Impact of vertical and horizontal advection on nutrient distribution in the southeast Pacific

Barceló-Llull, Bàrbara; Mason, Evan; Capet, Arthur; Pascual, Ananda

doi:https://doi.org/10.5194/os-12-1003-2016

Articles | Volume 12, issue 4

https://doi.org/10.5194/os-12-1003-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/os-12-1003-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 12, issue 4

Research article

|

25 Aug 2016

Research article |

| 25 Aug 2016

Impact of vertical and horizontal advection on nutrient distribution in the southeast Pacific

Bàrbara Barceló-Llull, Evan Mason, Arthur Capet, and Ananda Pascual

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Final revised paper (published on 25 Aug 2016)
Preprint (discussion started on 29 Sep 2015)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC C1277: 'review of the manuscript entitled "Impact of vertical and horizontal advection on nutrient distribution in the South East Pacific" by B. Barcelo-Llull, E. Mason, and A. Pascual', Xavier Capet, 04 Dec 2015
RC C1289: 'Review of "IMPACT OF VERTICAL AND HORIZONTAL ADVECTION ON NUTRIENT DISTRIBUTION IN THE SOUTH EAST PACIFIC"', Peter Gaube, 10 Dec 2015
EC C1292: 'a recommendation to post a short Response now, before revising', Matthew Hecht, 10 Dec 2015
AC C1341: 'Response to the editor', Bàrbara Barceló-Llull, 23 Dec 2015
- EC C1424: 'Comments regarding plans to revise, from Referee #1', Matthew Hecht, 08 Jan 2016

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Bàrbara Barceló-Llull on behalf of the Authors (30 Apr 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (05 May 2016) by Matthew Hecht

RR by Xavier Capet (28 May 2016)

Suggestions for revision or reasons for rejection

This is a review of the manuscript entitled "Impact of vertical and horizontal advection on nutrient distribution in the South East Pacific" by B. Barcelo-Llull, E. Mason, and A. Pascual. This study is mostly about examining the significance of mesoscale vertical velocities. The significance test chosen by the authors involves a comparison of the roles played by horizontal and vertical mesoscale velocities in the evolution of an initially smooth subsurface nitrate field.

The general idea of the manuscript is interesting and the subject of vertical velocities is of major importance. The manuscript is also well-written and clear for the most part. The revision carried out by the authors allows them to address the main concerns I had expressed. The messages in the text have however been insufficiently modified to reflect the new results. There are also many polishing issues. I therefore recommend a minor revision of the manuscript before publication.

Major comments:

1) In the abstract, the authors still say: “the Lagrangian results show that the impact of vertical advection on nutrient passive tracer distribution is 30% of the contribution of horizontal advection”. I believe this statement is at best confusing and the 30% presented as such has no particular meaning. It arises from the arbitrary choice of a 30 days integration. If for example you had run the experiment longer you would have obtained an even larger value. For sufficiently long times of integration any minor additional process added to U and V would lead to important differences.

2) A more useful diagnostic when trying to estimate the importance of vertical velocity is the additional experiment where drifters are released on isopycnals and followed for a year. This new experiment is overall well designed (despite some missing pieces of information, see below) but my impression is that its conclusions are orthogonal to those of the first experiment set. Indeed Fig. 9 and 10 suggest that the effect of vertical velocities is relatively limited. In particular, lateral advection is responsible for the overwhelming fraction of nutrient uptake. 30% increase mentioned in the text take place where nutrient uptake is small, e.g. just west of the high uptake taking place at the eastern domain boundary. I understand that a “positive” conclusion is preferable but the analyses do not support the conclusions which should be reformulated and nuanced.

what do you do for boundary conditions ? Can we trust the large uptake values taking place on the eastern and northern domain boundaries ? Why does there seem to be a discontinuity at the eastern boundary (with only one line of pixels with large positive uptakes for both experiments) ?

- l170: Baseline value is not provided for the nitrate uptake rate u. Could the authors give us some qualitative insight into the corresponding time scales (say for a 2 um/kg concentration and a few irradiance values at release depth) ? This would also help us gain insight into the parameter range explored in Figure 10.

Minor comments:

- l148: random distribution on the vertical between 150 and 250 m. Captions in Figure 6 and 7 mention 190m and 210 m depth.

- at line 250, expand a bit on the transition to make sure that the reader realises that the trajectories analysed below are those with release at the 2umol/kg.

- indicate the 2umol/kg isolines in figure 2

- units for nitrate concentrations are systematically wrong. A volume (l) or mass (kg) unit is missing.

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ED: Publish subject to minor revisions (Editor review) (10 Jun 2016) by Matthew Hecht

AR by Bàrbara Barceló-Llull on behalf of the Authors (01 Jul 2016) Author's response Manuscript

ED: Publish subject to minor revisions (Editor review) (12 Jul 2016) by Matthew Hecht

Dear Dr. Barceló-Llull and co-authors,

There is one point that remains a concern, regarding the way in which the 30% increase is discussed. Reviewer Xavier Capet’s comment was:

“2) A more useful diagnostic when trying to estimate the importance of vertical velocity is the additional experiment where drifters are released on isopycnals and followed for a year. This new experiment is overall well designed (despite some missing pieces of information, see below) but my impression is that its conclusions are orthogonal to those of the first experiment set. Indeed Fig. 9 and 10 suggest that the effect of vertical velocities is relatively limited. In particular, lateral advection is responsible for the overwhelming fraction of nutrient uptake. 30% increase mentioned in the text take place where nutrient uptake is small, e.g. just west of the high uptake taking place at the eastern domain boundary. I understand that a “positive” conclusion is preferable but the analyses do not support the conclusions which should be reformulated and nuanced.”

Your reply, in the exchange with the Reviewer, is very reasonable, acknowledging that these big increases of order 30% are occurring in places where the level of uptake is low:

“mesoscale activity is an important support for nitrate uptake (hence productivity) in this region where uptake is rather low”

But while this discussion puts the enhanced uptake into an accurate context, the manuscript still fails to do so.

I must insist that both abstract and main text be changed to qualify this salient result, acknowledging that the increase of up to 30% is generally found in areas of low uptake.

Sincerely Yours, Matthew Hecht

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AR by Bàrbara Barceló-Llull on behalf of the Authors (18 Jul 2016) Author's response Manuscript

ED: Publish as is (08 Aug 2016) by Matthew Hecht

AR by Bàrbara Barceló-Llull on behalf of the Authors (09 Aug 2016)

Short summary

Vertical velocity in the ocean makes an important contribution to the modulation of marine ecosystems through its impact on fluxes of nutrients and phytoplankton. Here, we estimate full 3-D current velocity fields from an observation-based data product. The 3-D currents are used to force a set of particle-tracking (Lagrangian) experiments. The Lagrangian results show that vertical motions induce local increases in nitrate uptake reaching up to 30 %.