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.

[1] Repeat trans-Pacific hydrographic observations along the pathway of Lower Circumpolar Deep Water (LCDW) reveal that bottom water has warmed by about 0.005 to 0.01°C in recent decades. The warming is probably not from direct heating of LCDW, but is manifest as a decrease of the coldest component of LCDW evident at each hydrographic section. This result is consistent with numerical model results of warming associated with decreased bottom water formation rates around Antarctica.

[1] Using high-quality data sets obtained about a decade apart, we examined the changes of dissolved inorganic carbon in the Pacific Ocean, separating anthropogenic and natural CO 2 . Observations along three transoceanic sections along 47°N, 179°E, and 17°S showed both decadal increases (>20µmol kg , except in the western subarctic Pacific. In contrast, in subtropical regions of both hemispheres, we found an increasing trend of >10 µmol kg -1 in oceanic uptake of anthropogenic CO 2 , reflecting accumulation in mode waters. Along 17°S, increases of anthropogenic CO 2 were >20µmol kg ) in the subtropical regions of both hemispheres and low values in the tropical Pacific. This distribution pattern is similar to previous estimates for the Anthropocene, implying that the redistribution processes of anthropogenic CO 2 have not changed on a basin scale over the last decade. We estimated the total anthropogenic and natural CO 2 storage in the Pacific Ocean to be 8.4 ± 0.5 and 0.6 ± 0.4 Pg carbon decade -1 , respectively.

[1] We conducted a trans-Pacific hydrographic section along 24°N in 2005 to investigate the ocean structure and its changes from previous observations in 1985. We detected significant basin-average water property changes from 1985 to 2005. Apparent oxygen utilization increased below the thermocline by up to 6 mmol kg À1 around the density of the central mode water (around 600 m). It appeared that the North Pacific intermediate water (around 800 m) was less dense in 2005 than in 1985 because of warming. From the decrease of the zonal gradient of the temperature and salinity around the North Pacific deep water (2500-4000 m) and lower circumpolar deep water (<4000 m), we suggest that northward bottom water and southward deep water transports became weaker from 1985 to 2005, consistent with the speculation from the observed temperature increase of the bottom water along its main path in previous studies. Although these water property changes suggest a slowdown of the meridional overturn in the North Pacific and large transport changes in the deep layers (below 4000 m) are estimated from an inverse method, significant heat transport changes were not detected. The estimated temperature transport change of 0.1 PW between the two sections was mainly due to shallow overturn changes, especially changes in the Kuroshio. To describe variability due to the Kuroshio changes, we estimated mass and heat transport changes from long-term observations of the Kuroshio in the Okinawa Trough, and we determined decadal variability of temperature transports, which was consistent with the variability estimated from sea-surface flux data sets.

A long‐term data synthesis experiment was conducted for the period 1957–2011 using a modified quasi‐global four‐dimensional variational data assimilation system that was originally developed to improve the representation of the deep ocean, including a unique method for anomaly assimilation. The overall characteristics of the resulting ocean state estimate, which is dynamically consistent without any artificial sources or sinks for heat and salt, are evaluated in the Pacific Ocean. It is shown that the data set better represents the comprehensive ocean state: the mean state of the water mass distribution and volume transport and components of temporal variability from the sea surface to the bottom on interannual to multidecadal timescales. This suggests that the data set can be used to examine interactions between temporal variations throughout the entire depth range and is useful for understanding ocean physics and its role in the climate system.

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