• European Marine Science
• Journal of Geophysical Research Oce... close

A radar scatterometer operates by transmitting a pulse of microwave energy toward the ocean's surface and measuring the normalized (per-unit-surface) radar backscatter coefficient (σ°). The primary application of scatterometry is the measurement of near-surface ocean winds. By combining σ° measurements from different azimuth angles, the 10 m vector wind can be determined through a Geophysical Model Function (GMF), which relates wind and backscatter. This paper proposes a mission concept for the measurement of both oceanic winds and surface currents, which makes full use of earlier C-band radar remote sensing experience. For the determination of ocean currents, in particular, the novel idea of using two chirps of opposite slope is introduced. The fundamental processing steps required to retrieve surface currents are given together with their associated accuracies. A detailed description of the mission proposal and comparisons between real and retrieved surface currents are presented. The proposed ocean Doppler scatterometer can be used to generate global surface ocean current maps with accuracies better than 0.2 m/s at a spatial resolution better than 25 km (i.e., 12.5 km spatial sampling) on a daily basis. These maps will allow gaining some insights on the upper ocean mesoscale dynamics. The work lies at a frontier, given that the present inability to measure ocean currents from space in a consistent and synoptic manner represents one of the greatest weaknesses in ocean remote sensing. International audience

The ocean bottom pressure records from eight stations of the Cascadia array are used to investigate the properties of short surface gravity waves with frequencies ranging from 0.2 to 5 Hz. It is found that the pressure spectrum at all sites is a well-defined function of the wind speed U10 and frequency f, with only a minor shift of a few dB from one site to another that can be attributed to variations in bottom properties. This observation can be combined with the theoretical prediction that the ocean bottom pressure spectrum is proportional to the surface gravity wave spectrum E(f) squared, times the overlap integral I(f) which is given by the directional wave spectrum at each frequency. This combination, using E(f) estimated from modeled spectra or parametric spectra, yields an overlap integral I(f) that is a function of the local wave age inline image. This function is maximum for f∕fPM = 8 and decreases by 10 dB for f∕fPM = 2 and f∕fPM = 30. This shape of I(f) can be interpreted as a maximum width of the directional wave spectrum at f∕fPM = 8, possibly equivalent to an isotropic directional spectrum, and a narrower directional distribution toward both the dominant low frequencies and the higher capillary-gravity wave frequencies. International audience

AbstractThis paper presents the first decadal reanalysis simulation of the biogeochemistry of the North West European shelf, along with a full evaluation of its skill, confidence, and value. An error‐characterized satellite product for chlorophyll was assimilated into a physical‐biogeochemical model of the North East Atlantic, applying a localized Ensemble Kalman filter. The results showed that the reanalysis improved the model simulation of assimilated chlorophyll in 60% of the study region. Model validation metrics showed that the reanalysis had skill in matching a large data set of in situ observations for 10 ecosystem variables. Spearman rank correlations were significant and higher than 0.7 for physical‐chemical variables (temperature, salinity, and oxygen), ∼0.6 for chlorophyll and nutrients (phosphate, nitrate, and silicate), and significant, though lower in value, for partial pressure of dissolved carbon dioxide (∼0.4). The reanalysis captured the magnitude of pH and ammonia observations, but not their variability. The value of the reanalysis for assessing environmental status and variability has been exemplified in two case studies. The first shows that between 325,000 and 365,000 km2 of shelf bottom waters were vulnerable to oxygen deficiency potentially threatening bottom fishes and benthos. The second application confirmed that the shelf is a net sink of atmospheric carbon dioxide, but the total amount of uptake varies between 36 and 46 Tg C yr−1 at a 90% confidence level. These results indicate that the reanalysis output data set can inform the management of the North West European shelf ecosystem, in relation to eutrophication, fishery, and variability of the carbon cycle.

Twenty‐five years of high‐resolution (1/12°) ocean reanalysis are used to examine the Confluence of the Malvinas Current (MC) with the Brazil Current (BC) from synoptic to interannual time scales. The model transports of the MC (38.0 Sv ± 7.4 Sv 57 at 41°S) and the BC (23.0 Sv ± 11 Sv at 36°S) agree with observations. The model shows the branching of the MC near the Confluence with an offshore branch returning south and an inner branch sinking below the BC and managing to continue northward along the continental slope. Northward velocities associated with the subsurface inner branch peak at 40 cm/s at 36°S at 700‐m depth. The model documents the migrations of the Subantarctic (SAF) and Subtropical Fronts (STF) at the Confluence. The SAF and STF positions vary over a large range at synoptic (800 km) and interannual scale (300 and 200 km, respectively) compared to the rather small seasonal migrations of the STF (150 km) and SAF (50 km). While trends in the MC are small over the 25 years of the reanalysis, the BC becomes more intense (12.5 cm/s), saltier (0.37 psu), and warmer (2.5°C) in the upper 1,000 m. These trends are accompanied with a southward displacement of the STF and the SAF of 150 and 50 km. International audience

AbstractThe shallow meridional overturning cells of the Atlantic Ocean, the subtropical cells (STCs), consist of poleward Ekman transport at the surface, subduction in the subtropics, equatorward flow at thermocline level and upwelling along the equator and at the eastern boundary. In this study, we provide the first observational estimate of transport variability associated with the horizontal branches of the Atlantic STCs in both hemispheres based on Argo float data and supplemented by reanalysis products. Thermocline layer transport convergence and surface layer transport divergence between 10°N and 10°S are dominated by seasonal variability. Meridional thermocline layer transport anomalies at the western boundary and in the interior basin are anti‐correlated and partially compensate each other at all resolved time scales. It is suggested that the seesaw‐like relation is forced by the large‐scale off‐equatorial wind stress changes through low‐baroclinic‐mode Rossby wave adjustment. We further show that anomalies of the thermocline layer interior transport convergence modulate sea surface temperature (SST) variability in the upwelling regions along the equator and at the eastern boundary at time scales longer than 5 years. Phases of weaker (stronger) interior transport are associated with phases of higher (lower) equatorial SST. At these time scales, STC transport variability is forced by off‐equatorial wind stress changes, especially by those in the southern hemisphere. At shorter time scales, equatorial SST anomalies are, instead, mainly forced by local changes of zonal wind stress.

Abstract 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. International audience

We constructed annual cycles of National Centers for Environmental Prediction air‐sea fluxes and temporal oceanic heat content change from Seward Line hydrographic surveys to quantify the different contributions to the oceanic heat budget within the Alaska Coastal Current (ACC) on the northern Gulf of Alaska shelf. The deficit between air‐sea fluxes and the temporal change in oceanic heat content throughout the cooling season (October–April) varies from ~40 to 110 W m−2 and is balanced by ocean heat flux convergence. Cross‐shelf heat flux convergence is insignificant on annual average, and the nearshore heat budget is likely entirely balanced by the ACC, which resupplies ~15%–50% of the heat removed by air‐sea fluxes during the cooling season. Furthermore, we estimated spatial heat flux gradients and conclude that air‐sea fluxes increase from east to west and from offshore to onshore. The cross‐shore gradients are governed by wind speed gradients, likely due to ageostrophic nearshore wind events during the cooling season, while the along‐shelf heat flux gradients are governed by the occurrence of low‐pressure systems in the northern GOA that result in cold northerly winds over the northwestern GOA. These results underline the ACC's role as the dominant oceanic heat source to the northern GOA shelf and further imply an increased cooling rate of the ACC west of the Seward Line. Furthermore, our analysis showed that nearshore regions, particularly waters in the ACC, are subjected to stronger winter cooling than the middle and outer shelves.