Non-tidal Sea Level: NTSL, as the name suggests, is sea level minus the tide. It includes the wind and pressure-driven changes of sea level as well as many other (non-tidal) causes of variation such as El Nino and ocean eddies.

Non-tidal Sea Level Anomaly: NTSLA is the anomaly of NTSL (with the long-term mean subtracted). See the Adjusted Sea Level info button for details.

Terminology: Non-tidal Sea Level Anomaly is sometimes referred to as either 'tidal residual' or 'non-tidal residual'. Both (confusingly) refer to the signal left over once a tidal prediction or analysis, which usually includes the mean, is subtracted.

Notes:

1. It is not possible to make maps of NTSLA directly from altimeter observations because NTSLA changes so rapidly compared to altimetry coverage. So OceanCurrent's maps of NTSLA are made by adding the 'static' Inverted Barometer response at 6h intervals to our daily maps of Adjusted Sea Level Anomaly made from a combination of altimeter and coastal tide gauge observations of adjusted sea level anomaly. The static inverse barometer is a fairly good approximation of the transient inverse barometer response, and is P'/(rho g) where P' is atmospheric pressure minus the daily, global over-ocean mean, rho is the average density of sea water and g is gravity.
2. Our maps of NTSLA do not include some of the processes that can be important in shallow (<~30m) areas, nearshore (<1km) areas, or over short (<12h) time intervals, such as wave setup, seiches or tsunamis. For this reason we only show NTSLA for the Australia-wide maps.