General comments

The manuscript deals with the complex surface and deep ocean circulation in the Iberian-Biscay-Ireland (IBI) region of the Northeast Atlantic Ocean, highlighting the importance of examining the currents for various applications such as climate research, marine navigation, fisheries and marine renewable energy. The study proposes a methodology for the creation of ocean monitoring indicators (OMI) to assess the interannual variability of the main ocean currents in this region.

The methodology involves defining specific monitoring windows based on existing literature, dynamic vertical density boundaries, and using Copernicus Marine products and ensemble methods to calculate annual volume transport anomalies within these windows.

The results show the variability of regional currents, with positive anomalies indicating either intensification or weakening of currents. Despite the uncertainties, some OMIs successfully detect periods of high or low current activity, and reveal significant trends.

The work is clearly presented and illustrated. As far as I know, this substantial work appears to take into account the main ocean currents in the region, and the whole seems worth publishing. However, a number of questions remain concerning the definition of the boundaries of certain sections, and these need to be clarified. In particular, it would be essential to specify the extent to which transport uncertainties for certain sections are linked to the choice of vertical and horizontal boundaries.

Specific comments

Figure 2: To which reanalysis (es) and for which period do the current fields shown in this figure correspond?

Figure 2 and table 2: The upper isopycnal limits chosen correspond roughly to the main thermocline. Have you tested the sensitivity of transport to the choice of this isopycnal value?

How was the outer (offshore) boundary of the different currents defined? The geographical limits seem rather arbitrary. Do they result solely from information based on the literature, or – which would seem more appropriate here – from an analysis of the variability of current intensity or transport for the different reanalyses? This is the case, in particular, for the Celtic-Armorican Slope section and the Armorican Slope Poleward current.

Figure 3: What do the different traces (lines) in the 𝜃-S diagrams represent? Monthly climatological average? Results from one or more reanalyses? Please clarify.

Discussion and Figure 4. Just out of curiosity, regarding the interpretation of interannual variability, is it by any chance the case that some of the significant signals you have highlighted have also been listed in the literature?

Technical corrections

Line 31 : change « Fricourt et al., 2007 » by « Friocourt et al., 2007 ».

Figure 2: The AC current on the Madeira section is barely legible. The contrast should be increased.

The following reference in the bibliography is not cited in the text : Cavagnaro, R. J., Copping, A. E., Green, R., Greene, D., Jenne, S., Rose, D., Overhus, D.: Powering the blue economy: Progress exploring marine renewable energy integration with ocean observations. Marine Technology Society Journal, 54(6), 114-125, https://doi.org/10.4031/MTSJ.54.6.11, 2020.

Review of “Monitoring main ocean currents of the IBI region”

By Á. de Pascual Collar, R. Aznar, B. Levier, M. García Sotillo.

The authors propose a methodology to monitor several surface and subsurface ocean current transports of the Iberia-Biscay-Ireland region, which allows to investigate their interannual/decadal variability or long-term trends.

Defining the lateral (longitude/latitude) and vertical boundaries (density range) of specific ocean currents is a tricky task. The choice of those boundaries might - sometimes often - lead to significant uncertainties. The uncertainties are assessed by comparing a set of five Copernicus products.

The methodology is clear, the figures adequate, and the manuscript well written. The manuscript is worth publishing after the minor changes listed below.

Main comments:

• I would provide the mean transport values in the results section (even the seasonal variability). Figure 4 shows the annual anomalies. It is however useful to know the mean values. It is very different to have a 1Sv anomaly for a 1Sv mean current (100%) or a 1Sv anomaly for a 10Sv mean current (10%). It is ok to keep the y-axis in Sv and not in % of the mean value.
• Either in the Introduction or Results sections, it would be interesting to compare the mean values of these transports to estimated values based on in-situ data from the literature.

Specific comments:

Lines 49-51: Needs to be rephrased slightly. “… is the lack of …”, “… data is scarce.”, “… observations are lacking.” (repetition/redundancy)

Line 52: What do you mean by “since modeling data is gridded”?

Line 103-125: Reorganize the list of sections following Figure 2: RT – CAS – WIP – ABB – MA – AS – LPP. This paragraph is also missing the Algerian-Balearic Basin (ABB) section.

Figure 2: Provide the data set product used in Figure 2.

Line 130: The Algerian Current (ALC) is missing

Line 134-145: I understand that the authors define the vertical boundaries based on the T/S diagrams, but how do the authors choose the density ranges? For example, The ALC is defined as the ocean transport integrated from the surface to the 27.25 isopycnal. Based on Figure 2, this isopycnal seems to be a little bit shallow, and thus part of the ALC transport is missing.

Line 155: Why is the OMI aimed to focus on interannual variability? How do the mean and seasonal variabilities compare between the different products?

Lin 220: I am not sure how the journal handles the acknowledgment section. If applicable, do not forget to add the acknowledgment section (fundings, …)