Abstract
Assimilation methods, meant to constrain divergence of model trajectory from reality using observations, do not exactly satisfy the physical laws governing the model state variables. This allows mismatches in the analysis in the vicinity of observation locations where the effect of assimilation is most prominent. These mismatches are usually mitigated either by the model dynamics in between the analysis cycles and/or by assimilation at the next analysis cycle. However, if the observations coverage is limited in space, as it was in the ocean before the Argo era, these mechanisms may be insufficient to dampen the mismatches, which we call shocks, and they may remain and grow. Here we show through controlled experiments, using real and simulated observations in two different ocean models and assimilation systems, that such shocks are generated in the ocean at the lateral boundaries of the moored buoy network. They thrive and propagate westward as Rossby waves along these boundaries. However, these shocks are essentially eliminated by the assimilation of near-homogenous global Argo distribution. These findings question the fidelity of ocean reanalysis products in the pre-Argo era. For example, a reanalysis that ignores Argo floats and assimilates only moored buoys, wrongly represents 2008 as a negative Indian Ocean Dipole year.
Highlights
That may cause such spurious effects are moored buoys in the ocean which are anchored at fixed locations[14]
A set of Observation System Experiments (OSEs) is conducted on a global ocean data assimilation system INCOIS-GODAS12,15 that comprises of the numerical ocean model MOM4.016 and 3D-VAR assimilation scheme[17] which can assimilate temperature and salinity profiles
We report degradations in the assimilated variables as well
Summary
A set of OSEs is conducted on a global ocean data assimilation system INCOIS-GODAS12,15 that comprises of the numerical ocean model MOM4.016 and 3D-VAR assimilation scheme[17] which can assimilate temperature and salinity profiles. The free model run (FRR) presents large deviations (~1 °C) in sea surface temperature anomaly (SSTA) on the eastern and western side of the Equator in the Pacific Ocean when compared to the TMIAMSRE24 (merged product based on TMI and AMSRE satellite SST measurements) observations (figure not shown) These differences are largely eliminated once moored buoys are assimilated (MBR experiment). Since all the above systems including ours use a version of MOM as the numerical model, these degradations could be model specific Another possibility is that fewer number of salinity observations compared to temperature from the moored buoys in the Pacific Ocean might have generated imbalances in the ocean state[26]. OSSEs offer several advantages compared to OSEs - e.g., knowledge of truth and complete control in generating observations at intended spatio-temporal scales
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