Historical Ocean Subsurface Temperature Analysis with Error Estimates

Abstract

An objective analysis of monthly ocean subsurface temperatures from 1950 to 1998 is carried out. The analysis scheme and the results with estimated analysis errors are presented. The analysis domain is global with a horizontal grid of 1° ? 1° and 14 vertical levels in the upper 500 m. Subsurface temperature observations used in the objective analysis are archived by the National Ocean Data Center of the National Oceanic and Atmospheric Administration, together with those collected through the global telecommunication system and domestic communication lines in Japan. All the observations are preprocessed by quality control and data selection procedures developed in this study. Together with these observations, three-dimensional fields of the upper-ocean temperature are optimally estimated using a variational technique. To ensure smooth and continuous vertical temperature profiles, a constraint term is introduced to the cost function that is minimized in the analysis. In addition, the analysis scheme is formulated to constrain mixed layer temperatures to become close to sea surface temperatures produced by the Met Office. The three-dimensional structure of thermal anomalies is represented by the objective analysis. Interannual variations of temperature anomalies in the northern and tropical Pacific are presented and examined with the estimated errors. For the purpose of verification against independent observations of the objective analysis, dynamical heights estimated from the analyzed temperatures and climatological salinity are compared with tide gauge and sea surface height observations. An investigation of analysis errors and signal-to-noise (S?N) ratio reveals that the reliability increases in the tropical Pacific since the 1970s and the S?N ratio for seasonally averaged temperatures in a 3° latitude ? 6° longitude box at 100-m depth is 2.5 in the 1990s. This is not only due to the increase in data sampling but also to an increase in interannual variances of subsurface temperature. At midlatitudes of the Northern (Southern) Hemisphere, the S?N ratio is above (below) unity over the whole period of the objective analysis. The changes are very small in these 50 yr, although recent observational networks cover the global oceans better and the observations are more homogeneously distributed than those of the previous decades.