the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Aug 2024
Current structure, circulation and transport in the Central Baltic Sea observed by array of moorings and gliders
Abstract. An array of moored current meters and gliders were deployed across the Central Baltic Sea. Weak and low persistent currents driven by the wind-driven Ekman transport were observed in the upper layer in the offshore area. Stronger currents with higher persistency were observed near the coast, presumably driven by the local sea level gradient. The kinetic energy was largely observed to be in the low-frequency band (timescales >36 h), but higher kinetic energy on shorter timescales was observed in the areas, where the halocline was close to the seafloor. A strong and highly persistent gravity current was observed in the near bottom layer of the Fårö sill. Circulation pattern, current structure, and meridional transport varied on synoptic and seasonal timescale due to changes in local forcing and caused changes in the water column habitats. A low persistent, cyclonic gyre in the upper layer and meridional transport to the north in the deep layer prevailed in the seasonal (3–4 months) timescale. Circulation patterns with higher persistency formed under stable forcing. A cyclonic circulation in the upper layer was supported by southerly winds and reversed by northerly winds. Northward transport in the deep layer was intensified by northerly winds and reversed by southerlies. Due to the seasonality in the meridional wind component, the pattern associated with northerly (southerly) wind is more common in spring and summer (autumn and winter).
The deep-water transport is approximately 400 km3 y-1 towards the Northern Baltic Proper, which is almost 1/3 of the total deep-water volume in the Northern Baltic Proper, Gulf of Finland, and Western Gotland Basin. This transport mostly occurs during spring and summer and brings denser, oxygen-depleted water to the deep layer of these areas.
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RC1: 'Comment on egusphere-2024-2272', Anonymous Referee #1, 17 Nov 2024
The manuscript "Current structure, circulation and transport in the Central Baltic Sea observed by array of moorings and gliders" is a description of one year of mooring and glider observations in the Central Baltic across roughly 58.4N. The paper is descriptive of the physical oceanography of the region. This domain displays estuarine dynamics since it is a shallow (~250 m or less), semi-enclosed, saltwater body.
Abstract:
-combine to make one paragraph
- "changes in water column habitats" is mentioned in the abstract but not discussed in the text. please remove from the abstractIntroduction:
- The study "addresses the knowledge gap regarding the current structure and circulation in the Central Baltic Sea." The authors need to provide a better scientific background and reason for completing the study. In the last paragraph of the introduction, the authors describe 4 specific scientific goals of the study. Please describe why you are studying these four items into the first paragraph of the introduction.
- Please remove the sentence "On the one hand, the pycnoclines determine the current shear maxima, but on the other hand, the current structure shapes the pycnoclines" - this statement is vague and not necessary in the introduction.
- The introduction would benefit from a schematic showing what is known about the Central Baltic from modeling studies (and some observations). It would be relevant to the paper to synthesize what is known for the specific study region of the paper.
- Similarly, the paper does not label the Faro Sill on the maps in a way that is clear to me. Please show me the location and bathymetry of Faro Sill. (Could be a part of the schematic)
- On figure 1, what is FD, ND, GD?Methods:
- Please merge the paragraphs that are only one sentence.Results:
- As part of figure 2, include a plot of wind direction
- Line 216 - since you describe there being 3 water masses, you should plot the three water masses on a T-S diagram
- In figure 2, what glider observations were used to make the temperature, salinity, and N^2 plots? All the data? How are they averaged? (in space? in time?)
- Line 226 - A three layer pattern due to the plots of Fig 3 and 4 is not clear. 1) the plots do not show full water column ADCP velocity for most of the mooring sites 2) There is variable bottom depth of the different mooring locations compared to the glider locations to which you are comparing. 3) M5 shows no high shear squared region. I suggest removing this analysis because I do not think that it is strong.
- Line 228 - You state that the current is weaker offshore, but this is not true at M2 and M3 where the current is stronger compared to M4 and M5.
- Line 249 - change peed to "speed"
- Line 250 - Since you compare the speeds of the mean profiles, also plot the standard deviation of the mean speed profiles in figure 5.
- figure 9 - Clearly label the figure's color bar limits that describe the value of correlation. Alternately, make the colorbar go from zero to one.
- Figure 12 - is there a weighted mean employed to achieve the plots from the glider observations? Describe in the methods if too much detail to put in the figure caption.Conclusions:
- Move the first sentence "The first results of the CABLE study were presented" to the last paragraph in the conclusion.Citation: https://doi.org/10.5194/egusphere-2024-2272-RC1 -
RC2: 'Comment on egusphere-2024-2272', J. H. Reißmann, 29 Nov 2024
general comments:
The authors investigate the structure and properties of the current field in the Central Baltic Sea using current data from an array of eight moorings in a line at the latitude of the Fårö Sill, seven equipped with a profiling ADCP and one with a classical current meter at a fixed level. These data were supplemented with ERA5 reanalysis wind data to take the forcing into account as well as some bottom temperature and salinity data from three of the moorings and CTD data from several glider surveys in that region to describe the hydrographical situation and calculate salt transports.
In detail, the authors present and examine stratification, current shear, profiles of mean current speeds and persistency, the seasonal variation of current velocity profiles, i.e. dependence on seasonal stratification, profiles of HKE spectra and profiles of HKE content in specific frequency ranges and their share in total HKE, profiles of complex correlation (magnitude and phase) between current and wind, profiles of mean current vectors in periods of nearly constant wind forcing, i.e. dependence on wind direction, to characterise the current field. Additionally, they calculate volume and salt transports across the line of moorings below 70 m and show temperature and salinity sections and mean velocities from glider sections along and across Fårö Sill to illustrate the overflow over the sill.
With this investigation the authors provide some substantial facts on the basis of measurements beyond the existing knowledge from simulations, while the presentation of the results and in particular their discussion unfortunately remain largely rather descriptive than quantitatively explanatory. However, the manuscript covers the primary aims of this work given by the authors in the introduction, which are all of more or less descriptive nature, very well.
The language is good and reads fluently. Some figures would benefit from minor improvements, see specific comments.
specific comments:
Figure 1: The coloured and dashed lines in the left map are hardly to distinguish. I suggest to rework this figure to make it clearer.
line 128: I think it would be good to give also the spatial resolution of the ERA5 data here, i.e. the area for which the used grid point is meant to be representative for.
Table 1: From Figures 3 and 4, I guess the starting day of the deployment period of M2 should be somewhat later than 09 May, which seems to be correct for M3.
line 154: This has to be lined out more explicitly. Why are the data low-pass filtered in general? What has been done exactly, in particular, for the calculations of HKE? For example, if the data is low-pass filtered, miscalculations of HKE in particular in the BSD are the consequence. Here you state mainly inertial oscillations, tides and seiches are suppressed by your filtering. In line 180 it says these are the main contributors to the BSD. This confuses the reader. I am sure this is not what you did. Please elaborate your filtering (What is done for which purpose and calculation?) in more detail to avoid confusion.
line 184: I think it is useful for the less experienced readers to explicitly give the definition of N2.
line 190: ‘zonal’ instead of ‘meridional’
line 190: ‘black’ instead of ‘dashed’, see text under Figure 1 and take account of the comment on it.
line 196: The regression of the volume transport with respect to the near bottom meridional velocity at M3 can be justified assuming a similar (meridional) current distribution over the considered transect cross-section. In addition, the same regression of the salt transport needs the assumption of similar distribution of salinity over the transect cross-section. Can you show to which extend these assumptions are valid and estimate the errors which are introduced by the supposed deviations from the assumptions?
line 250: This statement is certainly correct also for M10, but not very meaningful for the point measurement there. As it is also not an acoustic measurement and therefore implicitly excluded from the list in line 249, I would simply remove it explicitly from the list in line 249.
line 262: According to Figure 5, the persistency at 58 m in M9 is rather 54 % than 61 % to me.
line 263: According to Figure 5, the persistency at the thermocline in M9 is rather 49 % than 51 % to me.
line 268: I think I know what you mean, but for better reading you should briefly explain in which respect the cyclonic circulation is reflected by the mean velocity profiles. Else, for some readers, it may be hard to understand what exactly you mean here.
line 269: I am afraid I have a problem with the wording here. What is the difference between ‘interior basin’ and ‘central part’? Do you mean ‘basin rim’ and ‘basin centre’, respectively? Please clarify this wording.
line 303: Beside the technical correction to this line, I have a problem to understand what you want to say with this sentence. Do you mean something like: ‘A somewhat higher energy at the bottom in the BSD and BD bands was revealed at the two stations where the sea depth was in the range of the halocline depth (M2, M9).’ This is what I interpret. Please clarify this.
line 305: Similar to the preceding comment, this sentence would make much more sense to me if it started, for example, with ‘A significantly higher energy …’ in addition to the change proposed in the respective technical correction.
Figure 7: In the text to the figure it says in brackets that HKE spectra equal HKE spectral density multiplied by the frequency. I would rather expect that HKE spectra equal HKE spectral densities multiplied by the used frequency step or interval like a finite integration. Please check this and correct if necessary.
line 328: This is quite a simplification. Correct is that the vertical integrated transport is to the right of the wind vector. So, this should be reformulated somehow.
line 338: This is not directly visible from Figure 9 as the depicted vector sticks only show the relative angle between current and wind. Therefore, this statement should be explained some more.
Figure 9: In the first sentence, I suggest to replace ‘current direction’ and ‘wind direction’ by ‘current vector’ and ‘wind vector’ as the wording is in the second sentence to avoid confusion as the wind direction is opposite to the wind vector. Furthermore, I would add ‘α’ to ’mean angles’ and ‘correlation strength’ to the variable name ‘ρ’.
Figure 9: I am not sure whether the information of plots would be better or easier to get if the vector sticks are coloured instead of an extra row of dots with the colour information. Maybe the correlation strength could also be shown as the length of the vector sticks if a suitable scale can be found, or a combination of both, i.e. coloured vector sticks with variable length. I think it is worth to try this out.
line 402: Like in Figure 9, I would add ‘correlation strength’ to the variable name ‘ρ’.
line 415: Unit of cumulative wind stress should be [N m-2 d] instead of [N m-2 d-1] like in Figure 2.
line 422: According to section 2.2. it should be 22 April instead of 24 April and the link is somewhat different there. Please equalise both or clarify.
line 427: How does this relate to the Baltic residence times of about 30 years give elsewhere in literature? Is it an extraordinary high transport to that area observed in that year or does the water reside somewhere else before entering of after leaving that region for the rest of the time?
line 483: Why was something similar in contrast possible for the surface layer in Section 3.4., lines 360 to 362? What is the difference in consideration and interpretation?
line 502: I suggest to write ‘southwesterly’ instead of ‘southerly and westerly’, because that is what was investigated.
Line 511: Like before, I suggest to write ‘northeasterly’ instead of ‘northerly’, because that is what was investigated.
line 538: Should be ‘NNW-SSE’ instead of ‘WNW-SSE’ I guess, because that are opposite directions and the resulting orientation fits.
line 538: This statement is wrong what can easily be seen from Gauss’s Law as we certainly have a divergence-free current field. Or in other words, the higher current velocity along the channel would be exactly compensated by the smaller cross-section perpendicular to it in comparison to the larger zonal cross-section in combination with the smaller meridional current velocity.
line 597: I do not see a reversal of the cyclonic circulation in the upper layer in the results presented in section 3.4. for the first (southwesterly wind) and the second (north-northeasterly wind) period considered. The current reverses from north to south in the eastern part of the array from the first to the second period. But in the in the west at M2 in both periods the current is to the west.
technical corrections:
line 106: ‘… deployed at the same …’ instead of ‘… deployed to the same …’
line 115: ‘… were recorded at …’ instead of ‘… was recorded in …’
line 163: Equation 1: I suggest to remove ‘/n’ in the denominator and to put the fraction ‘1/n’ in front of the summation in the denominator instead for better readability.
line 171: Emery and Thomson (2004) is missing in the references.
line223: add unit ‘m’ to ’60-80’
Figure 4: add unit (1/s2) to the colourbar.
line 249: typo ‘speed’ not ‘peed’
line 298: better: The HKE share of the BD band at the Fårö Sill … , but the HKE in the LF band …
line 300: ‘… energy in the LF band …’ instead of ‘… energy at the LF band …’
line 301: ‘… energy in the BSD and BD bands in …’ instead of ‘… energy at BSD and BD in …’
line 303: ‘… energy in the BSH and BD bands was …’ instead of ‘… energy at the BSH and BD was …’
line 305: ‘… energy in the BSD band was also revealed …’ instead of ‘… energy at the BSD was revealed also …’
line 376: better to understand and less irritating: ‘vice versa’ instead of ‘opposite’
Figure 12: Exchange ‘0.1 g kg-1’ and ‘0.1 °C’ to make their order respective to ‘temperature and salinity’ before.
Citation: https://doi.org/10.5194/egusphere-2024-2272-RC2 -
RC3: 'Comment on egusphere-2024-2272', Anonymous Referee #3, 11 Dec 2024
Review of “Current structure, circulation and transport in the Central Baltic
Sea observed by array of moorings and gliders” by Liblik et al.
This manuscript describes the first analysis of an extensive set of observations in the Baltic Sea. Several ADCP mooring and observations from long term glider deployment. The observations is much needed to improve knowledge of the Baltic Sea oceanography.
I am a bit divided on my impression on the manuscript. On the one hand it is based on an new extensive set of observations that is/will be very useful, The analysis is comprehensive and detailed (although sometimes difficult to follow). On the other side, the main impression is that the manuscript is fragmented without a clear thread. After re-reading the manuscript several times I still am uncertain what I have learned form the manuscript, and the message remains unclear.
My suggestion is that the authors try to include more discussion on results in section 3 Results. I am aware of different views on how to divide results and discussion on results. But as of now I find that the description/discussion on figures are not very enlightening and does not provide much guidance what figures shows, and how they can interpreted. I find section 3.2 (which is a key section) rather difficult to read and to extract useful information on how the system works/operates. Much information is provided, but very little guidance for the reader. (There is a useful discussion on some parts, but in general I find it difficult to get a clear idea how results compare with other studies and what I can learn from present study)-
Major specific points.
- Avoid one sentence paragraphs. Try also to avoid paragraphs with few sentences. It gives a fragmented impression of the study.
- I notice that there are no comparison with results from other studies in the results section. This implies that it is rather difficult to get a clear picture how important the result from the study is, does it agree or disagree with earlier studied. Including some discussion in result section would improve readability ad impact of the study.
- Figures are difficult to read:
- Figure 1: The stations are difficult to read in the plot.
- Figures 2, 3, 9: The colorbars are too small to read (at least on paper where you cannot zoom in).
- Figures 6, 9: The arrows seem to describe current speed and direction. Please clarify what is shown.
- Figure 6: What is x-axis (guess stations but ...).
- Figure 3, 4 are hardly discussed but take up significant space. Please provide a relevant discussion. It is not clear to what the purpose what the purpose of Figure 4. Would readability increase with a log colorscale?
- Open access to data. I understand that the team wants to use the observation for more studies. It is stated that observations will be used for e.g. validating ocean models etc. It would be great to write out if data will be publicly available at some point (in, say two years).
Minor specific points
Line 115. How many time series and at what depth? (mentioned later, but should be clarified here).
Table 1: Table should be adjusted to show date in clearer way.
Table 1: Define what EMDB293, any reference to cruise report (assuming it is a cruise).
Line 157: Define what IGRF stands for.
Line 158: Not sure what 2-sided derivative algorithm is. Do you mean central numerical scheme. Please clarify.
Line 187 and following paragraph. It is difficult to read. Perhaps using bullet points improve readability.
Line 207: High P (79-97%) of wind. It is unclear to me, do you mean that wind has high persistence?
Figure 2: I do not see the point in having the wind speed “attached” to this figure. It would connect better to figure 3.
Line 249: Spelling peed.
Line 250: “acoustic measurements”. I presume it is ADCP measurements.
Line 291. You mention “diurnal tidal constitutents”, but could not 24 h period reflect daily cycle (in, e.g. wind forcing etc).
Figure 7: It would help if the most important time “periods” is illustrate din the figure (e.g as lines/tics in the “x-axis”.
Line 414: Please explain how the volume fluxes are consistent with cumulative wind stresses. What assumptions do you make, and what result do you get? Please enlighten the reader.
Citation: https://doi.org/10.5194/egusphere-2024-2272-RC3
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