The Nansen Center will host a lecture series in accoustic Oceangraphy during November and December, titled "Measuring mode-1 internal tides by ocean acoustic tomography and satellite altimetry: Towards global predictions of the mode-1 internal tide”.

Dr. Brian Dushaw is a visiting Fulbright exchange scientist at the Nansen Center for 6 months. He is a senior oceanographer at Applied Physics Laboratory, Seattle with expertise in applications of acoustic tomography to explore the ocean processes in integration with ocean models and observations.

He will give a series of 3 lectures at the Nansen Center during November and Decmber. Location of the lectures will be in the meeting room – ground floor at Nansen Center, Thormøhlensgt 47, Bergen.

Lecture 1: “Observation of mode-1 internal tides by acoustic tomography”
23^rd November 2011 @ 13:15-14:00

Abstract: Travel times of reciprocal 1000-km range acoustic transmissions, determined from the 1987 Reciprocal Tomography Experiment are used to study barotropic tidal currents and a large-scale, coherent baroclinic tide in the central North Pacific Ocean. The difference in reciprocal travel times determines the tidal currents, while the sum of reciprocal travel times determines baroclinic tide displacement of isotachs (or equivalently isotherms). The barotropic tidal current accounts for 90% of the observed differential travel time variance. The measured harmonic constants of the eight major tidal constituents of the barotropic tide and the constants determined from current meter measurements agree well with global tidal models. The amplitudes and phases of the first-mode baroclinic tide determined from sum travel times agree with those determined from moored thermistors and current meters. The baroclinic tidal signals are consistent with a largescale, phase locked internal tide, which has apparently propagated northward over 2000 km from the Hawaiian Ridge. The amplitude, phase, and polarization of the first mode M2 baroclinic tidal displacement and current are consistent with a northward propagating internal tide. The ratio of baroclinic to barotropic energy determined using range-averaging acoustic transmissions is about 8%, while a ratio of 26% was determined from the point measurements. The large-scale internal tide energy flux, presumed northward, is estimated to be about 180 W/m.

Reference: Dushaw, et al., 1995, Barotropic and baroclinic tides in the central North Pacific Ocean determined from long-range reciprocal acoustic transmissions, J. Phys. Oceanogr., 25, 631-647.

Lecture 2: “Resonant diurnal internal tides in the western North Atlantic”
30^th November 2011 @ 13:15-14:00

Abstract: Using a large acoustical array located midway between Puerto Rico and Bermuda, enhanced diurnal tidal signals associated with the lowest internal-wave mode were observed. These signals result from a diurnal internal wave resonantly trapped between the shelf just north of Puerto Rico and the turning latitude, a distance of 1100 km. The data obtained using the large acoustical array are consistent with the predicted Airy-function variation with latitude of diurnal internal waves near the turning latitude. The existence of this wave is a striking demonstration of the long spatial and temporal coherence of oceanic internal tides. In general, the energy radiated by such waves is a loss of energy from the earth-moon system and a source of energy for mixing in the deep ocean.

References: Dushaw, and Worcester, 1998, Resonant diurnal internal tides in the North Atlantic, Geophys. Res. Lett., 25, 2189-2193.
Dushaw, 2006, Mode-1 internal tides in the western North Atlantic. Deep Sea Res., 53, 449-473.  

Lecture 3: “On the predictability of mode-1 internal tides"
7^th December 2011 @ 13:15-14:00

Abstarct: A frequency-wavenumber tidal analysis for deriving internal-tide harmonic constants from TOPEX/Poseidon (T/P) measurements of sea-surface height (SSH) has been developed, taking advantage of the evident temporal and spatial coherence and the weak dissipation of internal tides. Previous analyses consisted of simple tidal analysis at individual points, which gave inconsistent harmonic constants at altimeter track crossover points. Such analyses have difficulty in distinguishing between the effects of interference, incoherence, and dissipation. The frequencywavenumber analysis provides an objective way to interpolate the internal tides measured along altimetry tracks to any arbitrary point, while leveraging all available data for optimal tidal estimates. Tidal analysis of T/P data from 2000 to 2007 is used to predict in situ time series measured during the 2001-2002 Hawaiian Ocean Mixing Experiment (HOME), the 1987 reciprocal tomography experiment (RTE87), and the 1991 acoustic mid-ocean dynamics experiment (AMODE), demonstrating both the temporal coherence and the lack of incoherent elements to this wave propagation. After correcting for changes in background stratification, the amplitude of the mode-1 internal tide was found to decrease by less than 20% over the 2000 km between the Hawaiian Ridge and 40N. A significant fraction of the variability of internal waves, that component associated with mode-1 internal tides, appears to be predictable over most of the world's oceans, using harmonic constants derived from satellite altimetry.

Reference: Dushaw et al., 2011, "On the predictability of mode-1 internal tides", Deep-Sea Research I, 58, 677-698.  

The full program is attached.