Maki Nagasawa

Spatial and temporal distribution of the wind-induced internal wave energy available for deep water mixing in the North Pacific

Using a three-dimensional multilevel numerical model, we examine the distribution of the wind-induced near-inertial internal wave energy in the North Pacific. Energetic low vertical mode near-inertial internal waves are excited at 30°-45°N in the western and central North Pacific by traveling midlatitude storms during winter and at 10°-30°N in the western North Pacific by tropical cyclones during fall. Thus excited internal waves propagate equatorward down to 5°-15°N, where their frequencies are twice the local inertial frequencies. Parametric subharmonic instability can then transfer their energy across the local internal wave vertical wavenumber spectrum to small dissipation scales. The calculated results show that low vertical mode double-inertial frequency internal waves are very weak at the times and locations of previous microstructure measurements, which suggests that the observed value of diapycnal diffusivity of ∼10 -5 m 2 s -1 , an order of magnitude lower than required to satisfy the large-scale advective-diffusive balance of the thermohaline circulation, may not be representative.

Turbulence Estimation Using Fast-Response Thermistors Attached to a Free-Fall Vertical Microstructure Profiler

AbstractEstimation of turbulence intensity with a fast-response thermistor is examined by comparing the energy dissipation rate from a Fastip Probe, model 07 (FP07), thermistor with from a shear probe, both of which are attached to a free-fall microstructure profiler with the fall rate of 0.6–0.7 m s−1. Temperature gradient spectra corrected with previously introduced frequency response functions represented by a single-pole low-pass filter yields with a bias that strongly depends on turbulence intensity. Meanwhile, the correction with the form of a double-pole low-pass filter derives less bias than of single-pole low-pass filter. The rate is compatible with when the double-pole correction with the time constant of 3 × 10−3 s is applied, and 68% of data are within a factor of 2.8 of in the wide range of = 10−10–3 × 10−7 W kg−1. The rate is still compatible with even in the anisotropy range, where the buoyancy Reynolds number is 20–100. Turbulence estimation from the fast-response thermistor is thus confir...

Comparison of Turbulence Intensity from CTD-Attached and Free-Fall Microstructure Profilers

AbstractTurbulence intensity estimated from fast-response thermistors is compared between conductivity–temperature–depth (CTD)-attached and free-fall microstructure profilers, conducted at the same location within 2 h. The agreement is generally good but anomalously overestimated values, deviating from a lognormal distribution, appear sporadically in the CTD-attached method. These overestimated outliers are evident as spiky patches in the raw temperature gradient profiles. They often occur when the fall rate of the CTD frame W (m s−1) is small and its standard deviation Wsd is large. These overestimated outliers can be efficiently removed by rejecting data with the criteria of Wsd > 0.2 W − 0.06, where W and Wsd are computed for a 1-s interval. After the data screening, thermal and energy dissipation, χ and e, from CTD-attached and free-fall profilers are consistent within a factor of 3 in the ranges of 10−10 < χ < 10−7°C2 s−1 and 10−10 < e < 10−8 W kg−1, respectively, for 50-m depth-averaged data. Energy...