extreme sea level drops in the western tropical pacific— causes, coastal impacts, and future...
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Extreme sea level drops in the western tropical Pacific—Causes, coastal impacts, and future projections
Aerial view of Olosega Village, courtesy
National Park of American Samoa
Matthew J. WidlanskyInternational Pacific Research Center
In collaboration with: Axel Timmermann (IPRC)Mark Merrifield (JIMAR, UHSLC)Shayne McGregor (UNSW Sydney)Malte Stuecker (UH Met. Dept)Wenju Cai (CSIRO Australia)
During strong El Niño events, sea level drops around some tropical western Pacific islands by up to a
foot (30 cm)
Samoans call these events ‘taimasa’ (pronounced [kai’ ma’sa])
in reference to the foul smelling tide caused by coral die-offs
Future extreme low sea level events may become more frequent
Snapshot of a key climate feature
Image from 8 February 2012MTSAT-2 visible channel, Digital Typhoon, National Institute of Informatics
South Pacific Convergence
Zone(SPCZ)
Recent literature on the SPCZ
(Widlansky et al., 2013 Nature Climate Change)
(mm day-1)
28°C26°C
(2011 Climate Dynamics)
Observed rainfall and SST climatology during DJF
NOAA SSTGPCP rainfall
• Tropical SPCZ adjacent the meridional SST gradient (equatorial)
• Subtropical SPCZ transects the meridional SST gradient (mid-latitudes) and is west of maximum zonal SST gradient
(e.g., Lindzen and Nigam, 1987 J. Atmos. Sci.)
Underlying SST gradients influence the SPCZ
Interannual variability of the SPCZ(mm day-1)
28°C26°C
Observed rainfall and SST climatology during DJF
La Niña
El Niño
Extreme El Niño
(2011 Climate Dynamics)
NOAA SSTGPCP rainfall
Some islands experienced
droughts while others more tropical
cyclones
Extreme zonally oriented SPCZ event:4 January 1998GMS-5IR water vapor6.70-7.16 μm
Katrina(28 days)
Susan(125 kts)
Ron(Tonga: 67% damaged)
In Samoa, prolonged low
sea levels exposed
shallow reefs
Very low sea levels, or ‘taimasa’, affect South Pacific islands mostly during strong El Niño events
Tide gauge observations (tropical western Pacific)
Interhemispheric sea level seesaw (r = 0.60) at lag 6 months
UH Sea Level Center data
(Widlansky et al., in review)
Shallow reef response to sea level variability
Normal conditions El Niño Taimasa
d > h d < h
Flat top Porites coral
photo courtesy National Park of American Samoa
Top portions of coral heads die off, creating what are known as microatolls (e.g., Woodroffe and McLean, 1990 Nature)
What causes extreme sea level drops?
Wind-stress variability associated with ENSO
26%
15%
Equatorial Pacific (10°S-10°N, 100°E-60°W)
wind stress anomaly(McGregor et al., 2012 J. Climate)
• 3 highest peaks of PC1 classified strong El Niño events, matching lowest sea levels in the Southwest Pacific
• PC2 abruptly switches from negative to positive, especially after 1982/83 and 1997/98 El Niño peaks
Southward shifted westerly wind anomaly east of Dateline
(McGregor et al., 2012 J. Climate)
Regressions: Sea surface height and wind stress
Zonal sea level gradient Meridional sea level gradient
Shading: Sea surface height (ECMWF ORAs4)Vectors: Wind stress (ERA interim)Blue contours: Negative wind-stress curl (SH cyclonic)
Vectors: Wind stress (ERA interim)Contours: Wind-stress curl (negative, SH cyclonic)Vectors: Wind stress (ERA interim)Shading: Sea surface height (ECMWF ORAs4)Tide Gauge Stations: (UHSLC)
Most pronounced during strong El Niño(e.g., Alory and Delcroix, 2002 JGR)
Canonical sea level response to El Niño(e.g., Wyrtki, 1984 JGR)
Equatorially symmetric wind stress pattern associated with ENSO
(Stuecker et al., 2013 Nature Geoscience)
Sea levels remain depressed south of 5°N; i.e., the meridional seesaw.
(Alory and Delcroix, 2002 JGR)
Regressions: Near-surface current anomalies reverse
Zonal sea level gradient Meridional sea level gradient
Shading: Sea surface height (ECMWF ORAs4)Vectors: Near-surface current, 5–56 m average (ORAs4)
Current anomalies reverse, sea levels return to normal in northwestern Pacific
Strengthened Equatorial Counter Current, drainage of West Pacific Warm Pool
(e.g., Wyrtki, 1984 JGR)
Regressions: SPCZ collapses equatorward
Zonal SPCZ events are associated with prolonged extreme sea level drops
(PC1 & PC2 > 0)
Shading: Rainfall (GPCP)Blue contours: Pacific rainbands enclosed by 5 mm d-1 rainfall annual climatology
Asymmetric western Pacific sea level response
r = 0.74 at lag 3 months
Combination-mode: ENSO (fE) + Annual Cycle (1)
~15 months
~9 months2–7 years
Near-annual combination tones appear in PC2 of surface winds (Stuecker et al., 2013 Nature Geoscience)
& western Pacific sea level gradient
Shallow-water model (1.5-layer) hindcast simulations
‘Zonal seesaw’ of tropical Pacific thermocline depth and sea levels
associated with ENSO
‘Meridional seesaw’ characterized by 9 and 15 month spectral energy
(Wang, Wu, & Lukas, 1999 J. Meteorol. Soc. Jpn.)
Combination-mode and prolonged sea level drops
PC2 experiment forced with the southward westerly wind shift—essentially nonlinear interaction between annual cycle & ENSO—
sufficient to prolong below-normal sea levels
Southwest Pacific
PC2 correlated with central Pacific sea level
Central Pacificr = 0.73
at lag 1 month
Correlation with observed sea surface height
Key Points
1) Extremely low sea levels—capable of damaging shallow coral reefs—persist long after termination of strong El Niño events in the tropical southwestern and central Pacific.
2) Sea level drops are related to interaction of El Niño with the forced annual cycle and associated seasonal development of the South Pacific Convergence Zone.
3) Hindcast experiments suggest potential predictability of future extreme sea level drops once El Niño has developed to a certain intensity threshold.
How will strong El Niño events and extreme sea level drops respond to climate change?
(Nature 2012)
Recent work by Cai & coauthors…
Observed climatology (1981–2005)Shading: Rainfall (GPCP)Blue contours: Rainfall > 5 mm d-1
Green contours: Warm pool > 27.5°CRed line: Zonal SPCZ position
CMIP5 projection (2074–2098 minus 1981–2005)RCP8.5 W m-2 (31 models)
Blue contours: CTRL Rainfall > 5 mm d-1
Green contours: CTRL Warm pool > 27.5°CShading: Warming Vectors: Surface wind change
Zonal SPCZ
Defining a “zonal SPCZ event”:
GPCP rainfall
Moderate El Niño
La Niña
Neutral
PC1 > 1 and PC2 > 0
First principal component
Sec
ond
prin
cipa
l com
pone
nt
CMIP5 projectionsConsidering only models able to simulate nonlinear
behavior of the SPCZ (12 out of 31 models)
Climate Change?
Number of zonal SPCZ events increases from Control to Climate Change period
(mm day-1)
28°C26°C
Observed rainfall and SST climatology during DJF
Meridional SST gradient & zonal SPCZ events
= [Box 1 SST – Box 2 SST]
Box 1
Box 2
1997/98El Niño
~ GPCP rainfall~ NOAA SST
SST trend pattern (departure from tropical mean)21st century projection (RCP 4.5 W m-2, 20 models)
Smaller future meridional SST gradient
Box 1
Box 2
Maximum equatorial warming is a robust response to greenhouse warming
(e.g., Xie et al., 2010 J. Climate)
Pacific island communities experience extreme weather –droughts or floods, tropical cyclones, & sea level
drops– during zonal SPCZ events
Simulated sea surface heights
RCP 4.5
MIROC5 ocean-atmosphere GCM
21st century projection?*very preliminary (1 model/1 run)
Sout
hwes
t Pac
ific
Sout
hwes
t Pac
ific
Should future increased frequency of El Niño Taimasa occur, a higher likelihood of prolonged low sea level events is
perceivable
1) Hindcast experiments to confirm seasonal predictability of El Niño Taimasa events Use a sophisticated coupled ocean-atmosphere model (e.g., NCAR CESM)
Simple shallow-water ocean model
2) Assess changing frequency of El Niño Taimasa Examine ensemble of CMIP5 future climate change experiments
Future simulation from one model
3) Biogeographic characterization of near-shore reef and community relevance • Compile bathymetric data• Partner with coral experts to determine growth
behaviors in response to sea level variability
Sketch outline of a reef flat
Plans for further study:
Last idea: Simulate shallow reef growth & decayNormal conditions El Niño Taimasa
(d > h) = 1
Non-branching coral (Porites)
(d < h) = 1
Parametric coral model
Height change
Growth Decay or erosion
Coral growth constraints:1) Water temperature below critical temperature (Tc – T)… warming2) Aragonite saturation state (arag)… ocean acidification3) Water depth above coral (d – h)… tides, El Niño, & sea level rise
Species specific parameters:1) Growth rate constant (0)2) Height dependent growth function (f)3) Decay rate of exposed coral ()
Collaborate with coral experts to simulate future reef vulnerability caused by climate extremes & communicate “Taimasa threat index”
2) Increased frequency of zonal SPCZ events
Increased number of zonal SPCZ eventsFlux adjusted perturbed physics experiments with HadCM3 model
(12 out of 17 experiments considered)
1 2
Greenhouse warming is likely to cause:1) More summers with small meridional SST gradients