Estimation of regional departures from global-average sea-level rise around New Zealand from AOGCM simulations
Quantifying future increases in sea level around New Zealand arising from climate change is a key factor for determining decisions on adaptation, yet most projections are expressed in terms of a global-mean rise in sea level. The work presented here makes use of the data from the Atmosphere-Ocean General Circulation Models (AOGCM) used in the 4th Intergovernmental Panel on Climate Change Assessment Report (IPCC AR4) of 2007 to estimate the possible regional changes in absolute sea level for the SW Pacific for the middle and end of this century for three future climate SRES scenarios (A1B, A2, B2). The AR4 models are initially assessed against satellite altimetry data and while the time mean characteristics of the dynamic topography are represented well, the representation of higher frequency variability is poor. Regional steric changes, determined from averaging several AR4 climate-ocean models, indicate departures of up to an additional 0.05 m, relative to the global average rise, may be applicable to the New Zealand region by the end of the century (2080–2099 relative to 1980–1999). The highest departures from the global mean rise in steric sea level would occur to the west of New Zealand (Tasman Sea) with the lowest departures in the deeper ocean areas (not the coast) to the south-east of the South Island (south of Chatham Rise). Of the three scenarios considered, the largest projected regional departures in sea-level rise around New Zealand for 2030–2049 occur in the A1B scenario and for 2080–2099 in the A2 scenario. However, the analysis presented here shows the projected regional increase in eustatic sea level will be relatively small compared to the global mean sea-level rise and supports earlier statements about departures in the region in more detail. Additionally, further regional contributions to sea-level rise will be influenced by future spatial changes in the gravitational field as melting of the Earth’s polar ice sheets accelerates, producing a south-to-north gradient in sea-level rise for New Zealand from this process, with a lower contribution in the south relative to the north. It remains unclear what effects climate change will have on oceanic current circulation in the New Zealand region, and therefore on regional and local changes in sea-surface height. Locally, around the New Zealand coast, relative sea-level rise will also be influenced by spatial and temporal variations in rates of vertical land motion arising primarily from tectonic processes, such as the subsidence currently being experienced in the Wellington region.