On the interpretation of short-period fluctuations in the thermocline obtained from a towed sensor

Se presentan algunas consideraciones relacionadas con la interpretación de los datos de las oscilaciones de la temperatura en la termoclina obtenidos con ayuda de un sensor en movimiento. En un modelo sencillo de relación de dispersión para ondas lineales internas con una disminución de energía en el espectro, isotrópica en cuanto a la dirección y uniforme por la frecuencia, se muestra que con determinadas velocidades de remolque del sensor es posible la aparición de un pico ficticio de densidad espectral en el espectro de frecuencias medido. También se analizan los resultados del estudio de la dependencia de la densidad espectral en los espectros de las ondas internas con respecto a la velocidad de movimiento del sensor, el carácter de la disminución de la energía y el número de onda para condiciones hidrológicas específicas. Se discute como evitar o disminuir las deformaciones Doppler en los espectros medidos. Aspects related to the interpretation of data from temperature oscillations in the thermocline obtained from a towed sensor are presented. A simple model of dispersion relation for linear internal waves with a decrease in energy in the spectrum, isotropic with regard to direction and uniform in frequency, shows that, at determined tow speeds of the sensor, a fictitious spectral density peak may appear in the frequency spectrum measured. The results of the study on the dependency of the spectral density in the interna1 wave spectra with respect to the tow speed of the sensor, the nature of the decrease in energy and wave number for specific hydrological conditions are also analyzed. How to avoid or decrease Doppler deformations in the spectra measured is discussed

Tsunami waves on the shelf near the west coast of Mexico (October 9, 1995)

The materials obtained by submerged buoy stations on the shelf of the west coast of Mexico were analyzed. The stations were equipped with autonomous sensors of temperature, electric conductivity, and pressure. The parameters of tsunami waves in the open sea were measured, and their interaction with the internal tide was registered

Thermic structure and intense internal waves on the narrow continental shelf of the Black Sea

Measurements results of the thermic structure taken from the waters of the narrow continental shelf of the south coast of Crimea are discussed. Continuous observations were performed in July-August 1993, from a stationary oceanologic platform (depth 30 m) using an antenna with three temperature distribution sensors and two temperature gradient distribution sensors situated at the platform comers (in a 19 x 22 m plane) covering the thermocline completely. Probes were also made of the water layer down to the bottom using a CTD, and also the currents at 5, 10 and 20 m were observed. A continuous register of the sea level waves, wind and atmospheric pressure was taken. The thermic structure showed oscillations in three time-scales: synoptic, related to the surge phenomena (3-5 days); inertial (17-18 h) and short period (less than 1 h). The thermocline was completely extended with a strong negative setup, and was close to the bottom with a positive setup, possessing vertical gradients of up to 2°C/m. Inertial movements showed up in the current oscillations and also produced vertical displacements in the thermocline up to 8-10 m, happening constantly while the measurements were taken. These displacements were usually related to groups of intense internal short period waves with amplitudes of 4-9 m, periods of 14-28 min and wavelengths of 250-360 m. An intense movement, with diurnal period related to the breeze processes on the coast, was also found in the currents. The relationship between the parameters of the intense waves in groups and those of the medium dispersion and non-linearity as well as the parameters of the calculated solitons in the Korteweg and de Vries equation, are analyzed. (C) 2000 Elsevier Science B.V

On the interpretation of short-period fluctuations in the thermocline obtained from a towed sensor [Acerca de la interpretaci�n de las fluctuaciones de corto periodo de la termoclina obtenidas por medio de un sensor en movimiento]

Aspects related to the interpretation of data from temperature oscillations in the thermocline obtained from a towed sensor are presented. A simple model of dispersion relation for linear internal waves with a decrease in energy in the spectrum, isotropic with regard to direction and uniform in frequency, shows that, at determined tow speeds of the sensor, a fictitious spectral density peak may appear in the frequency spectrum measured. The results of the study on the dependency of the spectral density in the internal wave spectra with respect to the tow speed of the sensor, the nature of the decrease in energy and wave number for specific hydrological conditions are also analyzed. How to avoid or decrease Doppler deformations in the spectra measured is discussed

Spatial structure of the temperature and salinity fields in the presence of internal waves on the continental shelf of the states of Jalisco and Colima (Mexico)

A method to filter fluctuations in the hydrophysical properties of the continental shelf, caused by intense internal tides, is proposed. The method is based on a smoothing of the field of the properties analyzed with a filter, the parameters of which are determined by the shape of the spatial correlation function of the field pulsations. The filtering method was tested on data from a fast oceanographic survey, conducted in an area of the monitoring polygon of the waters around Barra de Navidad. The survey was conducted on 25 and 26 November 1995, with an undulating CTD. In one day, 86 vertical profiles of temperature and salinity were obtained from nine transects perpendicular to the coast (survey area of 100 x 25 km) to a depth of 100 m. The filtering algorithm of the internal waves is studied in detail. The field measurements present an analytical shape obtained from an equation in which the elements describe the components: low frequency, pulsating and related to daily behavior. The results of the analysis of the initial temperature and salinity fields with respect to homogeneity and isotropy are discussed, as well as the structural peculiarities of the estimates of the spatial correlation functions. It is observed that the temperature and salinity fields along the shelf, within the limits of the polygon scale, are neither homogenous nor isotropic. The correlation functions of the field pulsations analyzed are more clearly observed in the thermocline. The characteristic scales perpendicular to the coast are smaller than along it. Examples of smoothing the temperature and salinity fields in the thermocline are given

On the interpretation of short-period fluctuations in the thermocline obtained from a towed sensor [Acerca de la interpretación de las fluctuaciones de corto periodo de la termoclina obtenidas por medio de un sensor en movimiento]

Aspects related to the interpretation of data from temperature oscillations in the thermocline obtained from a towed sensor are presented. A simple model of dispersion relation for linear internal waves with a decrease in energy in the spectrum, isotropic with regard to direction and uniform in frequency, shows that, at determined tow speeds of the sensor, a fictitious spectral density peak may appear in the frequency spectrum measured. The results of the study on the dependency of the spectral density in the internal wave spectra with respect to the tow speed of the sensor, the nature of the decrease in energy and wave number for specific hydrological conditions are also analyzed. How to avoid or decrease Doppler deformations in the spectra measured is discussed

A note on the effects of an individual large rainfall event on saline Lake Alchichica, Mexico

The present study reports on perturbations of the water column by large rainfall at Lake Alchichica, a saline lake in Central Mexico. Alchichica is located in the "Llanos de San Juan," a high-altitude plateau with a minimum elevation of 2,300 m above sea level. The climate is arid with annual precipitation less than 400 mm and annual evaporation of 500-600 mm. A single day large rainfall event delivered 1,810,000 m3 of water to the basin, raising the lake's water level by about 1 m. Temperature and salinity profiles showed an atypical temperature inversion up to 1�C in the upper layer accompanied by salinity decrease up to 0.5 g l-1. Transparency and pH were slightly altered, but dissolved oxygen, nutrients and chlorophyll a concentrations were not changed. In spite of the heavy rainfall and associated wind, the effects of the event were limited to the upper half of the epilimnion. After 2 days, the lake water level returned to its original level. The rapid leakage of the runoff minimized any long-term effects of the large rainfall. � 2007 Springer-Verlag

Spectrum of nonlinear internal waves in horizontally inhomogeneous shallow sea

A spectral model of nonlinear internal gravity waves (IGW) in a shallow sea with smooth horizontal heterogeneities is analyzed. It is shown that self-action produces harmonics of carrier frequency in the wave field and universal power asymptotics as ω-3 in the spectrum. A comparison of theoretical conclusions with field observations of IGW over the Mexican shelf in the Pacific Ocean is carried out. Copyright © 2005 by Pleiades Publishing, Inc

Climate of the Seasonal Cycle in the North Pacific and the North Atlantic Oceans

Time series of monthly sea-surface temperature (SST), air temperature (AT) and sea level pressure (SLP) were constructed from merged releases of the Comprehensive Ocean-Atmosphere Data Set (COADS). The time series were decomposed into seasonal and non-seasonal (short and long-term) components. The contribution of the seasonal cycle to the total variance of SST and AT exceeds 80% in the mid and in some high latitude locations and reaches its peak (> 95%) in the centres of subtropical gyres. In most cases, a combination of annual and semiannual harmonics accounts for more than 95% of the seasonal variability. Amplitudes of SST and AT annual cycles are highest near the western boundaries of the oceans; annual phases of SST and AT increase toward the eastern tropical oceans, revealing a southeastern propagation of the annual cycle over the Northern Hemisphere oceans. The annual cycle of AT leads that of SST by 1-3 weeks. The largest phase differences are observed in the regions of western boundary currents in the North Pacific and the North Atlantic oceans. This is consistent with spatial patterns of integral air-sea heat fluxes. Annual phases of SST increase along the Gulf Stream and the Kuroshio Current. This points to the importance of signal transport by the major ocean currents. The lowest annual amplitudes of SLP are observed along the equator (0�- 10�N) in both oceans. There are three distinct areas of high annual amplitudes of SLP in the North Pacific Ocean: Asian, Aleutian and Californian. Unlike the North Pacific, only one such area exists in the North Atlantic centred to the west of Iceland. A remarkable feature in the climate of the North Pacific is a maximum of semiannual SLP amplitudes, centred near 40�N and 170�W. It is also an absolute maximum in the entire Northern Hemisphere. Analysis of phases of harmonics of SLP seasonal cycle has revealed the trajectories of propagation of the annual and semiannual cycles. Analysis of semiannual to annual amplitudes ratio has revealed the regions of semiannual cycle dominance. Copyright � 2001 Royal Meteorological Society

Internal tides in the Northern Gulf of California

The characteristics of the internal tide in the Northern Gulf of California are described using data from two moored arrays of temperature and current sensors, one for summer and one for winter, located between Angel de la Guarda Island and the mainland. From the summer six-sensor mooring it was found that: (1) the current fluctuations are dominated by the semidiurnal frequency band, while the quarterdiurnal frequency dominated the temperature fluctuations. (2) The baroclinic semidiurnal horizontal current fluctuations are aligned with the gulf axis, and have amplitudes of 10-15 cm s-1; the vertical displacements reached 4 m in this frequency band. (3) The vertical modal structure for the temperature and velocity oscillations was dominated by the first and third modes. (4) The energy of the semidiurnal internal tide is 45% of that of the barotropic tide. (5) Vertical wave number spectra showed slightly asymmetric peaks in the high wave number components, indicating that their downflowing energy is larger than that flowing upward. From the winter two-sensor mooring, it was found that the vertical oscillations were mainly semidiurnal, with root mean square amplitudes of 7 m