A Simple Ocean Data Assimilation analysis of the global upper ocean 1950-95. Part II: Results

The authors explore the accuracy of a comprehensive 46-year retrospective a nalysis of upper-ocean temperature, salinity, and currents. The Simple Ocea n Data Assimilation (SODA) analysis is global, spanning the latitude range 62 degrees S-62 degrees N. The SODA analysis has been constructed using opt imal interpolation data assimilation combining numerical model forecasts wi th temperature and salinity profiles (MBT, XBT, CTD, and station), sea surf ace temperature, and altimeter sea level. To determine the accuracy of the analysis, the authors present a series of comparisons to independent observ ations at interannual and longer timescales and examine the structure of we ll-known climate features such as the annual cycle, El Nino, and the Pacifi c-North American (PNA) anomaly pattern. A comparison to tide-gauge time series records shows that 25%-35% of the va riance is explained by the analysis. Part of the variance that is not expla ined is due to unresolved mesoscale phenomena. Another part is due to error s in the rate of water mass formation and errors in salinity estimates. Com parisons are presented to altimeter sea level, WOCE global hydrographic sec tions, and to moored and surface drifter velocity. The results of these com parisons are quite encouraging. The differences are largest in the eddy pro duction regions of the western boundary currents and the Antarctic Circumpo lar Current. The differences are generally smaller in the Tropics, although the major equatorial currents are too broad and weak. The strongest basin-scale signal at interannual periods is associated with El Nino. Examination of the zero-lag correlation of global heat content sho ws the eastern and western tropical Pacific to be out of phase (correlation -0.4 to -0.6). The eastern Indian Ocean is in phase with the western Pacif ic and thus is out of phase with the eastern Pacific. The North Pacific has a weak positive correlation with the eastern equatorial Pacific. Correlati ons between eastern Pacific heat content and Atlantic heat content at inter annual periods are modest. At longer decadal periods the PNA wind pattern l eads to broad patterns of correlation in heat content variability. Increase s in heat content in the central North Pacific are associated with decrease s in heat content in the subtropical Pacific and increases in the western t ropical Pacific. Atlantic heat content is positively correlated with the ce ntral North Pacific.