Sea-level change and storm surges in the context of climate change R G Bell1 BE(Hons), PhD, MIPENZ D G Goring2 BE(Hons), MS, PhD W P de Lange3 BSc, MSc(Hons), PhD This paper reviews the latest research in New Zealand surrounding the issues of sea-level rise and extreme sea levels in the context of global warming and variability in the Pacific-wide El Niño– Southern Oscillation (ENSO). Past records of climate, sea level (excluding tides) and sea and air temperatures have shown that they are continuously fluctuating over various long-term timescales of years, decades and centuries. This has made it very difficult to determine whether the anthropogenic effects such as increased levels of “greenhouse” gases are having an accelerating effect on global sea levels or an increased incidence of extreme storms. Over the past century, global sea level has risen by 10–25 cm, and is in line with the rise in relative sea level at New Zealand’s main ports of +1.7 mm yr –1. What has become very clear is the need to better understand interannual (year-to-year) and decadal variability in sea-level, as these larger signals of the order of 5–15 cm in annual-mean sea level have a significant “flow-on” effect on the long-term trend in sea level. The paper describes sea level variability in northern New Zealand—both long- and short-term—involved in assessing the regional trends in sea level. The paper also discusses the relative contributions of tides, barometric pressure and wind set-up in causing extreme sea levels during storm surges. Some recent research also looked at a related question—Is there any sign of increased storminess, and hence storm surge, in northern New Zealand due to climate change? The paper concludes that, while no one can be completely sure how sea-level and the degree of storminess will respond in the near future, what is clear is that interannual and decadal variability in sea level is inextricably linked with Pacific-wide ENSO response and longer inter-decadal shifts in the Pacific climate regime, such as the latest shift in 1976. Keywords: sea-level rise — storm surge — climate change — El Niño–Southern Oscillation — tides 1

National Institute of Water & Atmospheric Research, NIWA, PO Box 11-115, Hamilton. E-mail: [email protected] Web-site: http://www.niwa.cri.nz/pgsf/CASHCANZ/ 2 National Institute of Water & Atmospheric Research, NIWA, PO Box 8602, Christchurch. E-mail: [email protected] 3 Coastal Marine Group, Department of Earth Sciences, University of Waikato, Private Bag 3105, Hamilton. E-mail: [email protected] After peer review, this paper, which was originally presented at the 1999 IPENZ Conference, was received in revised form on 25 May 2000.

1. Introduction Knowledge of rising and extreme sea levels is very important for engineers and planners, because coastal developments, such as ports, marinas, subdivisions and coastal protection works, are invariably controversial. A number of regional councils and territorial local authorities now have procedures within coastal or land zoning plans which explicitly require sea-level rise to be taken into account when designing structures or have been incorporated into building floor levels for new coastal subdivisions. Therefore, questions about the effect of longterm rise in sea level and the likelihood of extreme sea levels always arise during the course of any coastal development project. Rising sea level and shifts in the frequency or intensity of storms have important socioeconomic and environmental implications for the longterm stability of New Zealand’s long 11 000 km coastline, particularly populated areas adjacent to the coastal margin. Even more at risk are some of our Pacific Island neighbours. Scientists studying climate change are also IPENZ Transactions, 2000, Vol. 27, No. 1/Gen

concerned with global sea-level rise as a means of calibrating ocean-climate models to improve prediction of the effects of global warming. Various comprehensive analyses of worldwide sea-level records have demonstrated that over the past century eustatic (or global) sea level has been rising at a rate of +1.8 mm yr–1 with a range of uncertainty of 1–2.5 mm yr–1 (1)(2)(3). Further, there has been no compelling evidence of any acceleration in sea-level rise since the 1850s (4), when major harbours in Europe first began to install tide gauges (5). However, placed in a wider historical context of the past two millennia, it appears that the 0.1–0.25 m rise in sea level last century represents a comparatively recent acceleration, interpreted as the restoration of sea level following the drop caused by the Little Ice Age from AD 1500 to 1850 (1)(2)(6). An analysis by Hannah (7) of sea-level trends from 1900 to 1988 from tide-gauge data at New Zealand’s four main ports (Auckland, Wellington, Lyttelton, Dunedin) produced similar rates to the global trend with an average 1

Mokohinau Island

Open-Coast Sea-Level Sites >5 years 0.15 m as a result of the persistent offshore winds from the SW–W. Further analyses of sea-level records around other parts of New Zealand is proceeding as longer sea-level records become available.

4. Conclusions An analysis of sea-level data for northern New Zealand from an open-coast gauge at Moturiki and the nation’s longest port record at Auckland has highlighted the nonstationary behaviour of sea level at interannual and decadal timescales. Interannual contributions explain around 25% of the total variance in monthly sea level (excluding tides) and mainly arise from ENSO effects. To what extent lowfrequency variability in coastal and oceanic currents (e.g. East Auckland Current) contribute to interannual variability in sea level is not clear due to lack of long-term oceanographic data. The linear rise in secular sea level last century at the Port of Auckland of 1.3 mm yr–1, falls within the 1–2.5 mm yr–1 range for the global sea-level rise that century. However there has been an almost static trend in sea level since the mid-1970s caused by a “flow-on” effect from the unusually lengthy period of predominantly El Niño episodes which have kept sea levels lower than normal. This regional response has masked any ongoing global rise in sea level caused by thermal expansion of seawater and ice melt. It is likely within the next few years that the The Institution of Professional Engineers New Zealand

FIGURE 7: Cumulative exceedance probability for the higher and extreme tide heights from a 100-year forecast of tidal heights at hourly intervals for Moturiki (Bay of Plenty). The overall range in tide heights for 100 years is -1176 to +1147 mm, relative to the mean level of the sea (MSL). To place the extreme tides in context, Mean High Water Spring is 830 mm above MSL (or 880 mm above Moturiki Datum–1953).

Inter-decadal Pacific Oscillation switches to a “cool” phase (relative to the Eastern Pacific), which is likely to enhance La Niña episodes, and therefore raise regional sea level. Consequently inter-decadal variability is an important consideration in assessing the long-term rise and any acceleration in regional sea level around the New Zealand coast. There is a fundamental need to better understand the causes of very low frequency variability in sea level in various regions of the Pacific. To this end, the recent completion of extensive networks of open-coast sea-level gauges set up by the National Tidal Facility in Australia and Pacific Islands and by NIWA in New Zealand (Fig. 1) should provide the necessary high-quality sea-level data to further investigate interannual and decadal variability in sea level. It will also considerably improve our poor understanding of storm surges and their frequency distributions, in particular the effects of inverted barometer, wind set-up and ENSO climate variability. Research is also underway to model the physics of low-pressure systems to gain a better understanding of how various storm characteristics, such as storm speed, minimum central pressure and winds combine to cause a response in coastal sea levels around New Zealand. Estimates of the projected global rise in sea level by the 1995 IPCC report (2) indicate a “best estimate” rise of 20 cm by 2050 (range of uncertainty 7–39 cm) and 49 cm by 2100 (range of uncertainty 20–86 cm). We await with interest the third IPCC reportdue to be released in March 2001, for any revision of these estimates, although indiIPENZ Transactions, 2000, Vol. 27, No. 1/Gen

cations are that projections for sea-level rise will remain similar to those issued in 1995. Ongoing research on tides, storm surges and tsunami and a stronger commitment by national and regional government agencies to long-term monitoring of sea level will markedly improve future predictions of extreme sea levels and determine which coastal areas are potentially vulnerable to the ongoing rise in relative sea level. This will pave the way for much more informed forecasting methods for storm surges in critical areas and long-term forecasts of regional sea-level rise around New Zealand.

5. Acknowledgements The authors acknowledge the assistance of Prof. John Hannah (Univ. of Otago) supplying annual MSL data up to 1988 for Auckland; RNZN Hydrographic Office for monthly-mean sea-level data for Auckland; Dale Hansen (Northland Regional Council) providing Marsden Point data; and Westgate Transport Ltd. providing Port of Taranaki data.

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