Altimeter observations

The studies of the BSGC with the help of assimilation of shipborne and satellite altimeter data in the full hydrothermodynamic model [42] began simultaneously with the above-described adaptation calculations. In the course of the studies it was realized that the optimal variant of assimilation with respect to its efficiency and simplicity is the above-mentioned introduction of terms with relaxation to observations into the thermodynamic equations [43]. The main goals of the studies became the simulation of the detailed climatic annual cycle of the BSGC with the help of assimilation of climatic monthly temperature and salinity fields and the simulation of the synoptic variability of the BSGC with the use of the SLE acquired by the altimeter observations from the TOPEX/Poseidon and ERS-1,2 satellites. 

Satellite altimeter observations were assimilated in two BSGC models 1.5-layer model with a reduced gravity acceleration [46] and in the slightly modified model [44,45] considered above. In the former model, the altimeter sea level was assimilated directly into the equations of continuity at each time step. In the latter model [47], the assimilation was similar to that in [44,45], where the differences between the model and observed temperature and salinity fields were retrieved from the level increments with the use of corresponding coefficients of proportionality. These coefficients depended on the depth and were determined from the relations between the SLE and the thermohaline fields obtained in [43] from the modeling results. Selected simplifications in the model physics helped to decrease the horizontal step of the grid in both of the calculations down to 7 km. 

The authors of [50] regarded the eddies as manifestations of Rossby waves modified by the bottom topography. The parameters of similar waves obtained from the data of altimeter observations (see Sect. 2.4), except for the period, are close to the model values. The annual wave period, which prevails in the observations, is absent in the model this is related to the forcing of the model BSGC by a constant mean annual wind field. 

EOS The Earth-observing system including Terra, Aqua, Aura, AM-1, and other satellites. It is equipped with sensors to record data on clouds and ERB, altimeter, acoustic atmospheric lidar, laser wind gauge, and gives information about volcanic eruptions. 

Thus, field observations of the currents provide certain ideas about the general features of the BSGC and its seasonal, synoptic, and interannual variabilities mostly in the upper 300-m layer. In the nearest future, the knowledge about a more detailed spatial pattern is possible only for the surface currents with the use of drifter and altimeter data. Reliable data about the BSGC in deeper layers from observational data represent a matter of the relatively remote future. 

The synergy of data from SSMR and SSM/1 sensors (since 1978) as well as a combination of simultaneous data from active and passive instruments onboard the T/P satellite (since 1992) allowed researchers to obtain series of dates of the first and last observations of ice cover, duration of ice season as well as ice extent for 1979-2002 [26]. In the paper [7], data from the T/P satellite were complemented by observations from radar altimeters onboard the Geosat follow-on (GFO) (operating since January 2000) and ENVISAT (since November 2002) satellites. Ground tracks for these satellites covered the eastern part of the Large Sea and Small Sea... 

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