Understanding Sea Level Change in the South Pacific During the Late 20th Century and Early 21st Century

A spatially nonuniform sea level rise was observed in the South Pacific with high trends of up to 8 mm/year in the southwest subtropical Pacific and much lower values in the eastern ocean during the 1993 to 2015 altimetry data period. Negative trends were observed further south. In recent decades, these trends have been interpreted as a spin-up of subtropical gyre circulation. Here an analysis of altimetry data and data from the Coupled Model Intercomparison Project Phase 5 is presented, which attempts to separate natural climate variability from sea level changes. In order to quantify the impact of natural variability, a multiple linear regression was performed between sea level and dominant climate indices in the region, with the latter representing interannual to interdecadal climate variability. Our results indicate that the observed pattern cannot completely be explained by the climate modes but that a large residual trend remains. Plain Language Summary Sea level change is important in the context of climate change. The global mean sea level has risen about 20 cm in the last century. However, sea level change is not globally uniform but varies regionally due to different factors. These factors are a mixture of natural and human-induced contributions. Satellite data provide a global picture of sea level change. These observations started in the early 1990s. During the period 1993 to 2015 for which the satellite data are available, a spatial nonuniform trend has been observed in the South Pacific, with a maximum in the western part, east of New Zealand. The main objective of this work is to analyze whether this trend can be explained with natural climate variability or whether it is human influenced. Apart from the observations, model data that simulate the climate of the past century are used in order to answer that question. The results of this work indicate that the observed sea level pattern during 1993 to 2015 cannot be explained by natural variability alone.