Adjusted Sea Level: We used to call this Gridded Sea Level but changed the name in Sep 2021. Adjusted Sea level (like GSL) is sea level minus two rapidly-varying sea level signals mostly due to barotropic dynamics: astronomical tides and the ocean's response to atmospheric pressure (which is to rise about 10 cm for each 10 hPa fall of pressure). Adjusted Sea Level is thus a measure of the slow modes of the ocean. The slow modes are largely in geostrophic balance, so we can estimate the near-surface velocity from the gradients of ASL. ASL is also a measure of depth-integrated density (and thus ocean heat content), as shown on our Argo pages where ASL is compared with in-situ determinations of steric height anomaly.

Terminology: The once-traditional term 'adjusted' is commonly omitted by many agencies and oceanographers. We did this, too, then regretted it, because dropping the 'adjusted' leaves no short name for 'unadjusted sea level', which is the quantity most relevant to users interested in coastal inundation (see our Australia/NZ maps of Non-tidal Sea Level). Another candidate name for Adjusted Sea Level is ocean dynamic sea level. Related names include: dynamic height, dynamic topography, and subsurface pressure.

Adjusted Sea Level Anomaly: By 'anomaly', we mean the departure from the long-term (1993-2012) mean. We (like most users of altimetry) estimate ASLA by subtracting the long-term mean of ASL from the altimeter observations. This must be done in order to remove the ~100m sea level highs and lows (with respect to a smooth surface) mostly due to gravity perturbations associated with sea floor topography. We do it using a correction supplied by the space agencies, which is the Mean Sea Surface (MSS, evaluated along the precise track of the altimeter). Unfortunately this also removes the ~1m scale highs and lows of the Mean Dynamic Topography (MDT) associated with the long-term mean of the ocean circulation (that causes the Coral Sea off Queensland to be about 1m higher than waters off Tasmania with respect to the geoid). We add the Bluelink ocean model estimate of the MDT to our altimetry-derived maps of ASLA (formerly known as GSLA) to produce ASL, from which we estimate velocities that include the mean. Most users, however, are more interested in the anomaly of sea level (adjusted or not) than the more abstract concept of sea level with respect to the geoid. Showing the anomaly also allows use of a more restricted colour bar, and to show the along-track altimetry data closer to its supplied form.

Centile rankings of (daily, detrended) Adjusted Sea Level Anomaly: This is a way of seeing 'how anomalous' the anomaly at a particular place and time is, compared with past anomalies at the same place. But sea level anomalies are not randomly distributed about a mean value, because sea level has a significant trend; about 100mm in the last 28 years (~3.7mm/year). Nor does it have a very regular annual cycle [see time series plot of the Australasia-region average]. So we have chosen to show the daily centile rankings of ASLA after subtracting the Australasia-region trend, but not the average annual cycle, in contrast to how we rank anomalies of Sea Surface Temperature. Otherwise, the interpretation of the centile maps is the same: if a point on the map is red, it means that the adjusted sea level that day, at that location, falls within the top few percent of all observed anomalies (detrended as described above). Note that we have used a non-linear colour scale, in order to show more discrimination at the high and low ends.

Centile levels of the 28-year, detrended data set: This is the reference data set against which the detrended anomalies for a particular day are compared. For efficiency, it is not re-computed every day. Consequently, and because eddies do not follow in each other's tracks, new observations fall outside the range of the centile levels of the reference data set fairly frequently. We are currently preparing a revised, longer reference data set, so using it will change this behaviour.