Yujiro Kitade

Widespread freshening in the Seasonal Ice Zone near 140°E off the Adélie Land Coast, Antarctica, from 1994 to 2012

Long-term water mass changes during 1994-2012 are examined from nine repeat hydrographic sections in the Seasonal Ice Zone along 140°E, off Antarctica. Significant freshening trends are detected within most of the water masses from the bottom to surface. Bottom Water freshened by 0.008-0.009 decade -1 below isopycnal surfaces and its layer thickness decreased by 120-160 dbar decade-1 throughout the study period. In addition to general thinning, the layer thickness was anomalously thin in 2012, suggesting a possible link with the sudden calving of the Mertz Glacier Tongue and subsequent reduction in sea-ice production. Winter Water freshened by 0.03 decade-1 throughout the study period, with significant interannual variability. In the offshore region, a long-term increase in precipitation can explain a substantial portion of the freshening trend. The Lower Circumpolar Deep Water on the continental slope underwent freshening at the same rate as the Bottom Water during the last two decades. Modified Shelf Water also shows robust freshening at a rate of 0.03 decade-1. Combined with the freshening of near-surface and Bottom Water masses in this region, these data indicate freshening of the entire water column over the continental slope. This widespread freshening is broadly consistent with the enhancement of the global hydrological cycle, together with a possible acceleration of land ice melting. Key Points Antarctic Bottom Water freshened off Adelie Coast, Antarctica, during 1994-2012 Precipitation increase can cause a significant part of Winter Water freshening The freshening is consistent with the enhancement of global hydrological cycle ©2013. American Geophysical Union. All Rights Reserved.

Coastal-Trapped Waves with Several-Day Period Caused by Wind along the Southeast Coast of Honshu, Japan

Generation and propagation of several-day period fluctuations along the southeast coast of Honshu, Japan, were investigated by analyzing sea level data and by using a numerical model. The sea level data obtained at twelve stations from Choshi to Omaezaki in fall in 1991, showed energy peaks at the 3–6 day period at the eastern stations in this coast. Time lags of the 3–6 day period fluctuations between station and station indicate westward propagation along the coast. However, the energy level of the 3–6 day period fluctuations suddenly decreased south of the Izu Peninsula. Numerical experiments using a two-layer model were performed to clarify the generation and propagation mechanism of the several-day period fluctuations by periodical wind in fall. The amplitude distributions of observed sea level were qualitatively explained by a coastal-trapped wave (CTW) in the numerical experiment. From the discussions on propagation of a free wave, CTW with the characteristics of a shelf wave generated by the wind along the northeast of the Boso Peninsula was separated into two types of wave at the southeast of the peninsula. One is an internal Kelvin wave with large interface displacement and the other is the shelf wave propagating over the northern part of the Izu Ridge. The sudden decrease in the surface displacement with the 3–6 day period observed at the western stations is considered to be due to the local effect of the wind and phase relation between the internal Kelvin wave and shelf wave.

Antarctic Bottom Water production from the Vincennes Bay Polynya, East Antarctica

One year moorings at depths greater than 3000m on the continental slope off Vincennes Bay, East Antarctica, reveal the cold (<-0.5 degrees C) and fresh (<34.64) signals of newly formed Antarc ...

Distribution of overturn induced by internal tides and thorpe scale in Uchiura Bay

The vertical mixing process induced by internal tides was investigated by repeated conductivity, temperature, and depth (CTD) measurements and bottom-mounted acoustic Doppler current profiler (ADCP) in Uchiura Bay from July 24 to 25, 2001. Internal tides were observed with a wave height of 40 m and a horizontal current of 0.3 ms−1. Density inversions were found in the CTD data, and the method of Galbraith and Kelley (1996) was applied to the data to identify overturns and to calculate Thorpe scale. Most of the overturns distributed in the region of low Richardson number, so that they were considered to be caused by shear instability associated with the internal tides. Thorpe scale was calculated to be 0.48 m. From the Thorpe scale, the vertical eddy diffusivity due to internal tides in Uchiura Bay was estimated as Kρ ∼ 10−4 m2s−1.

SDP and LP Fluctuations Observed along the Coast of Sagami Bay

Episodic and subtidal variations of current velocity and temperature are found in the mooring data obtained at seven stations in Sagami Bay during the summer-fall period, 1991. A detailed description of the episodic variation, which occurred from late July to 1 August, shows the intrusion of warm and saline water into Sagami Bay through Oshima east-channel, which causes the rise of sea level along the coast. The power spectra of the current and temperature records in the subtidal frequency range show that significant energy is found at the period of 2–7 days (SDP) and longer than twenty days (LP). The phase relation between eastward current and temperature fluctuations for SDP is different from that for LP. At the bay head, the temperature rise for LP is accompanied by a westward current in the surface layer. For SDP fluctuation, the phase relation between the near-surface, alongshore current and subsurface temperature implies the westward propagation of internal gravity waves. The SDP temperature fluctuations are larger along the bay coast than in the center of the bay, which is a characteristic of internal Kelvin waves. The SDP fluctuations propagate from east to west with the speeds of 0.32–1.16 m s−1, the propagation speed being larger at the eastern corner of the bay head than in the other area. A cross-correlation analysis between the local wind and temperature and/or current at the bay head show that the SDP fluctuations of temperature and current are induced by the local wind. The wind leads the temperature and/or current by about 12 h for the 2–3 day period (SDP2) fluctuations, and by about 2.5 days for the 5–6 day period (SDP5) fluctuations. A theoretical analysis using a two-layer model has been conducted to clarify the response of these SDP fluctuations to the periodic wind forcing. The SDP2 fluctuations observed in Sagami Bay are considered to be mostly generated along the east coast of the bay as a forced internal Kelvin wave by the local wind. In contrast, most of SDP5 fluctuations are caused by internal Kelvin waves propagating into Sagami Bay from outside, having an amplitude 2.5 times that of the forced internal Kelvin wave under local wind forcing.

Dynamics and Energetics of Trapped Diurnal Internal Kelvin Waves around a Midlatitude Island

AbstractThe generation of trapped and radiating internal tides around Izu‐Oshima Island located off Sagami Bay, Japan, is investigated using the three-dimensional Stanford Unstructured Nonhydrostatic Terrain-following Adaptive Navier–Stokes Simulator (SUNTANS) that is validated with observations of isotherm displacements in shallow water. The model is forced by barotropic tides, which generate strong baroclinic internal tides in the study region. Model results showed that when diurnal K1 barotropic tides dominate, resonance of a trapped internal Kelvin wave leads to large-amplitude internal tides in shallow waters on the coast. This resonance produces diurnal motions that are much stronger than the semidiurnal motions. The weaker, freely propagating, semidiurnal internal tides are generated on the western side of the island, where the M2 internal tide beam angle matches the topographic slope. The internal wave energy flux due to the diurnal internal tides is much higher than that of the semidiurnal tides ...

The role of turbulent mixing in the modified Shelf Water overflows that produce Cape Darnley Bottom Water

The mixing process associated with modified Shelf Water (mSW) overflows that eventually mix to form Cape Darnley Bottom Water (CDBW) was investigated by hydrographic and microstructure observations off the Cape Darnley Polynya (CDP), East Antarctica, in January 2009. Closely spaced microstructure observations revealed that mSW properties varied considerably within a distance of ∼4 km across the shelf edge. Near the bottom, the rate of turbulent kinetic energy dissipation was enhanced to values greater than 10−7 W kg−1, and the vertical scale of the bottom boundary layer (BBL) was on the order of 10 m. The observed BBL around the shelf edge was characterized by strong vertical mixing with turbulent eddy diffusivities of ∼O(10−3−10−2) m2 s−1. A geostrophically balanced density current, which resulted from the presence of mSW over the continental shelf, is considered the primary energy source for the turbulent mixing in the BBL. This turbulent mixing transforms the overflowing mSW through mixing with ambient water masses, specifically with the overlying modified Circumpolar Deep Water. The BBL is also thought to partly contribute to the gradual descent of mSW down the continental slope through bottom Ekman transport. We conclude that turbulent mixing, primarily caused by a density current, plays an important role in CDBW formation, by modifying the mSW overflowing from the CDP.