AOS14-P01

Japan Geoscience Union Meeting 2016

The effect of diurnal cycle of surface heat flux on the temperature structure in the ocean
surface boundary layer*Yusuke Ushijima1, Yutaka Yoshikawa1
1.Graduate School of Science, Kyoto University
It is well known that solar radiation at the surface induces diurnal variation in both atmosphericand oceanic boundary layers. Despite of this evident feature, however, little attention has been
paid to the diurnal cycle of upper ocean and its impact on longer-scale atmospheric and oceanic
variability through the interaction between them. Recently, Large and Carbon (2015) performed
atmospheric general circulation model and showed that diurnal cycle of SST affects the longer timescale variability of sea surface heat flux and basin scale climate. This demonstrates significanceof the diurnal cycle in upper ocean on longer time scale variability. However, detailed processesof upper ocean response to the diurnal cycle is not well investigated. In this study, we performed
Large-eddy simulation of upper ocean under the diurnal cycle of the surface heat flux in order tounderstand the diurnal cycle effects on the upper ocean variability. Here two sets of simulations;
one with diurnal cycle of solar radiation and the other without it, are performed for spring
season. While heat gained at surface was distributed over the whole of wind-driven mixed layer inthe experiment without the diurnal cycle, in experiment with the diurnal cycle, heat was trappednear the surface in daytime and it was distributed over mixed layer in night time. This induces thediurnal cycle of SST and increases daily mean SST. The diurnal cycle of the surface heat flux also
affects the mixed layer depth. Noteworthy is that this effect depends on latitude; the diurnal
cycle makes MLD deeper (shallower) at lower (higher) latitude. The dependence on the latitude will
also be discussed.
Keywords: Diurnal Cycle, Mixed Layer, Sea Surface Heat Flux

©2016. Japan Geoscience Union. All Right Reserved.

- AOS14-P01 -AOS14-P02

Japan Geoscience Union Meeting 2016

Response of upper ocean cooling off northeastern Taiwan to typhoon passages*zhewen Zheng1
1.Institute of Marine Environmental Science and Technology, National Taiwan Normal University
In this study, all upper ocean responses to typhoons striking Taiwan from 2005 to 2013 weresimulated based on Regional Oceanic Modeling System to provide a comprehensive investigation on theprocess of typhoons induced upper ocean responses off northeastern Taiwan. Previous study indicatesthat the strong northeast wind, accompanied by a typhoon, could trigger a Kuroshio intrusion (KI)event that would promote the upwelling of the Kuroshio’s subsurface water onto the shelf and thus
causing cooling off northeastern Taiwan. In addition to this scenario, this study indicates anothermechanism of wind-current resonance (WCR) over the continental shelf of East China Sea that canalso trigger a distinct cooling (through entrainment mixing) within this region. Besides, statistic
results based on 17 typhoon cases indicate that the processes of typhoon passage leading todistinct cooling NET are not as common as expected. Actually, they are conditional phenomena. By
executing a series of sensitivity experiments and systematic analysis on the behaviors andbackground conditions (in both atmospheric and oceanic frames) of 17 typhoon cases, key criteriadetermining the occurrences of cooling NET through both mechanisms (KI and WCR) were elucidated
individually. Once the rotation rate of local sensed wind forcing (depending mainly on moving
track, translation speed, and RMWs of typhoons) off northeast Taiwan over the continental shelf of
ECS is comparable to the turning rate of wind-driven local inertial motions (it is about 27.4 hours
off northeast Taiwan), TCNET will be triggered through WCR. Occurrence of TCNET through themechanism of KI is determined mainly by intensity/strength of northeast wind within local NET. Bothprocesses are dominated by wind forcing rather than oceanic conditions. Finally, according to thepossible dynamic linkage between local SST off NET and regional weather system raised in recent
studies, the results elucidated in this study are believed to provide a possible advancement on
improving regional weather prediction surrounding NET.
Keywords: air-sea interaction, typhoons, modeling, remote sensing©2016. Japan Geoscience Union. All Right Reserved.

- AOS14-P02 -AOS14-P03

Japan Geoscience Union Meeting 2016

Variabilities of currents and turbulence in the Tokara Strait*Eisuke Tsutsumi1, Tomoharu Senjyu1, Keunjong Lee1, Hirohiko Nakamura2, Ayako Nishina2, Takeshi
Matsuno1
1.Research Institute for Applied Mechanics, Kyushu University, 2.Faculty of Fisheries, Kagoshima
UniversityTokara strait is a site through which the Kuroshio Current enters the deep western north Pacificfrom the shallow East China Sea, and considered to be an important region for the water massmodification of the Kuroshio water due to strong mixing. Although numerical studies have indicatedthe Tokara Strait is a hot spot of internal tide generation and mixing, observational works ofinternal tide and turbulence in the strait are only a few. In this study, results of turbulencemeasurements around the Tokara Strait are presented from a 7-day cruise in November 2015. We
performed a survey of current velocity and microstructure along two sections across the Kuroshio
Current using shipboard Acoustic Doppler Current Profiler (ADCP) and a free-falling microstructure
profiler, TurboMAP-L. In addition, a mooring array with an upward-looking 75-kHz ADCP was deployedbeneath the pathway of the Kuroshio for about 6 days to capture temporal variabilities of theKuroshio Current as well as tidal currents. Elevated vertical shear (S2 > 10-4 s-2) and dissipation
rate of turbulent kinetic energy (epsilon > 10-7 Wkg-1) were obtained near abrupt topographies andin the downstream side of the strait. The depth-averaged shear and dissipation rate were well2

correlated (R ~0.8). Current data from the moored ADCP showed that the vertical shear is dominatedby baroclinic tidal currents while mean flow at the site is dominated by the eastward Kuroshio
Current (~1 ms-1), indicating turbulent mixing in the strait was mainly induced by internal tideprocesses. We discuss the relation between enhancement of internal tide shear and the Kuroshio.
Keywords: turbulence, vertical shear, internal tide, Kuroshio, Tokara Strait©2016. Japan Geoscience Union. All Right Reserved.

- AOS14-P03 -AOS14-P04

Japan Geoscience Union Meeting 2016

Availability of turbulence measurements using a microstructure profiler attached to a CTD
frame*Yasutaka Goto1, Ichiro Yasuda1, Maki Nagasawa1
1.Atmosphere and Ocean Research Institute The University of Tokyo
Turbulence observations have been limited because of difficulty in microstructure measurements. Inorder to efficiently obtain much more turbulence data down to the ocean floor without spending
extra ship-time, we propose a new method, a microstructure profiler attached to a CTD-frame. Since
microstructure measurements of velocity shear are sensitive and fragile to vibration of theinstruments, measurements have been performed with free-fall or free-rise instruments. The profiler
attached to the CTD-frame can’t suppress vibrations. So the authors choose fast-response
thermistors to measure micro temperature fields, less sensitive to vibrations than velocity fields.
However, turbulence data from thermistors have not been common due to their insufficient temporal
resolutions: High frequency components of a temperature spectrum are attenuated. In the present
study, to overcome this deficiency, correction procedures for thermistor observations are firstlydevised by comparing concurrently obtained energy dissipation rate εestimated from thermistors andvelocity shear probes attached to a free-fall profiler. εestimated from thermistors by applying
frequency correction assuming a single-pole low-pass filter function has bias which stronglydepends on turbulence intensity. The correction with the form of double-pole low-pass filter
derives less bias, and 3×10-3[s] of the time constant is found to be the best match withε from theshear probe. Next, this correction is applied to temperature spectra obtained from thermistors
attached to the CTD-frame, and the turbulence intensity is compared with data from the free-fallprofiler conducted at the same locations within 2 hours. Most of them are compatible, however, someεfrom the CTD-attached method overestimate when the variation of the fall speed of the CTD-frame,(dW/dz)/W, is large. Large (dW/dz)/W corresponds to violation of the shape of the temperature
gradient spectrum in high wavenumber ranges, which makes spectrum peak obscure and possibly causes
overestimation. This result indicates that large (dW/dz)/W collapses the Taylor hypothesis andturbulent eddies can’t be resolved. Turbulence intensity estimated from free-fall and CTD-attachedthermistors reasonably agree by rejecting spectra with unclear peaks, and spectra with(dW/dz)/W>0.3, where violations of spectra expand to peaks at higher wavenumbers. In future,
turbulence observations are expected to expand widely by applying the present method to shipobservations and to floats equipped with thermistors.
Keywords: physical oceanography, turbulent mixing, turbulence observation, micro temperaturefluctuation©2016. Japan Geoscience Union. All Right Reserved.

- AOS14-P04 -AOS14-P05

Japan Geoscience Union Meeting 2016

Numerical investigation on effects of ocean surface turbulence on particle's sinking*Yutaka Yoshikawa1, Takahiro Mannen1
1.Graduate School of Science, Kyoto University
Biogenic small particles in the ocean can sink due to its own gravity. Marine-snow is a good
example of the particle sinking in the ocean. Because dissolved carbon in the ocean is absorbed
into the biogenic particle (phytoplankton) through photosynthesis, the particle sinking inducestransfer of the carbon from the ocean surface to deeper ocean. This vertical transfer process ofcarbon is referred to as biological pump, and is considered as one important pathway of carbon fromatmospheres into deeper oceans.
Fluid motion can accelerate and/or decelerate their sinking speed. Particle sinking in moving fluid
is also found in atmosphere where aerosol-particles are the sinking particles. Several previous
studies reported that effects of fluid motion on particle's sinking velocity depend largely on
nature of turbulence and properties of particles (e.g., Cargnelutti and Portela 2007), but forparticles with small inertia such as biogenic particles in the ocean and for steady isotropicturbulence, fluid motion does not affect the sinking velocity (Maxey 1987). On the other hand,numerical experiments for particles trapped in a ocean mixed layer (Noh et al. 2006) showed that
ocean surface turbulence, particularly Langmuir turbulence, traps more particles in the mixedlayer, leading to the conclusion that the turbulence decreases particle's sinking speed.
Here we performed numerical experiments of ocean surface turbulence and particle motion toinvestigate effects of turbulence on particle sinking in steady state. Large-eddy simulations are
performed for wind-induced and Langmuir turbulence in which particles are released and tracked. In
this study, particles sunk below the mixed layer base are removed and re-deployed at the surface.
This approach, unlike the previous study (Noh et al. 2006), allow us to investigate effects of
turbulence on the particle sinking in a steady state.
Our findings are (1) the particle's sinking speed is accelerated when a ratio of a terminal
velocity (sinking speed of the particle in fluid of rest) to the RMS of fluid vertical velocity,referred to as the velocity ratio, is O(0.1), (2) the particle's sinking speed is decelerated whenthe velocity ratio is O(10), and (3) the deceleration is amplified for Langmuir turbulence.
Keywords: Ocean Suraface Turbulence, Particle Sinking©2016. Japan Geoscience Union. All Right Reserved.

- AOS14-P05 -