David Griffin (CSIRO, Australia)

The IMOS-OceanCurrent project (oceancurrent.aodn.org.au) provides a service to members of the general and scientific communities interested in current and past conditions of the ocean around Australia. The project also supports scientific research (via the website and other services) and provides expertise in satellite altimetry data within Australia. One of the main products of the IMOS-OceanCurrent website are daily maps of adjusted sea level anomalies (SLA) and surface geostrophic currents for waters around Australia. Users of the website include sea-going oceanographers, marine biologists, professional and recreational fishermen, students, and the Royal Australian Navy. Some of our users might not be aware of the effect that the ever-changing atmospheric pressure has on the height of the sea surface, and that this effect is not included in the maps of isostatically-adjusted SLA we show on the website. The discrepancies between the oceanographers’ preferred maps of adjusted SLA and the sea level observed at the beach may be cause for confusion. Therefore, we felt the need to include maps of ‘observable’ SLA, with values that include the wind and pressure-driven changes of sea level as well as many other (non-tidal) causes of variation such as the El Niño and ocean eddies. We call this product ‘non-tidal SLA’, because it is calculated from sea level values that include everything, except the tides. IMOS-OceanCurrent’s maps of non-tidal SLA are made by adding the static inverse barometer response at 6-h intervals to our daily maps of adjusted SLA. The latter is built from a combination of altimeter and coastal tide gauge observations of adjusted SLA. The static inverse barometer is a fairly good approximation of the transient inverse barometer response. Including the maps of non-tidal SLA to the website gave us the opportunity to raise attention to the corrections done to the adjusted SLA maps that we have shown for over 10 years. In addition, these maps allow the users to better understand anomalous sea levels that they observe at the beach. For example, a series of atmospheric lows passed south of Australia in July 2021, resulting in the highest non-tidal sea level in the region for many years. This anomalously high sea level was not evident in the maps of adjusted SLA, but was observed at the beach, and in the maps of non-tidal SLA.