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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.

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Presentation on theme: "Extreme sea level drops in the western tropical Pacific— Causes, coastal impacts, and future projections Aerial view of Olosega Village, courtesy National."— Presentation transcript:

1 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. 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)

2 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 Image from 8 February 2012 MTSAT-2 visible channel, Digital Typhoon, National Institute of Informatics South Pacific Convergence Zone (SPCZ)

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

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

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

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

8 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)

9 Shallow reef response to sea level variability Normal conditionsEl Niño Taimasa d > hd < 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)

10 What causes extreme sea level drops?

11 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

12 Southward shifted westerly wind anomaly east of Dateline (McGregor et al., 2012 J. Climate) Regressions: Sea surface height and wind stress Zonal sea level gradientMeridional 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)

13 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 gradientMeridional 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)

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 r = 0.74 at lag 3 months

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

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 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

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

20 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?

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

23 CMIP5 projections 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 (mm day -1 ) 28°C 26°C Observed rainfall and SST climatology during DJF Meridional SST gradient & zonal SPCZ events = [Box 1 SST – Box 2 SST] Box 1 Box /98 El Niño ~ GPCP rainfall ~ NOAA SST

25 SST trend pattern (departure from tropical mean) 21 st 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

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

27 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:

28 Last idea: Simulate shallow reef growth & decay Normal conditionsEl Niño Taimasa  (d > h) = 1 Non-branching coral (Porites)  (d < h) = 1 Parametric coral model Height change GrowthDecay or erosion Coral growth constraints: 1)Water temperature below critical temperature (T c – T)… warming 2)Aragonite saturation state (  arag )… ocean acidification 3)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”

29 Thank you Matthew Widlansky

30 2) Increased frequency of zonal SPCZ events Increased number of zonal SPCZ events Flux 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


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