Water mass analysis along 22 °N in the subtropical North Atlantic for the JC150 cruise (GEOTRACES, GApr08)
Graphical abstract
Introduction
Oceanic water masses store and transport considerable amounts of energy, water and chemical elements in the earth's surface. These water masses impact the atmosphere through interactions at the air/sea interface. Water mass analysis, which consist in studying the formation, spreading, and mixing of water masses, is therefore essential to understand the role of oceans in climate processes. The methods used for water mass analysis have evolved from classical descriptions of oceanic circulation based on hydrographic properties to the determination of water mass formation regions, transport pathways, and mixing length scales from numerical models and novel tracer data (Tomczak, 1999). An example of such development is the introduction of the optimum multiparameter analysis (OMPA, Tomczak, 1981). This method enables estimating the contributions of different water masses defined in specific locations (end-members) to a measured ocean section based on a range of hydrographic parameters. This method demonstrates a significant amount of improvement compared to previous methods and has been widely used (Álvarez et al., 2014; García-Ibáñez et al., 2018; Jenkins et al., 2015; Pardo et al., 2012; Peters et al., 2018). However, the results of OMPA are strongly dependent on the choice of water mass end-members that possibly impact the ocean section, and OMPA cannot provide any information related to surface layers. Moreover, a water mass analysis conducted only with OMPA does not provide direct information on the water mass pathways between their formation region and the measured section. Therefore, the water mass analysis proposed in this study combines, for the first time to the best of our knowledge, an extended OMPA with a Lagrangian particle tracking experiment (LPTE) to better constrain the end-members and provide information on water mass pathways. LPTEs are widely used in recent times to investigate several aspects of ocean sciences, such as oceanic circulation (eg. Spence et al., 2014) or biogeochemistry (eg. Cetina-Heredia et al., 2016).
The present water mass analysis was conducted for the JC150 “Zinc, Iron and Phosphorus co-Limitation” GEOTRACES process study (GApr08). This cruise departed Point-à-Pitre, Guadeloupe on June 27, 2017 and arrived at Santa Cruz, Tenerife on August 12, 2017. The transect is located at the southern end of the North Atlantic Subtropical gyre (Fig. 1) on both sides of the Mid-Atlantic Ridge (MAR, ~22 °N, ~58–31 °W). The JC150 section was specifically studied to understand how a low phosphate environment could lead to zinc-phosphorus and iron-phosphorus co-limitation on nitrogen fixation (Browning et al., 2017; Mahaffey et al., 2014; Moore et al., 2009; Snow et al., 2015; Wu et al., 2000). In this context, the trace metals iron, zinc and aluminum, were measured. The aim of the present water mass analysis is two-fold. Firstly, it aims to provide a detailed understanding of the contribution and distributions of the water masses that exist along the zonal section as well as new constraints in water mass circulation in the subtropical North Atlantic that might be of general interest. Secondly, it aims to provide the tools to efficiently combine this hydrodynamic knowledge with the biogeochemical knowledge from the GEOTRACES program. To achieve this objective, all the OMPA end-members were chosen from GEOTRACES cruises with available zinc, iron, and aluminum concentrations. This enables the estimation of transport and mixing of these elements. Such a choice is a first to the best of our knowledge, and it is now possible thanks to the great extent of the GEOTRACES program.
This study presents the hydrographic properties measured during JC150, including potential temperature, salinity, and the concentration of oxygen and nutrients (θ, S, O2, NO3−, PO43−, and Si(OH)4) along with a water mass analysis based on an OMPA and a LPTE.
Section snippets
Hydrography and nutrients
The samples for nutrients, oxygen, and salinity analyses were collected using 24, 10 L trace metal clean Teflon-coated OTE (ocean test equipment) bottles with external springs, mounted on a titanium rosette and deployed on a Kevlar-coated conducting wire. A SeaBird 911plus CTD recorded the temperature, conductivity, and pressure at 24 Hz with an accuracy of ±0,001 °C, ± 0,0003 S/m, and ±0,015%, respectively. An SBE43 oxygen sensor measured the dissolved oxygen concentration. Standard SeaBird
Water mass analysis: results and discussion
The hydrographic properties measured during JC150, θ, S, AOU, and concentrations of O2, PO43−, NO3−, and Si(OH)4, are presented in this study for the first time. They are shown as property/property plots in Fig. 2, and as section in Fig. 3.
The discussion is organized in three parts. Firstly, the surface waters shallower than 200–300 m (where an eOMPA cannot be performed, because water properties are constantly changing due to ocean-atmosphere exchange) are discussed using satellite data and
Conclusions
Based on i) the hydrographic data (θ, S, concentrations of O2, NO3−, PO43−, and Si(OH)4), ii) an eOMPA, and iii) an LPTE conducted in an eddy-resolving ocean circulation model, a water mass analysis has been presented for the 2017 JC150 GEOTRACES process study (GApr08) in the subtropical North Atlantic along 22 °N.
This is the first time to the best of our knowledge that a water mass analysis combined an eOMPA with an LPTE. This approach demonstrated several advantages:
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In addition to a thorough
Author contributions
CM was the chief scientist of the cruise. CM, MCL, NJW, and EMSW participated in the sampling on board. They participated in the temperature, salinity, dissolved oxygen, and nutrient concentration data production along with JH. SvG produced the Lagrangian particle tracking experiment under the supervision of YD with the contribution of LA and FL. LA and FL produced the optimum multiparameter analysis and conducted the interpretation work. LA drafted the manuscript under the supervision of FL
Funding source
French Ministry of Higher Education, Research and Innovation (MESRI) public funding awarded to Lise Artigue (University of Toulouse).
French National Centre for Scientific Research (CNRS) public funding awarded to François Lacan (LEGOS).
NERC with reference NE/N001979/1 awarded to Claire Mahaffey (University of Liverpool) and Malcolm Woodward (PML) and NE/N001125/1 awarded to Maeve Lohan (University of Southampton).
Mercator Ocean International for Simon van Gennip and Yann Drillet.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We dedicate this work to Pr. Matthias Tomczack for his invaluable contributions to the Oceanographic community. As part of his large body of work, Matthias Tomczack introduced and developed the optimum multiparameter analysis. We particularly thank him for his thoughtful answers to our questions and his helpful explanations of his work.
We sincerely thank Dr. Johannes Karstensen for providing the OMP Analysis Package for MATLAB Version 2.0 with a clear manual of utilization on the website: //omp.geomar.de/
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