The NCEP Climate Forecast System Reanalysis - NOAA National ...

scale is twice the level thickness. The variance is set proportional to the square-root ofthe local vertical temperature gradient computed from the forecast. The observationalerrors are assumed to be uncorrelated, so that R is a diagonal matrix of the estimated errorvariances of the observations. The errors assigned to a temperature profile vary withdepth according to the square root of the vertical temperature gradient and are scaled tohave values between 1 o C and 2.5 o C. The standard error assigned to a salinity profile is aconstant 0.1 psu at all depths.Temperature and salinity profiles are assimilated at 6-hour intervals using allobservations from the previous 10-day interval. The more distant a profile is in time, theless weight it receives in the assimilation. This approach allows relatively sparse oceanobservations to have a greater impact on the model state (Derber and Rosati, 1989;Behringer et al. 1998).An exception to the assimilation procedure is made in the top level of the model (5meters) where the ocean analysis is accomplished by simple relaxation to gridded fieldsof temperature and salinity. The purpose is to provide a stronger constraint on the oceanat the interface with the atmosphere. This is done by relaxing the sea surface temperature,every 6 hours, to the daily mean ¼ o SST, described in section 4.1.4.4 The Ocean Observations4.4.1 Temperature ProfilesThe temperature observations used for assimilation are profiles from expendablebathythermographs (XBTs), from fixed mooring arrays: the Tropical Atmosphere Ocean /Triangle Trans Ocean Buoy Network (TAO/TRITON) (McPhaden et al., 1998) in thePacific Ocean, the Prediction and Research Moored Array in the Tropical Atlantic(PIRATA) (Bourlès et al, 2008) and the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) in the tropical Indian Ocean andfrom the Argo profiling floats (The Argo Science Team, 2001). The XBT observationscollected prior to 1990 have been acquired from the NODC World Ocean Database 1998(Conkright et al., 1999), while XBTs collected subsequent to 1990 have been acquired- 31 -