A Synergetic Approach for the Space-Based Sea Surface Currents Retrieval in the Mediterranean Sea
Abstract
:1. Introduction
2. Data
- The CMEMS (Copernicus Marine Environment Monitoring Service) operational delayed-time sea surface geostrophic velocities for the Mediterranean Sea derived from satellite altimetry [28]. These are gap-free daily data with nominal 1/8 horizontal resolution computed by the Sea-Level Thematic Assembly Center (TAC). The 2012–2016 time series was extracted. CMEMS data ID: SST-MED-SST-L4-REP-OBSERVATIONS-010-021;
- The CMEMS global operational sea surface currents provided by the MERCATOR model (first output at 0.49 m depth) [25]. These are daily data with nominal 1/12 horizontal resolution (data available from January 2016 to present). CMEMS data ID: GLOBAL-ANALYSIS- FORECAST-PHY-001-024;
- The CMEMS near-surface currents given by the operational forecasting model for the Mediterranean Sea, i.e., the Mediterranean Forecasting System (first output at 1.47 m depth) [31]. These are daily data with nominal 1/24 horizontal resolution. The data are available from January 2016 to present. CMEMS data ID: MEDSEA-ANALYSIS-FORECAST-PHY-006-013;
- The EMODNet (European Marine Observation and Data Network) surface currents in the Malta-Sicily channel given by the operational HF-RADAR observing system (CALYPSO Project, http://oceania.research.um.edu.mt/cms/calypsoweb/). These constitute hourly data with 1/37 horizontal resolution. The radar observing system is given by three antennas, two of which are placed on the Malta and Gozo Islands and a third one is located in the southern Sicily coast (Pozzallo) (see also Figure 12 [32]. The data are available from 2016 to present;
- The OGS (Istituto Nazionale di Oceanografia e di Geofisica Sperimentale) drifting buoys-derived sea surface currents and SST in the Mediterranean Basin. These lagrangian observations have six-hourly temporal resolution and the dataset coverage spans from 1986 to late 2016 (data doi=10.6092/7a8499bc-c5ee-472c-b8b5-03523d1e73e9);
- The CNR (Consiglio Nazionale delle Ricerche) Sea Forecast operational model for the computation of the three-dimensional surface currents, temperature, salinity, SSH, heat&water fluxes and wind stress in the Central Mediterranean (Tyrrhenian Sea and Sicily Channel). We were kindly provided with the monthly averaged data with nominal 1/48 horizontal resolution by the G3O-Operational Oceanography Group. More details available at http://www.seaforecast.cnr.it/forecast/index.php/en/forecast/sicily-strait-region/);
- The CMEMS Global Blended Mean Wind Fields. These are gap-free 6-hourly global data with 1/4 horizontal resolution computed at CERSAT/Ifremer. The data are available from January 2015 to present [33]. CMEMS data ID: WIND-GLO-WIND-L4-NRT-OBSERVATIONS-012-004;
- Two datasets of Sea Surface Salinity (SSS):
- The daily, 1/20 maps of L4 SSS obtained from the multifractal fusion applied to the L3 Soil Moisture and Ocean Salinity (SMOS) observations. The data are distributed by the Barcelona Expert Center (BEC) [34]. The data are available from 2011 to 2016;
- The 4-daily, 1/4 LOCEAN L3 SSS estimates derived from a combination of SMOS ascending and descending orbits measurements [35] and are available from 2010 to 2017.
3. Methods
3.1. Derivation of the Optimal Currents
3.2. The Forcing Term F and the Computation of the Associated Error h
- The lack of long and high-resolution time series of observed forcing terms estimates;
- The forcing term F of the SST heat conservation equation includes the vertical advection, the diffusion, the atmospheric heat fluxes and the entrainment velocity. At the first order, the main contributors to F are the atmospheric heat fluxes. In turn, the fluxes can be estimated as the large scales of the SST temporal derivatives, the smaller scales being related to the currents advection. This is also consistent with the analyses described in Rio et al. 2016 [24], who analyzed one year of MERCATOR model outputs at global scale (see, e.g., Figure 10 of Rio et al. 2016).
3.3. The Uncertainties on the Geostrophic Velocities (, )
4. Results
4.1. Case Study 1: Vortex Dynamics
4.2. Case Study 2: Coastal Upwelling in the Sicily Channel
- The presence of an overall flow divergence area (DIV > 0) off the western cape of Sicily, around the Egadi Islands, highlighted in light green in Figure 6, with some convergence areas intrusions found in both the MFS and the Optimal Currents estimates;
- The alternating flow convergence (DIV < 0) and divergence (DIV > 0) patterns inside the cold SST filament extending almost vertically from 3736N–1220E to 3700N–1220E. Such adjacent patterns are mostly found around 3706N and are highlighted in black in Figure 6. They are common to all the current fields, though looking smoother in the MERCATOR estimates.
4.3. Validation with the In-Situ Measured Currents
4.4. Comparative Analysis: Optimal Currents Validation in the Malta–Sicily Channel
5. Discussion and Conclusions
- During the period 2012–2016, we computed daily gap-free maps of Optimal currents. Using the OGS in-situ measured surface currents as a benchmark, we compared the performances of the Optimal currents and the altimeter-derived estimates. We found that the Optimal reconstruction method yields the largest improvements (locally up to 30%) in the western section of the Mediterranean basin and for the meridional component of the motion. Such improvements generally increase with the increasing SST spatial gradients and proved to be slightly higher during summertime. This is mostly due to the larger spatial SST gradients that can occur during summer, especially in areas of upwelling. This effect is particularly enhanced in the Western Mediterranean and in the Sicily Channel area, where you have very active upwelling regions (not shown);
- The Optimal reconstruction method is able to retrieve both the small scale geostrophic and ageostrophic circulation, obtaining a surface field characterized by a non-zero divergence. This was shown for the Sicily Channel. In this area, during summertime, it is frequent to observe patterns of ageostrophic circulation. Indeed, computing the Optimal currents during a coastal upwelling event (on 23 July 2016), we found that the surface divergence of the Optimal currents is comparable with the predictions of the CMEMS operational models, providing an estimate of the total (i.e., with non-zero divergence) surface-current field. The values of the Optimal currents divergence also proved to be in line with the monthly values predicted by the submesoscale-permitting CNR-Sea Forecast operational model for the Sicily Channel;
- The Optimal reconstruction, unlike the SQG method, allowed to overcome the ambiguity in retrieving the surface circulation when SST is not a direct proxy of the surface circulation. In this study, this was proved for a specific eddy dynamics case study in the Western Mediterranean area.
- In the Malta-Sicily channel, using the HFR surface currents as our benchmark, the Optimal currents improved the mean RMSE and BIAS compared to the numerical model outputs and to the satellite-derived currents (except for the bias in the retrieval of the zonal flow, where the MFS model exhibited the best performance).
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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ZONAL | RMSE (cm/s) | BIAS (cm/s) |
---|---|---|
OPTIMAL | 9.72 | 4.04 |
GV | 11.23 | 5.11 |
MERCATOR | 12.50 | 4.30 |
MFS | 12.70 | 2.60 |
MERIDIONAL | RMSE (cm/s) | BIAS (cm/s) |
---|---|---|
OPTIMAL | 9.00 | 1.40 |
GV | 9.12 | 1.30 |
MERCATOR | 12.65 | 3.60 |
MFS | 13.23 | 3.19 |
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Ciani, D.; Rio, M.-H.; Menna, M.; Santoleri, R. A Synergetic Approach for the Space-Based Sea Surface Currents Retrieval in the Mediterranean Sea. Remote Sens. 2019, 11, 1285. https://doi.org/10.3390/rs11111285
Ciani D, Rio M-H, Menna M, Santoleri R. A Synergetic Approach for the Space-Based Sea Surface Currents Retrieval in the Mediterranean Sea. Remote Sensing. 2019; 11(11):1285. https://doi.org/10.3390/rs11111285
Chicago/Turabian StyleCiani, Daniele, Marie-Hélène Rio, Milena Menna, and Rosalia Santoleri. 2019. "A Synergetic Approach for the Space-Based Sea Surface Currents Retrieval in the Mediterranean Sea" Remote Sensing 11, no. 11: 1285. https://doi.org/10.3390/rs11111285