**Appendix A. Free Cloud Pixel Probability**

The availability of IR observations is limited by the cloud coverage. Thus, it may be a key parameter to take into account when selecting the regions to study. Figure A1 is included as supporting information to select studied regions.

**Figure A1.** Probability of clear pixel for the time series (1982–2011) considered in the climatology of the maximum gradient of SST from University of Rhode Island (URI) Pathfinder 9 km frontal database (courtesy of Peter Cornillon Graduate School of Oceanography, University of Rhode Island (URI)).
