See attached file.

The manuscript by Lorente et al. investigates an alternative index for tracking the presence and strength of coastal upwelling and downwelling circulation patterns based on observed and modelled surface currents. In the case of the observed currents, analyses are presented from two coastal regions with time series extending over several months in each case. For the case of the modelled currents, an analysis is presented for one of the geographic regions for an entire 12-month period.

The surface-current-based coastal upwelling index (CUI) is compared against the traditional wind-based index. In that sense, there is no available ground-truth observation of vertical upwelling current. Independent evidence of upwelling circulation is provided in the form of sea surface temperature and chlorophyll observations whose spatial and temporal patterns match those predicted by the large events in the CUI indices.

The proposed surface-current-based CUI utilizes observations from various networks of high frequency (HF) radar installations. The availability of those observations is growing as more coastal HF radar sites are being added in many parts of the world. Extending the utility of HF radar observations is, therefore, of interest to a wide range of marine scientists and resource managers. This manuscript is generally well written and documented and the results support the use of a surface-current-based CUI. For these reasons, I recommend the manuscript for publication with only minor corrections.

Overall, the manuscript is well motivated and documented with references from the community. If anything, the Introduction could be condensed because it is slightly repetitive and long compared with the results section. The main results are well documented and they support the idea that surface-current-based CUI can be used as an alternative to an overwater wind based CUI. I do think that the conclusions section could focus more on why a surface-current-based CUI is advantageous. I am a little skeptical that using a numerical circulation model to obtain surface currents to then estimate a CUI is better than simply using the winds that drive the model.

The main benefit of a surface-current-based CUI that is suggested in the manuscript is the possibility to create a 2-D map of the CUI. For that additional level of CUI fidelity to be meaningful there should be some discussion of the relevant divergence scale that is controlling the upwelling process. The traditional wind-based CUI assumes a very large horizontal scale with surface currents diverging from the coastal boundary being responsible for the upwelling circulation. Two-dimensional observations of surface currents from HF radar (or a numerical circulation model) can, in theory, expose horizontal divergence in the flow field and the associated upwelling patterns. Such direct observations of divergence are very sensitive to errors and I’m not convinced by the results in this manuscript that the two-dimensional variations in CUI are meaningful. There should be, at a minimum, some discussion of scale and that fact that the mapping results are suggestive at best.

MINOR COMMENTS:

Line 12: “ecosystems, impacting on” should be “ecosystems, which has impacts on”

Line 28: “As the interface” should be “The interface”

Line 45: “process denominated Ekman” should be “process referred to as Ekman”

Line 74: “hence two” should be “two”

Line 160: “those situ” should be “those in situ”

Line 178: “CUI-HFR which” should be “CUI-HFR, which”