Comment on “Reviews and syntheses: Physical and biogeochemical processes associated with upwelling in the Indian Ocean”

by Puthenveettil Narayana Menon Vinayachandran, Yukio Masumoto, Mike Roberts, Jenny Hugget, Issufo Halo, Abhisek Chatterjee, Prakash Amol, Garuda V. M. Gupta, Arvind Singh, Arnab Mukherjee, Satya Prakash, Lynnath E. Beckley, Eric Jorden Raes, Raleigh Hood

General comments:

This review provides a comprehensive overview on the physical dynamics in upwelling regions and the resulting interplay with biogeochemical processes in the entire Indian Ocean based on observations and model results. Variability of upwelling has a strong impact on climate-relevant trace gases as well as biogeochemical processes both having an effect in many ways on one of the most densely populated regions. Upwelling processes in the Indian Ocean are still less well understood than in the other major oceans, therefore the topic is highly relevant.

The manuscript is well organized, structured and (mainly) easy to read. The authors address main upwelling regions in the Indian Ocean giving a historical background, an overview about recent observations (characterstic of upwelling and impact on physical parameter), present status of modelling these upwelling systems and their upwelling mechanisms, and their impact on marine ecosystems and biogeochemistry. My main concerns are related to the figures, which lack in care and precision and make the text sometimes hard to follow. I would also highly recommend a schematic figure for the different monsoon phases, that summarizes the main upwelling regions together with the monsoonal winds.

I recommend publication of this manuscript after minor revision. I will leave my comments below in the specific comments.

Specific comments:

-line992: Upwelling cannot be driven by primary production, it is the other way round.

Figure 2: Port Elizabeth (PE), Port Alfred (PA) and Agulhas Bank are mentioned in the figure caption but not in the figure. It would be easier to follow the descriptions in the text, if some geographical points would be marked in the figure or the latitude-longitude needs to be mentioned in the text.

Figure 3: “C.T” and “P.E.” are marked in the figure but not explained in the text/figure caption.

Figure 4: X-axes (longitude) and y-axes (latitude) should be labelled. The font/scales of the colorbars are hard to read/recognize. Acronyms “LHS” and “RHS” should be explained.

Figure 5: Please mark the Transkei shelf in the figure.

Figure 6: I would recommend to label the x-axes of the lower panels as well.

Figure 8: The titles of the figures are hard to read and a scale vector for the wind stress is missing. I also would recommend to mark the respective months (“JAN”, “JUL”, “APR”, “OCT”) in the figures. The colorbar is very tiny as well as the font/scales. The unit for the wind stress curl is missing as well.

Figure 9: This figure would improve a lot and the paragraph relating to this figure would be much easier to follow if some geographical points would be marked in the figure such as: Angoche (or Nampula), Sofala Bank (or Ponta Zavora), Inhambane, Delagoa Bright, Maputo.

Figure 10: Please mark in the figure the geographical points you are referring to in the text.

Figure 11: Please mark in the figure the geographical points (Tanzania, Kenya) you are referring to in the text. Please label the currents also in the lower panel.

Figure caption: correct “southeast” to “southwest”. Please refer to the contour lines.

Figure 12: 12a, b: What does the color shade show? A scale vector for the surface currents is missing. 12b: correct “SEM” to “SWM”.

12c, d: Please also mark the respective monsoon phase (NEM) in the figures as in 12a,b. The text would be easier to follow if country borders are drawn in the figures. I would also recommend to label the colorbars with the respective unit.

Figure 13: Please mark a-d in the figure as you are referring to in the figure caption and in the text.

Figure 15: The x-axes of 15c should have the same scale as 15a-b, d-e.

Figure 17: The font of the colorbar is hard to read and the unit is missing.

Figure 18: Could you give the approximate latitude of the hydrographic sections? “approximately midway of the east coast” is not very precise.

Technical corrections:

-line 121: typo: correct “Northeasterly” to “northeasterly”

-l156: typo: correct “IIOE-2 (2015-2010)”

-l236: typo: “day1” should say “day^-1”

-l289: typo: correct “Current” to “current”

-l429: Do you mean “vertical suction”?

-460: The acronym MC has not been explained yet.

-l482: typo: correct “water” to “Water”

-l495: The terms “northeast orientation” and “southeast orientation” are misleading. Better: “northeasterly winds” and “southeasterly winds” or “southwest directions” and “northwest direction”

-l515: The acronym EMC has not been explained yet.

-l565: typo: correct “southeast” to “southwest”

-l566 and more: correct “SEM” to “SWM”

-l594: typo: correct “North-East Monsoonal” to “northeast monsoonal” or use the acronym “NEM”

-l635: correct “move” to “moves”

l636: correct “recirculate” to “recirculates”

-l643: correct “forms” to “form”

-l654: correct “migrate” to “migrates”

-l664: correct “result” to “results”

-l680: typo: delete “the” in “the another”

-l702: typo: correct “study” to “studies”

-l710: correct “of” to “off”

-l747: correct “crosshore” to “cross-shore”

-l788: typo: correct “21oC”

-l790: correct “in the shelf” “on the shelf”

-l930: The term “peak” is associated with a maximum, maybe “minima” is a better term.

-l998: The paragraphs have the wrong order: paragraph 2.7.5 precedes 2.7.4.

-l1044: “Figure 16” should be in bold.

-l1225: typo: correct “Joshnson” to “Johnson”

-l1229: correct “Oxygen minimum zone” to “oxygen minimum zone” or “OMZ”

-l1249: “belief” – I would prefer “assumption”

-l1294: correct “Srilanka”

-l1356: correct “appear” to “appears”

-l1425: “Other early studies…” This sentence is very long and hard to follow, please rephrase.

-l1635: Figure 26 does not show wind.

-l1740: correct “tolarger”

-1747: correct the semicolon

-l1752: correct “are caused” to “is caused”

-l1771: What do you mean with “high temporal and temporal resolution”, please rephrase.

Reviews and syntheses: Physical and biogeochemical processes associated with upwelling in the Indian Ocean

Vinayachandran et al. described twelve upwelling regions in the Indian Ocean. The description starts with upwelling systems associated with coastal currents in the southwestern part of the Indian Ocean, followed by those largely driven by the Asian Monsoon and ends with those associated with coastal currents off Australia in the southeastern part of the Indian Ocean. Additionally, the authors discussed the Seychelles-Chagos Thermocline Ridge (SCTR), which is an upwelling system that develops in the open ocean between 5 and 15°S in the western part of the Indian Ocean.

Writing a review and synthesis paper on upwelling systems in the Indian Ocean is a great idea and I am very pleased by the efforts of the authors. However, to my understanding, a review and synthesis paper should provide a comprehensive overview without overloading the reader with details and aspects, which are interesting but not directly related to the topic. Considering this demand, the individual chapters of the paper reveal pronounced disparities. For instance, there are chapters such as the Introduction and chapter 2.6 which do not reflect the state of the art whereas e.g. chapters 2.1, 2.9, and 2.12 include so much additional information that they could most be stand alone papers on the Agulhas Current, the Bay of Bengal and the oceanography along the Australian west coast. Hence, to my understanding, the current version of the manuscript needs a major revision.

A revised version of the paper should include an Introduction clarifying terms, which thereafter are used coherently throughout the manuscript. Furthermore, chapters on the individual upwelling regions should focus on upwelling and exclude regional characteristics, which are not directly related to upwelling. It would also be helpful to summarize discussions on upwelling mechanisms since after reading about it from two different perspectives (modeling vs. observations), the general conclusion often remains unclear. In sum, I believe that uniform subdivision of all chapters into three sections such as Background, Mechanisms, and Productivity and Ecosystem, as seen e.g. in chapter 2.4, would help to shorten the manuscript, which is with 128 pages quite long. Simultaneously, it would increase the readability of the manuscript.

In the following I will provide arguments and suggestions whereas I will start with the Instruction and continue, thereafter, with chapter 2.

Introduction

The authors subdivided the Introduction into three sections describing upwelling (i), biogeochemical implication of upwelling (ii) and the study area (iii). This is a convincing structure but the section on upwelling contains only two references; one from 1905 and the other one from 1937. This is insufficient to introduce the reader into our current understanding of upwelling, which includes the formation of coastal parallel jets, meso-scale eddies and offshore-directed filaments, as well as interactions of these features with remotely and locally trigged waves. After reading an introduction to upwelling, I would expect that the reader exactly knows what the authors mean with ‘classical upwelling dynamics’ (see lines 745 – 746). Furthermore, it is crucial to define the following terms in order to better understand the description of the individual upwelling regions and, in particular, the differences between these regions:

Ekman upwelling, wind-driven coastal upwelling, core upwelling, shelf-edge upwelling, slope upwelling, divergent upwelling, dynamic upwelling, topographic-driven upwelling, dynamic boundary upwelling, current-driven upwelling, upwelling wedge, current-induced upwelling, upwelling Kelvin waves and upwelling node.

To emphasize the role of upwelling on marine ecosystems and ecosystem services such as fisheries, the authors refer to Eastern Boundary Upwelling Systems (EBUS) in the second section of the Introduction. This is convincing since it emphasizes the relevance of upwelling in general, as well as the uniqueness of the Indian Ocean where such an EBUS is not established. However, this approach suffers from the poor discussion of EBUS. In the following, a couple of references are listed which provide to my understanding nice overviews on EBUSs:

Kämpf, J. & Chapman, P. in Upwelling Systems of the World     (Springer International Publishing, 2016).

Messié, M., Chavez, F.P., 2015. Seasonal regulation of primary production in eastern boundary upwelling systems. Progress In Oceanography, 134, 1-18.

Chavez, F.P., Messie, M., Pennington, J.T., 2011. Marine Primary Production in Relation to Climate Variability and Change. Annual Review of Marine Science, 3, 227-260

Messié, M., Ledesma, J., Kolber, D.D., Michisaki, R.P., Foley, D.G., Chavez, F.P., 2009. Potential new production estimates in four eastern boundary upwelling ecosystems. Progress In Oceanography, 83, 151-158.

Carr, M.-E., 2001. Estimation of potential productivity in Eastern Boundary Currents using remote sensing. Deep Sea Research Part II: Topical Studies in Oceanography, 49, 59-80.

Carr, M.-E., Kearns, E.J., 2003. Production regimes in four Eastern Boundary Current systems. Deep Sea Research Part II: Topical Studies in Oceanography, 50, 3199-3221.

Bakun, A., 2017. Climate change and ocean deoxygenation within intensified surface-driven upwelling circulations. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 375, 20160327.

Brady, R.X., Lovenduski, N.S., Alexander, M.A., Jacox, M., Gruber, N., 2019. On the role of climate modes in modulating the air–sea CO2 fluxes in eastern boundary upwelling systems. Biogeosciences, 16, 329-346.

Zuidema, P., Chang, P., Mechoso, C.R., Terray, L., 2011. Coupled ocean-atmosphere-land processes in the tropical Atlantic. Joint edition of the newsletter of the Climate Variability and Predictability Project (CLIVAR) exchanges and the CLIVAR variability of the American Monsoon System Project (VAMOS), 55, 12 - 14.

Chavez, F.P., Toggweiler, J.R., 1995. Physical estimates of global new production: The upwelling contribution. In C.P. Summerhayes, K.-C. Emeis, M.V. Angel, R.L. Smith, B. Zeitschel (Eds.), Upwelling in the ocean, modern processes and ancient records (pp. 313-320). Chichester: Wiley & Sons.

Regarding the study area, I would suggest to go beyond mentioning papers and describe climate, ocean currents, and climate anomalies such as Madden-Julian Oscillation, El Nino Southern Oscillation (ENSO) and the Indian Ocean Dipole.

Furthermore, it would be extremely helpful to include maps showing the general circulation and primary production as well as the location of the upwelling region, which the authors discuss in chapter 2. Also, in this context it is crucial to define and standardize the used terms. For instance, select only one term for: northeast monsoon; NE monsoon; northeast Monsoon; NEM; North-East Monsoon which should also indicate whether it refers to boreal or austral regime. This is just one of many examples making it difficult to follow the description of the upwelling systems.

Chapter 2 Coastal Upwelling Systems

General remarks:

Since the Seychelles-Chagos Thermocline Ridge (SCTR) is an upwelling system that develops in the open ocean, I would suggest adding an additional chapter on upwelling systems in the open ocean. In this chapter the SCTR could be discussed, as well as the equatorial upwelling in the Indian Ocean. So far this upwelling region has not sufficiently been considered by the authors, even though same papers on this upwelling system have been cited.

Instead of describing the remaining elven upwelling regions in dependence of their geographical location, it would also be helpful for the reader if the author could subdivide chapter 2 into sections, which relate to e.g. driving forces. Such overarching sections could be: (i) monsoon-driven upwelling systems, (ii) upwelling systems associate with coastal currents and (iii) upwelling systems strongly influenced by freshwater fluxes such as those in the Bay of Bengal and off Sumatra/Java. This is just a suggestion, but I am convinced that defining overarching types of upwelling will help the reader to better understand upwelling in the Indian Ocean.

Within chapters 2.5 to 2.9 contents are mixed up. For instance, the authors discussed iron-limitation in chapter 2.5 line 697 – 699 and cited Naqvi et al. 2010. According to the reference list, this is a paper entitled ‘Marine hypoxia/anoxia as a source of CH4 and N2O’ but I guess the authors mean:

Naqvi, S.W.A., Moffett, J.W., Gauns, M.U., Narvekar, P.V., Pratihary, A.K., Naik, H., Shenoy, D.M., Jayakumar, D.A., Goepfert, T.J., Patra, P.K., Al-Azri, A., Ahmed, S.I., 2010. The Arabian Sea as a high-nutrient, low-chlorophyll region during the late Southwest Monsoon. Biogeosciences, 7, 2091-2100.

This paper discusses data measured off the Arabian Peninsula and along a transect across the central Arabian Sea. Hence, if the authors want do discuss iron limitation, it belongs to chapter 2.6. Furthermore ‘iron limitation’ is a controversially discussed issue. Similar to results derived from numerical models, also studies based on observations are coming to opposing conclusions regarding iron limitation. To my understanding, such opposing conclusion should have also been considered by the authors. See e.g. Rixen, T., Goyet, C., Ittekkot, V., 2006. Diatoms and their influence on the biologically mediated uptake of atmospheric CO2 in the Arabian Sea upwelling system. Biogeosciences, 3, 1 - 13.

In chapter 2.6 (line 806) the authors wrote: ‘a detailed discussion on biophysical interactions along this coast is discussed in Section 2.7.5.’ It makes really no sense to discuss biophysical interactions along this Arabian Coast in a chapter on upwelling along the Indian coast! Please spilt chapter 2.7.4 and 2.75 and discuss biogeochemical implications in association with the respective upwelling regions. In chapter 2.9.2 the authors discuss general biogeochemical characteristics of the Bay of Bengal, but it remains open to which extent this relates to upwelling! To my understanding this is also a main problem of chapters 2.1 and 2.11 as mentioned already before.

In the following, I will focus on chapter 2.6 and 2.8 because these chapters suffer from missing contents and describe upwelling system, which are comparable to EBUS. Upwelling in the western Arabian Sea is, perhaps in addition to the one off Somalia, the only major upwelling system in the Indian Ocean with an impact on marine productivity, which is comparable with those of EBUS. The upwelling region off Indonesia, in turn, is the only large upwelling system associated with an eastern boundary current in the Indian Ocean.

The authors introduce chapter 2.6 with a hint to the enormous work, which was carried out in the framework of JGOFS with respect to upwelling in the western Arabian Sea. This included modelling and fieldwork whereas the fieldwork had also a strong focus on biogeochemical implications of upwelling. I guess that > 6 ‘Arabian Sea special issues’ have been published. Sharon Smith edited many of these special issues and worked a lot on zooplankton dynamics in upwelling system. Other groups studied phytoplankton successes including their link to nutrient dynamics in surface waters, primary production and export production, as also recently summarized by Rixen et al. 2019a,b and references given therein. Hence, moving this topic to chapter 2.7.5 is incomprehensible and weakens the discussion on ecosystem shifts. To my opinion, ecosystem shifts as well as overfishing, are among the largest threads to  upwelling systems and aspects which should be considered in the discussion of each upwelling system in case the availability of data allows it. Goes and Gomes published a number of papers on ecosystem shifts, and the one belonging to chapter 2.6 have been cited in chapter 2.7.. Please see also their latest work: Goes et al. 2020.

Goes, J.I., Tian, H., Gomes, H.d.R., Anderson, O.R., Al-Hashmi, K., deRada, S., Luo, H., Al-Kharusi, L., Al-Azri, A., Martinson, D.G., 2020. Ecosystem state change in the Arabian Sea fuelled by the recent loss of snow over the Himalayan-Tibetan Plateau region. Scientific Reports, 10, 7422.

Rixen, T., Gaye, B., Emeis, K.-C., 2019. The monsoon, carbon fluxes, and the organic carbon pump in the northern Indian Ocean. Progress In Oceanography, 175, 24-39.#

Rixen, T., Gaye, B., Emeis, K.C., Ramaswamy, V., 2019. The ballast effect of lithogenic matter and its influences on the carbon fluxes in the Indian Ocean. Biogeosciences, 16, 485-503.

Chapter 2.10. According to authors ‘The upwelling off the southern coasts of Sumatra and Java Islands is a remarkable eastern boundary upwelling system (EBUS) in the Indian Ocean’. To my understanding this is not correct. Even though upwelling associated with an eastern boundary current develops off Indonesia, it reveals fundamental differences to other EBUS going beyond aspects discussed by the authors. The development of EBUSs is associated with the development of deserts on land. In contrast to this coevolution, Indonesia is characterized by extremely high river discharges. Freshwater fluxes establish e.g. capping effects similar to those in the Bay of Bengal and an ENSO-driven seesaw linking upwelling off Indonesia in the Indian Ocean to those in the equatorial Pacific Ocean. These are important aspects, which, to my understanding, should be considered in a review paper.