Review PaperClimate change impacts on tropical cyclones and extreme sea levels in the South Pacific — A regional assessment
Introduction
There can be few locations in the world that are more vulnerable to the impacts of climate change than the low-lying atoll island nations of the Pacific (Fig. 1). Even at current mean sea levels, vulnerability to extreme sea levels is large. For instance, in 1972, Cyclone Bebe caused great damage to Funafuti, leaving behind a large rampart of coral debris (Maragos et al., 1973). In 1990, Cyclone Ofa struck Samoa, with a high storm tide inundating many low-lying coastal regions. In 1997, the storm surge from Cyclone Gavin breached sea walls on the north coast of Vanua Levu island in Fiji, causing extensive flooding of Labasa town, the largest urban area on the island. In 2004, large waves accompanying Cyclone Heta caused significant damage to Niue, with much of the infrastructure on the island destroyed (Terry, 2007). Other significant impacts have occurred, highlighting the vulnerability of this region to inundation from extreme sea levels.
Predicted rising sea levels will lead to increases in inundation that put the future viability of human habitation on a number of Pacific islands at risk (Mimura et al., 2007, Nicholls and Cazenave, 2010). Increases in extreme sea levels are arguably the most significant threat that this region faces from anthropogenic climate change (Mimura et al., 2007), yet our ability to predict the future vulnerability of this region is limited. The main reason is that the climate of the tropical Pacific is already extremely variable due to the strong influence of the El Niño Southern Oscillation (ENSO) phenomenon, which has its source in this region (Sarachik and Cane, 2010). This makes it difficult to detect any trends above the existing large interannual variability. In addition, predictions of future tropical cyclone incidence in this region generally indicate substantial decreases in numbers, but current interannual variability of cyclone incidence is also very large (e.g. Chand and Walsh, 2009).
The recent publication of authoritative review papers on the effects of climate change on ENSO (Collins et al., 2010), climate change on tropical cyclones (Knutson et al., 2010a) and sea level rise and variability (Church et al., 2010b) provides an opportunity to reassess the possible impacts of climate change on extreme sea levels in the South Pacific, since in the current climate the interannual variation of both tropical cyclone behaviour and extreme sea levels is dominated by ENSO.
Extreme sea levels are the result of several coinciding processes, including astronomical tides and severe weather events such as tropical cyclones, which generate elevated coastal sea levels through storm surge and high waves (Fig. 2). Storm surges from tropical cyclones are well-known and documented events along many tropical continental shelf regions, where destructive sea levels are caused by the inverse barometer effect (IBE) and wind stress over the shelf seas (e.g. Fritz et al., 2007, Dube et al., 2009, Harper et al., 2009). Sea levels along the open coast may be further elevated by wave breaking processes. Paradoxically, the shelf conditions that favour high storm surges tend to attenuate the contribution of waves, and the shelf conditions that act to attenuate storm surges allow a larger contribution of waves. This is because wide and gently sloping continental shelves cause waves to steepen, break and lose energy and height further offshore, thereby reducing the wave heights at the coast. Steep shelf margins such as those that surround many islands and atolls in the Pacific, on the other hand, are less conducive to high storm surges but may instead support larger wave-induced contributions to sea level anomalies, which have been reported at Midway Atoll in the northern tropical Pacific by Hoeke (2010).
Tropical cyclone-induced storm surges tend to be localised and concentrated in the region of maximum onshore winds close to the cyclone centre. While potentially devastating, they are rare at any given location and therefore contribute to the extreme tails of the distribution of sea level anomalies. The waves generated by such systems, on the other hand, can propagate for thousands of kilometres in the deep ocean as swell with little loss of energy, potentially affecting a larger number of more distant shorelines. Therefore they may contribute to more frequent sea level anomalies, although these may be of smaller magnitude than the combined impacts of storm surge and local storm waves. Tidal extremes vary on predictable time scales due to the relative movement of the Earth, Sun and Moon. These short term increases in sea level may occur on a background of longer term changes brought about by modes of variability such as ENSO and climate change. While tectonic disturbances may generate tsunamis that may also cause extreme coastal sea levels, they are not addressed in this review as they are not climate-related.
The effect of anthropogenic climate change on mean sea levels, tropical cyclone and ENSO behaviour provides several avenues by which extreme sea levels may also be affected in the future. Rising sea levels due to thermal expansion of the warming oceans and the melting of glaciers, ice caps and the large ice sheets of Antarctica and Greenland will increase the background sea levels on which extreme events occur. Climate change may also cause changes to the behaviour of tropical cyclones by modifying their frequency, intensity and preferred regions of occurrence (Sugi et al., 2009, Bender et al., 2010, Lavender and Walsh, 2011). It may also affect modes of climate variability such as ENSO which in turn may influence background sea levels (e.g. Church et al., 2006, Lowe et al., 2010) and cyclone behaviour (e.g. Knutson et al., 2010a). Finally, the relative contribution of local storm surge and wave forcing, along with remotely generated swell, to positive sea level anomalies at a given location will determine how climate-driven changes to meteorological forcing will affect future changes to sea level extremes. The interactions between climate change and the various drivers of sea level anomalies are illustrated in Fig. 3.
The future vulnerability of tropical and subtropical islands in this region to extreme sea levels is therefore the topic of this review, whose purpose is to provide the most up-to-date quantitative estimates of future impacts over the Pacific Climate Change Science Program (PCCSP)1 region delineated in Fig. 1. A summary of vulnerability for the near-equatorial islands of the North Pacific is thus also included, due to their proximity to the centre of action of ENSO, the main influence on climate in the South Pacific. In Section 2 of this paper, we give an overview of global trends and projections in meteorological and oceanographic conditions that influence the incidence of extreme sea levels in the PCCSP region. Section 3 contains a summary of these observed trends in the PCCSP region and Section 4 details projections of future trends. Section 5 gives an analysis of the reasons for the projected trends, along with a summary of the reasons for remaining uncertainties, while Section 6 provides concluding remarks.
Section snippets
ENSO
There are numerous summaries of the effects of global climate and climate variability on the tropical Pacific (Rasmusson and Carpenter, 1982, Philander, 1990, Larkin and Harrison, 2001, Sarachik and Cane, 2010). This paper will not repeat this work but will focus on the main global drivers of variability in tropical cyclone incidence and extreme sea levels in the PCCSP region.
ENSO is the most important global mode of climate variability, with influences worldwide but with strongest effects in
ENSO
There were substantial trends in surface pressures and associated changes in circulation in the Pacific during the last century. Fig. 4 shows trends in sea level pressures over the equatorial Pacific and Indian oceans from 1871 to 1992. There has been a clear change in the mean state during this time, with pressures rising over the Maritime Continent2 and northern Australia and falling over much of the tropical
ENSO
Collins et al. (2010) review our current understanding of projected global trends in ENSO. As they note, observed changes in east–west Pacific tropical pressure gradients are clear (Fig. 4) and are consistent with a weakening of the Hadley and associated Walker circulations (Vecchi and Soden, 2007). This weakening is projected to continue into the future but is not likely to be associated with SST anomaly patterns that are similar to present-day El Niño anomaly fields. This concept is important
Analysis of the problem
In the previous section, it was shown that almost all models predicted a decrease in tropical cyclone numbers in a warmer world in the South Pacific region. This is also true globally. At least two main hypotheses have been proposed to explain this result. The first is that while an increase in SST is usually associated with an increase in PI, the tropical atmosphere is usually more stable in a warmer world than in the current climate, thus making it more difficult for tropical cyclones to
Research recommendations and implications for adaptation
As discussed in this paper, future changes to sea level extremes in the Pacific due to anthropogenic global warming will be affected by changes to tropical cyclones, and by future differences in climate variability, including the El Nino Southern Oscillation phenomenon. Sea level extremes will also be affected by wave climate change and rising sea levels. There remain significant barriers to reducing the uncertainties associated with future projections of these factors. These uncertainties have
Acknowledgements
The authors would like to thank the Pacific Climate Change Science Program of the Australian Department of Climate Change and Energy Efficiency for partially funding this research. The authors would also like to thank their respective institutions for supporting this work. They are grateful to Dr Neil White for valuable discussions and for providing Fig. 7.
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