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Lee-Lueng Fu

Abstract

The Argentine Basin of the South Atlantic Ocean is a region of complicated ocean dynamics involving a wide range of spatial and temporal scales. Previous studies reported the existence of a basin mode of topographic barotropic Rossby waves with a period close to 25 days in the region. Using observations of sea level anomalies from satellite altimeter measurements, the present study provides evidence of interaction between the large-scale 25-day waves and the energetic mesoscale variability of the region. The amplitude of the 25-day waves is highly intermittent with dominant periods in the range of 110–150 days. Within this period band, the wave amplitude is coherent with the energy level of the mesoscale variability: when the mesoscale energy level goes down, the wave amplitude goes up, and vice versa, suggesting an exchange of energy between the two scales. This coherence is linked to the first three empirical orthogonal functions (EOFs) of the sea level anomalies. The spatial patterns of these EOFs are characterized by eddies and meanders associated with the Brazil–Malvinas Confluence. The findings of the study suggest a mechanism of energy exchange at work between the mesoscale variability and the large-scale waves in the Argentine Basin.

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Lee-Lueng Fu

Abstract

The forcing of the equatorial Indian Ocean by the highly periodic monsoon wind cycle creates many interesting intraseasonal variabilities. The frequency spectrum of the wind stress observations from the European Remote Sensing Satellite scatterometers reveals peaks at the seasonal cycle and its higher harmonics at 180, 120, 90, and 75 days. The observations of sea surface height (SSH) from the Jason and Ocean Topography Experiment (TOPEX)/Poseidon radar altimeters are analyzed to study the ocean’s response. The focus of the study is on the intraseasonal periods shorter than the annual period. The semiannual SSH variability is characterized by a basin mode involving Rossby waves and Kelvin waves traveling back and forth in the equatorial Indian Ocean between 10°S and 10°N. However, the interference of these waves with each other masks the appearance of individual Kelvin and Rossby waves, leading to a nodal point (amphidrome) of phase propagation on the equator at the center of the basin. The characteristics of the mode correspond to a resonance of the basin according to theoretical models. For the semiannual period and the size of the basin, the resonance involves the second baroclinic vertical mode of the ocean. The theory also calls for similar modes at 90 and 60 days. These modes are found only in the eastern part of the basin, where the wind forcing at these periods is primarily located. The western parts of the theoretical modal patterns are not observed, probably because of the lack of wind forcing. There is also similar SSH variability at 120 and 75 days. The 120-day variability, with spatial patterns resembling the semiannual mode, is close to a resonance involving the first baroclinic vertical mode. The 75-day variability, although not a resonant basin mode in theory, exhibits properties similar to the 60- and 90-day variabilities with energy confined to the eastern basin, where the SSH variability seems in resonance with the local wind forcing. The time it takes an oceanic signal to travel eastward as Kelvin waves from the forcing location along the equator and back as Rossby waves off the equator roughly corresponds to the period of the wind forcing. The SSH variability at 60–90 days is coherent with sea surface temperature (SST) with a near-zero phase difference, showing the effects of the time-varying thermocline depth on SST, which may affect the wind in an ocean–atmosphere coupled process governing the intraseasonal variability.

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Lee-Lueng Fu

Abstract

Six hydrographic sections were used to examine the circulation and property fluxes in the South Indian Ocean from 10° to 32°S. The calculations were made by applying an inverse method to the data. In the interior of the South Indian Ocean, the geostrophic flow is generally northward. At 18°S, the northward interior mass flux is balanced by the southward Ekman mass flux at the surface, whereas at 32°S the northward interior mass flux is balanced by the southward mass flux of the Agulhas Current. There is a weak, southward mass flux of 6 × 109 kg s−1 in the Mozambique Channel. The rate of water exchange between the Pacific Ocean and the Indian Ocean is dependent on the choice of the initial reference level used in the inverse calculation. The choice of 1500 m, the depth of the deep oxygen minimum, has led to a flux of water from the Pacific Ocean to the Indian Ocean at a rate of 6.6 × 109 kg s−1.

Heat flux calculations indicate that the Indian Ocean is exporting heat to the rest of the world's oceans at a rate of −0.69 × 1015 Wat 18°S and −0.25 × 1015 Wat 32°S (negative values being southward).The geostrophic component of the heat flux is dominated by its barotropic component. There is a convergence of freshwater flux in the area between 18° and 32°S, but the magnitudes of the freshwater fluxes are less than hydrological estimates by an order of magnitude. The requirement of a freshwater flux as large as the hydrological estimates drives abnormal horizontal and vertical mass fluxes.

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Lee-Lueng Fu

Abstract

Using hydrographic data of the IGY and METEOR expeditions, the inverse method of Wunsch (1978) has been applied to the subtropical South Atlantic Ocean to determine its general circulation and meridional heat transport. The method is based on the conservation of mass and salt in a number of isopycnal layers. Results of the meridional circulation of the two data sets are pretty much the same: a northward transport (∼20 × 106 metric tons per second) of waters above the North Atlantic Deep Water (NADW) and a comparable southward transport of deep waters. Details of the horizontal circulation of various water masses can be quite different between the two data sets; nonetheless, some gross common features have been found: the northward transport of the Surface Water is basically carried by the Benguela Current, the South Equatorial Current and the North Brazilian Coastal Current. The Antarctic Intermediate Water is carried northward by the Benguela Current as opposed to flowing all the way northward along the South American Coast. The southward flowing NADW is deflected from the South American Coast into the mid ocean by a seamount chain near 20°S. There is no significant net meridional transport of waters below the NADW in this region.

The computed total heat transport (geostrophic plus Ekman) is equatorward with a magnitude of about 0.8 × 1015 W near 30°S and indistinguishable from zero near 8°S. Forcing the total heat transport across 30°S to be poleward would result in an unrealistic circulation scheme.

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Lee-Lueng Fu

Abstract

Seven years' worth of sea level observations from the TOPEX/Poseidon altimeters and wind observations from the European Remote-Sensing Satellites (ERS-1/2) scatterometers were used to investigate the dynamics of large-scale intraseasonal sea level variability at mid- and high latitudes. Coherent patterns of sea level variability with spatial scales of 1000 km and timescales of 20 days to 1 year are identified in three particular regions: the Bellingshausen Basin west of Drake Passage, the Australian–Antarctic Basin, and the central North Pacific Ocean (30°–50°N) near the date line. Significant coherence is found between wind stress curl and the sea level variability. A simple barotropic vorticity equation, in which the time rate of relative vorticity variation is balanced by the wind stress curl and a damping term, is used to simulate the sea level anomalies with a significant degree of correlation with the observations. Although the coherence between the simulation and the observation is significant from periods of 30 days to 1 yr, the variance of the simulation is too low at periods shorter than 100 days. This is probably caused by the errors in the wind forcing as well as the other terms neglected in the vorticity equation. The simulation requires a damping timescale generally longer than 20 days, consistent with theoretical estimates of the dissipation timescales of a frictional bottom boundary layer. In the Bellingshausen Basin, the phase of the coherence between sea level and wind stress curl also shows dependence on frequency according to the dissipation mechanism and estimated damping timescales. However, the results at the other two locations are less conclusive.

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Lee-Lueng Fu

Abstract

In the open ocean away from the equator, westward propagation is a ubiquitous characteristic of oceanic variability. The objectives of the study are to investigate the latitudinal dependence of the frequency of westward-propagating variability. Two-dimensional Fourier analysis in frequency and zonal wavenumber is applied to time–longitude records of sea surface height data obtained from the Ocean Topography Experiment (TOPEX)/ Poseidon mission. The focus of the study is placed on spatial scales larger than the mesoscale. The frequency of westward propagation is generally lower than the critical frequency of the first-mode baroclinic Rossby waves, as expected from the conventional theory of Rossby waves in a midlatitude ocean. However, westward propagation with frequency of up to 2 times the critical frequency is also observed at most latitudes. This supercritical propagation can be explained by the effects of the vertical shear of the mean flow at midlatitudes and by the effects of the equatorial wave guide at the tropical latitudes. Westward propagation with frequency much higher than the critical frequency (by a factor of 5–10) is also observed at certain latitudes in all oceans. The most energetic cases are found along the latitudes of strong zonal jets, including the Brazil/Malvinas Confluence, the Agulhas Return Current, and the Gulf Stream Extension, with decreasing variance in the order. The high-frequency westward propagation exhibits the frequency and wavenumber characteristics of barotropic Rossby waves.

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Yongsheng Xu and Lee-Lueng Fu

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The wavenumber spectra of sea surface height from satellite altimeter observations have revealed complex spatial variability that cannot be explained by a universal theory of mesoscale turbulence. Near the edge of the core regions of high eddy energy, agreement is observed with the prediction of the surface quasigeostrophic (SQG) turbulence theory, which has fundamental differences from that of the traditional quasigeostrophic (QG) turbulence theory. In the core regions of high eddy energy, the spectra are consistent with frontogenesis that is not fully accounted for by the SQG theory. However, the observations in the vast ocean interior of low eddy energy exhibit substantial differences from the predictions of existing theories of oceanic mesoscale turbulence. The spectra in these regions may reflect the ocean’s response to short-scale atmospheric forcing and air–sea interaction. The observations presented in this paper serve as a test bed for new theories and ocean general circulation models.

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Lee-Lueng Fu and Jorge Vazquez

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Radial orbit error, defined as the uncertainty in the geocentric altitude of a satellite, is the dominant error in the measurement of sea surface height by a satellite altimeter. Apart from a geographically dependent component (a function of latitude and longitude only), this error has been proven to be recoverable by analyzing the difference in altimetric measurement of sea surface height at ground track intersections (crossover differences). An effective approach to the problem is to model orbit error in terms of a Fourier series with the Fourier coefficients determined by minimizing the residual crossover difference in a least-square sense. The solution of the coefficients, however, is known to be singular due to the presence of geographically dependent orbit errors that cause no crossover differences. To obtain a unique solution, an a priori constraint is usually imposed (e.g., conforming the resulting ocean topography to a geoid model). In this paper we demonstrate that the problem can be solved by the use of singular value decomposition, without the need for any a priori constraint. The essence of the method is to leave the geographically dependent errors unchanged and make only those corrections that are warranted by the information contained in crossover differences, thus leaving the resultant ocean topography free from any undue distortion that may otherwise be incurred by an a priori constraint. The method will be particularly useful for application to high-accuracy altimetric missions such as Topex/Posceidon, because the orbit error can be reduced without compromising the accuracy of the measured mean ocean topography.

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Yongsheng Xu and Lee-Lueng Fu

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The wavenumber spectrum of sea surface height (SSH) observed by satellite altimetry was analyzed by Xu and Fu. The spectral shape in the wavelength range of 70–250 km was approximated by a power law, representing a regime governed by geostrophic turbulence theories. The effects of altimeter instrument noise were assumed insignificant at wavelengths longer than 70 km. The authors reexamined the assumption in the study. Using nearly simultaneous observations made by Jason-1 and Jason-2 during their cross-calibration phase, this study found that the white noise level of altimetry measurement was best estimated from the spectral values at wavelengths from 25 to 35 km. After removing a white noise level based on such estimate from the SSH spectrum, the spectral slope values changed significantly over most of the oceans. A key finding is that the spectral slopes are generally steeper than k −2 (k is wavenumber) poleward of the 20° latitudes, where flatter spectral slopes in some regions have previously caused problems for dynamic interpretations. The new results indicate that the spectral slopes in the core regions of the major ocean current systems have values between the original geostrophic turbulence theory and the surface quasigeostrophic theory. The near k −4 spectrum suggests that the sea surface height variability at these wavelengths in the high eddy energy regions might be governed by frontogenesis.

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Lee-Lueng Fu and Clement Ubelmann

Abstract

Conventional radar altimeter makes measurement of sea surface height (SSH) in one-dimensional profiles along the ground tracks of a satellite. Such profiles are combined via various mapping techniques to construct two-dimensional SSH maps, providing a valuable data record over the past two decades for studying the global ocean circulation and sea level change. However, the spatial resolution of the SSH is limited by both coarse sampling across the satellite tracks and the instrument error in the profile measurements. A new satellite mission based on radar interferometry offers the capability of making high-resolution wide-swath measurement of SSH. This mission is called Surface Water and Ocean Topography (SWOT), which will demonstrate the application of swath altimeter to both oceanography and land hydrology. This paper presents a brief introduction to the design of SWOT, its performance specification for SSH, and the anticipated spatial resolution and coverage, demonstrating the promise of SWOT for fundamental advancement in observing SSH. A main objective of the paper is to address issues in the anticipated transition of conventional profile altimetry to swath altimetry in the future—in particular, the need for consistency of the new observing system with the old for extending the existing data record into the future. A viable approach is to carry a profile altimeter in the SWOT payload to provide calibration and validation of the new measurement against the old at large scales. This is the baseline design of SWOT. The unique advantages of the approach are discussed in the context of a new standard for observing the global SSH in the future.

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