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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

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

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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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
and
Jorge Vazquez

Abstract

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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Jinbo Wang
and
Lee-Lueng Fu

Abstract

The Surface Water and Ocean Topography (SWOT) mission will measure the sea surface height (SSH) using a Ka-band radar interferometer (KaRIn) over a swath off the nadir of the satellite tracks. The mission requires calibration and validation (CalVal) of the SSH wavenumber spectrum at wavelengths between 15 and 1000 km. The CalVal in the short-wavelength range (15–150 km) requires in situ observations. In the long-wavelength range (150–1000 km), the CalVal will use the onboard Jason-class nadir altimeter. Using a high-resolution global ocean simulation, this study identifies the spatial scales beyond which the nadir and off-nadir observations can be considered comparable. Our results suggest that the ocean signals at nadir can represent off-nadir ocean signals at wavelengths longer than 120 and 70 km along the midswath and the inner edge of the KaRIn grid, respectively, indicating that the nadir altimeter is able to fulfill its goal to validate the long-wavelength KaRIn measurement. The wavelength along the inner edge is limited around 70 km because the onboard nadir altimeter cannot resolve spatial scales longer than ~70 km. These wavelengths provide a reference point for the required spatial coverage of the SWOT SSH in situ CalVal.

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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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Lee-Lueng Fu
and
Richard D. Smith

The sea surface elevation relative to the geoid, a dynamic boundary condition for the three-dimensional oceanic pressure field, is being determined over the global ocean every 10 days by a precision radar altimeter aboard the TOPEX/POSEIDON satellite. This is the most accurate altimeter data stream to date for the study of the ocean general circulation and its variability. The authors compare results from 2 years (October 1992–October 1994) of the satellite observations to computer simulations for the same period using a state-of-the-art ocean general circulation model driven by realistic winds from an atmospheric weather-prediction model. The average horizontal resolution of the model is 1/5° (varying from 30 km at the equator to 6 km at the polar latitudes), the highest for a global simulation performed to date. Comparisons of the mean circulation, the mesoscale variability, the amplitude, and phase of the annual cycle, as well as intraseasonal and interannual changes show that the simulations and observations agree fairly well over a broad range of temporal and spatial scales. However, the sea level variance produced by the model is generally less than the observation by a factor of 2, primarily in the eddy-rich regions. Comparison of wavenumber spectra indicates that even higher spatial resolution is needed to fully resolve the mesoscale eddies. The absolute dynamic topography determined from either the data or the model has an error on the order of 10 cm at wavelengths larger than 2000 km. Geoid errors are the limiting factor for the utility of the altimeter data at smaller scales. Heat flux forcing is a key factor in determining the simulated annual cycle of the ocean. Improved agreement with the observation is achieved when the model is driven by heat flux forcing instead of being nudged to sea surface temperature climatology. The temporal evolution of the intraseasonal fluctuations at mid- and high latitudes as well as the interannual variability of the tropical oceans, both are primarily wind driven, is simulated fairly well by the model.

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