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A dynamically consistent state estimate is used for the period 1992-2011 to describe the changes in oceanic temperatures and heat content, with an emphasis on determining the noise background in the abyssal (below 2000 m) depths. Interpretation requires close attention to the long memory of the deep ocean, implying that meteorological forcing of decades to thousands of years ago should still be producing trendlike changes in abyssal heat content. Much of the deep-ocean volume remained unobserved. At the present time, warming is seen in the deep western Atlantic and Southern Oceans, roughly consistent with those regions of the ocean expected to display the earliest responses to surface disturbances. Parts of the deeper ocean, below 3600 m, show cooling. Most of the variation in the abyssal Pacific Ocean is comparatively featureless, consistent with the slow, diffusive approach to a steady state expected there. In the global average, changes in heat content below 2000 m are roughly 10% of those inferred for the upper ocean over the 20-yr period. A useful global observing strategy for detecting future change has to be designed to account for the different time and spatial scales manifested in the observed changes. If the precision estimates of heat content change are independent of systematic errors, determining oceanic heat uptake values equivalent to 0.1 W m−2 is possibly attainable over future bidecadal periods. [ABSTRACT FROM AUTHOR]

Historical bathythermograph datasets are known to be biased, and there have been several efforts to model this bias. Three different correction models of temperature bias in the historical bathythermograph dataset are compared here: the steady model of Hanawa et al. and the time-dependent models of Levitus et al. and Wijffels et al. The impact of these different models is examined in the context of global analysis experiments using the Simple Ocean Data Assimilation system. The results show that the two time-dependent bias models significantly reduce warm bias in global heat content, notably in the 10 years starting in the early 1970s and again in the early 1990s. Overall, the Levitus et al. model has its greatest impact near the surface and the Wijffels et al. model has its greatest impact at subtropical thermocline depths. Examination of the vertical structure of temperature error shows that at thermocline depths the Wijffels et al. model overcompensates, leading to a slight cool bias, while at shallow levels the same model causes a slight warm bias in the central and eastern subtropics and at thermocline depths on the equator in the Pacific Ocean as a result of reduced vertical entrainment. The results also show that the bias-correction models may alter the representation of interannual variability. During the 1997/98 El Niño and the subsequent La Niña the Levitus et al. model, which has its main impact at shallow depths, reduces the 50-m temperature anomalies in the eastern equatorial Pacific by 10%%--20%% and strengthens the zonal currents by up to 50%%. The Wijffels et al. correction, which has its main impact at deeper levels, has much less effect on the oceanic expression of ENSO. [ABSTRACT FROM AUTHOR]