Surface Heat Budget over the North Sea in Climate Change Simulations

Round 1

Reviewer 1 Report

This is a review report for the paper entitled by “Surface Heat Budget over the North Sea in Climate Change Simulations” which was submitted to MDPI Atmosphere. Main author is Dr. Christian Dieterich who belongs to SMHI. The aim of this study is to investigate projected changes in heat budget of the North Sea. To achieve this, authors designed numerical experiments using regional climate models, RCA4-NEMO coupling ocean and atmosphere. Then, they employed 5-GCMs from CMIP5 with 3-scenarios used in IPCC AR5. For downscale to a regional model, boundary conditions are changed depending on these GCMs and scenarios. A half part of results described in this paper is for validation of model experiments. Results of hindcast were compared with some observational data sets. The rest part contains result of heat budget of the North Sea and its climate change signals. Results say that the North Sea is getting warmer with increasing of several degrees of SST, but net surface heat budget is not so large. Therefore, other factors are discussed. Interestingly, it was pointed out that the total heat uptake did not show large change, but the heat balance was quite changed. It is also shown that this is related to the change in temperature difference between the ocean and land. The strategy of experiments looks good. It is clear that the interpretation using an ensemble has some advantages over the interpretation of the results of a single model. The manuscript was well written, and no major questions arose, except for the following parts. I recommend a minor revision at this time.

Major comments:

1. Significance of modeled surface heat flux changes

I have a question about significance of surface heat flux (including its spatial patters). It looks quite small value. Is this statistically significant? Authors indicated the time series of annual mean only for SST (Figure 6).  I think that It is better to see time series of heat fluxes. It is also better to show significant area in the spatial pattern of the change. I agree that the effects of the decadal change will be small on an average for 30 years, but in many cases the results will depend on how you choose the period and the magnitude of the fluctuation. We recommend you consider the above in the light of the need for a more prudent explanation to the reader.

2. Description of experiments

Abstract and summary say that combinations of 3 RCP scenarios and 5 GCMs were used, but in fact, the results of SRES scenarios and 1 GCM are also conducted (Table 1). What is this meaning? How does it relate to the main results?

3. Description of downscaling to RCM

Because I am very beginner for doing numerical model, I could not well understand for the detail of downscaling. I would like to know (briefly) the data flow of atmospheric and ocean coupling in GCM and regional model especially when downscaling is done. I think it is better if you put some schematics and easy understandable explanation about this. It may help reader except experts.

4. Validation with other comprehensive observed flux

Authors conducted a nice validation work for the modeled or its ensemble SST. But for the surface heat budget, I think there is room for improvement. While authors used only two field observation studies (i.e. LVE1 and OWSF), why don’t you use satellite estimates of fluxes? HOAPS3 which was used for validation of SST is one candidate. IFREMER, GSSTF, JOFURO will be able to provide nice data suite for this.

Minor comments:

P6. L267: The SST in the North Sea is mainly determined locall by atmospheric flux…

I think that things is depending on time scales. Please show evidence or cite any reference. 

The word “atmospheric flux” is vague. I think that a word “air-sea flux”/“surface flux” is better.

Table 3

I think Qns is not familiar definition. Does this mean Qns = (Qlw + Qsh + Qlh)? If so, please state. Net heat flux should be better use of Qn/Qnet than Qa (Qa is usually used for acronym on air humidity).

Author Response

Please see attached file answers1.pdf

Author Response File: Author Response.pdf

Reviewer 2 Report

The topic is important, the paper is informative. After the minor corrections I suggest the manuscript for publication.

The text of the manuscript is too long. Please make the text shorter and reduce the length of sentences. Please give more concentrate information. Please qualify the results in the text if it is possible.

Please use my comments in separate file.

Comments for author File: Comments.pdf

Author Response

Please see attached file answers2.pdf

Author Response File: Author Response.pdf

Reviewer 3 Report

This manuscript is a modeling study to validate and analyze the projection of climate change in the North Sea using an ensemble of regional atmosphere-ice-ocean model runs. A fully-coupled modeling system allows the interaction between each model component, which is a more realistic representation of the Earth system. However, biases in sea surface temperature are reported from various coupled modeling systems. This work is valuable in the sense that it uses an ensemble method to demonstrate the potential of coupled regional climate models in improving climate projection. 

I found the paper to be overall well written and much of it to be well described. Therefore, I recommend this paper for publication for a minor revision. 

Minor Comments:

1) In the model description section, could you please describe the data fields been exchanged between each model component and how often does the exchange happen?

2) L 269 As I understand, the coupled model has a negative bias of net heat flux at the North Sea. If so, does it comes from parametrization schemes in the atmosphere model? Can an ensemble of different physical schemes reduce this bias?

3) In the summary and conclusions section, could you please briefly describe what is the advantage of using coupled model compared to the ocean-only model.

Author Response

Please see attached file answers3.pdf

Author Response File: Author Response.pdf