Katsuro Katsumata

Simulated Rapid Warming of Abyssal North Pacific Waters

Warming the Deep The coldest ocean waters are located at the bottoms of the major ocean basins, and, because it takes a long time for water to sink from the surface to these regions, they are relatively isolated from the warming trends that are now occurring at shallower depths. However, warming in these deep waters has recently been observed, sooner than anticipated. Masuda et al. (p. 319, published online 24 June) performed computer simulations of ocean circulation and found that internal waves are able to transport heat rapidly from the surface waters around Antarctica to the bottom of the North Pacific, which can occur within four decades, rather than the centuries that conventional mechanisms have suggested. Computer simulations suggest a possible reason for the warming of North Pacific bottom water during the past four decades. 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.

Seismic reflection imaging of a Warm Core Ring south of Hokkaido

A multi-channel seismic reflection (MCS) survey was conducted in 2009 to explore the deep crustal structure of the Pacific Plate south of Hokkaido. The survey line happened to traverse a 250-km-wide Warm Core Ring (WCR), a current eddy that had been generated by the Kuroshio Extension. We attempted to use these MCS data to delineate the WCR fine structure. The survey line consists of two profiles: one with a shot interval of 200 m and the other with a shot interval of 50 m. Records from the denser shot point line show much higher background noise than the records from the sparser shot point line. We identified the origin of this noise as acoustic reverberations between the sea surface, seafloor and subsurface discontinuities, from previous shots. Results showed that a prestack migration technique could enhance the signal buried in this background noise efficiently, if the sound speed information acquired from concurrent temperature measurements is available. The WCR is acoustically an assemblage of concave reflectors dipping inward, with steeper slopes (~2°) on the ocean side and gentler slopes (~1°) on the coastal side. Within the WCR, we recognised a 30-km-wide lens-shaped structure with reflectors on the perimeter.

Cold wind warms Southern Ocean

Well-mixed water around 500 m depth in the Southern Ocean has been warming. Now research reveals how strengthening wind increases the volume of the warm water.

An improved simulation of the deep Pacific Ocean using optimally estimated vertical diffusivity based on the Green's function method

An improved vertical diffusivity scheme is introduced into an ocean general circulation model to better reproduce the observed features of water property distribution inherent in the deep Pacific Ocean structure. The scheme incorporates (a) a horizontally-uniform background profile, (b) a parameterization depending on the local static stability and (c) a parameterization depending on the bottom topography. Weighting factors for these parameterizations are optimally estimated based on the Greens function method. The optimized values indicate an important role of both the intense vertical diffusivity near rough topography and the background vertical diffusivity. This is consistent with recent reports that indicate the presence of significant vertical mixing associated with finite-amplitude internal wave breaking along the bottom slope and its remote effect. The robust simulation with less artificial trend of water properties in the deep Pacific Ocean illustrates that our approach offers a better modeling analysis for the deep ocean variability.

Heat content and steric height change in the Pacific Ocean

Changes in the heat content and the steric height in the Pacific Ocean were studied by comparing results from ship-based basin-scale repeat hydrographic surveys mainly conducted in the 2000s in and previous surveys conducted mostly in 1990s. The layer from the surface to a depth of 1000 m accounted for 90% of the increased heat content. In this layer, heat content increased in the western Pacific, with large increases in the western Pacific warm pool and near New Zealand, and decreased in the eastern Pacific. In the bottom layers, below 5000 m the heat content increased in almost entire Pacific. There is a notable heat content increase along the pathway of Lower Circumpolar Deep Water and we can also see the trend toward the northwest. The steric height below 2000 dbar increased in the western Pacific and decreased in the eastern Pacific. The largest increase was seen in the Southern Oceans and as well as the western boundary region off the coast of Japan.