Western boundary circulation and coastal sea-level variability in northern hemisphere oceans

Diabaté, Samuel Tiéfolo; Swingedouw, Didier; Hirschi, Joël Jean-Marie; Duchez, Aurélie; Leadbitter, Philip J.; Haigh, Ivan D.; McCarthy, Gerard D.

doi:https://doi.org/10.5194/os-2021-24

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https://doi.org/10.5194/os-2021-24

Preprints

06 Apr 2021

Review status: this preprint is currently under review for the journal OS.

Western boundary circulation and coastal sea-level variability in northern hemisphere oceans

Samuel Tiéfolo Diabaté¹, Didier Swingedouw², Joël Jean-Marie Hirschi³, Aurélie Duchez³, Philip J. Leadbitter⁴, Ivan D. Haigh⁵, and Gerard D. McCarthy¹ Samuel Tiéfolo Diabaté et al.

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¹ICARUS, Department of Geography, Maynooth University, Maynooth, Co. Kildare, Ireland
²Environnements et Paleoenvironnements Oceaniques et Continentaux (EPOC), UMR CNRS 5805 EPOC-OASU-Universite de Bordeaux, Allée Geoffroy Saint-Hilaire, Pessac 33615, France
³National Oceanography Centre, Southampton, UK
⁴University of East Anglia, Norwich, UK
⁵University of Southampton, Southampton, UK

Received: 15 Mar 2021 – Accepted for review: 29 Mar 2021 – Discussion started: 06 Apr 2021

Abstract. The northwest basins of the Atlantic and Pacific oceans are regions of intense Western Boundary Currents (WBC), the Gulf Stream and the Kuroshio. The variability of these poleward currents and their extension in the open ocean is of major importance to the climate system. It is largely dominated by in-phase meridional shifts downstream of the points where they separate from the coast. Tide gauges on the adjacent coastlines have measured the inshore sea level for many decades and provide a unique window on the past of the oceanic circulation. The relationship between coastal sea level and the variability of the western boundary currents has been previously studied in each basin separately but comparison between the two basins is missing. Here we show for each basin, that the inshore sea level upstream the separation points is in sustained agreement with the meridional shifts of the western boundary current extension over the period studied, i.e. the past seven (five) decades in the Atlantic (Pacific). Decomposition of the coastal sea level into principal components allows us to discriminate this variability in the upstream sea level from other sources of variability such as the influence of large meanders in the Pacific. This result suggests that prediction of inshore sea-level changes could be improved by the inclusion of meridional shifts of the western boundary current extensions as predictors. Conversely, long duration tide gauges, such as Key West, Fernandina Beach or Hosojima could be used as proxies for the past meridional shifts of the western boundary current extensions.

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Samuel Tiéfolo Diabaté et al.

Status: final response (author comments only)

RC1:
'Comment on os-2021-24', Anonymous Referee #1, 28 Apr 2021

This manuscript examined interannual to decadal coastal sea-level variability associated the Kuroshio and the Gulf Stream variations. The authors showed that the first EOF modes of coastal sea level variability both in the North Atlantic and North Pacific are associated with the meridional shifts of the western boundary currents. In contrast, the second EOF mode in the North Pacific is related to the large meander of the Kuroshio, which is clearly different from the second EOF mode in the North Atlantic. The topic of the manuscript is important, and the quality of the analyses is good. I have two comments.

General comment:

The manuscript is well written, but the novelty of this study is unclear. In the abstract, there are only two sentences about the results of the present study (L6-10). As the authors cited in the manuscript, there are many studies that examined coastal sea level variability associated with the Kuroshio and the Gulf Stream variability. Also, recent review paper (Woodworth et al. 2019) compared coastal sea level variability between the Kuroshio and the Gulf Stream regions. What are the new findings in the present study? Please more clarify this point.

Specific comment:

As the authors pointed out in section 3.2.1, the sea surface velocity changes associated with the first EOF mode in the upstream region of the separation point is different between the North Atlantic and North Atlantic (L341-349). The Kuroshio was shifted on-shoreward, but the Gulf Stream was shifted off-shoreward in the positive phase of the first EOF mode (Fig. 5), although the corresponding costal sea level anomalies are positive. However, the detailed difference has not been discussed in the manuscript. It is interesting to compare the sea level change in the across-shore direction between the two regions.

Citation: https://doi.org/10.5194/os-2021-24-RC1
- CC1:
  'Reply on RC1', Samuel T. Diabate, 31 May 2021
  
  Dear Anonymous Referee,
  Thank you for your comments. Here are my answers. Please find the attached file which contains also the figures and references mentioned.
  Best Regards,
  Sam
  --
  
  I want to thank Anonymous Referee #1 for their comments. They are greatly appreciated and will help improve the manuscript.
  Comment 1:
  “The manuscript is well written, but the novelty of this study is unclear. In the abstract, there are only two sentences about the results of the present study (L6-10). As the authors cited in the manuscript, there are many studies that examined coastal sea level variability associated with the Kuroshio and the Gulf Stream variability. Also, recent review paper (Woodworth et al. 2019) compared coastal sea level variability between the Kuroshio and the Gulf Stream regions. What are the new findings in the present study? Please more clarify this point.”
  Response:
  In the Pacific, the coastal sea-level upstream of the Kuroshio has been shown to co-variate with, on the one hand, the location of the Kuroshio as it approaches the Izu-Ogasawara Ridge (Kuroda et al., 2010), and on the other hand, with the atmospheric regime shifts in central North Pacific (Senjyu et al., 1999). These different results have been nicely tied up together by Sasaki et al. (2014) and Yasuda and Sakurai (2006), who have argued that Rossby wave, breaking as coastally trapped wave at arrival at the western margin, bring the central Pacific signal to the coast of Japan, while modifying the latitude of the Kuroshio. Our results compliment these findings, as we show using altimetry and subsurface temperature that the main mode of sea-level variability reflects change in the Kuroshio Extension location. However, findings of Sasaki et al. (2014) were limited to 1993 – 2011, and those of Yasuda and Sakurai (2006) to the model world, whereas our results hold for 1968 – 2019 and are solely based on observations.
  In the Atlantic, to the contrary—and to the best of my knowledge—no study (including the recent work of Woodworth et al. 2019) had previously associated the location of the Gulf Stream Extension with the upstream sea level. That the agreement between the upstream sea level and the meridional shifts of the extension holds in both ocean separately is a new result, and our main finding. This finding is new and of great importance for the community interested in the sea-level variability of the North Atlantic western margin, and to those interested in the Gulf Stream North Wall, while in the same time it brings further evidence to the community working on the Japanese sea-level and/or on the Kuroshio Extension.
  We understand from the comment of Anonymous Referee #1 that the novelty of the study is not straightforwardly shown in the manuscript. We have identified places where the manuscript could be modified (abstract, introduction, conclusion) to present more accurately than at present what has been summarised in the two above paragraphs. An improved version of the manuscript will be presented in the final response.
  Comment 2:
  “As the authors pointed out in section 3.2.1, the sea surface velocity changes associated with the first EOF mode in the upstream region of the separation point is different between the North Atlantic and North Atlantic (L341-349). The Kuroshio was shifted on-shoreward, but the Gulf Stream was shifted off-shoreward in the positive phase of the first EOF mode (Fig. 5), although the corresponding costal sea level anomalies are positive. However, the detailed difference has not been discussed in the manuscript. It is interesting to compare the sea level change in the across-shore direction between the two regions.”
  Response:
  Indeed, the upstream patterns of sea surface velocity are different in each basin. If significant, this discrepancy could be due to different interactions in each basin between the upstream jet and the bathymetry. It suggests that some more involved mechanisms than a pure Kelvin wave are at work. Across-jet analysis of the sea surface height (SSH) variability is feasible for the region southeast of Japan, because, there, the lateral shifts of the jet are large (Figure 5.a of the manuscript). Figure 1 below shows such analysis, where the 138.875°E was used. Agreement between the leading principal component of the tide-gauge obtained sea-level anomaly and the meridional shifts of the Kuroshio at 138.875°E is visible. Overall this analysis produces the same results than the sea surface velocity (SSV) analysis presented in the manuscript, and no different conclusion can be made.
  For the upstream Atlantic situation, an across-jet SSH analysis does not produce satisfactory results and is hence not shown. Comparatively to southeast Japan, where the change of path of the Kuroshio produces large SSV change (Figure 5.a of the manuscript), SSV changes across the Florida Current are small and have small zonal extent (see Figure 5.b of the manuscript), which largely complicates an across-jet analysis. Nonetheless, that the Florida Current shifts off-shoreward in the positive phase of the first EOF mode is not impossible. A similar situation is seen in the Pacific, southeast of Kyūshū (Figure 5.a of the manuscript). Indeed, when the Kuroshio shifts off-shoreward at 138.875°E, it moves in-shoreward southeast of Kyūshū (See Figure 2 below). The upstream situation in the Atlantic hence doesn’t necessarily conflicts with the patterns seen in the Pacific. In any case, and most importantly, conclusions drawn from SSH analysis, as those obtained with SSV analysis, are only valid from 1993 onwards due to the altimetry data availability. Contrarily to the region of the Gulf Stream and Kuroshio extensions, we cannot validate altimetry obtained results for the upstream Gulf Stream and Kuroshio with subsurface temperature indices extending further back in time. Hence, this topic was not developed furthermore in the manuscript.
  We understand from the comment of Anonymous Referee #1 that a more developed discussion on the behaviour of the upstream jet would be beneficial to the paper. While I agree with Anonymous Referee #1, I must also admit that what we can bring to the discussion is limited, because the altimetry results for the upstream situation cannot be cross-validated with subsurface temperature (at least, not with the data we have produced). A detailed investigation of the difference between the two upstream situation (Pacific and Atlantic) and of the involved mechanisms is beyond the scope of this work. Nonetheless, any changes to the manuscript will be presented in the final response. Any advices on this very interesting subject are welcome.
  Again, I want to thank Anonymous Referee #1 for their comments.
  Samuel Tiéfolo Diabaté, lead author
  samuel.diabate.2020@mumail.ie
  
  Citation: https://doi.org/10.5194/os-2021-24-CC1
  - RC2: 'Reply on CC1', Anonymous Referee #1, 21 Jun 2021
    
    I have satisfied the authors' reply. I hope that the authors adequately refrect our discussion in the manuscript.
    
    Citation: https://doi.org/10.5194/os-2021-24-RC2
RC3: 'Comment on os-2021-24', Tal Ezer, 19 Jul 2021

The comment was uploaded in the form of a supplement: https://os.copernicus.org/preprints/os-2021-24/os-2021-24-RC3-supplement.pdf

Citation: https://doi.org/10.5194/os-2021-24-RC3

Samuel Tiéfolo Diabaté et al.

Supplement

https://doi.org/10.5194/os-2021-24-supplement

Data sets

Spreadsheets for GSNW, KEI and tide gauge derived sea-level modes Diabaté, Samuel Tiéfolo; Swingedouw, Didier; Hirschi, Joël Jean-Marie; Duchez, Aurélie; Leadbitter, Philip J.; Haigh, Ivan D.; McCarthy, Gerard D. https://doi.org/10.5281/zenodo.4659318

Samuel Tiéfolo Diabaté et al.

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Short summary

The Gulf Stream and the Kuroshio are major currents of the North Atlantic and North Pacific respectively. They transport warm water northward and are key components of the Earth climate system. For this study, we looked at how they affect the sea level of the coasts of Japan, USA and Canada. We found that the inshore sea level covary with the north to south shifts of the Gulf Stream and Kuroshio. In the paper, we discuss the physical mechanisms that could explain the agreement.


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