1. Extreme sea level drops in the western tropical Pacific—
Causes, coastal impacts, and future projections
Matthew J. Widlansky
International 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)
Aerial view of Olosega Village, courtesy
National Park of American Samoa

2. Samoans call these events ‘taimasa’ (pronounced [kai’ ma’sa])
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

3. Snapshot of a key climate feature South Pacific Convergence Zone
(SPCZ)
Image from 8 February 2012
MTSAT-2 visible channel, Digital Typhoon, National Institute of Informatics

4. Recent literature on the SPCZ
(Widlansky et al., 2013 Nature Climate Change)

5. Observed rainfall and SST climatology during DJF
Underlying SST gradients influence the SPCZ
Observed rainfall and SST climatology during DJF
(mm day-1)
28°C
26°C
NOAA SST
GPCP 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.)
(2011 Climate Dynamics)

6. Observed rainfall and SST climatology during DJF
Interannual variability of the SPCZ
Observed rainfall and SST climatology during DJF
(mm day-1)
28°C
26°C
Extreme El Niño
El Niño
La Niña
NOAA SST
GPCP rainfall
(2011 Climate Dynamics)

7. Extreme zonally oriented SPCZ event:
4 January 1998
GMS-5
IR water vapor μm
Some islands experienced droughts while others more tropical cyclones
Ron
(Tonga: 67% damaged)
Susan
(125 kts)
Katrina
(28 days)
In Samoa, prolonged low sea levels exposed shallow reefs

8. Interhemispheric sea level seesaw (r = 0.60) at lag 6 months
Tide gauge observations (tropical western Pacific)
UH Sea Level Center data
Interhemispheric sea level seesaw (r = 0.60) at lag 6 months
Very low sea levels, or ‘taimasa’, affect South Pacific islands mostly during strong El Niño events
(Widlansky et al., in review)

9. photo courtesy National Park of American Samoa
Shallow reef response to sea level variability
Normal conditions
El Niño Taimasa
d > h
d < h
Top portions of coral heads die off, creating what are known as microatolls (e.g., Woodroffe and McLean, 1990 Nature)
Flat top Porites coral
photo courtesy National Park of American Samoa

10. What causes extreme sea level drops?

11. (McGregor et al., 2012 J. Climate)
Wind-stress variability associated with ENSO
Equatorial Pacific
(10°S-10°N, 100°E-60°W)
wind stress anomaly
(McGregor et al., 2012 J. Climate)
26%
15%
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

12. Zonal sea level gradient Meridional sea level gradient
Regressions: Sea surface height and wind stress
Zonal sea level gradient
Meridional sea level gradient
Canonical sea level response to El Niño
(e.g., Wyrtki, 1984 JGR)
Most pronounced during strong El Niño
(e.g., Alory and Delcroix, 2002 JGR)
Equatorially symmetric wind stress pattern associated with ENSO
(Stuecker et al., 2013 Nature Geoscience)
Southward shifted westerly wind anomaly east of Dateline
(McGregor et al., 2012 J. Climate)
Vectors: Wind stress (ERA interim)
Contours: Wind-stress curl (negative, SH cyclonic)
Shading: Sea surface height (ECMWF ORAs4)
Vectors: Wind stress (ERA interim)
Blue contours: Negative wind-stress curl (SH cyclonic)
Vectors: Wind stress (ERA interim)
Shading: Sea surface height (ECMWF ORAs4)
Tide Gauge Stations: (UHSLC)

13. Zonal sea level gradient Meridional sea level gradient
Regressions: Near-surface current anomalies reverse
Zonal sea level gradient
Meridional sea level gradient
Strengthened Equatorial Counter Current, drainage of West Pacific Warm Pool
(e.g., Wyrtki, 1984 JGR)
Current anomalies reverse, sea levels return to normal in northwestern Pacific
Sea levels remain depressed south of 5°N; i.e., the meridional seesaw.
(Alory and Delcroix, 2002 JGR)
Shading: Sea surface height (ECMWF ORAs4)
Vectors: Near-surface current, 5–56 m average (ORAs4)

14. 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

15. Asymmetric western Pacific sea level response
at lag 3 months

16. Combination-mode: ENSO (fE) + Annual Cycle (1)
Near-annual combination tones appear in PC2 of surface winds
(Stuecker et al., 2013 Nature Geoscience)
& western Pacific sea level gradient
~15 months
~9 months
2–7 years

17. 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.)

18. Combination-mode and prolonged sea level drops
Southwest Pacific
PC2 experiment forced with the southward westerly wind shift
—essentially nonlinear interaction between annual cycle & ENSO—
sufficient to prolong below-normal sea levels

19. Correlation with observed sea surface height
PC2 correlated with central Pacific sea level
Correlation with observed sea surface height
Central Pacific
r = 0.73
at lag 1 month

20. Key Points
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.
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.
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?

21. Recent work by Cai & coauthors…
(Nature 2012)
Observed climatology (1981–2005)
Shading: Rainfall (GPCP)
Blue contours: Rainfall > 5 mm d-1
Green contours: Warm pool > 27.5°C
Red 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°C
Shading: Warming Vectors: Surface wind change

22. Defining a “zonal SPCZ event”: PC1 > 1 and PC2 > 0
GPCP rainfall
Zonal SPCZ
La Niña
Second principal component
Neutral
Moderate El Niño
First principal component

23. CMIP5 projections Climate Change?
Considering 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

24. Observed rainfall and SST climatology during DJF
Meridional SST gradient & zonal SPCZ events
Observed rainfall and SST climatology during DJF
(mm day-1)
28°C
26°C
Box 1
Box 2
~ GPCP rainfall
~ NOAA SST
1997/98
El Niño
= [Box 1 SST – Box 2 SST]

25. Maximum equatorial warming is a robust response to greenhouse warming
Smaller future meridional SST gradient
Maximum equatorial warming is a robust response to greenhouse warming
(e.g., Xie et al., 2010 J. Climate)
Box 1
Box 2
SST trend pattern (departure from tropical mean)
21st century projection (RCP 4.5 W m-2, 20 models)
Pacific island communities experience extreme weather –droughts or floods, tropical cyclones, & sea level drops– during zonal SPCZ events

26. MIROC5 ocean-atmosphere GCM
Simulated sea surface heights
MIROC5 ocean-atmosphere GCM
Southwest Pacific
21st century projection?
*very preliminary (1 model/1 run)
RCP 4.5
Southwest Pacific

27. Plans for further study:
Should future increased frequency of El Niño Taimasa occur, a higher likelihood of prolonged low sea level events is perceivable
Plans for further study:
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

28. Parametric coral model Non-branching coral (Porites)
Last idea: Simulate shallow reef growth & decay
Normal conditions
El Niño Taimasa
(d > h) = 1
(d < h) = 1
Parametric coral model
Height change
Growth
Decay or erosion
Coral growth constraints:
Water temperature below critical temperature (Tc – T)… warming
Aragonite saturation state (arag)… ocean acidification
Water depth above coral (d – h)… tides, El Niño, & sea level rise
Non-branching coral (Porites)
Species specific parameters:
Growth rate constant (0)
Height dependent growth function (f)
Decay rate of exposed coral ()
Collaborate with coral experts to simulate future reef vulnerability caused by climate extremes & communicate “Taimasa threat index”

29. Thank you Matthew Widlansky mwidlans@hawaii.edu
Thank you

30. 2 1 Increased number of zonal SPCZ events
Flux adjusted perturbed physics experiments with HadCM3 model (12 out of 17 experiments considered) 2 1
Greenhouse warming is likely to cause:
More summers with small meridional SST gradients
2) Increased frequency of zonal SPCZ events