Fabienne Gaillard

Quality Control of Large Argo Datasets

Abstract Argo floats have significantly improved the observation of the global ocean interior, but as the size of the database increases, so does the need for efficient tools to perform reliable quality control. It is shown here how the classical method of optimal analysis can be used to validate very large datasets before operational or scientific use. The analysis system employed is the one implemented at the Coriolis data center to produce the weekly fields of temperature and salinity, and the key data are the analysis residuals. The impacts of the various sensor errors are evaluated and twin experiments are performed to measure the system capacity in identifying these errors. It appears that for a typical data distribution, the analysis residuals extract 2/3 of the sensor error after a single analysis. The method has been applied on the full Argo Atlantic real-time dataset for the 2000–04 period (482 floats) and 15% of the floats were detected as having salinity drifts or offset. A second test was per...

Overview of the First SMOS Sea Surface Salinity Products. Part I: Quality Assessment for the Second Half of 2010

Multi-angular images of the brightness temperature (TB) of the Earth at 1.4 GHz are reconstructed from the Soil Moisture and Ocean Salinity (SMOS) satellite sensor data since end 2009. Sea surface salinity (SSS) products remote sensing from space is being attempted using these data over the world oceans. The quality of the first version of the European Space Agency operational Level 2 (L2) SSS swath products is assessed in this paper, using satellite/in situ SSS data match-ups that were collected over the second half of 2010. This database reveals that 95% of the SMOS L2 products show a global error standard deviation on the order of ~ 1.3 practical salinity scale. Simple spatiotemporal aggregation of the L2 products to generate monthly SSS maps at 1° ×1° spatial resolution reduces the error down to about 0.6 globally and 0.4 in the tropics for 90% of the data. Several major problems are, however, detected in the products. Systematically, SMOS SSS data are biased within a ~ 1500 km wide belt along the world coasts and sea ice edges, with a contamination intensity and spread varying from ascending to descending passes. Numerous world ocean areas are permanently or intermittently contaminated by radio-frequency interferences, particularly in the northern high latitudes and following Asia coastlines. Moreover, temporal drifts in the retrieved SSS fields are found with varying signatures in ascending and descending passes. In descending passes, a time-dependent strong latitudinal bias is found, with maximum amplitude reached at the end of the year. Errors in the forward modeling of the wind-induced emissivity and of the sea surface scattered galactic sources are as well identified, biasing the sss retrievals at high and low winds and when the galactic equator sources are reflected toward the sensor.

Circulation patterns and transport of the Azores Front-Current system

In this paper, different types of new data sets (hydrography, Lagrangian and Eulerian current measurements) and Quasi-Geostrophic model results, obtained after altimetric data assimilation, are used to study the structure and variability of the Azores Current and Frontal system. The Azores Current was observed to transport between 11.0 and 18.0 Sv eastwards and the associated Counter Current some 2.0–10.0 Sv to the west, resulting in a net value of about 8–9 Sv. Furthermore, both data and model results revealed a meandering Azores Current, where some freely rotating eddies were also identified. These hydrographic and Lagrangian results exhibit space and time scales that agree fairly well with the dynamics shaped by baroclinic instability. Current meters moored across the Azores Current system delineated a mean Counter Current flowing westwards with a maximum subsurface core of about 2.0 cm/s at a mean depth of 800 dbar. This is in excellent agreement with previous studies, which explains this Azores Counter Current as the rectification process of the geostrophic turbulence occurring north of the main Azores Current stream. A new scheme is proposed for the formation mechanism of the freely rotating cyclonic eddies observed south of the Azores. It is shown that the north–south contrast thickness of the 17–19°C water layer across the Azores Front decreases downstream. This will create, in turn, a downstream increase of the most unstable wavelength, in a non-linear baroclinic instability context. As a consequence, both large cyclonic and anticyclonic features are able to form at the eastern side of the Azores Current (around 19°W), while at the western side (around 35°W) only large anticyclones will survive (western-generated cyclones will be small enough to be quickly dissipated). This means that the eastern cyclones of the Azores Current may live longer than the shorter western ones. However, because longer-lived cyclonic eddies propagate westwards with a mean speed of 2.5 km/day, they may be observed south of Azores and of the main stream several months later, although they were not formed there.

Mean full-depth summer circulation and transports at the northern periphery of the Atlantic Ocean in the 2000s

A mean state of the full-depth summer circulation in the Atlantic Ocean in the region in between Cape Farewell (Greenland), Scotland and the Greenland-Scotland Ridge (GSR) is assessed by combining 2002–2008 yearly hydrographic measurements at 59.5°N, mean dynamic topography, satellite altimetry data and available estimates of the Atlantic–Nordic Seas exchange. The mean absolute transports by the upper-ocean, mid-depth and deep currents and the Meridional Overturning Circulation (MOCσ = 16.5 ± 2.2 Sv, at σ0 = 27.55) at 59.5°N are quantified in the density space. Inter-basin and diapycnal volume fluxes in between the 59.5°N section and the GSR are then estimated from a box model. The dominant components of the meridional exchange across 59.5°N are the North Atlantic Current (NAC, 15.5 ± 0.8 Sv, σ0 27.55) east of the Reykjanes Ridge, the northward Irminger Current (IC, 12.0 ± 3.0 Sv) and southward Western Boundary Current (WBC, 32.1 ± 5.9 Sv) in the Irminger Sea and the deep water export from the northern Iceland Basin (3.7 ± 0.8 Sv, σ0 27.80). About 60% (12.7 ± 1.4 Sv) of waters carried in the MOCσ upper limb (σ0 27.55) by the NAC/IC across 59.5°N (21.1 ± 1.0 Sv) recirculates westward south of the GSR and feeds the WBC. 80% (10.2 ± 1.7 Sv) of the recirculating NAC/IC-derived upper-ocean waters gains density of σ0 27.55 and contributes to the MOCσ lower limb. Accordingly, the contribution of light-to-dense water conversion south of the GSR (∼10 Sv) to the MOCσ lower limb at 59.5°N is one and a half times larger than the contribution of dense water production in the Nordic Seas (∼6 Sv).

Observation of a deep convection regime with acoustic tomography

In the winter of 1991–1992 a convection experiment was conducted in the western Mediterranean Gulf of Lions, combining a variety of observational techniques. An essential component was an acoustic tomography array, consisting of six moorings, designed to observe the time evolution of the large-scale processes believed to be relevant in a convection regime. Here two-dimensional slice inversions in three directions from the central mooring are used to estimate the volume of convected water and the mean convection depth and to observe the preconditioning and restratification processes before and after the convection. The near-surface layer is well sampled by the acoustics, which show cooling and subsequent entrainment of the warmer Levantine Intermediate Water (LIW) from below, in agreement with mixedlayer calculations. During the 2 months prior to the main convection event the total heat loss of the large-scale field is in approximate agreement with the surface heat fluxes, showing that little net warm-water advection takes place from outside the convection region. The implied confinement of water by the local circulation should be an important factor in setting the location and extent of the deep convection patch. The volume of water modified by convection in this winter is estimated to correspond to an area of order 60 km radius and 1500 m depth. It is argued that the homogenized area implies an annual mean deep water replenishment of 0.3 Sv. The restratification, by the return of less dense water in the surface and LIW layers, occurs first by rapid capping in the near-surface region. Thereafter, the southern and eastern parts of the region restratify in the deeper layers on a 40-day timescale, while a dense core of 50–60 km diameter remains in the northwest until the end of the experiment.

Open-ocean deep convection explored in the Mediterranean

Open-ocean deep convection is a littleunderstood process occurring in winter in remote areas under hostile observation conditions, for example, in the Labrador and Greenland Seas and near the Antarctic continent. Deep convection is a crucial link in the “Great Ocean Conveyor Belt” [Broecker, 1991], transforming poleward flowing warm surface waters through atmosphere-oceaninteraction into cold equatorward flowing water masses. Understanding its physics, interannual variations, and role in the global thermohaline circulation is an important objective of climate change research. In convection regions, drastic changes in water mass properties and distribution occur on scales of 10–100 km. These changes occur quickly and are difficult to observe with conventional oceanographic techniques. Apart from observing the development of the deep-mixed patch of homogeneous water itself, processes of interest are convective plumes on scales <1 km and vertical velocities of several cm s−1 [Schott et al., 1994] that quickly mix water masses vertically, and instability processes at the rim of the convection region that expedite horizontal exchanges of convected and background water masses [e.g., Gascard, 1978].

SMOS salinity in the subtropical north Atlantic salinity maximum: 1. Comparison with Aquarius and in situ salinity

Sea surface salinity (SSS) measured from space by the Soil Moisture and Ocean Salinity (SMOS) mission is validated in the subtropical North Atlantic Ocean. 39 transects of ships of opportunity equipped with thermosalinographs (TSG) crossed that region from 2010 to 2012, providing a large database of ground truth SSS. SMOS SSS is also compared to Aquarius SSS. Large seasonal biases remain in SMOS and Aquarius SSS. In order to look at the capability of satellite SSS to monitor spatial variability, especially at scales less than 300 km (not monitored with the Argo network), we first apply a monthly bias correction derived from satellite SSS and In Situ Analysis System (ISAS) SSS differences averaged over the studied region. Ship SSS averaged over 25 km is compared with satellite and ISAS SSS. Similar statistics are obtained for SMOS, Aquarius, and ISAS products (root mean square error of about 0.15 and global correlation coefficient r of about 0.92). However, in the above statistics, SSS varies due to both large-scale and mesoscale (here for scales around 100 km) variability. In order to focus on mesoscale variability, we consider SSS anomalies with respect to a monthly climatology. SMOS SSS and Aquarius SSS anomalies are more significantly correlated (r > 0.5) to TSG SSS anomaly than ISAS. We show the effective gain of resolution and coverage provided by the satellite products over the interpolated in situ data. We also show the advantage of SMOS (r = 0.57) over Aquarius (r = 0.52) to reproduce SSS mesoscale features.

In Situ–Based Reanalysis of the Global Ocean Temperature and Salinity with ISAS: Variability of the Heat Content and Steric Height

AbstractThe In Situ Analysis System (ISAS) was developed to produce gridded fields of temperature and salinity that preserve as much as possible the time and space sampling capabilities of the Argo network of profiling floats. Since the first global reanalysis performed in 2009, the system has evolved, and a careful delayed-mode processing of the 2002–12 dataset has been carried out using version 6 of ISAS and updating the statistics to produce the ISAS13 analysis. This last version is now implemented as the operational analysis tool at the Coriolis data center. The robustness of the results with respect to the system evolution is explored through global quantities of climatological interest: the ocean heat content and the steric height. Estimates of errors consistent with the methodology are computed. This study shows that building reliable statistics on the fields is fundamental to improve the monthly estimates and to determine the absolute error bars. The new mean fields and variances deduced from the ...

A synthesis of the POMME physical data set: One year monitoring of the upper layer

The Programme Ocean Multidisciplinaire Meso Echelle (POMME) experiment was designed to describe and quantify the role of mesoscale processes in the subduction of mode waters in the northeast Atlantic. During 1 year ( September 2000 - October 2001), in situ measurements were conducted over a 8 degrees square area centered on 18 degrees W, 42 degrees N. We present the synthesis of the physical data set collected during this experiment. To improve time and space coverages, these measurements are combined with satellite information from sea surface temperature and altimetry. Daily fields of temperature, salinity, and stream function are produced over a seasonal cycle with a simplified Kalman filter. We analyze the annual cycle of the upper layer. The 1 year mean circulation in the upper 400 m resembles the scheme proposed by Paillet and Mercier ( 1997). The meridional component of the flow carries cold water southward, thus contributing to cool the POMME area. The annual mean heat budget shows that this advection by the mean current nearly balances the warming by the surface heat fluxes. The mixed layer maximum depth is reached in March. It increases, in zonal averages, from 100 m south of 38 degrees N to more than 270 m north of 45 degrees N and, remarkably, is shaped by the mesoscale. The subpolar mode water formed in the north of the POMME area has a lower density (26.8 - 27.0) than the mode water formed in the years 1988 - 1990 that Paillet and Arhan ( 1996) found in the density range ( 27.0 - 27.1). This mode water is continuously advected southward across the 42 degrees N latitude, leading to an annual mean transport of 1.4 10(6) m(3) s(-1).

Observation of spiciness interannual variability in the Pacific pycnocline

Monthly gridded fields predominantly based on global Argo in situ temperature and salinity data are used to analyze the density-compensated anomaly of salinity (spiciness anomaly) in the pycnocline of the subtropical and tropical Pacific Ocean between 2004 and 2011. Interannual variability in the formation, propagation and fate of spiciness anomalies are investigated. The spiciness anomalies propagate on the isopycnal surface sq = 25.5 along the subtropical-tropical pycnocline advected by the mean currents. They reach the Pacific Western Tropics in about 5–6 years in the Southern Hemisphere and about 7–8 years in the Northern Hemisphere. Their amplitude strongly diminishes along the way and only very weak spiciness anomalies seem to reach the equator in the Western Tropics. A complex-EOF analysis of interannual salinity anomalies on sq = 25.5 highlights two dominant modes of variability at interannual scale: i) the former shows a variability of 5–7 years predominant in the Northern Hemisphere, and ii) the latter displays an interannual variability of 2 to 3 years more marked in the Southern Hemisphere. The significant correlation of this second mode with ENSO index suggests that spiciness formation in the southeastern Pacific (SEP) is affected by ENSO tropical interannual variability. A diagnosis of the mechanisms governing the interannual generation of spiciness in the SEP region leads the authors to suggest that the spiciness interannual variability in the sub-surface is linked to the equatorward migration of the isopycnal outcrop line sq = 25.5 into the area of maximum salinity. Quantitative analysis based on Turner angle reveals the dominance of the spiciness injection mechanism occurring through convective mixing at the base of mixed layer.