el niño diversity, cross-equatorial winds, and the intertropical … · 2018. 11. 9. · extreme...
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El Niño diversity, cross-equatorial winds, and the intertropical convergence zone
Shineng Hu
Scripps Institution of Oceanography, UC San Diego
In collaboration with Alexey Fedorov (Yale U.)
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Cross-equatorial winds control decadal changes in El Niño diversity and ITCZ
Shineng Hu
Scripps Institution of Oceanography, UC San Diego
In collaboration with Alexey Fedorov (Yale U.)
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A La Niña-like mean-state shift over recent decades
McGregor et al. (2014)
(1992-2011)Decadal trend in SST and SLP (contours)
Source: JISAO
PD
O index
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Weakened El Niño activity since 2000(1
980-1
999)
(2000-2
010)
El N
iño c
om
posi
tes
E. Pacific
(EP) El Niño
C. Pacific
(CP) El Niño
ENSO
reduction
McPhaden et al. (2011);
Ashok et al. (2007); Kug et al. (2009); Kao and Yu (2009);
Yeh et al. (2011); Takahashi and Dewitte (2015); etc.
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ITCZ stopped crossing the equator since 2000
Southward migration of ITCZ during El Niño
Nino3
Rainfall anomaly averaged over eastern Pacific (150°W-90°W)
(°C)
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A summary of recent changes in the tropical Pacific
Major climate features since 2000, as compared to 1980s & 1990s:
• A La Niña-like mean state shift (“negative PDO”).
• Weakened ENSO variability with more frequent CP El Niño;
• ITCZ barely crossing the equator even at the peak of 2015 extreme El Niño;
What is the cause of these changes?
Externally forced or internally generated?
Capotondi et al. (2015); Yu et al. (2015); Meehl et al. (2016); Levine et al. (2017); and many more others
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Tropical Pacific mean state
Colors: SST (°C) Contours: precipitation (mm/day) Arrows: surface winds (m/s)
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Multidecadal trends in SST, surface winds & precipitation (contours)
1982-2015 trend
(°C/dec)
• The strengthening of cross-equatorial winds is a robust feature found in satellite,
reanalysis, and in-situ wind products.
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Cross-equatorial winds (red) versus Nino3 (black): weakly correlated
• The multidecadal strengthening of cross-equatorial winds is, most likely, not
caused by ENSO changes.
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Origin of cross-eq. wind strengthening: local component
TEPy = Northern Box SST – Southern Box SST
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Origin of cross-eq. wind strengthening: local component
TEPy = Northern Box SST – Southern Box SST
Residual after linear regression
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Origin of cross-eq. wind strengthening: remote component
VEP-residual = Residual of VEP after regressing onto TEPy
TTNA = Tropical North Atlantic SST anomaly
• Our coupled simulations suggest that the
observed TNA warming and the
subsequent coupled response could
explain 60-70% of the decadal
strengthening of cross-equatorial winds
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Imposing observed wind trend to a coupled model
Imposed meridional wind stress• Community Earth System Model (CESM)
• Ocean-atmosphere coupled
• 2°atmosphere, 1°ocean
• Flux adjustment over tropical Pacific
• Two 500-yr simulations to detect robust
changes in ENSO (Wittenberg 2009)
• CTL: Control
• CrE: Imposed cross-equatorial winds
(~30 years of multidecadal trend)
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Ocean currents response to cross-equatorial winds
Surface zonal currents (cm/s)
• Consistent with Philander and Pacanowski (1981)
Arrows: surface wind stress (10-2 N/m2)
Surface meridional currents (cm/s)
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Tropical Pacific mean state response to imposed cross-equatorial winds
SST (colors), precipitation (contours), and surface winds
Model
(°C/dec)
Observation
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ENSO response to cross-equatorial winds
Model Observation
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Mixed-layer heat budget of Niño3 in El Niño development phase
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Extreme El Niño response to cross-equatorial winds
• The probability of extreme El Niño events decreases by about 40–60%, depending
on whether extreme events are defined via SST or precipitation.
SST anomaly based Precipitation based
Cai et al. (2014); Power et al. (2013)
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Is El Niño changing?
El Niño
La Niña
(Fedorov and Philander 2000; Capotondi and Sardeshmukh 2017; and many more others)
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Cross-equatorial winds in CMIP5 modelsR
CP8.5
min
us
his
tori
cal
Observed range
• Too-weak cross-equatorial wind bias
found in most climate models, closely
related to the double-ITCZ bias (Li and
Xie 2014);
• Huge uncertainties in future projections
of cross-equatorial winds;
• Factors that can affect cross-equatorial
winds: state of tropical North Atlantic;
interhemispheric asymmetry; AMOC;
land-sea distribution; aerosols; etc
(Frierson and Hwang 2012; Wang et al.
2016).
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Summary
• A multidecadal strengthening of cross-eq. winds is observed over E. Pacific since the 1980s,
a robust feature across in-situ, satellite, and reanalysis wind products.
• The cross-eq. wind strengthening is unlikely caused by ENSO changes, and is at least partly
forced by the warming of tropical North Atlantic.
• The cross-eq. wind strengthening could explain many observed multidecadal changes in the
tropical Pacific since the 1980s, including (1) a La Niña-like mean state shift, (2) ITCZ
crossing the equator less frequently, (3) weakened ENSO variability, and (4) a shift of
ENSO type from EP to CP.
• The representation of cross-eq. winds in climate models needs to be improved, and their
future changes need to be constrained for reliable projections of ENSO changes.
Hu, S., and Fedorov, A. V. (2018) Cross-equatorial winds control El Niño diversity and change. Nat. Clim. Change, 8, 798-802.
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Satellite In-situ buoyERA interim
Surface cross-equatorial
(meridional) wind
anomalies averaged within
the equatorial band
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Contrasting extreme El Niño of 1997 and 2015
1997 extreme El Niño
2015 extreme El Niño
Colors: SST
Contours: precipitation
Arrows: surface winds
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Model bias before and after flux adjustment
Before
After
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ENSO response to cross-equatorial winds
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Atlantic impacts on ENSO
• TNA warming suppresses ENSO
variability in the eastern Pacific, consistent
with previous studies (Timmermann et al.
2007; Yu et al. 2015; Levine et al. 2017).