What are the CMEMS ‘phy_assim’ Products?

The Sea Level Anomaly (SLA) variable has been present in the DUACS products for many years. It is obtained from the altimeter measurements after applying various geophysical corrections as discussed in the DUACS processing page. This means that different geophysical signals are removed from the measurements. They include the ocean tide signal and the Dynamic Atmospherical (DAC) signal induced by wind and pressure forcing. Additionally, a residual Long Wavelength Error (LWE) signal is also removed from altimeter measurement. The LWE is an empirical correction that aims at removing residual ocean tide and DAC signal as well as residual orbit error (residual signals induced by the imperfection of the solution used for these corrections)

Whereas the physical content of the SLA variable delivered is useful for many applications, it may not fit specific needs. For this reason, the “phy_assim” product was introduced in the CMEMS catalogue in 2014. It allows the users to easily modify the physical content of the SLA.

Different physical variables are available in the “phy_assim” product (see table below). While the SLA (filtered or not) field is by default corrected from different geophysical effects, some of them (namely, the ocean tide, dynamical atmospherical correction and Long wavelength Error correction) are included in the “phy_assim” product in order to let the user compute the SLA field without these corrections. A SLA field including the ocean tide effects and the DAC effect can be obtained that way:

[sla not corrected from ocean tide and dac effect ] = [sla from product]+[dac]+[ocean_tide]-[lwe]

The user can also decide to include in the SLA field only the ocean tide effect and/or the DAC effect.

Note that the LWE correction is not a geophysical correction. However, as it aims at reducing the residual signal of dac and ocean tide (see discussion above), we recommend to use it when reintroducing dac and ocean_tide in the SLA field.

 

Physical variable name description
sla_filtered Sea Level Anomaly, filtered from high frequency signal considered as noises (see DUACS processing). SLA referenced to the [1993, 2012] period
sla_unfiltered Row Sea Level Anomaly (i.e. not filtered). SLA referenced to the [1993, 2012] period
dac Dynamic Atmospheric correction
Ocean_tide Ocean tide signal
lwe Long wavelength correction
mdt Mean dynamic topography referenced to the [1993, 2012] period

 

The phy_assim product also includes a raw SLA field, i.e. a SLA including high frequency signal and noise measurement. At the opposite, the filtered SLA is obtained after low-pass filtering in order to reduce noise measurement effects  (see DUACS processing).

A Mean Dynamic Topography (MDT) field is also included. It can be used to compute the Absolute Dynamic Topography (ADT) as discussed in the altimeter reference period page.

 

Phy_assim product application example:

SLA assimilation in a numerical ocean model forced by wind and atmospheric pressure and/or including ocean tide signals. (e.g. Benkiran et al, 2014)