[1] A four-dimensional variational (4D-VAR) data assimilation system using a coupled ocean-atmosphere global model has been successfully developed with the aim of better defining the dynamical states of the global climate on seasonal to interannual scales. The application of this system to state estimations of climate processes during the 1996-1998 period shows, in particular, that the representations of structures associated with several key events in the tropical Pacific and Indian Ocean sector (such as the El Niño, the Indian Ocean dipole, and the Asian summer monsoon) are significantly improved. This fact suggests that our 4D-VAR coupled data assimilation (CDA) approach has the potential to correct the initial location of the model climate attractor on the basis of observational data. In addition, the coupling parameters that control the air-sea exchange fluxes of mass, momentum, and heat become well adjusted. Such an initialization using the 4D-VAR CDA approach allows us to make a roughly 1.5-year lead time prediction of the 1997-1998 El Niño event. These results demonstrate that our 4D-VAR CDA system has the ability to enhance forecast potential for seasonal to interannual phenomena.

Recent observational surveys have shown significant oceanic bottom-water warming. However, the mechanisms causing such warming remain poorly understood, and their time scales are uncertain. Here, we report computer simulations that reveal a fast teleconnection between changes in the surface air-sea heat flux off the Adélie Coast of Antarctica and the bottom-water warming in the North Pacific. In contrast to conventional estimates of a multicentennial time scale, this link is established over only four decades through the action of internal waves. Changes in the heat content of the deep ocean are thus far more sensitive to the air-sea thermal interchanges than previously considered. Our findings require a reassessment of the role of the Southern Ocean in determining the impact of atmospheric warming on deep oceanic waters.

[1] We calculated basin-scale and global ocean decadal temperature change rates from the 1990s to the 2000s for waters below 3000 m. Large temperature increases were detected around Antarctica, and a relatively large temperature increase was detected along the northward path of Circumpolar Deep Water in the Pacific. The global heat content (HC) change estimated from the temperature change rates below 3000 m was 0.8 × 10 22 J decade; a value that cannot be neglected for precise estimation of the global heat balance. We reproduced the observed temperature changes in the deep ocean using a data assimilation system and examined virtual observations in the reproduced data field to evaluate the uncertainty of the HC changes estimated from the actual temporally and spatially sparse observations. From the analysis of the virtual observations, it is shown that the global HC increase below 3000 m during recent decades can be detected using the available observation system of periodic revisits to the same sampling sections, although the uncertainty is large.

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