Ocean Currents 1

Surface winds derived from earth Observation satellites are increasingly required for use in operational monitoring and forecasting of the ocean. A drawback of space-borne wind observing systems, such as scatterometers, is that they provide time and space coverage unsuitable for, among others, high-resolution ocean model forcing.

Recent attempts of combining scatterometer data and numerical weather prediction (NWP) outputs, blended ocean forcing products, allows for an increased temporal resolution (e.g., daily) but generally only resolves NWP spatial scales of ~200 km. Therefore, information on wind-current interaction, on the diurnal wind cycle and on wind variability in moist convection areas is lost in such products. Moreover, known systematic NWP model (parameterization) errors are in fact propagated at times and locations where no scatterometer winds are available. The alternative, direct forcing from NWP results in even more extensive physical drawbacks. We propose to maintain the increased temporal coverage in a gridded wind and stress product, but also to maintain most beneficial physical qualities of the scatterometer winds, i.e., 25-km spatial resolution, wind-current interaction, variability due to moist convection, etc., and, at the same time avoid the large-scale NWP parameterization and dynamical errors. Additionally, we correct these winds for the effects of atmospheric stability and mass density, using stress equivalent 10 m winds, U10S.

In fact, collocations of scatterometer and global NWP winds show these physical differences, where the local mean and variability of these differences are rather constant in time and thus could be added to the ERA-interim time record in order to better represent physical interaction processes and avoid NWP model errors. Correction of either the wind vector biases and wind vector variability is expected to affect ocean forcing. Information on the scatterometer wind sampling error is provided by these collocations.

Accurate estimate of ocean surface currents is both a challenging issue and a growing end-users requirement. Advancing the quantitative estimation of ocean surface currents from satellite sensor synergy and demonstrating the impact in user-led scientific, operational and commercial applications is the main objective of the GlobCurrent project, a Data User Element (DUE) from the European Space Agency (ESA). In the framework of this study, a global reanalysis of 12 years (2002-2014) of global ocean currents at two depths (surface and 15m) has been calculated as the sum of the geostrophic and Ekman components. The geostrophic component is based on the SSALTO-DUACS multimission altimeter maps of velocity and the CNES-CLS13 Mean Dyanmic Topography. Ekman currents at two levels (surface and 15m) are calculated from an empirical model. The model parameters (amplitude and angle) have been derived as to minimize the misfit between wind stress data and the ageostrophic component extracted from in-situ drifter velocities (SVP drifters at 15m depth, Argo floats at the surface). Parameters have been fitted by month, longitude and latitude. They feature a clear seasonal cycle in good agreement with the ocean stratification. Also, the surface and 15m Ekman currents show a spirale-like response to wind stress in good qualitative agreement with the theory. Currents have been calculated over a 12 years period (2002-2014) and an estimate of the current accuracy is provided together with the current values. Finally, a number of tests have been performed to investigate the impact of taking into account other information as the Mixed Layer Depth (from an Argo floats based climatology), effective wind stress, and Stokes drift from the WaveWatchIII model in a future empirical model formulation, showing that the present model may be further improved.

The GlobCurrent project (http://www.globcurrent.org) funded under the ESA Data User Element (DUE) from 2014-2016 aims to: (i) advance the quantitative estimation of ocean surface currents from satellite sensor synergy; and (ii) demonstrate impact in user-led scientific, operational and commercial applications that, in turn, will improve and strengthen the uptake of satellite measurements.

Within the GlobCurrent project global and regional observation-based surface currents from 2002-2014 are now available at an interpolated spatial resolution of 10 km and temporal resolution of 3 hours. In addition delay Doppler altimetry from Sentinel-3, giving improved resolution and precision in the retrieval of SSH and the surface geostrophic current will gradually become available together with range Doppler derived surface current from Sentinel-1 for selected intense surface current regimes. Moreover, the ocean color images from the Korean geostationary GOCI sensor of the East China Sea and Sea of Japan allow the Maximum Cross Correlation technique to produce surface current advection from sequences of high frequency (hourly) image acquisitions. These fields will be combined with Sentinel-3 ocean color and SST retrievals. An important activity of the GlobCurrent project is the validation and quality assurance of the surface current fields and higher-level products using independent observations and information. Assessments are performed by intercomparison to model fields with and without assimilation, direct in-situ observations (e.g., Argo, surface drifters, ship drift logs), and high-resolution remote sensing (e.g., satellite optical glitter and land-based high frequency radars). In addition, the value of the GlobCurrent products for ship navigations are investigated. These assessments, moreover, are complemented and extended by a range of validation studies pursued by independent user led teams. In this presentation we demonstrate how the GlobCurrent multisensor synergy based analyses approach can stimulate and strengthen research and development to better understand the 2-dimensional surface expressions in the context of the upper ocean 3-dimensional dynamics. 

In the last two decades, satellite altimetry has emerged as the main tool for the investigation of surface ocean dynamics. Multiple altimeter observations can be combined to produce global maps of surface velocities capable of resolving structures of the order of 100 km. However, they cannot provide information at the smaller scales (mesoscale and sub-mesoscale), which in recent years have been recognised to be critical for the ocean energy budget and global biogeochemical cycles. The signatures of (sub)mesoscale processes can be retrieved from along track observations, characterized by higher resolution than the gridded products. However, they can only provide estimates of the velocity component perpendicular to the track.

Methods based on multi-sensor satellite synergies represent a promising approach for retrieving the full velocity components. In particular, maps of surface tracers, such as ocean colour and temperature, can be used to identify the location and direction of the main ocean fronts. As these features are primarily originated by the stirring induced by horizontal surface dynamics, their direction provides a good indication of the direction of the main currents. Therefore, information on front directions can be combined with altimeter information on the flow perpendicular to the track to produce a section of 2-D velocities.

This synergy between along-track altimetry with ocean colour has been recently tested in the southern Madagascar region. The area is particularly favourable for the application of such approach because of the strong surface signature of the East Madagascar Current which flows poleward along the eastern edge of the Madagascar and crosses TOPEX/Jason track 196 tracks almost perpendicularly. The marked gradients in the surface tracer fields favour the formation of strong frontal features, providing a good test location for this method. The reconstructed currents were compared to the velocity time series of three moorings deployed along this track from February 2005 to April 2006. Although with some limitations, the approach provided promising results.

Here we will further extend this analysis, by developing an analogous approach based on the surface fronts obtained from sea surface temperature. The validation of the reconstructed velocities will be also extended by including the time series of total velocities observed at a Jason-1 crossing point further offshore than the moorings. Comparing the results for the different surface tracers will provide key insights to identify the specific conditions (temporal and dynamical) under which one tracer performs better than the other, and to better understand the overall limitations of such an approach. The general concepts at the base of the proposed approach can be easily extended to other ocean regions and would be especially applicable when ocean colour and SST and altimeter observations are co-localized on the same satellite, as will be the case for the Sentinel-3 mission.

The global database of salinity observations is rapidly expanding now as a result of new satellite measurement capabilities (ESA’s Soil Moisture Ocean Salinity, or SMOS, and the NASA Aquarius and SMAP missions) and other enhancements to the global observing system (e.g., the Argo float array, TSG ...). However, improved understanding of the processes modulating SSS is critical to use of SSS as an indicator of air-sea exchange in the hydrological cycle. Besides air-sea exchange of water, SSS is also fundamentally affected by oceanic advection and mixing.

            The new satellite SSS data can now be used to improve understanding of the physical processes that influence upper-ocean salinity and ocean-atmosphere interactions in a precipitation-dominated regime with net freshwater and buoyancy fluxes into the ocean. The surface freshening induced by rain, or river plumes, stabilizes the surface layer of the ocean. In high-precipitation regions, the vertical structure of salinity can be more important than that of temperature in setting upper-ocean stratification and mixed-layer depth to form barrier layers (BL). BL suppresses vertical turbulent exchanges and limits the penetration depth of the wind-induced turbulent mixing to a thin surface layer. Under the action of winds over such a fresh near-surface layer, the stable upper-ocean starts to slip (horizontal advection and spreading) over the underlying water, practically without friction. The advected surface layer can then be called a "slippery layer". The combined effects of barrier layers and slipping results in forming more intense near-surface currents, into the transport of freshwater by this enhanced upper ocean circulation and in reducing upper ocean cooling. In this context, a key question central to the scientific use of the new satellite SSS data (probed in the first-centimeter below the air-sea interface) is: How does the ocean SSS impact the feedbacks on the atmosphere and on the ocean surface current generation? While sea surface temperature feedbacks on the ocean-atmosphere interactions have been studied for years thanks to satellite measurements, very little is known still on the SSS feedbacks on the atmosphere and on ocean surface circulation. Consequently, there is a need to evaluate the impact of properly resolving the variability of SSS in high precipitation region, in terms of momentum and heat fluxes and surface current generation.

            An ideal test-case region to perform such analysis is the eastern tropical Pacific Ocean under the inter-tropical convergence zone (ITCZ), where the intensity of surface water flux is largest in summer and generates a very large-scale fresh water pool at the ocean surface, and where SPURS-2 US campaign will take place in 2016.

            In this talk we shall present an analysis of the Eastern Pacific Fresh Pool (EPFP) 3D dynamics during the 6 year period covering 2010-2015 based on SMOS salinity data combined with rain, wind, currents, altimetry, SST and in situ observations.

            Intraseasonal SSS variability in the EPFP is investigated with a particular focus on the changes in the expansion rate of the volume and area of the freshpool together with the advection speed of the SSS fronts at its boundaries. An effective upper layer advection estimate of the EPFP is evaluated and compared to other estimates (geostrophy, ekman...) in order to identify possible physical mechanisms responsible for the pool variability such as barrier layers, slippery layer effects, anomalous E-P local effects or buoyancy surface fluxes.

            Interannual SSS variability in the EPFP correlated with ENSO will also be shown, giving some new insights of the salinity role in the recent El niño phase.