Regional circulation around New Caledonia from two decades of observations

Cravatte, Sophie; Kestenare, Élodie; Eldin, Gérard; Ganachaud, Alexandre; Lefêvre, Jérôme; Marin, Frédéric; Menkès, Christophe E.; Aucan, Jérôme

doi:10.1016/j.jmarsys.2015.03.004

Cited by 30 publications

(42 citation statements)

References 47 publications

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“…The next step is to understand the quantitative aspect linking the Trichodesmium abundances to N 2 fixation rates, including their vertical distribution even when Trichodesmium filaments/colonies are spread out in the water column. Another important field of interest is to be able to understand phytoplankton functional types using satellites including Trichodesmium (de Boissieu et al, 2014). At present, we do not know any such study that included Trichodesmium but we have hopes that with our new in situ database and our understanding of the mat shapes detected in the present study, and the development of performing statistical methods such as machine learning, advances can be made in that regard, a prospect for future work.…”

Section: Discussionmentioning

confidence: 88%

Remote sensing of <i>Trichodesmium</i> spp. mats in the western tropical South Pacific

et al. 2018

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Trichodesmium is the major nitrogen-fixing species in the western tropical South Pacific (WTSP) region, a hot spot of diazotrophy. Due to the paucity of in situ observations, remote-sensing methods for detecting Trichodesmium presence on a large scale have been investigated to assess the regional-to-global impact of this organism on primary production and carbon cycling. A number of algorithms have been developed to identify Trichodesmium surface blooms from space, but determining with confidence their accuracy has been difficult, chiefly because of the scarcity of sea-truth information at the time of satellite overpass. Here, we use a series of new cruises as well as airborne surveys over the WTSP to evaluate their ability to detect Trichodesmium surface blooms in the satellite imagery. The evaluation, performed on MODIS data at 250 m and 1 km resolution acquired over the region, shows limitations due to spatial resolution, clouds, and atmospheric correction. A new satellite-based algorithm is designed to alleviate some of these limitations, by exploiting optimally spectral fea-tures in the atmospherically corrected reflectance at 531, 645, 678, 748, and 869 nm. This algorithm outperforms former ones near clouds, limiting false positive detection and allowing regional-scale automation. Compared with observations, 80 % of the detected mats are within a 2 km range, demonstrating the good statistical skill of the new algorithm. Application to MODIS imagery acquired during the February-March 2015 OUTPACE campaign reveals the presence of surface blooms northwest and east of New Caledonia and near 20 • S-172 • W in qualitative agreement with measured nitrogen fixation rates. Improving Trichodesmium detection requires measuring ocean color at higher spectral and spatial (< 250 m) resolution than MODIS, taking into account environment properties (e.g., wind, sea surface temperature), fluorescence, and spatial structure of filaments, and a better understanding of Trichodesmium dynamics, including aggregation processes to generate surface mats. Such sub-mesoscale aggregation processes for Trichodesmium are yet to be understood.

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Section: Discussionmentioning

confidence: 88%

Remote sensing of <i>Trichodesmium</i> spp. mats in the western tropical South Pacific

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“…Exploring the possible mechanisms of dissipation east of the islands, and of generation west of the islands are beyond the scope of this study, but should be investigated in a following study. Instabilities of the sheared currents found west of the islands (Cravatte et al, 2015) are one of the possible mechanisms. Alternatively, southeasterly trade winds prevailing in the region are modified by the presence of high mountains in Fiji, Vanuatu, and New Caledonia.…”

Section: Discussionmentioning

confidence: 99%

Observed Characteristics and Vertical Structure of Mesoscale Eddies in the Southwest Tropical Pacific

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In the Southwest Pacific Ocean, waters transit from the subtropical gyre before being redistributed equatorward and poleward. While the mean pathways are known, the contribution to the mixing and transport of the water from mesoscale eddies has not been comprehensively investigated. In this research, satellite altimetry data, combined with an eddy detection and tracking algorithm is used to investigate the distribution and surface characteristics of mesoscale eddies in this region of complex bathymetry (10°S–30°S, 140°E–190°E). Detected eddies are then colocalized with in situ data from Argo floats to determine their vertical structure and the effect of eddies on the water masses. The numerous islands affect the eddy behavior as most eddies are formed in the lee of islands, propagate westward and decay when encountering shallow bathymetry. Eddies are sparse and short‐lived in the tropical area north of Fiji, impacting only the top 200 meters of water. They do not appear to be able to trap and transport waters in this region. In the Coral Sea, a region of lateral shear between currents transporting waters of different origins, eddies are more numerous and energetic. They affect the water properties down to at least 500 m depth, and anticyclonic eddies trap water to ∼200 m, contributing to the upper thermocline waters mixing and transport. South of New Caledonia, mesoscale eddies are ubiquitous, with typical lifetimes longer than 5 months. They affect the temperature, salinity, and velocities down to ∼1,000 m depth, and weakly contribute to the mixing of lower thermocline waters.

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“…It is the deepest lagoon in New Caledonia with maximum depths of ~80 m, and is semienclosed by a coral reef barrier located between 5 to 25 km from the coastline. The lagoon is connected to the adjacent Pacific Ocean, through strait passages that allow the intrusion of oligotrophic oceanic waters forced by tides, trade winds, and circulation of the external Vauban Current [50]. On the continental side, there are many small and medium size rivers that contribute to the system, as well as several mining sites [10,51].…”

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confidence: 99%

Bio-Optical Characterization and Ocean Colour Inversion in the Eastern Lagoon of New Caledonia, South Tropical Pacific

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Abstract:The Eastern Lagoon of New Caledonia (ELNC) is a semi-enclosed system surrounded by an extensive coral reef barrier. The system has been suffering impacts from climate variability and anthropogenic activities, including mining exploitation. Satellite monitoring is thus an essential tool to detect such changes. The present study aimed to assess the bio-optical variability of the ELNC and examine the applicability of ocean colour algorithms, using in situ bio-optical and radiometric data, collected during the March 2014 CALIOPE 2 cruise. The chlorophyll a concentration (Chla) varied from 0.13-0.72 mg·m −3 , and the coastal stations were spectrally dominated by non-algal particles (NAP) and coloured dissolved organic matter (CDOM) (>80% of the total non-water absorption at 443 nm), due to the contribution of allochthonous sources. The phytoplankton specific absorption was generally lower (mean, 0.049 m 2 ·mg Chla −1 ) than typical values observed for the corresponding Chla range, as well as the spectral slopes of the absorption of CDOM plus NAP (a dg ) (mean, 0.016 nm −1 ) and of the particle backscattering coefficient (b bp ) (mean, 0.07 nm −1 ). The remote sensing reflectance obtained using two in-water approaches and modelled from Inherent Optical Properties (IOPs) showed less than 20% relative percent differences (RPD). Chla estimates were highly biased for the empirical (OC4 and OC3) and semi-analytical (GSM, QAA, GIOP, LMI) algorithms, especially at the coastal stations. Excluding these stations, the GSM01 yielded the best retrievals with 35-40% RPD. a dg (443) was well retrieved by all algorithms with~18% RPD, and b bp (443) with~40% RPD. Turbidity algorithms also performed reasonably well (30% RPD), showing the capacity and usefulness of the derived products to monitor the water quality of the ELNC, provided accurate atmospheric correction of the satellite data. Regionally tuned algorithms may potentially improve the Chla retrievals, but better parameterization schemes that consider the spatiotemporal variability of the specific IOPs are still needed.

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Regional circulation around New Caledonia from two decades of observations

Cited by 30 publications

References 47 publications

Remote sensing of <i>Trichodesmium</i> spp. mats in the western tropical South Pacific

Remote sensing of <i>Trichodesmium</i> spp. mats in the western tropical South Pacific

Observed Characteristics and Vertical Structure of Mesoscale Eddies in the Southwest Tropical Pacific

Bio-Optical Characterization and Ocean Colour Inversion in the Eastern Lagoon of New Caledonia, South Tropical Pacific

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Regional circulation around New Caledonia from two decades of observations

Cited by 30 publications

References 47 publications

Remote sensing of &lt;i&gt;Trichodesmium&lt;/i&gt; spp. mats in the western tropical South Pacific

Remote sensing of &lt;i&gt;Trichodesmium&lt;/i&gt; spp. mats in the western tropical South Pacific

Observed Characteristics and Vertical Structure of Mesoscale Eddies in the Southwest Tropical Pacific

Bio-Optical Characterization and Ocean Colour Inversion in the Eastern Lagoon of New Caledonia, South Tropical Pacific

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Remote sensing of <i>Trichodesmium</i> spp. mats in the western tropical South Pacific

Remote sensing of <i>Trichodesmium</i> spp. mats in the western tropical South Pacific