*4.1. Northwest Australia*

Guided by maps of clear sky probability (Figure A1) and tidal current maps (Figure 1), we focused first on a region located Northwest of Australia, with latitudes between 22◦S and 12◦S, and longitudes between 114◦E and 124◦E. We browsed the SST L2 archive (METOP, VIIRS and MODIS) from 2014 to 2016 and selected granules that accomplish at least 30 (100 pixels × 100 pixels) patches with 80% free cloud pixels. Table 1 summarizes the number of available granules for this region. The number of days with more than one L2 granule available is 303 days, which represents a 27% of the analyzed period. The SST corresponding to 8 September 2016 was in particular selected based on this criterium. Gradients of SST were computed and exhibit moderate values around 0.2 ◦C km<sup>−</sup>1.


**Table 1.** Number of L2 granules available from 1 January 2014 to 31 December 2016 in the Northwest Australia case study region.

In this area, semidiurnal internal tide currents (0.08–0.12 m s<sup>−</sup>1) are typical of what may be found in the open ocean (Figure 1). These currents are masked over the continental shelf because HRET is not reliable there [9]. Barotropic currents are amplified over the shelf with M2 tidal currents up to 0.6–0.8 m s<sup>−</sup>1.

Figure 3 illustrates the procedure to quantify the signature of M2 SST fluctuations for the Northwest Australia case study region. We first compute the Sobel gradient of the SST field then we retrieve M2 baroclinic and barotropic currents for regions. Finally the amplitude of SST fluctuations associated with baroclinic and barotropic currents are estimated by taking the product of SST gradients and tidal currents (Equation (7)).

Both baroclinic and barotropic tidal SST fluctuations are intensified on fronts and filaments. Barotropic fluctuations are largest on the shelf with values up to 0.3 ◦C and no significant signal in the open ocean (see Figure 3d). Baroclinic SST fluctuations are one order of magnitude smaller, being 0.03–0.04 ◦C (see Figure 3f).

**Figure 3.** (**a**) SST captured by VIIRS on the 8 September 2016. (**b**) Sobel gradient of SST. Note that the SST field shown here is composed of three L2 granules. The gradient is computed for each granule individually. Thus, the representation of the gradient presents two scan lines with missing data that correspond to the boundaries of the granules. (**c**) **M2 barotropic FES** amplitude current. Gray contour correspond to the bathymetry. (**d**) Estimation of the amplitude of M2 signature on SST. (**e**) **M2 baroclinic HRET** amplitude current for areas with a bathymetry deeper than 1000m. Gray contour lines correspond to the bathymetry. (**f**) Estimation of the amplitude of M2 IW signature on SST.
