SST spatial anisotropic covariances from METOP-AVHRR data
Introduction
Sea Surface Temperature (SST) is a key geophysical variable for many applications (see e.g. Donlon et al., 2002 and reference therein). For instance, global-scale, complete and realistic SST fields are key input data in the Ocean General Circulation Models (OGCMs). One may also cite the use of SST data to track, characterize and reconstruct mesoscale dynamics (cf. Isern-Fontanet et al., 2006, Klein et al., 2009, Tandeo et al., 2013). In this respect, satellite sensing data provide invaluable global-scale data with relatively high spatial and temporal resolutions. Such data are however vulnerable to various atmospheric contaminations (e.g., cloud cover, aerosols, heavy rains) and they may present large missing data rates. Therefore, spatial and temporal interpolation techniques are necessary to reconstruct the hidden information (see e.g. Reynolds et al., 2007). Understanding and modeling of mesoscale SST variability appear crucial to provide interpolation priors.
The temporal variability of the SST has been investigated for short time scales from in situ data (cf. Wainer, Clauzet, Servain, & Soares, 2003) and from remote sensing sources (cf. Tandeo, Ailliot, & Autret, 2011) but, its spatial variability, especially spatial anisotropy and non-stationarity, remains poorly explored. Whereas regional studies addressed the isotropic spatial variability of the SST (e.g. Gohin & Langlois, 1993 in the bay of Biscay), evidences for local spatial anisotropies have been reported for SST fields in some studies (e.g. Park & Chung, 1999 in the Sea of Japan) as well as for other geophysical variables such as ocean color (cf. Doney, Glover, McCue, & Fuentes, 2003). However, to our knowledge, no study has explored thoroughly at a global-scale these mesoscale spatial SST variabilities. This study relies on geostatistic tools to address these issues from the analysis of the Advanced Very High Resolution Radiometer (AVHRR) instrument on-board the METOP-A satellite (cf. Le Borgne et al., 2007, O&SI SAF Project Team, 2013). Using compact parametric model of the spatial covariance of the SST fields and the associated model fitting scheme, we produce observation-based atlases of the mesoscale spatial variability of the SST for the global Ocean, in terms of local anisotropy, spatial scales and residual variances. These atlases provide new insights on global and regional mesoscale ocean dynamics and new priors for missing data interpolation.
This paper is organized as follows. The METOP-AVHRR SST data and the geostatistical methodology are introduced in Section 2. We discuss the results obtained in the Malvinas current and the global Ocean in Section 3. Section 4 further discusses our key contributions and prospects for future work.
Section snippets
METOP-AVHRR data
METOP-A satellite has a high resolution coverage of the world Ocean. The data are delivered in satellite projection at full resolution and remapped onto a regular 0.05° × 0.05° global grid every 12 h (see O&SI SAF Project Team, 2013 for more details). METOP-AVHRR is one of the most efficient SST data source due to the high quality measurements and the number of observations (cf. Le Borgne, Legendre, & Marsouin (2007)). In this study, we only use night-time data to avoid the variability due to the
Results
The estimated parameters σ02 (nugget), σ2 (sill), ϕ (anisotropic angle), Lmin (minimum range) and Lmax (maximum range) are discussed in this section. They are computed for a 1° × 1° gridover the global Ocean. Locally, we can suppose that the spatial variability is second-order stationary. The computation of the empirical semivariograms proceeds as follows. For a given 1° × 1° area, we first compute daily empirical semivariograms: we sort all locations within the considered area and retrieve the
Conclusion
In this paper, we have presented a geostatistic method to characterize the non-stationarity of the spatial variability of the SST at mesoscale from 2008 METOP-AVHRR data. We chose an exponential model with an anisotropic parameterization to take into account the differences of the SST spatial variability along the directions. Five parameters were estimated in the global Ocean. They are available online as supplementary material. Among them, we find the stationary variance, the anisotropy
Acknowledgment
The data from the EUMETSAT Satellite Application Facility on Ocean and Sea Ice are accessible through the SAF's homepage: http://www.osi-saf.org/. We are grateful to Dr. Pierre Le Borgne for his expertise and valuable comments on this work.
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Both authors equally contributed to this work.