ENSO Teleconnections and the North Atlantic Ocean An Exercise in ...

ENSO Teleconnections and
the North Atlantic Ocean
An Exercise in Correlation
Ben Shaw
MPO 671
Fall 2008

Motivation
• We are familiar with the mean state:
• Strong trades push warm waters to the western
Pacific
• Cool water is upwelled in the eastern Pacific
• The thermocline slopes from deepest in west to
shallowest in east
• BUT: El Nino/Southern Oscillation (ENSO) can
occur, forcing the system into an anomalous
state.
• Trades weaken, westerly wind bursts push warm
water & rain to the east.
• Teleconnections have far-reaching effects.
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• By calculating the correlation between
timeseries, I hope to:
– Gain a better understanding of the
geophysical processes that allow ENSO
events to be felt thousands of kilometers
away.
– Make a few hypotheses in support of
these correlations
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Methodology
• If processes in the Pacific have an effect on
the North Atlantic, correlation between the
regions should be fairly robust.
• I compare the NINO3.4 SSTA timeseries with
timeseries of observations in the North
Atlantic.
(iri.columbia.edu)
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Correlation in Matlab
• 'corrcoef' outputs the results of a
normalized covariance function
• By applying this function to the NINO3.4 SSTA
and North Atlantic SSTA timeseries, we are
able to plot contour maps showing correlation
between the two.
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NINO3.4 & N. Atlantic SSTA Correlation
lag=0
lag=3
lag=6
#1
#3
#2
lag=9
lag=12
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Discussion
• What causes the three Areas of Interest (AOI)?
• AOI 1: Occurs simultaneously with NINO3.4
autocorrelation
• Rapid adjustment - likely the result of an
atmospheric process
• AOI 2: Lag correlation reveals the connection
• Slower adjustment - indicates an oceanic process
• Possibly resulting from propagation of AOI 1's
negative signal into a positive phase
• AOI 3: Appears at lag=3 months, lingers even up
to 12 months
• Perhaps the result of changes in the gyre
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AOI 1
• If it's an atmospheric adjustment, we might
be able to tell by calculating the correlation
coefficients between NINO3.4 and North
Atlantic wind speed...
lag=0
lag=4
...or maybe not
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• Maybe correlating against mean sea level
pressure will tell us something
Lag=0
(www.nssl.noaa.gov)
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NAO vs. ENSO
• ENSO and MSL pressure are positively
correlated at zero lag in high latitudes of
North Atlantic
• Indicates that ENSO is partially responsible for
causing the negative phase of NAO
• NAO's Negative Phase:
• Weaker Icelandic low
• Weaker Azores high
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AOI 2
• Follows a pattern similar to the “atmospheric
bridge” discussed in Lee et al. 2008
• Establishes itself following long-lasting ENSO
events (through JFM)
Lee et al
(2008)
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AOI 2 - Speculation
• Does a plot of winds at 200hPa really help us?
• Another hypothesis:
• As the negative correlation fades, it may evolve
into a positive phase
lag=0 lag=3 lag=9
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AOI 3
• Why does it sit so snugly up against the North
American coast? And why does it persist
beyond the other two phenomena?
• Location is just north of Gulf Stream
separation point (Cape Hatteras, NC)
• Possibly the result of changes in the Gulf
Stream retroflection as a result of the oceanic
adjustment that manifests itself as AOI 1
• Taylor et al. (1998) found that the Gulf Stream
separation point was sensitive to both the
NAO and ENSO, but at a lag time of 2 years
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Conclusions
• ENSO has areas of significant correlation
against SSTA in the North Atlantic
• ENSO doesn't seem to contribute significantly
to the NAO, instead may act to weaken it
• Atmospheric processes appear to generate a
negative SSTA correlation in the subtropical
North Atlantic
• This perturbation then evolves into a region of
positive correlation
• Gulf Stream dynamics are difficult to extract
from a 1-year lag study, especially if ENSO's
signal is felt at an average lag of 2 years
• Correlation studies are useful tools, but
require much more work and insight to isolate
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the responsible dynamics

References
NOAA's National Severe Storms Laboratory
http://www.nssl.noaa.gov/
NOAA's National Weather Service Climate Prediction Center
http://www.cpc.ncep.noaa.gov/
International Research Institute for Climate and Society
http://iri.columbia.edu/climate/ENSO/enso.html
Lee, S.-K., D. B. Enfield, C. Wang (2008), Why do some El
Ninos have no impact on tropical North Atlantic SST?
Geophys Res. Let., 35, L16705.
Taylor, A.H., M.B. Jordan, J.A. Stephens (1998), Gulf Stream
shifts following ENSO events. Nature, 393, 638.
Stewart, R.H., Introduction to Physical Oceanography, Dept.
of Oceanography Texas A & M University, 2006.
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