Extreme sea level variability due to El Niño Taimasa Matthew J. Widlansky Axel Timmermann1,2, Shayne McGregor3, Malte F. Stuecker4, Yoshimitsu Chikamoto1, Wenju Cai5 and Mark Merrifield2 1International Pacific Research Center, University of Hawai‘i at Mānoa 2Dept of Oceanography, University of Hawai‘i at Mānoa 3Climate Change Research Centre, University of New South Wales 4Dept of Meteorology, University of Hawai‘i at Mānoa 5CSIRO Marine and Atmospheric Research-Aspendale

taimasa [kai’ ma’sa] Samoan “foul smelling tide” microatoll Low tide limits upward coral growth Flat top Porites coral courtesy National Park of American Samoa

Shallow reefs respond to sea level variability Normal conditions El Niño Taimasa depth > height depth < height Top portions of coral heads die off, creating microatolls (e.g., Woodroffe and McLean, 1990 Nature)

Tide gauges in the tropical western Pacific

Very low sea levels, or ‘taimasa’, affect tropical western Pacific islands during strong El Niño UH Sea Level Center data Prolonged low sea levels in Southwest Pacific (r = 0.60 at lag 6 months)

Meridional sea-level seesaw of El Niño Taimasa Sea level gradient & PC2 wind stress correlated (r = 0.74 at lag 3 months)

Zonal sea level gradient Meridional sea level gradient Sea level responds to wind-stress variability 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) 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) Shading: Sea surface height (ECMWF ORAs4) Tide Gauge Stations: (UHSLC)

SPCZ South Pacific Convergence Zone— largest rainband in Southern Hemisphere Zonal SPCZ Equatorward collapse of rainband Observed rainfall and sea surface temperature during austral summer (mm day-1) 28°C 26°C Extreme El Niño GPCP rainfall NOAA SST (Vincent et al. 2011, Climate Dynamics; Cai et al. 2012, Nature)

Zonal SPCZ events are associated with prolonged sea level drops in Southwest Pacific (PC1 & PC2 > 0) Shading: Rainfall (GPCP) Blue contours: Pacific rainbands enclosed by 5 mm day-1 rainfall annual climatology

Key Points Extreme low sea levels persist after termination of strong El Niño Sea level drops are related to interaction of El Niño with seasonal development of the South Pacific Convergence Zone Hindcasts suggest potential predictability of future extreme sea level drops (J. Climate 2014) How will coastal sea levels and shallow reefs respond to the next El Niño?

El Niño Update http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_update/sstanim.shtml

Dynamical forecasts (June initialization) Blue are most recent forecasts

Dynamical sea-level forecasts NOAA CFSv2 (June initialization) November 2014: Guam 15 cm below normal Meridional sea-level seesaw (red minus blue) March 2015: Samoa 15 cm below normal

Retrospective forecast of the 1997/98 El Niño

Retrospective forecast of the 1997/98 El Niño

Strong El Niño phase-locked to annual cycle

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