The Global Ocean Surface Current defined here as the average current for the top 0 to 30m layer is derived from the synergistic use of three different satellite derived parameters. The first one is the Altimeter derived gridded Map of Absolute Dynamic Topography (MADT) generated from the suite of altimeters such as JASON2, SARAL/ALTIKA, Cryosat etc., the second data set is the gridded ocean surface vector winds derived ASCAT wind data and the last one is the gridded SST data derived from AVHRR. The methodology presented by Bonjean and Lagerloef, (2002) is used to derive ocean surface current by combining the geostrophic component from altimeter data and the ageostrophic component from scatterometer and radiometer data. The method of deriving the surface current is based on the resolution of quasisteady quasilinear momentum equations, neglecting local acceleration. Equatorial velocities are obtained by solving a weak formulation of the momentum equations using a basis set of orthogonal polynomials.
Data Access
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Data Version
 Version 1.0 (beta)
Data Sources
 The daily gridded map of Absolute Dynamic Topography data is obtained from
 AVISO/DUACS ftp site (ftp://ftp.aviso.oceanobs.com)
 The daily gridded ocean surface vector wind data from ASCAT is obtained from
 ftp://ftp.ifremer.fr/ifremer/cersat/products/gridded/MWF/L3/ASCAT/Daily
 The daily gridded SST data of Reynolds OISST is obtained from
 ftp://eclipse.ncdc.noaa.gov/pub/OIdailyv2/NetCDF
Data Citation
 This dataset may be cited as "MOSDAC (http://www.mosdac.gov.in)",(Sikhakolli et al., 2013). Sikhakolli, R., R. Sharma, R. Kumar, B. S. Gohil, A. Sarkar, K. V. S. R. Prasad and S. Basu, Improved determination of Indian Ocean surface currents using satellite data, Rem. Sens. Lett., 4, 335343, 2013.
Processing Steps
 Using the daily gridded data of MADT, Vector winds and SST data the Geostrophic, wind driven and buoyancy components of the ocean surface current respectively are first calculated for the offequatorial regions (3°N to 90°N and 3°S to 90°S).
 Using the polynomial expansion the equatorial currents (±3° latitude band) are derived.
 The daily ocean surface current at 0.25° resolution is then derived by applying a linear weighted average procedure to the equatorial and off equatorial current solutions with in the latitude band of ± 3° to ± 4° band.
References
 Bentamy A..; D. CroizeFillon, 2011: Gridded surface wind fields from Metop/ASCAT measurements. Inter. Journal of Remote Sensing. DOI 10.1080/01431161.2011.600348.
 Bonjean, F, and G. S. E. Lagerloef (2002), Diagnostic model and analysis of the surface currents in the tropical Pacific Ocean., J. Phys. Oceanogr., 32, 29382954.
 Reynolds, R. W., T. M. Smith,_C. Liu, D. B. Chelton, K. S. Casey, and M. G. Schlax (2007), Daily highresolutionblended analyses for sea surface temperature, J. Climate., 20, 54735496.
 Sikhakolli, R., R. Sharma, S. Basu, B. S. Gohil, A. Sarkar and K. V. S. Prasad, Evaluation of OSCAR ocean surface current product in the tropical Indian Ocean using in situ data , J. Earth Syst. Sci., 2013.
 Sikhakolli, R., R. Sharma, R. Kumar, B. S. Gohil, A. Sarkar, K. V. S. R. Prasad and S. Basu, Improved determination of Indian Ocean surface currents using satellite data, Rem. Sens. Lett., 4, 335343, 2013.
 SSALTO/DUACS user hand book: (M)SLA and (M)ADT Near Real Time and Delayed Time products, AVISO, Nov 2009.
Derivation Techniques and Algorithm
 The methodology follows the work of Bonjean and Lagerloef (2002). The basic equations are those of quasi linear and steady flow in a surface layer where the horizontal velocity U = (u,v) is allowed to vary with vertical coordinate z, and where vertical turbulent mixing is characterized by an eddy viscosity A uniform with depth. The vertical shear U_{z} reaches zero at a constant scaling depth z =  H. Using complex notations U(x,y,z,t) = u + iv and ∇ = ∂/∂x + i ∂/∂y , the basic equations are
ifU =  (1/ρ_{m}) ∇ p + AU_{z} (1a) (1/ρm) p_{z} = g + ∇θ (1b) ∇θ = g χ_{T} ∇SST , (1c)  with  H ≤ z ≤ 0, and subject to the following boundary conditions
U_{z} (z=0) = τ / A (2a) U_{z} (z = H) = 0 (2b)  The characteristic density is ρ_{m} = 1025 kg m ^{3},
 The acceleration due to gravity g = 9.8 m s^{2},
 and the coefficient of thermal expansion χT = 3 x 10^{4} K^{1},
 The vector field τ = τ^{x} + i τ^{ y} represents the surface wind stress divided by ρ_{m},
 H has been chosen to be 70 m,
 The parameter A is chosen by the empirical formulation as A = a (W/W1)^{b}  w 1 m s^{1} where W_{1} = 1 m s^{1} , a = 8 x 10^{5} m ^{2} s^{1} , and b = 2.2
 where W_{1} = 1 m s^{1} , a = 8 x 10^{5} m ^{2} s^{1} , and b = 2.2 .
 The equation for the velocity shear is
U_{z}  (if/A) U_{z} = (1/A)∇θ  (3) 

 which is a secondorder differential equation in velocity shear Uz , subject to the boundary conditions (2a,b). After solving for the shear profiles, one can find an expression for the velocity at the surface, which is
ifU_{0}= g ∇ζ + (1/H ) q(H/h_{e})τ + ((H/2)/q(H/2he))∇θ  (4) 

 Here the function q is defined by q (x) = x /tanh (x) and h_{e} = (A/if )^{1/2} is complex and its modulus is proportional to the Ekman depth h_{e} = sqrt( 2A/f).
 Using equation (4) and the datasets mentioned above, the daily ocean surface currents were generated. Equatorial velocities (±3° latitude band) are obtained by solving a weak formulation of the momentum equations using a basis set of orthogonal polynomials as described in Bonjean and Lagerloef (2002).
Limitations
 As the equatorial currents are estimated through an approximation procedure using polynomial coefficients the correlation with insitu currents in the equatorial region is observed to be relatively poorer especially for the meridional currents.
Known problems with data
 As these input satellite data are not very reliable near to the coast, the estimated currents very near to the coast may also have problems.
Related data collections
 OSCAR Currents: Bonjean, F, and G. S. E. Lagerloef (2002), Diagnostic model and analysis of the surface currents in the tropical Pacific Ocean., J. Phys. Oceanogr., 32, 29382954
 GEKCO Currents: Joel Sudre, Christophe Maes and Veronique Garcon (2013), On the global estimates of geostrophic and Ekman surface currents. Limnology and Oceanography. DOI: 10.1215/215736892071927
File Naming Convention
 The typical file name is 'ISRO_CURRENT_TOT_YYYYMMDD.nc' where

 'ISRO_CURRENT' signifies that this product is generated at SACISRO
 word 'TOT' signifies that this is the total current (Geostrophic + ageostrophic)
 'YYYY' corresponds to the year, ex: 2015
 'MM' corresponds to the month, ex: 09
 'DD' corresponds to the date, ex: 26
 All the data files are in NetCDF 4 format and the images are in gif format
MetaData
Sr. No  Core Metadata Elements  Definition  
1  Metadata language  English  
2  Metadata Contact  Dr. Rajesh Sikhakolli, GRD/AOSG/EPSA, Space Applications Centre (ISRO), Ahmedabad380015, India. Email: srajesh@sac.isro.gov.in  
3  Metadata date  September 15, 2015  
4  Data Lineage or Quality  Daily ocean surface currents derived from satellite data  
5  Title  Zonal and Meridional components of Ocean Surface Current (m/s)  
6  Abstract  Daily ocean surface currents(m/s) derived from the synergistic use of satellite derived Sea Level, Ocean surface vector winds and Sea surface temperature data.  
7  Dataset Contact  Dr. Rajesh Sikhakolli, GRD/AOSG/EPSA, Space Applications Centre (ISRO), Ahmedabad380015, India. Email: srajesh@sac.isro.gov.in  
8  Update frequency  Daily  
9  Access Rights or Restriction  Open Access  
10  Spatial Resolution  0.25° deg (or) ~25km  
11  Language  English  
12  Topic Category  Physical Oceanography  
13  Keywords  Ocean Currents, Ocean Circulation  
14  Date or period  Daily  
15  Responsible Party  Dr. Rajesh Sikhakolli, GRD/AOSG/EPSA, Space Applications Centre (ISRO), Ahmedabad380015, India. Email: srajesh@sac.isro.gov.in  
16  Organization  Space Applications Centre (ISRO), Ahmedabad, India  
16a  Org. role  Calculated Ocean Surface currents (m/s) for each day using daily map of absolute dynamic topography (MADT) data from AVISO (SSALTO/DUACS user hand book2009), gridded wind data from ASCAT (Bentamy et al., 2011) and SST data from AVHRR (Reynolds et al., 2007)  
16b  Individual Name  Dr. Rajesh Sikhakolli, GRD/AOSG/EPSA, Space Applications Centre (ISRO), Ahmedabad380015, India. Email: srajesh@sac.isro.gov.in  
16c  Position  Scientist/Engineer, GRD/AOSG/EPSA, SAC (ISRO), Ahmedabad380015, India. Ph: +91 79 2691 6052. Email: srajesh@sac.isro.gov.in  
17  Vertical Extent (minimumValue, maximumValue, unitOfMeasure, vertical datum)  Average ocean surface current for 0 to 30 m vertical layer in m/s  
18  Geographic Extent 


19  Geographic Name, Geographic Identifier  Global Ocean  
20  Bounding box 


21  Temporal Extent  Daily  
22  Distribution Information  Online download of data files in NetCDF format and images in GIF format  
23  Processing Level  Level 4  
24  Reference System  Projection  Cartesian Coordinate System 