Change of the Global Ocean Vertical Heat Transport over 1993–2010

A dynamically and data-consistent ocean state estimate during 1993–2010 is analyzed for bidecadal changes in the mechanisms of heat exchange between the upper and lower oceans. Many patterns of change are consistent with prior studies. However, at various levels above 1800 m the global integral of the change in ocean vertical heat flux involves the summation of positive and negative regional contributions and is not statistically significant. The nonsignificance of change in the global ocean vertical heat transport from an ocean state estimate that provides global coverage and regular sampling, spatially and temporally, raises the question of whether an adequate observational database exists to assess changes in the upper ocean heat content over the past few decades. Also, whereas the advective term largely determines the spatial pattern of the change in ocean vertical heat flux, its global integral is not significantly different from zero. In contrast, the diffusive term, although regionally weak except in high-latitude oceans, produces a statistically significant extra downward heat flux during the 2000s. This result suggests that besides ocean advection, ocean mixing processes, including isopycnal and diapycnal as well as convective mixing, are important for the decadal variation of the heat exchange between upper and deep oceans as well. Furthermore, the analyses herein indicate that focusing on any particular region in explaining changes of the global ocean heat content is misleading.