the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Jul 2023
Dependency of simulated tropical Atlantic current variability on the wind forcing
Abstract. The upper wind-driven circulation in the tropical Atlantic Ocean plays a key role in the basin wide distribution of water mass properties and affects the transport of heat, freshwater, and biogeochemical tracers such as oxygen or nutrients. It is crucial to improve our understanding of its long-term variability which largely relies on model simulations due to sparse observational data coverage especially before the mid-2000s. We applied two different forcing products to a high-resolution ocean model which resolves the complex zonal current field in the tropical Atlantic. Where possible, we compared the simulated results to long-term observations. We found that in simulations the strength of the wind stress curl above the upwelling regions of the eastern tropical North Atlantic is important to set the mean strength of the off-equatorial surface and subsurface currents north of the equator. Too strong wind stress curl above the upwelling regions seems to overestimate the subsurface currents resulting in unrealistic seasonal variability. The simulated decadal to multidecadal variability of the tropical Atlantic current field can, to a large extent, be explained by changes in the Sverdrup dynamics. The combination of both simulations and observations reveals that the recent strengthening of the EUC can be indeed interpreted as a recovery from a weak phase the current experienced since the late 1990s. Where it has become common place for models to explain processes behind ocean observations, we postulate that long-term observations, once they have reached a critical length, can be used to test the quality of wind-driven simulations. This study presents one step in this direction.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-1433', Anonymous Referee #1, 13 Aug 2023
Review of “Dependency of simulated tropical Atlantic current variability on the wind forcing” submitted for Ocean Science by Kristin Burmeister, Franziska U. Schwarzkopf, Willi Rath, Arne Biastoch, Peter Brandt, Joke F. Lübbecke, and Mark Inall (egusphere-2023-1433)
This paper investigates wind-driven variability of upper layer currents in the tropical Atlantic Ocean by comparing two simulations with a high-resolution ocean circulation model forced by two different atmospheric products provided for ocean-climate modeling. Long-term mean states are compared first, then seasonal cycle and long-term variability including trends are compared. Observational results are also used to validate the model where possible.
Results presented would be very useful for enhancing our comprehensive understanding of mean state and variability of upper layer currents in the tropical Atlantic Ocean. Methods used are appropriate, though they may not be necessarily novel. Publication would be recommended after some revisions.
Major points.
It would be very helpful for readers if the key drivers, all of which should not be necessarily wind-driven, for currents discussed in the paper are summarized in the early part of the manuscript. For example, the easterly wind, the Ekman divergence, and the meridional gradient of Sverdrup stream function for EUC.
Specific points.
- L7,69,366 etc.: It would be helpful where “the upwelling regions of the eastern tropical North Atlantic”, which is referred to many times in the manuscript, specifically represent is indicated somewhere in the manuscript. Figure 1c would be one of candidates.
- L43: Does the Ekman transport diverge or converge south of the ITCZ?
- L52–53: It would be worth mentioning that there are some resemblances of the structure of the zonal currents between the equatorial Atlantic and Pacific, as authors cited many papers that treated the tropical Pacific Ocean later in this paragraph.
- L92: This is a nice approach, but it would be appropriate to describe specifically how the time gap was filled between the CORE simulation and observations somewhere in the manuscript.
- L179: To what is the EUC transport reconstruction using the mooring compared?
- L204: Are there any known reason for the differences found among phase speeds of the first baroclinic mode?
- L365: It would be nice to show that the JRA55 wind forcing actually captures fine structures that force NEUC variability.
- L406–408, 577–578: Are only first two baroclinic modes used for the harmonic analysis shown in Figures 4 and 5? If so, please be specific about this.
- L412: Since long-term trends are treated here, it would be good to discuss why trends are relevant or whether any issues have been raised in relation to global warming.
- L543: “This agrees with the weakening easterly winds along the equator.” … What does "this" point to? Should "weakening" be replaced with "trend of"? Or should “in JRA” be appended at the end of this sentence?
Technical points.
L62,63: Please give definition for NBUC and NBC.
L95: It would be more appropriate to cite Large and Yeager (2009) for introducing the CORE v2 dataset.
L121: well-establish ---> well-established
L124: relative ---> relatively
L129,757: It would be appropriate to cite Kobayashi et al. (2015; JMSJ) for introducing JRA-55.
L136: I would suggest something like, “Meridional ship sections are taken between 21°W and 28°W for velocity (31 sections) and hydrography and oxygen (22 sections) between 2000 and 2018”.
Table 1, L157: It would be needed to indicate that Zu and Zl are determined by the depths of specific values of potential density.
L246: establish ---> established
L274: extend ---> extent
L275: reasonable ---> reasonably
L276: The transports of currents north of the equator from the two simulations diverge…
L296: 2°S and 2°S ---> 2°S and 2°N?
L344–345: Are only eastward components integrated? If so, please notify it.
L345: 1°12’S-1°12’N
L345: Furthermore
L376: force ---> forced
L389: created ---> create
L421: extent ---> extend
L439–440: It would be helpful if what is done here is explained using equations (e.g., U = ΨN – ΨS).
L447,481: anomalous ---> anomalously
L476: we find significant negative trends in current transport east of ~30°W in JRAsim.
L480: west of ---> east of (?)
L526–527: despite the distinct inter-simulation discrepancies of the NECC on multidecadal timescales, especially after year 2000.
L564: wester ---> western
L573: asses ---> assess
L605: strength the ---> strength and the
Figure 2 (the last line of the cation): as well as
Figure 3: Is the phase given in month of the year when zonal wind stress/wind stress curl is maximum?
Figure 8: Please add labels a through I to panels.
Figure 11 (caption, the line above the bottom): different ---> difference
Figure A5: wind stress curl (above) and Sverdrup transport (below)
L784: Perez et al. (2021) … Remove abstract.
Large, W. G. and Yeager, S. G.: The global climatology of an interannually varying air-sea flux data set, Clim. Dynam., 33, 341–364, https://doi.org/10.1007/s00382-008-0441-3, 2009.
Kobayashi, S., Ota, Y., Harada, Y., Ebita, A., Moriya, M., Onoda, H., Onogi, K., Kamahori, H., Kobayashi, C., Endo, H., Miyaoka, K., and Takahashi, K.: The JRA-55 reanalysis: general specifications and basic characteristics, J. Meteorol. Soc. Jpn., 93, 5–48, https://doi.org/10.2151/jmsj.2015-001, 2015.
Citation: https://doi.org/10.5194/egusphere-2023-1433-RC1 - AC2: 'Reply on RC1', Kristin Burmeister, 08 Dec 2023
-
RC2: 'Comment on egusphere-2023-1433', Anonymous Referee #2, 08 Sep 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1433/egusphere-2023-1433-RC2-supplement.pdf
- AC1: 'Reply on RC2', Kristin Burmeister, 08 Dec 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-1433', Anonymous Referee #1, 13 Aug 2023
Review of “Dependency of simulated tropical Atlantic current variability on the wind forcing” submitted for Ocean Science by Kristin Burmeister, Franziska U. Schwarzkopf, Willi Rath, Arne Biastoch, Peter Brandt, Joke F. Lübbecke, and Mark Inall (egusphere-2023-1433)
This paper investigates wind-driven variability of upper layer currents in the tropical Atlantic Ocean by comparing two simulations with a high-resolution ocean circulation model forced by two different atmospheric products provided for ocean-climate modeling. Long-term mean states are compared first, then seasonal cycle and long-term variability including trends are compared. Observational results are also used to validate the model where possible.
Results presented would be very useful for enhancing our comprehensive understanding of mean state and variability of upper layer currents in the tropical Atlantic Ocean. Methods used are appropriate, though they may not be necessarily novel. Publication would be recommended after some revisions.
Major points.
It would be very helpful for readers if the key drivers, all of which should not be necessarily wind-driven, for currents discussed in the paper are summarized in the early part of the manuscript. For example, the easterly wind, the Ekman divergence, and the meridional gradient of Sverdrup stream function for EUC.
Specific points.
- L7,69,366 etc.: It would be helpful where “the upwelling regions of the eastern tropical North Atlantic”, which is referred to many times in the manuscript, specifically represent is indicated somewhere in the manuscript. Figure 1c would be one of candidates.
- L43: Does the Ekman transport diverge or converge south of the ITCZ?
- L52–53: It would be worth mentioning that there are some resemblances of the structure of the zonal currents between the equatorial Atlantic and Pacific, as authors cited many papers that treated the tropical Pacific Ocean later in this paragraph.
- L92: This is a nice approach, but it would be appropriate to describe specifically how the time gap was filled between the CORE simulation and observations somewhere in the manuscript.
- L179: To what is the EUC transport reconstruction using the mooring compared?
- L204: Are there any known reason for the differences found among phase speeds of the first baroclinic mode?
- L365: It would be nice to show that the JRA55 wind forcing actually captures fine structures that force NEUC variability.
- L406–408, 577–578: Are only first two baroclinic modes used for the harmonic analysis shown in Figures 4 and 5? If so, please be specific about this.
- L412: Since long-term trends are treated here, it would be good to discuss why trends are relevant or whether any issues have been raised in relation to global warming.
- L543: “This agrees with the weakening easterly winds along the equator.” … What does "this" point to? Should "weakening" be replaced with "trend of"? Or should “in JRA” be appended at the end of this sentence?
Technical points.
L62,63: Please give definition for NBUC and NBC.
L95: It would be more appropriate to cite Large and Yeager (2009) for introducing the CORE v2 dataset.
L121: well-establish ---> well-established
L124: relative ---> relatively
L129,757: It would be appropriate to cite Kobayashi et al. (2015; JMSJ) for introducing JRA-55.
L136: I would suggest something like, “Meridional ship sections are taken between 21°W and 28°W for velocity (31 sections) and hydrography and oxygen (22 sections) between 2000 and 2018”.
Table 1, L157: It would be needed to indicate that Zu and Zl are determined by the depths of specific values of potential density.
L246: establish ---> established
L274: extend ---> extent
L275: reasonable ---> reasonably
L276: The transports of currents north of the equator from the two simulations diverge…
L296: 2°S and 2°S ---> 2°S and 2°N?
L344–345: Are only eastward components integrated? If so, please notify it.
L345: 1°12’S-1°12’N
L345: Furthermore
L376: force ---> forced
L389: created ---> create
L421: extent ---> extend
L439–440: It would be helpful if what is done here is explained using equations (e.g., U = ΨN – ΨS).
L447,481: anomalous ---> anomalously
L476: we find significant negative trends in current transport east of ~30°W in JRAsim.
L480: west of ---> east of (?)
L526–527: despite the distinct inter-simulation discrepancies of the NECC on multidecadal timescales, especially after year 2000.
L564: wester ---> western
L573: asses ---> assess
L605: strength the ---> strength and the
Figure 2 (the last line of the cation): as well as
Figure 3: Is the phase given in month of the year when zonal wind stress/wind stress curl is maximum?
Figure 8: Please add labels a through I to panels.
Figure 11 (caption, the line above the bottom): different ---> difference
Figure A5: wind stress curl (above) and Sverdrup transport (below)
L784: Perez et al. (2021) … Remove abstract.
Large, W. G. and Yeager, S. G.: The global climatology of an interannually varying air-sea flux data set, Clim. Dynam., 33, 341–364, https://doi.org/10.1007/s00382-008-0441-3, 2009.
Kobayashi, S., Ota, Y., Harada, Y., Ebita, A., Moriya, M., Onoda, H., Onogi, K., Kamahori, H., Kobayashi, C., Endo, H., Miyaoka, K., and Takahashi, K.: The JRA-55 reanalysis: general specifications and basic characteristics, J. Meteorol. Soc. Jpn., 93, 5–48, https://doi.org/10.2151/jmsj.2015-001, 2015.
Citation: https://doi.org/10.5194/egusphere-2023-1433-RC1 - AC2: 'Reply on RC1', Kristin Burmeister, 08 Dec 2023
-
RC2: 'Comment on egusphere-2023-1433', Anonymous Referee #2, 08 Sep 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1433/egusphere-2023-1433-RC2-supplement.pdf
- AC1: 'Reply on RC2', Kristin Burmeister, 08 Dec 2023
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Kristin Burmeister
Franziska U. Schwarzkopf
Willi Rath
Arne Biastoch
Peter Brandt
Joke F. Lübbecke
Mark Inall
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(18838 KB) - Metadata XML