the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Nov 2021
Characteristics of joint heavy precipitation and high sea level events on the Finnish coast in 1961–2020
Abstract. Simultaneous heavy precipitation with high sea level can lead to more severe flooding than if the hazards occur individually. In recent years, the significance of compound flooding has gained attention in many coastal areas, but studies on compound flooding in Finland are still lacking. In this paper, we investigate the co-occurrence of heavy precipitation and high sea level events (hereafter compound events) on the Finnish coast, because the simultaneous occurrence of these events can be considered as an indicator for compound flooding. We use daily sea level observations from tide gauges and gridded precipitation data to extract the days when both sea level and precipitation are extreme on the same day. Reanalysis data is used to derive composite maps of the compound events, and these maps are then qualitatively compared to the maps of non-compound events (i.e. when only one of the two variables is extreme alone). Finally, the dependence of the compound events on monthly indices of different atmospheric circulation patterns such as North Atlantic Oscillation is studied with correlation analysis. Our results show that the occurrence of the compound events is mostly controlled by sea level variability, with climatologically the most compound events taking place in autumn and winter. In addition to a large inter-annual variability of the compound events, statistically significant increasing trends were observed at some tide gauges, especially in the Bothnian Bay. The composite maps of meteorological variables indicate that the compound events are associated with strong extratropical cyclones which bring moisture and push the storm surge towards the coast. In average, the compound events are associated with stronger winds near the tide gauges than those events when only sea level or precipitation is extreme alone. Finally, we found that a negative phase of Scandinavian pattern (i.e. upper-level through over Scandinavia) is the most favorable atmospheric circulation pattern for the occurrence of compound events, in particular during autumn.
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Status: closed
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RC1: 'Comment on nhess-2021-314', Anonymous Referee #1, 23 Nov 2021
Although the results are promising, higher effort can be done to enrich the paper; the analysis is quite simple, and more information can be obtained from the data the authors show. Regarding methodology, information is missing, and overall, the it needs further explanation and can be greatly improved. Also, I’m not sure if the correlation between compounding effect in precipitation and sea level can be associated with climate patterns; can be an arbitrary association resulting from the low number of compound events found (something to discuss with the authors). Another aspect is the organization of the paper; the introduction and conclusions are mixed up. Finally, I have found that the grammar and language must be revised and notably improved before publishing. Therefore, I think the manuscript is not ready for publication; major revision.
- AC1: 'Reply on RC1', Mika Rantanen, 25 Feb 2022
-
RC2: 'Comment on nhess-2021-314', Anonymous Referee #2, 26 Nov 2021
This work presents a joint analysis between sea level and precipitation observations around the coastlines of Finland to investigate the occurrence of compound events that are relevant for coastal flooding episodes. The results are generally well described and presented, although the analyses are sometimes superficial. I am giving some suggestions below, followed by a list of minor issues that I have found in the manuscript. I believe that this work has the potential to be published, after the points below have been addressed.
Section 4.2: why is the timing of the compound events different between tide gauges in the end of the gulfs and those closer to the Baltic? what are the mechanisms explaining this difference? is it because extreme sea levels are caused by different processes in th two areas?
Section 4.3: The idea of the composite maps to investigate the patterns that lead to compound and non-compound events is interesting. But the robustness of the results shown in the maps in figures 6 and 7 should be discussed. Only 27 and 28 situations are used to produce these maps and there is a lack of information about their similarities. I think deviations from the mean fields are needed to improve the interpretation of these results. In addition, to increase the size of the sample I suggest redoing these maps using various nearby tide gauges, instead of only one. This limitation is noted later in section 5.1, but without further development.
There is no information on the predominant month when these situations take place. I guess, from the seasonal cycles in figure 4, that precipitation extremes will dominate in summer and sea level in winter. I wonder if the synoptic patterns of precipitation-only and sea level-only extremes during the fall/winter season are different from those shown in figures 6 and 7, when all seasons are included. I think it would be more interesting to select the season when coincidences are possible; otherwise, what we are seeing is only the dominant pattern in different seasons.
- 67: please, rewrite the sentence “that have been investigated on the Finnish coast not until the recent decade” as it is hardly understandable.
- 100: on the use of the maximum daily sea levels, is there any difference due to different samplings before and after 1971?
- 111: 21tg to 21st
- 142: the term “orthogonal” can be misleading in this context. If calculated with rotational EOFs over monthly data, NAO, SCA and other patterns are by construction orthogonal to each other (on a monthly basis, not on a seasonal or yearly basis). The meaning of orthogonal here seems to refer to the spatial structure of the MSLP right?
- 170: change were to was
- 214: change are to is
- Figures 6, 7: units are missing from the legends. Also in the caption, what does “average sea level height” means? Is it the average for the particular tide gauge during the selected episodes?
- Section 5.3: this section seems to me more part of the introduction. Just a suggestion…
Citation: https://doi.org/10.5194/nhess-2021-314-RC2 - AC2: 'Reply on RC2', Mika Rantanen, 25 Feb 2022
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RC3: 'Comment on nhess-2021-314', Anonymous Referee #3, 06 Dec 2021
General comments:
The manuscript deals with the concurrent occurrence of heavy rainfall and high water events along the Finish coast. The paper is well written, and what is presented is sound. The only thing I do not understand is, what is the point the authors want to make.
If they wanted to point out that these compound events create more severe flooding then in the case of non-compound events, well, in this case the authors failed miserably. There is no indication, and no statistics shown that can convince me that it is important to look at compound events in order to understand the severity of these events.
Actually, I am not sure why the authors have written this paper. The atmospheric circulation might be interesting, but me as an oceanographer am always looking at implications that some variables have for storm surge. I do not think that you need to analyze rain data in order to find out about flooding events.
Maybe I am missing something, maybe I was expecting too much. But as it stands now, this article is not very interesting. On these points I do reject the manuscript, but the editor might see it differently.
Specific comments:
Abstract: the authors want to correlate the rainfall and water level to compute compound flooding. Already in the abstract, they state that these events mostly are due to sea level variability (nothing surprising). I am not sure how the rainfall fits into this picture…
36-42: for this please see also Ferrarin et al, 2021 (Progress in Oceanography)
30: are you sure it is the precipitation, and not some other correlated variable like wind or atmospheric pressure that makes these events compound events?
98: how strong is this trend (numbers)?
105: please make clear if these data are observations or come from a meteorological model.
Table 1: have all points a MSL of 0?
123: I guess there is also rain data in ERA5. Did you use it and compare it to the observation data? If not, why not? If yes, how did the two data sets compare?
307-9: this is what I was fearing… that there is little correlation between rain and sea level. And for the whole article you are basically repeating it…
375-6: Well, I was eager to see the prove of this statement, but I guess the authors do not convince me
406-9: This is all hypothetical. Where can I verify this hypothesis?
431-2: It is maybe a proxy for compound flooding, but I am not sure if this is important, because you did not show the importance of compound flooding.
443-450: Well, you should have really shown that sea level is higher statistically during compound flooding then without compound flooding. I didn’t see this analysis, hopefully I didn’t miss it.
451-5: are these levels statistically different from non-compound events?
Citation: https://doi.org/10.5194/nhess-2021-314-RC3 - AC3: 'Reply on RC3', Mika Rantanen, 25 Feb 2022
Status: closed
-
RC1: 'Comment on nhess-2021-314', Anonymous Referee #1, 23 Nov 2021
Although the results are promising, higher effort can be done to enrich the paper; the analysis is quite simple, and more information can be obtained from the data the authors show. Regarding methodology, information is missing, and overall, the it needs further explanation and can be greatly improved. Also, I’m not sure if the correlation between compounding effect in precipitation and sea level can be associated with climate patterns; can be an arbitrary association resulting from the low number of compound events found (something to discuss with the authors). Another aspect is the organization of the paper; the introduction and conclusions are mixed up. Finally, I have found that the grammar and language must be revised and notably improved before publishing. Therefore, I think the manuscript is not ready for publication; major revision.
- AC1: 'Reply on RC1', Mika Rantanen, 25 Feb 2022
-
RC2: 'Comment on nhess-2021-314', Anonymous Referee #2, 26 Nov 2021
This work presents a joint analysis between sea level and precipitation observations around the coastlines of Finland to investigate the occurrence of compound events that are relevant for coastal flooding episodes. The results are generally well described and presented, although the analyses are sometimes superficial. I am giving some suggestions below, followed by a list of minor issues that I have found in the manuscript. I believe that this work has the potential to be published, after the points below have been addressed.
Section 4.2: why is the timing of the compound events different between tide gauges in the end of the gulfs and those closer to the Baltic? what are the mechanisms explaining this difference? is it because extreme sea levels are caused by different processes in th two areas?
Section 4.3: The idea of the composite maps to investigate the patterns that lead to compound and non-compound events is interesting. But the robustness of the results shown in the maps in figures 6 and 7 should be discussed. Only 27 and 28 situations are used to produce these maps and there is a lack of information about their similarities. I think deviations from the mean fields are needed to improve the interpretation of these results. In addition, to increase the size of the sample I suggest redoing these maps using various nearby tide gauges, instead of only one. This limitation is noted later in section 5.1, but without further development.
There is no information on the predominant month when these situations take place. I guess, from the seasonal cycles in figure 4, that precipitation extremes will dominate in summer and sea level in winter. I wonder if the synoptic patterns of precipitation-only and sea level-only extremes during the fall/winter season are different from those shown in figures 6 and 7, when all seasons are included. I think it would be more interesting to select the season when coincidences are possible; otherwise, what we are seeing is only the dominant pattern in different seasons.
- 67: please, rewrite the sentence “that have been investigated on the Finnish coast not until the recent decade” as it is hardly understandable.
- 100: on the use of the maximum daily sea levels, is there any difference due to different samplings before and after 1971?
- 111: 21tg to 21st
- 142: the term “orthogonal” can be misleading in this context. If calculated with rotational EOFs over monthly data, NAO, SCA and other patterns are by construction orthogonal to each other (on a monthly basis, not on a seasonal or yearly basis). The meaning of orthogonal here seems to refer to the spatial structure of the MSLP right?
- 170: change were to was
- 214: change are to is
- Figures 6, 7: units are missing from the legends. Also in the caption, what does “average sea level height” means? Is it the average for the particular tide gauge during the selected episodes?
- Section 5.3: this section seems to me more part of the introduction. Just a suggestion…
Citation: https://doi.org/10.5194/nhess-2021-314-RC2 - AC2: 'Reply on RC2', Mika Rantanen, 25 Feb 2022
-
RC3: 'Comment on nhess-2021-314', Anonymous Referee #3, 06 Dec 2021
General comments:
The manuscript deals with the concurrent occurrence of heavy rainfall and high water events along the Finish coast. The paper is well written, and what is presented is sound. The only thing I do not understand is, what is the point the authors want to make.
If they wanted to point out that these compound events create more severe flooding then in the case of non-compound events, well, in this case the authors failed miserably. There is no indication, and no statistics shown that can convince me that it is important to look at compound events in order to understand the severity of these events.
Actually, I am not sure why the authors have written this paper. The atmospheric circulation might be interesting, but me as an oceanographer am always looking at implications that some variables have for storm surge. I do not think that you need to analyze rain data in order to find out about flooding events.
Maybe I am missing something, maybe I was expecting too much. But as it stands now, this article is not very interesting. On these points I do reject the manuscript, but the editor might see it differently.
Specific comments:
Abstract: the authors want to correlate the rainfall and water level to compute compound flooding. Already in the abstract, they state that these events mostly are due to sea level variability (nothing surprising). I am not sure how the rainfall fits into this picture…
36-42: for this please see also Ferrarin et al, 2021 (Progress in Oceanography)
30: are you sure it is the precipitation, and not some other correlated variable like wind or atmospheric pressure that makes these events compound events?
98: how strong is this trend (numbers)?
105: please make clear if these data are observations or come from a meteorological model.
Table 1: have all points a MSL of 0?
123: I guess there is also rain data in ERA5. Did you use it and compare it to the observation data? If not, why not? If yes, how did the two data sets compare?
307-9: this is what I was fearing… that there is little correlation between rain and sea level. And for the whole article you are basically repeating it…
375-6: Well, I was eager to see the prove of this statement, but I guess the authors do not convince me
406-9: This is all hypothetical. Where can I verify this hypothesis?
431-2: It is maybe a proxy for compound flooding, but I am not sure if this is important, because you did not show the importance of compound flooding.
443-450: Well, you should have really shown that sea level is higher statistically during compound flooding then without compound flooding. I didn’t see this analysis, hopefully I didn’t miss it.
451-5: are these levels statistically different from non-compound events?
Citation: https://doi.org/10.5194/nhess-2021-314-RC3 - AC3: 'Reply on RC3', Mika Rantanen, 25 Feb 2022
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