International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004 
  
equation through dimensional analysis (Platt and 
Sathyendranath, 1993). The solution is of the form, 
Pant- (BDPAUK* CS Q (1h) -2 ON 
I pL )). 
where 
A = BDPm”/K 
B = phytoplankton biomass given as the concentration 
of chlorophyll-a(mg Chlm™) 
D = day length (h) 
Pm® =Assimilation number 
K = vertical diffuse attenuation coefficient 
Q = Weights for fifth —order polynomial fit (x=5) 
I. =dimensionless irradiance cateulatea AS 10" / Ix (Ix = 
a®/Pm® ) 
M = exp "^, is the optical transmittance of mixed layer 
of thickness Z,,. 
This approach assumes uniform biomass throughout the mixed 
layer, uniform diffuse attenuation coefficient and a sinusoidal 
variation of surface irradiance. When winter convection is 
active, mixed layer depth is greater than euphotic depth and 
hence, the assumption holds good and mixed layer production 
represents total production. 
2.1.1 Pre-processing of OCM data 
Atmospheric correction of OCM data was performed using long 
wavelength approach (Gordon and Wang, 1994, Chauhan et al., 
2001). 
Chlorophyll was estimated from the retrieved spectral water 
leaving radiance by the application of suitable bio-optical 
algorithms. An empirical algorithm also known as Ocean 
Chlorophyll 2 or OC2 (O’Reilly et. al., 1998) was used with 
OCM data, on the basis of results of a study done on inter- 
comparison of different bio-optical algorithms. It was found 
that this algorithm captures the inherent sigmoid relationship 
between R,490/ R555 band ratio and Chlorophyll 
concentration C, where R,, is remote sensing reflectance. The 
algorithm was shown to retrieve low as well as high chlorophyll 
concentration, which means a better retrieval even in case 2 
waters. 
2.1.2 Computation of light available at sea surface 
Light available at sea surface was computed using radiative 
transfer model. For the given location, date and time, solar 
elevation was calculated by standard procedures (Paltridge and 
Platt, 1976). The local noon surface irradiance I," under cloud 
—free skies was estimated using Bird’s model (Bird, 1984). 
Reflection losses at the air-sea interface were computed 
assuming Fresnel reflectance at a flat sea surface. 
2.1.3 Computation of parameter of light transmission or diffuse 
attenuation coefficient 
Vertical attenuation coefficient K was estimated from 
water leaving radiance of OCM bands using modified algorithm 
by Muller et. al, 1994. K(490) has been computed by 
assuming case —1 waters where changes in phytoplankton 
862 
biomass is considered to be the most important factor for 
changes in optical properties of water. 
2.1.4 Estimates of photosynthetic parameters and mixed layer 
depth 
The Hastenrath and Greisher (1989) climatology was used to 
define monthly mixed layer thickness. 
2.2 Wind speed 
Wind speed of the same area was calculated using MSMR data. 
The MSMR provides global brightness temperature 
measurements at 6.6, 10.65, 18 and 21 G Hz frequencies with 
dual polarisations. Weekly wind speeds were generated for the 
same time span at the resolution of 75 km. MSMR data were 
not available in 2002 for the same duration in February-March, 
therefore, Quick scat data were used for the latter period. 
IRS P4 OCM derived Primary Production and MSMR derived 
wind speed images were generated for the study area and 
corresponding duration. 
3. RESULTS AND DISCUSSION 
Large-scale increase in abundance of phytoplankton (winter 
bloom) occurs every year during specific period in February- 
March in NAS. Weekly averaged chlorophyll images, 
generated from OCM data of March 1* week, 2000-2004, 
representing typical high productivity pattern in deep waters of 
NAS are shown in Figure 1. 
    
  
Fig. 1 Chlorophyll pattern from Oceansat VOCM for 1* week of March, 
2000-2004 indicate accurrence of winter bloom every year in NAS 
Figure 1. Chlorophyll pattern from Oceansat [/OCM for j 
week of March, 2000-2004 indicating occurrence of winter 
bloom every year in NAS 
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