The Sea Level Seesaw of El Nino Taimasa
Total Page:16
File Type:pdf, Size:1020Kb
The Sea Level Seesaw of El Niño Taimasa During El Niño Taimasa, coral tops of Samoa’s fringing reefs become exposed to air at low tide. Image courtesy of the National Park of American Samoa. he huge swings in sea levels “Repeated exposure of shallow ated sea level drops,” says Widlansky, linked to the El Niño– South- reefs to air at low tide causes the top who conducts research on tropical ern Oscillation (ENSO) portions of coral heads to die off, of- climate variability during his postdoc- Tthreaten vulnerable island communi- ten creating what are known as micro- toral studies at the IPRC. “In spite of ties and coastal ecosystems in the trop- atolls on shallow reef flats,” explained these ecological and societal impacts ical western Pacific. Sea level in that the managers at the National Marine of El Niño-related sea level drops on region can rise during La Niña and Sanctuary of American Samoa to Pacific islands, little is known about drop during El Niño by up to 20–30 Matthew Widlansky, IPRC postdoc- their causes, regional manifestations, cm. (See ‘Klaus Wyrtki and El Niño’ toral fellow. and what will happen in the future with in IPRC Climate, vol. 6, no. 1, 2006, “Hearing accounts of flat-top coral further climate change.” for a historical perspective of detecting heads found throughout the western To explore why these sea level ex- these sea level seesaws.) Pacific coastal reef ecosystem moti- tremes occur and, hopefully, to improve The most extreme sea level drops vated me to further study the associ- prediction of future events, Widlansky are in the southwestern Pacific, which 25 by exposing the shallow reefs that 20 circle and protect the islands, lead to 15 catastrophic coral and fish die-offs. 10 1982/83 El Niño 1997/98 El Niño Samoans refer to very low sea level as 5 ‘taimasa’ (pronounced [kai’ ma’sa]; 0 translation from Samoan to English −5 is “foul smelling tide” referring to −10 the odor from decaying marine life). −15 ide gauge observations (cm) Luafata Simanu-Klutz, assistant pro- T −20 0.8 Northwest Pacific 0.4 −25 fessor of Samoan language and history (Apra Harbour, Guam) 0.0 −0.4 −30 Southwest Pacific −0.8 Lag (months) at the University of Hawai‘i, recalls (Pago Pago, American Samoa) Correlation coefficient −12 −8 −4 0 4 8 12 −35 times when the prevailing trade winds 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 2007 2009 2011 changed, sea level would drop, and Figure 1. Tide gauge observations in the Northwest Pacific (orange) and Southwest Pacific (blue) “you could walk far out on the reefs. with the linear trend in each region removed. Thick lines represent multi-station averages (see During the day, the stench was at times Fig. 2 for locations). Observations from Guam and American Samoa are highlighted. Insert shows quite unbearable...fish died in the heat.” the correlation coefficient between regions as a function of lag in months (Southwest Pacific lags Northwest Pacific; r = 0.60 at lag 6 months). Grey boxes highlight two strong El Niño events re- ferred to throughout. Data from the University of Hawai‘i Sea Level Center. 16 IPRC Climate, vol. 14, no. 1, 2014 partnered with IPRC Faculty Member Axel Timmermann, normal sea levels persist up to six months longer,” explains and they began collaborations with an international team of Timmermann. climate scientists and oceanographers. The most severe sea level drops, the team noted, took Using tide-gauge records throughout the tropical Pacific place during the strong El Niño of 1982/83 and 1997/98 (see compiled by Mark Merrifield, director of the University of Figure 1). During both, tide gauge levels initially fell far be- Hawai‘i Sea Level Center, the scientists analyzed past sea low normal in the tropical northwestern Pacific but remained level extremes. near normal around Samoa, creating a steep north-south “From the tide-gauge network, we saw that sea lev- sea level gradient across the equator (Figure 2a). Focusing els around Guam tend to recover quickly during the end on the sea level drops near Samoa, which occurred immedi- of a very strong El Niño events, but in Samoa the below- ately after the peak of these two El Niño events, the scientists a) Modes of wind-stress variability and sea surface height gradient 4.0 3.0 2.0 1.0 0 −1.0 Standardized indices −2.0 0.8 PC1 wind stress 0.4 −3.0 0 PC2 wind stress −0.4 −4.0 NWPAC minus SWPAC −0.8 Lag (months) 1982/83 El Niño 1997/98 El Niño sea surface height Correlation coefficient −8 −4 048 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 2007 2009 2011 b) PC1 regression: Sea surface height & wind stress (cm) PC2 regression: Sea surface height & wind stress Wind stress 10 Wind stress 20°N 0.02 kg m-1 s-2 8 0.02 kg m-1 s-2 6 Guam 10°N Malakal Guam Kwajalein 4 Malakal Kwajalein Majuro Majuro 2 Kapingamarangi 0° 0 Nauru Nauru Kapingamarangi Kanton −2 Kanton Honiara 10°S −4 Honiara Pago Pago Pago Pago −6 20°S −8 −10 120°E 135°E 150°E 165°E 180°W 165°W 150°W 120°E 135°E 150°E 165°E 180°W 165°W 150°W Figure 2. Two principal components (PC1 and PC2) of wind-stress and sea level variability in the tropical western Pacific based on empirical orthogo- nal function decomposition of the equatorial (10°S–10°N, 100°E–60°W) wind-stress anomaly. (a) PC1 (red) and PC2 (blue) respectively explain 26.3% and 14.9% of the equatorial wind-stress variance. Insert shows correlation between PC2 and Northwest Pacific minus Southwest Pacific sea surface heights (black) as a function of lag in months (sea level gradient lags PC2; r = 0.74 at lag 3 months). (b and c) Linear regressions of sea surface height (cm, shading) and wind stress (Kg m-1 s-2, vectors) onto PC1 and PC2. Black boxes represent the Northwest Pacific (5°N–15°N, 130°E–180°W) and Southwest Pacific (15°S–2°N, 150°E–165°W). Triangles indicate tide gauge stations (U.S.-affiliated Pacific Islands green). Wind-stress and sea surface height observations (1979–2011) from respectively the ECMWF interim reanalysis and operational Ocean Reanalysis system 4. International Pacific Research Center 17 discovered that these were periods winds (see Figure 2c) that is collocated El Niño Taimasa. Timmermann, how- when normal trade winds had returned with the more zonal positioning of the ever, cautions, “A definitive association in the North Pacific but remained SPCZ. They found this wind shift, as- is limited by the small sample of zonal weak south of the equator (compare sociated with ENSO and the seasonal SPCZ events.” Figure 2b-c). development of the SPCZ, causes the Given the well-established pre- Associated with these sea level sea level seesaw between the North and diction of the seasonal evolution of and wind changes, is a northeastward South Pacific during the termination of climate and sea level once a strong El shift of the South Pacific Conver- very strong El Niños. Niño is launched, the scientists think gence Zone, the largest rainband in “This interaction—or combina- that the long duration of extreme sea the Southern Hemisphere (see IPRC tion—between ENSO and the annual level drops at the end of such an El Climate, vol. 12, no. 2). Such a shift in cycle causes the prolonged sea level Niño may be highly predictable. Such the rainband causes droughts south of drops for Samoa and the abrupt re- predictability could warn island com- Samoa and sometimes triggers more covery around Guam” explains Malte munities several seasons in advance frequent tropical cyclones to the east. Stuecker, a meteorology doctoral stu- and give them the opportunity to adapt To describe this climate pattern, the dent at the University of Hawai‘i at at least to some of the detrimental con- scientists have coined the term El Niño Mānoa, who also participated in the sequences of taimasa events. Taimasa, a term that captures the dam- study. Widlansky and Timmermann aging effects on shallow reefs around “As the SPCZ and wind response say further observational studies are Samoa and nearby regions. is greatest during the strongest El Niño planned, along with sophisticated Widlansky collaborated with events,” McGregor notes, “we would hindcast experiments that are be- Shayne McGregor, a former IPRC not expect the extreme sea level drop ing conducted by IPRC Postdoctoral postdoctoral fellow and now at the Uni- during weaker El Niño events (includ- Fellow Yoshimitsu Chikamoto, to versity of New South Wales, to study ing Modoki events) or La Niña.” confirm predictability of future El how the prolonged low sea level in The ENSO-dependent position Niño Taimasa impacts. the South Pacific is linked to the well- of the SPCZ appears to explain the Though sea levels are likely to known southward shift of weak trade asymmetric sea level response during gradually rise with global warming, thereby perhaps offsetting somewhat the harm of extreme low sea level pe- riods, recent modeling evidence pre- sented by Wenju Cai (Commonwealth Scientific and Industrial Research Or- ganisation, Australia) suggests that strong El Niño events are very likely to occur more often, and thus El Niño Taimasa events could also become more frequent. This story is based on Widlansky, M.J., A. Timmermann, S. McGregor, M.F. Stuecker, and W.