Ocean Surface Phenomena

2 Ocean surface phenomena 2.1 Introduction This chapter summarizes those open ocean and floating ice properties that modify the surface and affect the emitted and reflected radiation. For the open ocean, these include wind-generated capillary and gravity waves, breaking waves, the generation and decay of foam, and the modulation of short waves by long waves and currents. Short waves are also suppressed by natural and human-generated slicks. At longer time periods and over larger spatial scales, ocean currents, eddies and Rossby and Kelvin planetary waves generate large scale changes in ocean surface elevation. Polar ice properties that affect the radiation include the areal extent and type of pack ice, and the presence and size of icebergs. In the following, Section 2.2 discusses the oceanic winds and the ocean surface wave properties important to remote sensing: in particular the difference between the short-period capillary waves and the longer-period gravity waves, the changes that occur in the gravity wave profile with increasing wave amplitude, the growth of capillary waves on the surface of the longer-period waves, the effect of wave breaking and the generation of foam. The section also discusses the distribution of wave surface slopes as a function of azimuth angle relative to the wind direction. Although this topic seems obscure, it is essential for determination of sun glint, which can overwhelm the satellite observations, and for the measurement of vector wind speeds at microwave frequencies. The section concludes with a discussion of surface slicks. Section 2.3 discusses the changes in sea surface height induced by ocean currents and long-period planetary waves, and Section 2.4 discusses sea ice. 2.2 Ocean surface winds and waves Surface winds play a dominant role in the modification of the temperate ocean surface. The most prevalent process is the wind generation of ocean waves. Another is that the surface wind stress and the atmospheric heat exchange drive the ocean circulation. Figure 2.1 shows the distribution of the global wind speeds over ice-free waters derived by two methods: first, from satellite passive microwave Special Sensor Microwave/Imager (SSM/I) observations of wind magnitudes using the techniques described in Chapter 9; second, from winds co- located with satellite observations and derived from National Center for Environmental 28 2.2 Ocean surface winds and waves 29 15 10 5 Number of occurrences (%) 0 0612 18 24 Windspeed (m s−1) Figure 2.1. Comparative histograms of the 10-m wind speed obtained from SSM/I data (shaded area) and from co-located NCEP pressure data (line). Both data sets consist of 6.8 × 1010 measurements taken between January 1992 and December 1997. The bin sizes for the winds are 1.2 m s−1 (Courtesy Remote Sensing Systems, Santa Rosa, CA, used with permission). Prediction (NCEP) gridded surface pressures. For this figure and throughout the book, the surface wind velocity corresponds to the wind measured at a 10-m height, called the 10-m wind speed U. An important difference between the description of ocean currents and atmospheric winds is that currents are described by their direction of flow; winds in terms of their upwind direction. Figure 2.1 shows that for both distributions, the peak in the wind speed distribution lies between 5 and 8 m s−1, where about 40% of the wind speeds lie in this range, with a mean wind speed of about 7 m s−1. Although wind speeds greater than 12 m s−1 strongly contribute to the generation of waves, foam and to the transfer of momen- tum to currents, they occupy only 10% of the histogram. As Phillips (1977) describes, the wind-driven wave amplitudes and range of excited wavelengths depend on the turbulent energy flux from the atmosphere to the sea surface. This flux in turn strongly depends on the temperature stratification above the sea surface; if the atmosphere is warmer than the surface, then the atmosphere is stably stratified so that for the same wind speed, the turbulent flux is less than for an unstable stratification. Consequently, for the same wind speed, a stronger flux yields more waves and roughness, while a weaker flux yields less. Slicks also affect the surface response. In summary, the 30 Ocean surface phenomena frequency and amplitude distribution of the wind-induced surface waves depends not only on U, but also on the ocean/atmosphere temperature difference and on the presence or absence of slicks. Winds excite waves ranging in length from less than a centimeter to hundreds of meters where, depending on the observational window, all lengths are important to remote sensing. Long ocean waves are dominated by gravity, but for centimeter-scale waves, the effects of surface tension or capillarity become important. For a surface tension appropriate to seawater, Figure 2.2 compares the phase speed of pure gravity waves with that of capillary- gravity waves (Phillips, 1977). The figure shows that the gravity wave phase speed increases with wavelength, while the capillary-gravity phase speed has a minimum at a wavelength of 1.8 cm. The figure also shows that for the same wavelengths, capillary-gravity waves propagate faster than gravity waves and that surface tension is important up to wavelengths of about 7 cm. Although these capillary-gravity waves are very short relative to long gravity waves, their presence and distribution relative to the wind direction strongly contribute to microwave remote sensing. 50 45 40 ) 1 − 35 Capillary-gravity waves 30 25 Gravity waves Wave phase speed (cm s 20 15 Phase speed minima 10 (1.8 cm) 5 0 2 4 6 8 10 12 Wavelength (cm) Figure 2.2. Comparison of the phase speed for capillary-gravity waves (dashed line) and for pure gravity waves (solid line) plotted versus wavelength for seawater. The vertical line marks the phase speed minimum for capillary waves. See text for additional information. 2.2 Ocean surface winds and waves 31 As observation of any pond or puddle shows and as Kawai (1979) demonstrates in his laboratory experiments, capillary-gravity waves with wavelengths close to the phase speed minimum immediately form and grow following the onset of a gust. These waves achieve an equilibrium distribution around the minimum wavelength within a few seconds of the wind onset, are independent of position, and rapidly decay when the wind ceases. If the wind continues blowing, the frequency of the largest amplitude or dominant wave shifts to lower frequencies and longer wavelengths. This is also the case for capillary wave growth on an existing swell field (Donelan and Pierson, 1987, p. 4975). The generation of ocean swell differs from capillary waves, in that ocean swell can be generated at great distances from the observation site where the swell properties are only slowly modified by changes in the wind speed. The evolution of long-period wind-generated waves can be described as a function of either time or fetch, where fetch is defined as the downwind distance from a coast. The time description applies to the case of a uniform wind turned on at a specific time over an initially flat water surface far from any coast. For this case, as time proceeds, capillary waves appear first, then gravity waves form at lower frequencies, longer wavelengths and with greater amplitudes, so that the width and size of the wave frequency distribution increases with time. This wave growth continues until the energy input from the wind equals the energy dissipation by breaking and viscosity, at which time an equilibrium is reached that is independent of position. In contrast, the fetch description applies to a steady wind blowing off a coast, where the wave spectra are independent of time and depend primarily on wind speed and fetch (Huang, Tung and Long, 1990, especially their Figure 1). Consequently, as the fetch increases, the waves increase in amplitude and length. At distances far from the coast, the wave spectrum again reaches a wind speed-dependent equilibrium. Seasonally, the strongest winds and largest waves in the Northern Hemisphere occur in the winter North Atlantic and North Pacific. In the Southern Hemisphere, the strongest winds and largest waves occur during the austral winter in the Southern Ocean, which is a circumpolar sea unobstructed by land masses. Kinsman (1984) states that a fetch of 1500 km is sufficient for the development of the largest observed storm waves. Of these, the largest peak-to-trough wave amplitude observed to date was about 34 m, as recorded by the USS Ramapo in 1934 in the central North Pacific (Kinsman, 1984, p. 10). Characteristic wavelengths within storms range from 150 m in the North Atlantic, to 240 m in the Southern Ocean. Long-period swell has been observed with lengths as long as 600 m (Kinsman, 1984). 2.2.1 Change in the wave profile with increasing amplitude Ocean surface waves are described in terms of their amplitude aw, which is defined as half the peak-to-trough wave height, their wavenumber kw = 2π/λw and their radian frequency ωw = 2π/Tw, where Tw is the wave period, λw is the wavelength, and the subscript w distinguishes these terms from those used to describe electromagnetic waves. If η is the wave height measured from the mean free surface and x is parallel to the wave propagation 32 Ocean surface phenomena Air 0.1 Water 0.2 η 0.4 Mean free surface Figure 2.3. Comparison of the profiles of a single frequency gravity wave for the different values of the wave slope awkw given above each wave. The vertical axis is exaggerated by 60% to emphasize the change in wave shape with amplitude.

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