Lorenz Magaard

Geographic and Seasonal Distributions of Brunt–Väisälä Frequency and Rossby Radii in the North Pacific and North Atlantic

Abstract Long-term mean temperature and salinity profiles, computed from an edited set of historical hydrographic data, have been used to calculate mean profiles of density and Brunt–Vaisala seasonal variations in internal Rossby radii are everywhere surprisingly small.

Annual Baroclinic Rossby Waves in the Central North Pacific

Abstract We analyze XBT data, collected in the central North Pacific under the NORPAX program (TRANSPAC), by means of inverse methods (cross-spectral fit). We separate the data into two parts: a fluctuation with length scales in the order of the dimensions of the North Pacific basin and a wavelike fluctuation with much shorter scales of a few hundred kilometers. At the annual frequency we find the wavelike fluctuation to consist of a random field of first-order baroclinic Rossby waves traveling basically northwest with wavelengths of ∼300 km and phase speeds of ∼1 cm s−1. The group velocity is directed mainly southwest and is of the order of 1 cm s−1. We describe the fields of temperature, pressure, velocity, sea level elevation, and horizontal and vertical particle displacement associated with the annual baroclinic Rossby waves.

Wasserstandsschwankungen und Seegang

Wasserstandsanderungen, die sich innerhalb von Zeitraumen von Monaten bis zu Jahrhunderten abspielen, wurden bereits in den Kapiteln 2 und 5 behandelt. Hier sollen nun Schwankungen des Wasserstandes betrachtet werden, die in Zeitbereichen von etwa einer Sekunde bis zu einem Monat stattfinden.

On the Generation of Baroclinic Rossby Waves in the Ocean by Meteorological Forces

Abstract The generation of baroclinic Rossby waves in a continuously stratified ocean by fluctuating fields of wind stress, buoyancy flux and atmospheric pressure at the sea surface is studied by means of boundary layer theory. The internal wave field has been represented analytically in term of the generating meteorological fields and the damping influence of bottom friction. A preliminary application to an example from the eastern Pacific shows that the influence of the atmospheric pressure is negligible compared to that of the other generating agents; on the other hand, fluctuations of the wind stress and the buoyancy flux could be strong enough to generate the waves observed by Emery and Magaard (1976). A more exacting application requires more knowledge about the meteorological fields at the sea surface and has to be left to a later investigation.

Semi-diurnal tidal currents at ‘site D’☆

Abstract Current meter records obtained in1969 at a site in the western North Atlantic were examined for tidal structure. The M2 semi-diurnal tide was found to be strongly in evidence and to be highly baroclinic. The results indicate a barotropic M2 tide with a maximum horizontal velocity of about 1 cm/sec approximately east-west. The baroclinic part of the M2 tide was found to change markedly over a two-month period and to be dominated by the first and second modes. No phase-locking was found between the barotropic tide and the baroclinic modes.

Rossby Wave Driven Eulerian Mean Flows Along Non-Zonal Barriers, with Application to the Hawaiian Ridge

Abstract We show that the reflection of baroclinic Rossby waves from a non-zonal barrier generates a Eulerian mean flow along the barrier. In order to produce a mean current of realistic magnitude, friction (in the form of Rayleigh damping) has to be incorporated into the theory. However, the wave-induced mean flow still exists in the absence of friction; the key requirement for mean flow production is the non-zonality of the reflecting wall. The theory is applied to the Hawaiian Ridge where the incident Rossby wave field is known for the 5° square 20–25°N, 155–160°W (Magaard, 1983). The mean flow produced by this incident wave field consists of 1) a narrow eastward coastal jet ∼20 km wide, and 2) a band-structured flow of alternating directions beyond the coastal jet. The width of each band in 2) is ∼50 km, and the envelope of the current oscillations decreases (from a maximum surface value of ∼0.75 m s−1) exponentially with distance from the Hawaiian Ridge. In the region between the western side of Maui...

On the Potential Energy of Baroclinic Rossby Waves in the North Pacific

Abstract Estimates of baroclinic Rossby wave potential energy spectra for various parts of the North Pacific were calculated from published material containing information about this energy in many different formats, definitions and units. The standardized results lead to frequency spectra of potential energy, ranging from 0.1 to 0.45 cpy (cycles per year), in 34 5° squares between 20 and 30°N; to energy spectra, ranging from 0.28 to 2.11 cpy, in eight subsections of the great circle route from Honolulu to San Francisco; and to a map of annual Rossby wave energy between 30 and 40°N. The most remarkable finding is a consistent sequence of interannual Rossby wave spectra, with a peak at 0.15 cpy (i.e., at a wave period of 6.7 years) covering the area 20–25°N and 175–130°W. The most complete information is available for a 5° square east of the Hawaiian Islands. There the spectrum shows, besides the 0.15 cpy peak, another broad peak ranging from about 0.4 to 1.4 cycles per year.

On Stable and Unstable Rossby Waves in Non-Zonal Oceanic Shear Flow

Abstract We derive linear equations for the study of baroclinic instability of a non-zonal oceanic shear flow whose direction is allowed to change with depth. These equations can be used to study unstable disturbances as well as stable Rossby waves in such a flow. We find an approximate analytic solution of the equations by using a two-level model. According to this solution a non-zonal shear flow is always unstable, and the length scales of the unstable disturbances are larger than the internal Rossby radius of deformation (corresponding wavelengths are larger than 2π times the internal Rossby radius of deformation). In all our examples of non-zonal shear flow, and strongly unstable zonal shear flow, the wavelength of the fastest growing disturbance is ∼10 times the internal Rossby radius of deformation. Only for weak westward unstable zonal shear flow do we find the fastest growing unstable disturbance to have a wavelength close to 2π times the internal Rossby radius of deformation. We also compute nume...

Stable and unstable Rossby waves in the North Pacific current as inferred from the mean stratification

Abstract We have determined free Rossby waves in the North Pacific Current by numerical methods. We have found only two stable solutions — the barotropic and first-order baroclinic Rossby shear modes. The influence of the current on the dispersion features of these waves is small for the barotropic shear mode, but is significant for the baroclinic shear mode. An explicit comparison of the dispersion relations for the baroclinic wave in case of vanishing and non-vanishing current is given. We have found at most one unstable solution per wave number. The unstable wave with largest growth rate has an e-folding time of 1.1 year. We have calculated vertical profiles of the stream function and the temperature for the various shear modes at various wave numbers. The temperature shear modes have been calculated for later usage in a Rossby wave model to be fitted to observed temperature data from the North Pacific Current area.

Rossby Wave Analysis of Subsurface Temperature Fluctuations along the Honolulu-San Francisco Great Circle

Abstract Seven years of XBT observations collected between Honolulu and San Francisco reveal subsurface temperature fluctuations with strong, in-phase vertical coherence and fair horizontal coherence over 1000 km distance. Their annual and semi-annual harmonics exhibit randomness of phase, suggesting that the fluctuations are modeled better as a stochastic process than as a deterministic signal. Cross-spectral fitting of a stochastic, first baroclinic mode Rossby wave model to the fluctuations demonstrates the Rossby waves could be producing 50–60% of the observed subsurface covariance at periods exceeding nine months. Phase is seen to propagate northwestward and energy westward and southwestward. The corresponding sea surface velocities and sea level fluctuations are of the order 1 cm s−1 and 1.5 cm.