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On the Modifications of Near-Inertial Waves at Fronts
Noname manuscript No. (will be inserted by the editor) On the modifications of near-inertial waves at fronts: Implications for energy transfer across scales Leif N. Thomas Received: date / Accepted: date Abstract In the ocean, wind-generated kinetic energy (KE) manifests itself primarily in balanced currents and near-inertial waves. The dynamics of these flows is strongly constrained by the Earth's rotation, causing the KE in bal- anced currents to follow an inverse cascade but also preventing wave-wave interactions from fluxing energy in the near-inertial band to lower frequencies and higher vertical wavenumbers. How wind-generated KE is transferred to small-scale turbulence and dissipated is thus a non-trivial problem. This article presents a review of recent theoretical calculations and numerical simulations that demonstrate how some surprising modifications to internal wave physics by the lateral density gradients present at ocean fronts allow for strong in- teractions between balanced currents and near-inertial waves that ultimately result in energy loss for both types of motion. Keywords fronts · internal waves · wave-mean flow interactions 1 Introduction Winds blowing over the ocean generate currents with a range of frequencies. The bulk of the kinetic energy (KE) in these flows is contained in mesoscale currents and eddies with low frequencies, which are strongly constrained by the Earth's rotation to follow the geostrophic balance. Winds also generate near-inertial waves (NIWs), which are oscillatory, unbalanced motions with frequencies close to the inertial frequency f = 2Ω sin φ (Ω is the Earth's angular velocity and φ latitude). The dissimilar temporal scales of the bal- anced, mesoscale currents and the NIWs suggest that the two types of motion should not interact very effectively, and that the fast waves should simply pass Leif N. -
A Tale of Three Waves
A brief introduction to oceanic waves April 16, 2015 Daniel Whitt Department of Applied Mathematics and Theoretical Physics University of Cambridge [email protected] http://www.damtp.cam.ac.uk/user/dw494 1 Introduction: A tale of three waves The first of two lectures to be delivered at Capital Normal University, Beijing March 30-April 3, 2015 Many textbooks treat oceanic wave motion extensively (e.g. Phillips, 1966; Leblond and Mysak, 1978; Lighthill, 1978; Gill, 1982; Pedlosky, 1986; 2003) and the ideas have been studied for many centuries (e.g. the tidal equations were first derived by Pierre-Simon Laplace in 1776). As a result, I can't possibly give either a particularly original or comprehensive treatment of oceanic wave motions in this first lecture. Previous researchers and teachers deserve much (if not all) of the credit for what appears here. Rather, my goal is to give a descriptive introduction to oceanic wave motions using both ob- servations and mathematical analysis. My hope is that after this lecture you will have some appreciation for 1) the ubiquity and diversity of wave phenomena in the ocean, and 2) the prin- ciple mathematical models and techniques that have been used to interpret and understand these motions in atmospheric science, oceanography and geophysical fluid dynamics. In the second lecture, I will build on these ideas by discussing some topics in ocean wave physics that are \special" in that I have worked on them and/or the topic particularly interests me. 1.1 Introduction The ocean can support a variety of classes of waves (which we loosely define as traveling perturba- tions to an equilibrium background state with a propagation speed that is typically different from the background flow speed). -
Effect of a Simple Storm on a Simple Ocean
Effect of a Simple Storm on a Simple Ocean Jeff Moehlis 1 Introduction: Near-Inertial Oscillations and Storms Horizontal motion of a free particle on the Earth's surface subject only to the Coriolis force is governed by the equations du dv p fv = 0; p + fu = 0; dt − p dt p where up and vp are respectively the eastward and northward components of the particle's velocity in the frame rotating with the Earth, f 2Ω sin φ is the Coriolis parameter, Ω is ≡ E E the frequency of the Earth's rotation, and φ is the latitude (see, e.g., [4]). This has solution ift up + ivp = e− (u0 + iv0), where u0 and v0 are the initial components of the velocity. This 2 2 1=2 corresponds to the particle's velocity describing a circle of radius (u0+v0) =f with frequency f. In the northern hemisphere, f > 0 and the particle rotates in a clockwise direction when viewed from above. The inertial frequency f is the low-frequency cutoff for internal waves in the ocean. An internal wave with frequency near f is called a near-inertial oscillation (NIO). About half of the total kinetic energy associated with internal waves in the ocean is contained in NIOs [5]. There is much observational evidence, starting with [17, 14], that wind from storms can excite near-inertial currents in the mixed layer of the ocean; recent observations include [8, 13, 15]. Simple models which treat the mixed layer as a solid slab have been quite successful at explaining the process by which wind generates such currents (see, e.g., [14, 5]). -
The Signature of Inertial and Tidal Currents in Offshore Wave Records
JUNE 2012 G E M M R I C H A N D G A R R E T T 1051 The Signature of Inertial and Tidal Currents in Offshore Wave Records JOHANNES GEMMRICH AND CHRIS GARRETT University of Victoria, Victoria, British Columbia, Canada (Manuscript received 2 March 2012, in final form 10 April 2012) ABSTRACT The roughness of the sea surface can be affected by strong currents. Here, long records of surface wave heights from buoy observations in the northeastern Pacific Ocean are examined. The data show the influence of tidal currents, but the first evidence of wave-height modulation (up to 20%) at a frequency that is slightly higher than the local inertial frequency is also found. This finding shows the effect on surface waves of near- inertial currents, which are typically the most energetic currents in the open ocean. The result has implications for wave forecasting but also provides valuable information on the frequency, strength, and intermittency of the associated near-inertial motions. 1. Introduction Data Management (http://www.meds-sdmm.dfo-mpo.gc. ca/isdm-gdsi/waves-vagues/index-eng.htm) and the Na- The height of ocean surface waves is largely a function tional Data Buoy Center (http://www.ndbc.noaa.gov/) of wind speed, duration, and fetch (Holthuijsen 2007), for Canadian and U.S. locations, respectively. Technical and potential long-term trends in wave height might be modifications of the buoy hardware and the data pro- a tool for monitoring climate change (Young et al. 2011). cessing present challenges for long-term trend analyses Ambient currents on various time scales can change but do not affect data quality over time scales of hours the amplitude, direction, and frequency of ocean sur- to days (Gemmrich et al.