Yoshiaki Toba

Dependence of Whitecap Coverage on Wind and Wind-Wave Properties

Using Phillips equilibrium range theory and observational data, we show first that the total rates of wave energy dissipation estimated by the Hasselmann and Phillips dissipation models are substantially consistent with each other, though their original forms are different. Both are proportional to the cube of air friction velocity, u*3, with a weak dependence on wave age. As a direct manifestation of the wave energy dissipation processes, we reanalyze previous observational data of whitecap coverage and find that it has greater correlation with the wind speed or friction velocity than the wave period or wave age. However, the data scatter decreases remarkably when the breaking-wave parameter RB= u*2/νωp is used, where ν is the kinematic viscosity of air, and ωp, the wind-wave spectral peak frequency. Physical interpretation of RB with some related issues, and a discussion of the probability models of whitecap coverage in terms of a threshold mechanism, are also presented. We conclude that RB is a good parameter to effectively express the overall wave breaking behavior for the case of wind-waves in local equilibrium with the wind. Since RB can be expressed as the product of u*3 and the wave age, this result demonstrates a stronger dependence of whitecap coverage on wave age than expected by the previous description by power-laws of u* and by the two theoretical models. Our conclusion suggests that current dissipation models should also be modified to represent full properties of wind-wave breaking.

Effect of wind waves on air-sea gas exchange: proposal of an overall CO2 transfer velocity formula as a function of breaking-wave parameter

A new formula for gas transfer velocity as a function of the breaking-wave parameter is proposed basedon correlating gas transfer with whitecap coverage. The new formula for gas transfer across an air—seainterface depends not only on wind speed but also on wind-wave state. At the same wind speed, ahigher gas transfer velocity will be obtained for a more developed wind-sea, which is represented by asmaller spectral peak frequency of wind waves.We suggest that the large uncertainties in the traditionalrelationship of gas transfer velocity with wind speed be ascribed to the neglect of the effect of windwaves. The breaking-wave parameter can be regarded as a Reynolds number that characterizes theintensity of turbulence associated with wind waves in the downward-bursting boundary layer (DBBL).DBBL provides an effective way to exchange gas across the air—sea interface, which might be relatedto the surface renewal.

A Spectral Approach for Determining Altimeter Wind Speed Model Functions

We propose a new analytical algorithm for the estimation of wind speeds from altimeter data using the mean square slope of the ocean surface, which is obtained by integration of a widely accepted wind-wave spectrum including the gravity-capillary wave range. It indicates that the normalized radar cross section depends not only on the wind speed but also on the wave age. The wave state effect on the altimeter radar return becomes remarkable with increasing wind speed and cannot be neglected at high wind speeds. A relationship between wave age and nondimensional wave height based on buoy observational data is applied to compute the wave age using the significant wave height of ocean waves, which could be simultaneously obtained from altimeter data. Comparison with actual data shows that this new algorithm produces more reliable wind speeds than do empirical algorithms.

Breaking of wind waves and the sea surface wind stress

In the conventional treatment of the coefficient of sea surface wind stress by plotting it against 10-m wind speed, there are inevitable discrepancies among results of various investigators. The reason is considered to lie primarily in the fact that the state of the sea surface or of waves is disregarded, which may have great influence on the sea surface wind stress.Former concepts concerning the conditions which control the sea surface wind stress are discussed, and it is shown that a more universal expression may be obtained by plotting the coefficient against a kind of roughness Reynolds number:Re2*=u*H/ν, whereu* is the friction velocity of air, ν the kinematic viscosity of air, andH the characteristic wave height.H is used here to treat some data in wind-wave tunnels, as a tentative variable, one step towards a more rigorous approach to the problem.This variableRe2*, orRe4*=u*w/L/vw=2πgu*w/vwn1, where the subscript ω represents values for water,L andn1 the characteristic wave length and frequency, respectively, is also the condition describing the air entrainment or the breaking of wind waves. In this case, these Reynolds numbers are interpreted as the quantity describing the intensity of turbulence of the water surface itself. It is shown, using data from our wind-wave tunnel experiments, that the breaking commences asRe2* reaches 1×103, or asRe4* reaches 3×103. Simultaneously, the stress-coefficient begins to increase sharply at this value ofRe2*. This phenomenon is understood as an increased momentum transfer from the air to the water through “boundary penetration of turbulence” caused by the breaking of wind waves. Further, it is suggested that there is a possibility that this excess momentum transfer does not increase wave momentum, but reinforces drift current.

Simultaneous remote sensing of chlorophyll, sea ice and sea surface temperature in the Antarctic waters with special reference to the primary production from ice algae

Abstract The primary production of the Antarctic Ocean south of 60°S was studied by using monthly chlorophyll- a concentration (Chl) and sea surface temperature (SST) from November 1996 to April 1997, as observed by the Ocean Color and Temperature Sensor (OCTS) aboard the Advanced Earth Observing Satellite (ADEOS). The area was divided into six sectors: (A) West and (a) East Pacific, (B) West and (b) East Atlantic, and (C) West and (c) East Indian Ocean. Sectors (A), (B), and (C) showed high Chl, and (a), (b), and (c) showed low Chl. The high Chl sectors were characterized by the wide ice covered area in spring and summer. The average Chl was 0.35 mg/m 3 . A high Chl of 0.6–2.1 mg/m 3 was found only at the ice melting area (SST below 0°) and probably originated from the liberation of the seasonal primary production by ice algae. The result shows a strong contribution of sea ice and ice algae to primary production and to the regionally heterogeneous Chl distribution by the drifting of Chl-containing ice to the melting spots. We propose a hypothesis of iron supply via snow covered sea ice as follows: iron is (1) collected from the atmosphere, (2) made soluble, (3) supplied vertically to the ice algae below, (4) taken up by diatom cells , and (5) transported to the ice melting areas.

Unusual Behavior of the Kuroshio Current System from Winter 1996 to Summer 1997 Revealed by ADEOS-OCTS and Other Data—Suggestion of Topographically Forced Alternating-Jet Instability

While the Advanced Earth Observing Satellite (ADEOS) was operating, the Kuroshio and the Kuroshio Extension, or the Kuroshio Current System, exhibited unusual behavior from the winter of 1996 to the summer of 1997. This behavior of the Kuroshio Current System has been closely studied using a time series of satellite observation images of SST and ocean color obtained by ADEOS-OCTS, reinforced by SST images obtained by NOAA-AVHRR. Our findings include (i) a long lasting, very southerly path of the Kuroshio Extension; (ii) a Kuroshio path very distant from Japan with the following alternating-jet-like north-south flow pattern of the Kuroshio Extension, which occurred twice, once in February and once in April 1997, as independent events and which was observed to be affected by the bottom topography of the Izu-Ogasawara Ridge and Trench, and of the Japan Trench; (iii) cutting off of a cold water mass after the February event; and (iv) the formation of a vortex pair after the April event. A new mechanism is suggested for the formation of the alternating-jet flow pattern: a topographically forced alternating-jet instability (AJI). An SST-Chlorophyll Diagram (T-Chl Diagram) generated using simultaneous data from a single satellite is useful for analyzing the water mass structure of this region, including biological processes.

Adjustment of Wind Waves to Sudden Changes of Wind Speed

An experiment was conducted in a small wind-wave facility at the Ocean Engineering Laboratory, California, to address the following question: when the wind speed changes rapidly, how quickly and in what manner do the short wind waves respond? To answer this question we have produced a very rapid change in wind speed between Ulow (4.6 m s−1) and Uhigh (7.1 m s−1). Water surface elevation and air turbulence were monitored up to a fetch of 5.5 m. The cycle of increasing and decreasing wind speed was repeated 20 times to assure statistical accuracy in the measurement by taking an ensemble mean. In this way, we were able to study in detail the processes by which the young laboratory wind waves adjust to wind speed perturbations. We found that the wind-wave response occurs over two time scales determined by local equilibrium adjustment and fetch adjustment, Δt1/T = O(10) and Δt2/T = O(100), respectively, in the current tank. The steady state is characterized by a constant non-dimensional wave height (H/gT2 or equivalently, the wave steepness for linear gravity waves) depending on wind speed. This equilibrium state was found in our non-steady experiments to apply at all fetches, even during the long transition to steady state, but only after a short initial relaxation Δt1/T of O(10) following a sudden change in wind speed. The complete transition to the new steady state takes much longer, Δt2/T of O(100) at the largest fetch, during which time energy propagates over the entire fetch along the rays (dx/dt = cg) and grows under the influence of wind pumping. At the same time, frequency downshifts. Although the current study is limited in scale variations, we believe that the suggestion that the two adjustment time scales are related to local equilibrium adjustment and fetch adjustment is also applicable to the ocean.

Unusual Behavior of the Kuroshio Current System from Winter 1996 to Summer 1997 Revealed by ADEOS-OCTS and Other Data (Continued): A Study from Broad External Conditions with Bottom Topography

In the previous paper (Toba and Murakami, 1998) we reported on an unusual path of the Kuroshio Current System, which occurred in April 1997 (April 1997 event), using the Ocean Color and Temperature Scanner (OCTS) data of the Advanced Earth Observing Satellite (ADEOS). The April 1997 event was characterized by the flow of the Kuroshio along the western slope (northward) and the eastern slope (southward) of the Izu-Ogasawara Ridge, a very southerly turning point at about 32°N, followed by a straight northward path up to 37°N of the Kuroshio Extension along the eastern flank of the Izu-Ogasawara and the Japan Trenches. Overlaying of depth contours on ADEOS-OCTS chlorophyll-a images at the April 1997 event demonstrates the bottom topography effects on the current paths. A new finding based on TOPEX/Poseidon altimeter data is that the sea-surface gradient across the Kuroshio/Kuroshio Extension diminished greatly in the sea area southeast of the central Japan, as a very temporary phenomenon prior to this event. This temporary diminishing of the upper-ocean current velocity might have caused a stronger bottom effect along the Izu-Ogasawara Ridge, and over the Izu-Ogasawara Trench disclosed a weak background, barotropic trench-flank current pattern, which existed otherwise independently of the Kuroshio Extension. The very southerly path of the Kuroshio Extension from winter 1996 to autumn 1998 corresponded, with a time lag of about 1.5 years, to the previous La Niña tendency with weaker North Equatorial Current. The April 1997 event occurred in accordance with its extreme condition.

New progress of the Kuroshio Current System research by using ADEOS-OCTS

Abstract The operation period of the Advanced Earth Observing Satellite (ADEOS) was rather short, new progress of the Kuroshio Current System research has been made, by use of data of the Ocean Color and Temperature Scanner (OCTS). One example is finding of unusual behavior of the Kuroshio Current System from the winter of 1996 to the summer of 1997, and it has led to a suggestion of a new dynamic mechanism of Topographically Forced Alternating-Jet Instability (AJI). Detailed paper is referred to Toba and Murakami (1998).

A method of simultaneous analysis of wind and temperature profile data over the water surface by use of the method of least squares

Priestley’s 1959 discussion concerning the estimations of momentum flux and of heat flux from profile observations of wind and temperature, stating that the closeness of the curve fitting is deceptive, is first critically re-examined. Then a method is proposed to estimate vertical flux of momentum and heat over the water surface, from the combined data of of wind speed and temperature are assumed to have similar log-linear profiles, and the most probable profiles are determined by applying the method of least squares simultaneously to wind and temperature data. Consequently, the most probable values of vertical flux of momentum and heat may be estimated to satisfy as much as possible observed data of both wind and temperature simultaneously.The coefficient of the linear term of the log-linear profile, which is treated as an indeterminate coefficient in this method, may be determined from each observed data as a function of stability length. By tentatively applying the method to Rider’s 1954 data it is found that the coefficient shows a characteristic behavior with the stability length.