Heat and Salinity Transport between the Mixed Layer and the Permanent Pycnocline Estimated from Argo Data

The ocean warming has already extended to the global abyssal waters. The circulation of heat, freshwater and materials absorbed into the ocean, however, has not been investigated enough. Subduction is one of the major processes by which properties of the upper layer facing the atmosphere are transferred into the ocean interior. The opposite process of subduction is “obduction”: water of the permanent pycnocline is entrained into the mixed layer when it deepens in winter. The obduction process is expected to be important for the atmosphere, especially its long-term variations, since it brings anomalies of temperature, carbon dioxide, and other properties in the ocean interior to the surface layer. The contribution of obduction to the ocean mixed layer budget has not been examined. The author quantitatively estimates the effect of obduction on the mixed layer budget on the basis of observations with the Eulerian definition. The Argo MOAA GPV dataset, which includes monthly 1° grided values, is utilized in this study.

The mean obduction and subduction rates estimated from the latest Argo dataset are consistent with previous studies. Although the effect of obduction is small compared with the other terms in the mixed layer budget, the heating rate due to obduction is up to 0.5 K/yr or more in places in the mid latitudes of the Atlantic and Indian Oceans. In some high-latitude areas where the dichothermal layer is formed, obducted water contributes to warming and salinizing the mixed layer. The interannual variation of the mixed-layer temperature is not affected by obduction very much. On the other hand, that of the mixed-layer salinity is closely correlated with that of salinity transport due to the obduction process in the high latitudes. The author also examines the interannual changes of properties of subducted water. The mean temperature of water subducted in the Gulf of Alaska in 2014-2015, for example, were higher by more than 2 K compared with normal years as a results of the "blob".