72 research outputs found
Using Acoustic Travel Time to Determine Dynamic Height Variations in the North Atlantic Ocean
There is often an approximately linear relationship between various water-column integrals, in particular between surface dynamic height anomaly ÎD and acoustic round-trip travel time Ï. Consequently, the record from an inverted echo sounder, which measures Ï, can be interpreted in terms of ÎD. Nevertheless, the slope m of this linear relation is not everywhere well defined, and varies from place to place. This study seeks to establish where, in the extratropical North Atlantic, one can reasonably assume a linear relation between ÎD and Ï, and for these regions compute m. Using climatological atlas data and historic hydrographic data, it is shown that a well-defined, linear relation exists between ÎD and Ï in a region centered on the Gulf Stream and extending from the northern Sargasso Sea almost to Ireland. Where m is well defined, it is negative, and its value is usually similar to that associated with first-baroclinic-mode excitation. Its magnitude generally decreases with increasing latitude. The value of m typically ranges from â40 dyn m sâ1 in the northern Sargasso Sea to â20 dyn m sâ1 in the North Atlantic Current. In the Gulf Stream it is typically between â30 and â35 dyn m sâ1
Simultaneous pressure, velocity and temperature measurements in the Florida Straits
We present a descriptive picture of the variability in the Florida Current as measured by a large number of current meters, temperature sensors, and bottom mounted pressure sensors in the period March-August, 1974. Because of the very high velocities, only measurements made in the near-bottom region were possible. The tidal regime is found to be somewhat more complex than postulated from earlier measurements...
Bottom Temperatures Related to Gulf Stream Displacement off the Southeast United States Shelf
Bottom temperature time series recorded beneath the Gulf Stream at 265 and 589 m depth off the Georgia coast are compared with simultaneous time series of main thermocline depth determined from inverted echo sounder and bottom pressure gauge records at the same sites. Bottom temperature is found to be coherent with vertical displacement of the thermocline, suggesting that bottom temperature under the Gulf Stream front is a potentially useful indicator of Gulf Stream displacement. Additional evidence is provided by the similarity of bottom temperature and thermocline depth coherences with longshore current at the shelf break. Bottom temperature at the deeper station appears to be the better indicator of Gulf Stream meandering for periods longer than five days
Model results of flow instabilities in the tropical Pacific Ocean
A twoâandâaâhalfâlayer model of the tropical Pacific Ocean is used to investigate the energy source for the intraseasonal dynamicâheight variability observed near 6°N. A simulation of equatorial circulation is produced by forcing the model with meanâmonthly windâstress climatology. Two westwardâpropagating waves appear in the upper layer in the central and eastern portion of the model basin. These two waves are distinguished by period and meridional structure. An offâequatorial wave with period of 30 days and wavelength of 1100 km has a meridional seaâlevel maximum near 6°N similar to that of the 30â50 day intraseasonal wave observed in the ocean. The meridional velocity signal also is asymmetric with respect to the equator, with maximum near 4°N. The second wave with period of 15 days has a strong meridional velocity signal centered on the equator. The seaâlevel and zonal velocity signals associated with this equatorial wave have maxima near 1.5°N and 1.5°S. The eddyâenergy budget reveals strong conversions from the meanâflow to eddy field through baroclinic and upperâlayer barotropic conversion terms. Conversion terms north of the equator exhibit a bimodal structure: one maximum between the equator and 3°N is dominated by upperâlayer barotropic conversion spatially coincident with the cyclonic shear along the equatorward edge of the South Equatorial Current (SEC), and a second smaller maximum between 3°N and 5°N is a combination of upperâlayer barotropic conversion along the poleward edge of the SEC (anticyclonic shear) and baroclinic conversion near the core of the SEC. The two peaks in the conversion terms, combined with similar structure in the fluxâdivergence terms in the model eddyâenergy budget, provide evidence that two wave processes are generated at the different source regions: one near the equator and a second between 2°N and 5°N
Using modern time series analysis techniques to predict ENSO events from the SOI time series
We analyze the monthly 1866â2000 Southern Oscillation Index (SOI) data to determine: whether the SOI data are sufficiently noise-free that useful predictions can be made from them, and in particular, whether future ENSO events can be predicted from the SOI data.
The âHilbert-EMDâ technique is used to aid the analysis. This new frequency-time algorithm, based on the Hilbert transform, may be applied to time series for which the conventional assumptions of linearity and stationarity may not apply.
With the aid of the EMD procedure, a cleaner representation of ENSO dynamics is obtained from the SOI data. A polynomial function is then used to predict SOI values. Using only the data from January 1866 through December 1996, this prediction correctly indicated a warm event in 1997â1998 and a cold event in 1999. Using all the data (through December 2000), this prediction shows no strong ENSO events (positive or negative) during the time period January 2001 through December 2004
Near bottom speed and temperature observations on the Blake-Bahama outer ridge
Speed and temperature measurement made in the bottom boundary layer (BBL) in the region of the Western Boundary Undercurrent at 28°22âČN, 74° 13âČW over an âŒ11 day period are presented. The observations suggest that the BBL structure is consistent with that of a turbulent Ekman layer formed in an initially stably stratified fluid over a uniform surface even though they were obtained in and above an abyssal furrow. The inferred friction velocities u*(Ć«* = 0.66 cm/s) generally are larger than those inferred by Weatherly (1972) under the Florida Current and at times sufficiently large to result in erosion of some of the finer cohesive sediments if the criterion for their erosion summarized in McCave (1978) is assumed to apply at the site of the observations
WindâCurrent Coupling on the Southern Flank of Georges Bank: Variation with Season and Frequency
Comparison of several years of current observations on the southern flank of Georges Bank with nearby wind data shows that the windâcurrent coupling is primarily between longshelf wind stress and longshelf current. The strongest windâcurrent coupling occurs in winter, when the water column is homogeneous. The weakest coupling is in late summer and early fall, when the water column is highly stratified. The coherence and transfer coefficient between longshelf wind and longshelf current is highest for periods between 4 and 12 days, decreasing both for longer periods (out to 56 days) and shorter periods (down to 2 days). Models of the windâcurrent coupling indicate that a highly damped resonance may exist on Georges Bank and that a smaller current response is expected when the water column is stratified. The observations also indicate that the wind-driven currents on Georges Bank are strongly controlled by friction. The near-surface current moves to the right of wind stress and there is a springâneap modulation of the windâcurrent transfer coefficient caused by the modulation of the bottom stress associated with the springâneap tidal cycle. The longshelf current is linearly related to wind stress and responds almost symmetrically to wind forcing
Ispravak u radu: âPlimne oscilacije u sjevernom Jadranu: opaĆŸanja, modeliranje varijacijskom asimilacijom podataka i linearna plimna dinamikaâ
A precision/round-off error has been discovered in the tidal analysis routines used in the paper âNorth Adriatic tides: observations, variational data assimilation modeling, and linear tide dynamicsâ by J. W. Book, H. Perkins, and M. Wimbush (2009, Geofizika, 26, 115â143). Tidal elevation phases for 12 of the 15 stations are, on average, too low by 3.9° for the diurnal constituents and 7.9° for the semidiurnal constituents in Tabs. 4 and 5. These tables have been corrected and are republished here.
The error also had an effect on the input data used for the linear variational data assimilation model, and combined with a nearest neighbor interpolation scheme produced an approximate 15 minute forward shift in time for 6 of the 43 synthesized tidal records. The error produced final model solutions that had tidal elevation phases 3.5° too high for M2, 1.6° too high for K1, and similar matching phase shifts for other semidiurnal and diurnal constituents.
The errors in the input data have been corrected, the interpolation scheme has been changed to a piecewise cubic spline method, and the model runs have all been redone. The new model results suggest a minor change in optimal friction parameter, which in turn alters model Q factors and dissipation. However, the original finding that these values are not well determined by this methodology remains true. Model and observational results originally shown in Figs. 5â7 and Figs. 13â14 have slightly changed and are republished here. The main conclusions from the original work regarding Kelvin waves and TRW dynamics for the North Adriatic basin remain unaltered by these corrections.Otkrivena je pogreĆĄka zaokruĆŸivanja u rutinama za plimnu analizu u radu J. W. Book, H. Perkins i M. Wimbush: âPlimne oscilacije u sjevernom Jadranu: opaĆŸanja, modeliranje varijacijskom asimilacijom podataka i linearna plimna dinamikaâ (Geofizika, 26, 2009, 115â143). Faze plimnih denivelacija, dane u tablicama 4. i 5., za 12 od 15 postaja podcijenjene su u prosjeku 3,9° za dnevne komponente te 7,9° za poludnevne komponente. Ovdje dajemo tablice s ispravljenim vrijednostima. GreĆĄka je utjecala na ulazne podatke koji su koriĆĄteni u linearnom modelu za varijacijsku asimilaciju podataka te je, u kombinaciji s interpolacijskom shemom najbliĆŸeg susjeda, uzrokovala vremenski pomak unaprijed od pribliĆŸno 15 minuta za 6 od ukupno 43 sintetizirana plimna zapisa. GreĆĄka je proizvela konaÄna modelska rjeĆĄenja koja su precijenila faze plimnih denivelacija za 3,5° za M2 komponentu, 1,6° za K1 komponentu, te fazne pomake sliÄnih iznosa kod drugih poludnevnih i dnevnih komponenti. GreĆĄke u ulaznim podacima su ispravljene, interpolacijska shema je promijenjena tako da koristi po dijelovima kubne spline-ove te su ponovno provedeni modelski raÄuni. Novi rezultati modela sugeriraju malu promjenu u optimalnom parametru trenja, koji dalje mijenja Q faktore modela i disipaciju. MeÄutim izvorni nalaz, da te vrijednosti nisu dobro odreÄene ovom metodologijom, ostaje nepromijenjen. Rezultati modela i opaĆŸanja, izvorno prikazani na slikama 5.â7. i 13.â14., malo su se promijenili i ovdje su nanovo prikazani. Glavni zakljuÄci iz izvornoga rada u vezi s Kelvinovim valovima i TRW dinamikom u sjevernom Jadranu nakon ovih ispravki ostaju isti
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