The Role of Sea Surface Temperature in Reanalysis

Abstract

With the aim of understanding the role of SST in the reanalysis for the preradiosonde, presatellite, and satellite eras, a number of observation system experiments were performed using the NCEP/Department of Energy (DOE) reanalysis system. Five pairs of experiments were conducted using observed and climatological SSTs for cases 1) without any observation, 2) surface pressure observation only with the observation density of 1915, 3) surface pressure observation with the observation density of 1997, 4) surface observation and radiosondes, and 5) all observations, including satellite retrievals. The analyses were run for 4 months in 1997 (strong El Niño) and 1993 (near-normal SST). The impact of SST and the various observation systems on the analysis of near-surface parameters, the upper-level field, and several diagnostic fields were compared against the control analysis with observed SST and all available observations. The most important finding of this study is that the impact of SST varies with the time scale of the analysis, which is most apparent in the surface-pressure-only observation experiments. The impact of SST is largest for the low-frequency (seasonal) analyses and smaller for the high-frequency (daily) analyses. This is particularly apparent for near-surface temperature and upper-level height field analyses. In the extreme case of the strong El Niño year, the simulation with observed SST without any observations [Atmospheric Model Intercomparison Project (AMIP)-type run] produced seasonal mean 2-m temperature (T2M) and 500-hPa height fields that agreed better with the control analysis than the analysis with surface pressure observation only with climatological SST. On the contrary, the impact of surface pressure observation is greater on higher-frequency analyses, and lower on low-frequency analyses. Generally speaking, accurate analysis of SST is important when limited atmospheric observation is available. But even for the full atmospheric observation system, climatological SST produces inferior analysis over ocean as well as land. The introduction of radiosonde data drastically reduces errors in the analyses and diagnostic fields; thus, radiosonde data are indispensable for accurate estimation of the atmospheric state in both short and long time scales.