Description: Global mean sea level has been rising and is attributed to anthropogenic climate change forcing. The causes are mainly thermal expansion due to ocean warming and addition of water mass into the ocean from melting of land ice. Regional sea levels can deviate significantly from global mean sea level due to different processes at various spatiotemporal scales. In this study, we examine Australian sea levels and underlying mechanisms based on observations from 1880s to current, including direct sea level measurements by in-situ tide gauges and satellite altimetry, as well as other relevant oceanic and atmospheric observations. We focus on three aspects to explain the regional distribution of Australian sea levels. Firstly, we examine the connection of sea levels with modes of climate variability (e.g., ENSO) and driving physical processes (e.g., wave propagation). Secondly, the regional sea level budget from 1966 to present from selected tide gauges are analysed to identify the dominant factors for sea level trends, including ocean dynamics, sea level fingerprints, vertical land movement and inverse barometer effect. Thirdly, the connection between Australian sea levels and ocean gyre circulation and boundary currents (such as the South Pacific subtropical ocean gyre and East Australian Current) are identified, focusing on coastal sea levels in response to strength and position of ocean gyres and boundary currents. Findings from this historical study improve our understanding of sea level changes and variability around Australia, which will help us to project future sea level changes in coming decades with more confidence and reliability.

Description: Sea-level rise integrates the responses of several components (ocean thermal expansion, mass loss from glaciers and ice sheets, terrestrial water storage). Before the satellite era, global sea-level reconstructions depend on tide-gauge records and ocean observations. However, the available global mean sea level (GMSL) reconstructions using different methods indicate a spread in sealevel trend over 1900-2008 (1.3~2.0 mm/yr). With the improved understanding of the causes of sea-level change, here we update the original Church and White (2011) reconstruction by using the latest observations, taking the time-evolving sea-level fingerprint, sterodynamic sea level (SDSL) climate change pattern and local vertical land motion (VLM) into account. The updated trend of GMSL of 1.6 ± 0.2 mm/yr (90% confidence level) over 1900-2019 is consistent with the sum of contributions of 1.5 ± 0.2 mm/yr, slightly lower than 1.8 ± 0.2 mm/yr from original reconstruction. The lower trend from the updated reconstruction is mainly due to including residual VLM correction. The trends at tide gauge locations from updated reconstruction agree better with the tide gauge observations, with comparable mean trend of 1.7 mm/yr (standard deviation; STD of 2.0 mm/yr) from observation and 1.7 mm/yr (STD of 1.2 mm/yr) from the updated reconstruction. The inclusion of sea-level fingerprint and SDSL climate change pattern are the dominant contributors for improved reconstruction skill on regional scales at tide gauge locations. This update leads to GMSL solution that are consistent with other reconstructions in terms of long-term trend and 30-year running rate.

Description: A rapid warming and freshening of the Southern Ocean have been observed over the past several decades and are attributed to anthropogenic climate change. We conducted ocean model perturbation experiments to separate roles of individual surface forcing in the Southern Ocean temperature and salinity changes. Model-based findings are compared with results from a theoretical framework including three idealized processes defined on the θ-S diagram. Under the future scenario of CO2 doubling, the heat flux forcing dominates the large-scale warming, deepening of isopycnals, and spiciness changes along isopycnals, which can be captured by an idealized pure warming process to represent the subduction of surface heat uptake. The poleward-intensifying westerly winds account for 24% of the enhanced warming between 35°and 50°S and would have comparable contribution as the heat flux forcing after removing the global ocean warming effect. In contrast, the widespread freshening in the Southern Ocean driven by increased surface freshwater input is largely compensated by the wind-driven saltening. The response to freshwater forcing could not be approximated as a similar pure freshening process as the induced cooling and freshening have comparable effects on density. The wind-driven changes are primarily through the local heave of isopycnals, thus resembling an idealized pure heave process, but contain considerable spiciness signals especially in the midlatitude Southern Ocean, resulting from anomalous northward transport and subduction of heat and salt that are largely density-compensating. These distinct signatures of individual surface forcing help us to better understand observed and projected changes in the Southern Ocean.

Description: Changes in sea level are mostly driven by internal climate variability, and anthropogenic forcing. Moreover, changes in stratospheric and tropospheric Ozone during the second-half of the 20th century also cause significant changes in ocean heat uptake. A recent study has shown that both stratospheric and tropospheric ozone have contributed to the Southern Ocean warming in the deep ocean. This study has shown that the 30% of Southern Ocean warming during 1955-2000 is driven by ozone in the upper 2000 m of the ocean. Of this 30%, 60% is attributed to stratospheric and 40% to tropospheric ozone changes. Changes in ocean heat uptake consequently affect sea level and we assess these changes due to ozone. From the analysis of four CMIP6 models with a total of 28 ensemble members, we find that thermosteric sea level increases between 40-60 S and decreases between 60 S and the Antarctic peninsula. Hence, there is a gradient in thermosteric sea level established around 60 S with higher sea level on the equatorward side and lower sea level on the poleward side. The increased sea level in the Southern Ocean is plausibly related to an increase in the strength of westerlies due to ozone forcing that has helped to deposit more heat due to the intense churning of the subtropical gyres. Further investigations are required to quantify the changes in sea level due to ozone in comparison to aerosols, greenhouse gases, and their combination (i.e., historical trend) to explain the observed sea level changes.