The manuscript provides a comprehensive review of the impacts of large-scale patterns of climate variability on the climate and environmental conditions in the Baltic Sea. In general, it is, in my opinion, a well-written review, and it gives a good overview of the state-of-the-art in this area of research.

Main points:

1) Perhaps unavoidable for a multiple-authorship manuscript, some sections appear less consistent with the rest of the manuscript. Other sections read more as a listing of results, where the reader would more strongly benefit from an interpretation of those results into a coherent picture. This could be amended with little effort. I give some specific examples below.

2) More effort, in my view, should, however, be placed into the Conclusion section. In a review paper, the conclusions section is probably more critical than in other types of manuscripts, and in this case it appears to me as too unspecific. After reading it several times, my take-home message was 'more needs to be done' and 'uncertainties still remain'. This is true for many fields of research, but a review manuscript should provide the reader with a more specific message. Perhaps the authors would like to restructure this section, including the grading of certainty. For instance, the section could be wrapped up with a paragraph of more certain findings, followed by less specific findings, and conclude with the questions that are still unresolved and really need targeted research.

Another point that sometimes confused me is the emphasis on 'internal variability'. This would, per se, not be wrong, but the reader is then prompted to expect a section on the impact of external forcing, either on the large-scale drivers, on the Baltic Sea itself, or on teleconnections. However, this does occur. I honestly did not see the need to highlight 'natural variability' in the manuscript. The only external forcings that are discussed in the manuscript are the rise in temperatures due to anthropogenic climate change and authentication. Both are very weakly related to 'teleconnections', if at all.

Particular points:

3) 'from the climatological reference state appear as geopotential height anomalies that can persist for up to 3 or 4 weeks.'

This seems to me to be a very long duration for a blocking situation. Please, check and eventually include a reference.

4) 'Traditionally, weather regimes are defined for winter and classified by four patterns..'

Do the authors consider weather regimes to be different from weather patterns or synoptic patterns? Perhaps a clarification would be helpful here. A search of 'seasonal weather regimes' yields a large number of published papers, so the authors may want to qualify the sentence.

5) 2.3.3 Sea level

' The sea level has a pronounced seasonal cycle with higher sea levels in autumn and winter than in summer and spring (Stramska, 2013)'

This sentence, while true, is oddly placed. The section is about interannual variations and how the Kattegat straits filter this variability. Then this sentence suddenly states that the most considerable variations (the annual cycle) are generated within the Baltic Sea itself (probably due to runoff).

6) ' Surface Air Temperature and Sea Temperatures Low winter SST is associated with negative NAO.... This wave train...'

what wave train?

7) '385 January and March, negative NAO phases (NAO < -0.5) are linked to substantially larger mean annual ice extent (259,000 kmš),'

The units did not render correctly when converting to pdf. This incorrect rendering of special symbols also occurs for some references, e.g., Matthäus, W. and Franck, H., 1992; Meier et al., 2022; Ostrowska et al., 2019; Patrizio et al., 2023. Please check the reference list carefully in the pdf file produced by the submission system.

8) 'but correlations can be of comparable magnitude to those of the Atlantic Oscillation,'

North Atlantic Oscillation

9) 'For the 20th century, common power ...'

common spectral power

10) 'about 20-40% of the shortwave variability '

shortwave radiation variability

11) The following two paragraphs appear inconsistent: First, it states that ' Wind conditions did not show a strong relationship with how often cyanobacteria blooms occurred, although winds mainly influenced variations from year to year (Kahru et al., 2020, their Figure 9)

Then, later: 'However, sea surface temperature and wind speed play a significant role in modulating decadal cyanobacteria blooms (Kahru et al., 2018)'.

What should the reader interpret from these results? Is wind responsible or not for cyanobacteria variability ?

Perhaps wind is not responsible for individual blooms but, somewhat, indirectly affects the conditions that modulate decadal variability; this needs an explanation.

12) This is an example of somewhat inconsistent writing.

'Yet, clear links between primary productivity and large-scale patterns, the relative importance of teleconnections on primary productivity and future primary productivity responses under an acceleration of warming remain unclear in the Baltic Sea.#

Clear links remain unclear ?

13) 'Because primary productivity forms

the base of the marine food web, understanding its link to natural variability and large-scale patterns is important to better predict its future and implications.'

But large-scale patterns are part of the natural variability or do the authors mean something else, like natural variability of the food web itself?

Summary:

The study highlights potential teleconnections of the North Atlantic Region to the Baltic Sea  and collects evidence from the available literature. Potentially impacted variables range from sea ice cover and oceanographic variables (such as temperatures or sea level elevations) to biogeochemistry. The manuscript considers various time scales (from synoptic to multi-decadal). Future research is encouraged and priority research needs are outlined. 

Major Comments:

I appreciate all  the effort that went into this manuscript. Overall, it is well written - however, at times the presentation would benefit from greater precision and clarity. For example, the phrase “Teleconnections between the North Atlantic and the Baltic Sea region” (Ln 1, 55) may be misleading, as it could be interpreted as implying a mutual or two-way interaction with the North Atlantic Ocean. Also, it would be helpful to clarify early on that variability in the Baltic Sea is predominantly governed by local atmospheric dynamics. These local synoptic systems are embedded within the large-scale circulation, which allows atmospheric patterns in the North Atlantic region to exert an indirect impact on regional atmospheric dynamics and, consequently, on the Baltic Sea. The mechanisms underlying these potential remote influences from the North Atlantic should be explained more explicitly, and known effects should be quantified whenever possible, including contemporary estimates of uncertainty. In its present form, the manuscript appears somewhat vague and lacks a clearly articulated objective. Previous findings are often described using rather unspecific terminology (e.g., “robust connections” (Ln 320), “fundamentally connected” (Ln 37), “closely connected” (Ln 65)), which makes it difficult to assess their precise meaning. Moreover, the discussion appears somewhat one-sided, with many processes being primarily attributed to the North Atlantic Oscillation (NAO) (e.g., Sect. 2.3).

Please refer to my specific points for additional suggestions.

Specific Comments:

Ln 1: This sounds like a mutual or two-way relationship. If so, this statement needs evidence.

Ln 2: It remains unclear what “in the region” refers to.

Ln 5: Shouldn’t this be “regional ocean model data” instead of “climate models data”? I don’t expect classical climate models can resolve the Baltic Sea in the level of detail needed for this study.

Ln 6: “other key indicators from synoptic to multidecadal time scales” is rather vague.

Ln 17: what does "their" refer to?

Ln 18: I think “modulating” should rather be “modulate”. Could you provide a reference?

Ln 27f: The flowing sentence is unclear and might be misinterpreted “The Baltic Sea represents a unique interface where Atlantic, Arctic and continental climate influences create a dynamic system that is governed by remote teleconnections.”. The Baltic Sea is mostly impacted by local atmospheric dynamics, which is certainly embedded in the large-scale atmospheric circulation of the Northern Hemisphere.

Ln 28-28: I find these paragraphs somewhat confusing. It does not become clear from the text which processes act on which time scales and how local and larger scale dynamics are related.

Ln 35: Please explain why these are considered key modes and add some explanation on their definition. I would expect specific local atmospheric pattern to be of interest (e.g. Lehmann et al. (2002))  and it would be interesting how these relate to the considered larger-scale modes.

Ln 40: It does not become clear what is meant by "these climate drivers". I would expect that local air temperature, local winds and precipitation are the key physical drivers for the Baltic Sea ocean dynamics. There might well be a relation to the major atmospheric modes in the North Atlantic – but it does not become clear how and how much these could explain all the local atmospheric phenomena.

Ln 43: “Permanent anoxic bottom waters have developed in the deep basins of the central Baltic Sea during the 20th century”.  Is this right? cf. Moros et al (2024).

Ln 46:  “Atmospheric ….” This sentence is completely unrelated to the foregoing sentence but gives somehow the impression that atmospheric Euro-Atlantic teleconnections are responsible that there has been only a limited success in improving eutrophication.

Ln 48ff: This paragraph is rather vague. Could you explain “the multiple interacting drivers” and the statement “coastal and sub-basin dynamics respond differently to various physical forcing” a bit more?

Ln 54: I don’t understand the special focus on biogeochemistry as it depends on local ocean dynamics and local nutrient dynamics. Please delete or clarify.

Ln 56: This last sentence implies that the aim is to understand climate variability while the following text is about weather pattern. It should be clarified throughout the manuscript which processes act on which timescales and which might be important. Also, I lack a clear relation to the title of the manuscript.

Ln 68: What is meant by “climate modes”? (e.g. the NAO is relevant mainly for winters and acts on various time scales; I would tend to call it something like atmospheric mode of variability).

Ln 70ff: The description of the jet dynamics on the various time scales ant it’s relation to the major large scale atmospheric modes could be more precise and systematic. How predictable is the jet dynamics?

Ln 84: Again, this sentence is very vague and it does not become clear which processes and dynamics it refers to:  “Given the complexity of mid-latitude dynamics, it is difficult to disentangle the individual impacts of these processes” . Please delete or be more specific.

Ln 84ff: It does not become clear which time scales are to be considered and why these where chosen.  E.g. major Baltic inflows play a major role and depend on very specific weather pattern -  not the NAO (e.g. Schimanke et al (2014); Schinke & Matthäus (1998), Matthäus & Franck (1992) and many more to follow).

Ln 91ff: It does not become clear to me how local blocking might be impacted by teleconnections and how or if it relates to 2.1.1. and what is the current state of knowledge.

Ln 113ff: The authors might consider to rename this section as it does not become clear what is meant by “Natural variability” in this context. In any case I would appreciate a clear distinction which time scales are considered.

Ln 122: I find it confusing to start the subsection “Synoptic, intraseasonal and seasonal time scales” with the sentence “The climate of the Baltic Sea region is driven by ….” as according to the common definition climate refers to averages of 30 years or more.

Ln 130: I would not count the NAO as a classical “weather regime” – rather as a leading atmospheric mode in an EOF analysis of sea level pressure anomalies on at least monthly time scales for a certain region and it is dominant during winter. All modes should be clearly introduced somewhere. Also, it should be mentioned that in most EOF-based NAO definitions, the Baltic Sea region is included in the analysis domain. So it’s not a teleconnection in a classical sense. Still, while the Baltic contributes variance it does not control the structure or phase of the NAO.

Ln 138ff: Please specify how exactly these regimes were identified. Also, it does not become clear how these regimes relate to the local atmospheric dynamics in the Baltic Sea region.

Ln 166: I would rather say something like this “the NAO reflects observed interannual variability in winter”  - it’s not really a driver.  Then it should be clarified what kind of variability.

Ln 174ff: This is interesting. Could you provide some more details? As I understood this also refers to the North Atlantic while the links to the Baltic Sea region remain unclear. Please correct me if I am mistaken.

Ln 183: What is meant by “multidecadal tendencies”.

Ln 200ff: It should be quantified how strong all the mentioned teleconnections are (within contemporary uncertainty bounds) and mention that the expected impact on the Baltic Sea region is presumably even weaker.

Ln 202: Better replace  “On decadal time scales, climate variability …” by  “On decadal time scales, atmospheric variability …”

Ln 224: Better add  “decadal atmospheric variability in winter time over the North Atlantic”

Ln 233:  I expect that most teleconnections were identified based on model results rather than “observed”.  Please be more precise or correct me. The NAO reflects the atmospheric variability over the North Atlantic during winters and I doubt that teleconnections could be established for individual synoptic systems (as the text might imply).

Ln 246: Why is that? “This highlights important room for improvement through model development”. Aren’t these feedbacks included in the models already? Most Baltic Sea models are forced by prescribed atmospheric boundary conditions which implicitly should include all the mentioned feedbacks. Please clarify.

Ln 247: Where do “North Atlantic freshwater anomalies” originate from and which specific region do the authors have in mind?

Ln 266 Consider to rename “Key regional climatic effects of natural variability”. It’s not clear which Atlantic pattern are “natural” or if these undergo shifts due to global warming. Also, this has so far not been subject of the manuscript.

Ln 267 I find it very confusing to talk about climate here after considering various time scales but not climate change in the Section above.

Ln 271: Again, I would suggest to be a bit more careful when it comes to cause and effect in this paragraph. There is information lacking how strongly the NAO relates to precipitation in the Baltic Sea region.

Ln 279ff: Doesn’t this rather belong to Section 3.2.2? Here I would rather expect some quantification how strong all these effects might impact the Baltic Sea region.

Ln 287ff:  Could you be more explicit how strong the described effects were suggested to be (opposed to locally driven dynamics)?

Ln 311: How strong is this correlation?

Ln 316: For which purpose has this index been defined and how does it relate to the major North Atlantic climate modes (e.g. correlation strengths)?

Ln 320: Please quantify “robust connections”.

Ln 325: How strong were these correlations. Why were these not mentioned in the respective subsections?

Ln 326: Doesn’t this also depend on region? E.g. the storm surge event in October 2023 brought exceptionally high water levels to the German Baltic Sea coast due to strong easterly winds across the Baltic Sea (i.e. apparently not NAO-driven). This section could be more objective and precise on the estimated impact of the NAO compared to other factors.

Ln 333: Please quantify “Low winter SST” and “high SST”. Where?

Ln 335: I find it somewhat confusing to introduce suddenly a new pattern that was never mentioned before.

Ln 340: Please quantify “SST is likely to increase during positive AMV state”. How likely? How much?

Ln 353: Is there evidence or is this speculation? “Consequently, atmospheric conditions resembling the positive phase of the NAO elevate the risk for MHWs in the Baltic Sea.” I am especially puzzled because the NAO is dominant mainly in winter.

Ln 361: Please quantify “strongly influenced”.

Ln 362: Please quantify how much zonal and meridional flows contribute. Can you specify the meridional flow and the relation to the leading atmospheric modes?

Ln 398ff: Please quantify the magnitude of all the mentioned effects.  

Ln 425: Please quantify how strong this effect is (relative to the mean seasonal signal).

Ln 426ff: “3 Biogeochemistry” It might be good to mention right at the beginning that this sections starts with general facts and while respective teleconnections will be explored later in Section 3.2.  

Ln 441: Are there indications that the very specific local atmospheric pattern leading to Major Baltic Inflows are impacted by teleconnections? What about local mixing - e.g. Reissmann et al (2009).

Ln 460ff: A classical impacting factor would also be upwelling (e.g. Lehmann & Myrberg (2008), Janssen et al (2004)).

Ln 487ff: I do not generally agree with the statement on cyanobacteria having a low salinity tolerance. Or do you mean that they can tolerate low salinities? Please double check (e.g. Munkes et al. (2021))

Ln 512: “is increasing with time” which time scale is considered here?

Ln 526:  How strong are these correlations?

Ln 529: There are many more studies (e.g. Karlson et al. 2008; Lips (2008), Löptien & Dietze (2023), Rakko & Seppäälä (2014), Wasmund (1997) and many more to follow)

Ln 601ff: These studies refer to the western Baltic where hypoxia are seasonal. I would expect a strong relation to late summer mixing as well as late summer temperatures. How can this we related to the (winter?) NAO.

Ln 728ff: Please provide examples including a quantification of the respective impacts.

Ln 733: Please quantify “strong”.

Ln 735: Please explain and quantify the “reshaping”.

Ln 736: I find this statement is rather vague: “Across time scales, the available literature defines different teleconnection patterns which may also be viewed through the lens of the Weather Regime framework”. It would be good to provide concrete examples to back it.

Ln 738/739 Which “Apparent differences described in the literature” appear?

Ln 720ff: I must admit that I am confused about the natural variability aspect in this paragraph. To me both local and remote atmospheric variability could well be impacted by climate change.

Ln 740ff: This omits all other aspects such as nutrient controls, species composition etc.: “Biogeochemical responses mirror the physical controls but …”

Ln 740: Not sure what is meant here “filtered by strong stratification” and where the strong stratification occurs.

Ln 741: How is this sentence supported? “This results in a system where teleconnections clearly matter,” I would like some evidence.

Ln 742: This is the first time global warming is mentioned. While I generally agree it comes a bit “out of the blue”.

Ln 750ff: If the physical controls of the biogeochemistry are so uncertain already, how can an analysis of teleconnections help?

Ln 755:  Is the “Possible predictability in the North Atlantic” the major reason for writing this manuscript? This would be a nice motivation, consistent with the title, and the authors might consider to mention this early on. However, especially for the biogeochemistry it would be good to then add some background information on the state-of-the-art of predictability, including some discussion on the major sources for uncertainties.

Ln 760: Which design do you suggest for a multi-model ensemble such that it is able to detect teleconnections? What is meant here with “external variability”?.

Ln 764: I cannot follow this argumentation from the above text. Please help me out.  A regional coupled model instead of using a forced model with prescribed atmospheric boundary conditions would introduce feedbacks from the Baltic Sea to the atmosphere. This seems unrelated to the teleconnections, considered in this manuscript.  

Ln 768: While I appreciate event-based modelling, I struggle to see the benefit for teleconnections.

Ln 769: Again – while I certainly appreciate high-frequency measurements I need some help to understand the benefit for advancing teleconnections.

References:

Janssen, F., Neumann, T., & Schmidt, M. (2004). Inter-annual variability in cyanobacteria blooms in the Baltic Sea controlled by wintertime hydrographic conditions. Marine Ecology Progress Series, 275, 59-68.

Karlson, B., Eilola, K., & Hansson, M. (2008). Cyanobacterial blooms in the Baltic Sea–correlating bloom observations with environmental conditions. In Proc 13th Int Conf on Harmful Algae (pp. 247-252).

Lehmann, A., Krauß, W., & Hinrichsen, H. H. (2002). Effects of remote and local atmospheric forcing on circulation and upwelling in the Baltic Sea. Tellus A: Dynamic meteorology and oceanography, 54(3), 299-316.

Lehmann, A., & Myrberg, K. (2008). Upwelling in the Baltic Sea—A review. Journal of Marine Systems, 74, S3-S12.

Lips, U. (2008) Abiotic factors influencing cyanobacteria bloom development in the Gulf of Finland (Baltic Sea). Hydrobiologia 614(1), 133–140.

Löptien, U., & Dietze, H. (2022). Retracing cyanobacteria blooms in the Baltic Sea. Scientific reports, 12(1), 10873.

Matthäus, W., & Franck, H. (1992). Characteristics of major Baltic inflows—a statistical analysis. Continental Shelf Research, 12(12), 1375-1400.

Munkes, B., Löptien, U., & Dietze, H. (2021). Cyanobacteria blooms in the Baltic Sea: a review of models and facts. Biogeosciences, 18(7), 2347-2378.

Moros, M., Kotilainen, A. T., Snowball, I., Neumann, T., Perner, K., Meier, H. M., ... & Schneider, R. (2024). Giant saltwater inflow in AD 1951 triggered Baltic Sea hypoxia. Boreas, 53(2), 125-138.

Rakko, A. & Seppäälä, J. (2014) Effect of salinity on the growth rate and nutrient stoichiometry of two Baltic Sea filamentous cyanobacterial species. Estonian J. Ecol. 63, 545–570.

Reissmann, J. H., Burchard, H., Feistel, R., Hagen, E., Lass, H. U., Mohrholz, V., ... & Wieczorek, G. (2009). Vertical mixing in the Baltic Sea and consequences for eutrophication–A review. Progress in Oceanography, 82(1), 47-80.

Schimanke, S., Dieterich, C., & Meier, H. M. (2014). An algorithm based on sea-level pressure fluctuations to identify major Baltic inflow events. Tellus A: Dynamic Meteorology and Oceanography, 66(1), 23452.

Schinke, H., & Matthäus, W. (1998). On the causes of major Baltic inflows—an analysis of long time series. Continental Shelf Research, 18(1), 67-97.

Wasmund, N. (1997) Occurrence of cyanobacteria blooms in the Baltic Sea in relation to environmental conditions. Int. Revue der gesamten Hydrobiologie und Hydrographie 82(2), 169–184.