ENSO Influence on Atlantic Hurricanes via Tropospheric Warming

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ENSO Influence on Atlantic Hurricanes via
Tropospheric Warming
Brian Tang and David Neelin Dept. of Atmospheric
and Oceanic Sciences, UCLA Institute of
Geophysics and Planetary Physics
Current affiliation MIT Dept. of Earth,
Atmospheric, and Planetary Sciences
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ENSO ? N Atlantic teleconnection theory
Tropical cyclogenesis and tropical cyclone
intensity theory
During an El Nino, wave dynamics spread
anomalously warm tropospheric temperatures
eastward over the tropical Atlantic. SST is
brought into equilibrium with the troposphere
after 4-5 months.
Warmer than normal tropospheric temperatures
relative to SST increases the moist static
stability of the atmosphere. Tropical
cyclogenesis and intensification are hindered.
If prolonged ENSO induced tropospheric
temperature anomalies occur over the tropical N
Atlantic without being compensated by SSTs,
seasonal tropical cyclone frequency and intensity
may be impacted.
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Indices
NAtlSST Seasonal (Jun.-Nov.) box averaged SST
anomalies NAtlTT Seasonal box averaged
tropospheric temperature anomalies ENSO Seasonal
Nino3.4 anomalies Frequency Seasonal number of
named N Atlantic tropical cyclones Intensity
Seasonal average maximum wind speed of all named
N Atlantic tropical cyclones
Boxed region used for area averaging. Contours
are correlations between NAtlSST and seasonal
tropospheric temperature at each grid point.
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Simple Linear Regression Frequency regressed
on NAtlSST ? Variance explained (R²) 31
Frequency regressed on NAtlTT ? Variance
explained (R²) 1 Multiple Linear Regression
Frequency regressed on NAtlSST NAtlTT ? R²
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NAtlTT contains information independent of
NAtlSST that is important to the physical
pathways that influence TC frequency intensity
in the N Atl.
Time series of NAtlTT and NAtlSST. Note the
collinearity!
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1st principal component EQ PC 90 of the
variance between NAtlTT and NAtlSST Measure of
equilibrium between SST and tropospheric
temperature 2nd principal component DEQ PC
10 of the variance between NAtlTT and NAtlSST
Measure of disequilibrium between SST and
tropospheric temperature, i.e. an empirical
measure of atmos. convective instability relative
to SST
Correlations of defined indices principal
components with N Atlantic TC frequency and
intensity
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DEQ PC tied to onsetting El Nino/La Nina events
Time series of DEQ PC and JunNov Nino3.4
anomalies
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Regression coef for DEQ PC K/K, Frequency
storms/K, Intensity knots/K, Aug-Oct
Nino3.4 K/K regressed on a sliding 3 mon
averaged Nino3.4 window. 0 lag corresponds to
middle of the contemporaneous hurricane season.
Frequency Intensity show a similar pattern as
DEQ PC except inverted. Supports a thermodynamic
pathway between ENSO and TC activity over the N
Atlantic.
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Regression coef for EQ PC K/K and Nov.-Jan.
Nino3.4 (8.5 month lead) K/K regressed on a
sliding 3 mon averaged Nino3.4 window.
EQ PC is strongly influenced by ENSO events from
the preceding fall/winter. By the time hurricane
season rolls around, equilibrium has been
reestablished between the sea surface
troposphere.
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Gridded seasonal (Jun.-Nov.) tropospheric
temperature anomalies regressed on DEQ PC (top)
and regressed on May-July Nino3.4 anomalies (2.5
month lead) (bottom). Regression coefficients
contoured K/K and 90, 95, and 99 significance
levels shaded.
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Gridded seasonal (Jun.-Nov.) SST anomalies
regressed on DEQ PC. Regression coef contoured
K/K and 90, 95, and 99 significance levels
shaded.
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Summary
DEQ PC is associated with departures from SST and
tropospheric temperature varying in equilibrium
can also be interpreted as an empirical measure
of convective instability relative to SST. DEQ PC
has high correlations to N Atlantic TC Frequency
and Intensity relative to both NAtlSST and
NAtlTT. DEQ PC is strongly correlated to ENSO
through the hurricane season and into the
following winter. Onsetting El Nino events create
a disequilibrium state in which NAtlTT is warmer
relative to its normal relationship to NAtlSST
creating a large scale environment which is less
conducive to TC intensification and tropical
cyclogenesis. EQ PC is related to the prior
winter ENSO, consistent with equilibrium being
reestablished after past ENSO forcing on a time
scale of several months.
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Tang, B. H., and J. D. Neelin (2004), ENSO
Influence on Atlantic hurricanes via tropospheric
warming, Geophys. Res. Lett., 31, L24204,
doi10.1029/2004GL021072.
Link http//www.agu.org/journals/gl/gl0424/2004GL
021072/
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Data
Time Period 1979 2003 SST HADISST
(1979-1981) and OISSTv2 (1982-2003) Tropospheric
Temp NCEP/NCAR Reanalysis (1979-2003) ENSO
Nino3.4 Index (5S-5N, 120W-70W) Hurricane Data
Best Track Reanalysis Data
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DEQ PC can be regarded as an empirical measure of
atmospheric convective instability relative to SST
-DEQ PC aNATLTT(pressure) b
(a)
95 significant
Regression coefficients, a, of -(DEQ PC)
regressed on layered NATLTT as a function of
pressure levels
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ENSO influences N Atlantic tropical cyclone
frequency and intensity
Scatter plots of Frequency index and Intensity
index with ENSO index
DEQ PC also influences N Atlantic tropical
cyclone frequency and intensity
Scatter plots of Frequency index and Intensity
index with DEQ PC
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R freq winds nino34 natlsst
natltt eqpc deqpc -------------------
-------------------------------------------------
PIpmin -0.6776 -0.1408 0.2574 -0.8321
-0.4192 -0.6688 0.6480