that actually fills the
whole troposphere
2.
Random fluctuations in baroclinic eddy activity in the midlatidude storm
tracks lead to random changes in momentum convergence, shifting the
jet axis north or south
Role of zonal asymmetries in creating the NAO
(from recent review paper by Vallis and Gerber, 2007)
1. Observational picture
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Climatological DJF Eady growth rate, a
measure of how baroclinically unstable
the atmosphere is. High growth rate
favours frequent development of
baroclinic storms
Climatological DJF eddy kinetic energy, a
measure of how much eddy activity there
actually is in the atmosphere
Climatological DJF eddy meridional
momentum flux. Positive values mean
momentum is transported northward.
Note the strong momentum convergence
over the center of the Atlantic basin.
The atmosphere is most unstable on the eastern seaboards of continents, but the biggest eddy activity is somewhat downstream in the
middle of the ocean basins. This is because eddies propagate as they develop, reaching maturity further downstream along the storm track.
Eddy momentum fluxes are strongest in roughly the same region.
Role of zonal asymmetries in creating the NAO
(from recent review paper by Vallis and Gerber, 2007)
2. Results from numerical model with zonally symmetric statistics
This figure shows the “teleconnection map” for surface pressure
in a numerical model where forcing and boundaries are zonally symmetric.
The base point is chosen randomly.
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According to the “annular modes” viewpoint, we would expect a rather ringlike patter to emerge, but instead we see something that is quite localized
and looks surprisingly like the NAO.
The leading EOF of surface pressure in this model is zonally-symmetric,
however.
The conclusion is that eddies do transport momentum and cause “wobbles”
in the jet position, but the eddy dynamics are quite local and so the jet
wobbles are local rather than global. In this model, such wobbles occur
randomly at all longitudes, and the EOF represents them as a zonallyuniform “annular mode”.
Role of zonal asymmetries in creating the NAO
(from recent review paper by Vallis and Gerber, 2007)
3. Results from numerical model with zonally asymmetric statistics
This shows results from the same model as before, except that:
-a thermal anomaly representing land-ocean heating contrast has been
inserted (magenta lines)
-a meridionally elongated mountain range representing the Rockies has
been inserted (cyan lines).
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These fixed surface asymmetries set up permanent asymmetries in the
atmospheric fields, leading to a zonally-confined storm track. The
meridional wobbles due to eddy momentum convergence fluctuations still
occur at all longitudes, but are stronger in the region of maximum eddy
kinetic energy. The leading EOF shows a corresponding region of
enhanced variability.
In conclusion, the NAO is due to the same dynamics hypothesized for the
annular modes, but these dynamics occur in a region that is zonally
confined because of the permanent asymmetries in the atmospheric
forcing.