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Refereed Publications 1991
Camargo, S. J. and I. L. Caldas, 1991: Average Magnetic-Surfaces in Tokamaks. Plasma Physics and Controlled Fusion, 33(6): 573-581.
An average invariant which describes average magnetic surfaces for a system without symmetry is obtained. The system is a Tokamak toroidal equilibrium perturbed by resonant helical windings. The magnetic field is a superposition of the magnetic fields of the equilibrium and the helical windings, and the corresponding vector potential is determined. An average vector potential is defined to obtain the average invariant. Analysis of the average surfaces showed that the magnetic islands move towards the plasma centre and decrease in width as the pressure increases. Dupenhoat, Y. and M. A. Cane, 1991: Effect of Low-Latitude Western Boundary Gaps on the Reflection of Equatorial Motions. Journal of Geophysical Research-Oceans, 96: 3307-3322.
The western tropical Pacific is thought to be an important zone for generating El Niño: reflections at the boundary make it a source region of equatorial Kelvin waves. Calculations of the effect of a gappy western boundary on the reflection process are carried out in the framework of the low-frequency limit of the shallow-water equations and thus are highly idealized. The method is also applied to a schematic version of the flow through the Indonesian seas from the western Pacific to the Indian Oceans. The results indicate some strong sensitivities to the location of the gap and to the structure of the incoming flows. In addition, the results can be quite different, depending on whether the zonal extend of the gap is assumed to be infinite or finite. (More precisely, the latter means that the extend of the gap is short compared with the zonal wavelength of the relevant free waves at that frequency.) In view of the complexity of the results for even such a simplified model, it will be very difficult to be confident of any modeling study of the Indonesian throughflow short of a highly resolved numerical calculation with a detailed representation of the geometry and bathymetry. Nonetheless, we offer tentative conclusions concerning the efficiency of the western Pacific boundary as a reflector. Our results suggest that the realistic boundary will not greatly alter expectations based on a simple solid boundary if the reflections important for El Niño are primarily in motion, represented by low-order Rossby modes. This is also consistent with observational evidence indicating no anomalous throughflow during El Niño events. Gordon, A. L. and K. T. Bosley, 1991: Cyclonic Gyre in the Tropical South-Atlantic. Deep-Sea Research Part A-Oceanographic Research Papers, 38: S323-S343.
A cyclonic gyre within the eastern tropical South Atlantic is resolved by an extensive oceanographic station array obtained in 1983 and 1984. The gyre is centered near 13-degrees-S and 5-degrees-E with a sea surface relief relative to 1500 decibars (db) of 8 dyn cm. The 500 db surface relative to 1500 db reveals a much diminished cyclonic circulation, shifted slightly to the south. The weak baroclinic expression of the cyclonic gyre is confined for the most part to the upper 300 db, with a surface characteristic speed of only 3 cm s-1. A transport of 5 x 10(6) m3 s-1 across a line from the gyre center to the African continental margin, including the Angola Current, may best depict the gyre 0-300 db transport (relative to 1500 db). Ship drift data for the region do not show the presence of the cyclonic gyre because the wind-induced Ekman layer masks the gyre. The thermocline of the cyclonic gyre is significantly saltier and lower in oxygen than the suspected source water: the main thermocline of the South Atlantic subtropical gyre. A strong front near 18-degrees-S, the Angola-Benguela Front, separates the cyclonic gyre regime from that of the subtropical gyre. Using the regional freshwater balance, gyre thermocline residence time is determined to be between 4.4 and 8.5 years, implying an oxygen utilization of 0.3-0.5 ml l-1 y-1. Below the thermocline there is some evidence for southward flow of North Atlantic Deep Water along the eastern boundary. It is inferred that this flow is fed by eastward spreading of North Atlantic Deep Water along the equatorial belt. Houghton, R. W., 1991: The Relationship of Sea-Surface Temperature to Thermocline Depth at Annual and Interannual Time Scales in the Tropical Atlantic-Ocean. Journal of Geophysical Research-Oceans, 96(C8): 15173-15185.
Expendable bathythermograph data derived from SEQUAL/FOCAL and TOGA sponsored volunteer observing ship programs in the tropical Atlantic Ocean are used to study the relationship between the sea surface temperature (SST) and the underlying thermocline. Time series up to 8 years long allow investigation of both the annual and interannual variability between 20N and 20S along selected shipping lanes. For the annual harmonic a warm SST is correlated with a shallow thermocline everywhere except in the eastern equatorial region. Near the intertropical convergence zone (ITCZ) the 180-degrees phase shift in the annual harmonic of both the SST and the thermocline depth occurs at slightly different latitudes. Over most of the tropical Atlantic the annual cycle of SST is linked to the phase of the net surface heat flux and only in the eastern equatorial and coastal upwelling regions does heat flux across the thermocline become a significant factor for the annual cycle of the SST. Canonical correlation analysis of the interannual variability reveal two distinct modes with structures similar to the annual cycle. The first, in which a warm SST is correlated with a shallow thermocline, is a dipole with centers of maximum amplitude off the equator at approximately 8S at 12N and a node at the ITCZ. The second mode, for which a warm SST is correlated with a deep thermocline, peaks at the equator. The predominance of the first mode suggests that for most of the tropical Atlantic Ocean the dynamical state beneath the surface mixed layer has only a limited influence on SST fluctuations. Munnich, M., M. A. Cane and S. E. Zebiak, 1991: A Study of Self-Excited Oscillations of the Tropical Ocean Atmosphere System .2. Nonlinear Cases. Journal of the Atmospheric Sciences, 48(10): 1238-1248.
We study the behavior of an iterative map as a model for El Nino and the Southern Oscillation (ENSO). This map is derived from a model that combines linear equatorial beta-plane ocean dynamics with a version of the Bjerknes hypothesis for ENSO. It differs from the linear model of Cane et al. only in that the coupling from ocean to atmosphere is idealized as a nonlinear relation tau-(h(e)) between a wind stress tau of fixed spatial form and h(e), the thermocline displacement at the eastern end of the equator. The model sustains finite amplitude periodic and aperiodic oscillations. A period doubling bifurcation leads from a period of less than 2 years to the 3-4 year one observed in nature. Other principal results are: the resulting period depends on the curvature of the function away from the unstable equilibrium at h(e) = 0, and not solely on its linear instability; at least two Rossby modes must be included in the model for aperiodic oscillations to appear; no stochastic term is needed for this aperiodicity, but it appears more readily if the model background state includes an annual cycle. Ou, H. W., 1991: Some Effects of a Seamount on Oceanic Flows. Journal of Physical Oceanography, 21(12): 1835-1845.
To demonstrate some effects of a seamount on oceanic flows, we have considered a uniform, two-layer flow passing a right circular cylinder of arbitrary height in a rotating fluid. In the case of vanishing stratification, we first generalize previous results of low obstacles to an obstacle of finite height, and then show how the frictional regime provides a transition from partial to total blocking as the obstacle top approaches the surface. Seager, R., 1991: A Simple-Model of the Climatology and Variability of the Low-Level Wind-Field in the Tropics. Journal of Climate, 4(2): 164-179.
A simple model of the low-level wind field in the entire tropics is presented. The dynamics are the same as those within the familiar Gill model, i.e., linear, steady state, contained within a single vertical mode and damped by Rayleigh friction. Convective atmospheric heating can occur if a lifted air parcel is buoyant relative to its surroundings, and the heating is computed with reference to the cloud model of Yanai et al. Radiative cooling is represented by a Newtonian cooling to an equilibrium lapse rate. The model is forced by surface temperature and humidity. A qualitatively correct representation of the climatological flow is achieved. The main differences between model and observations relate to the model's inability to reproduce the intensity and limited spatial scale of the convergence zones. Model simulations of anomalous circulations are subject to the same limitations. Problems related to the lack of an explicit boundary layer in the model, the poor representation of radiation, and the cumulus parameterization are discussed, together with suggestions for future work. Ting, M. F., 1991: The Stationary Wave Response to a Midlatitude SST Anomaly in an Idealized GCM. Journal of the Atmospheric Sciences, 48(10): 1249-1275.
The atmospheric stationary wave response to a midlatitude sea surface temperature (SST) anomaly is examined with an idealized general circulation model (GCM) as well as steady linear model, in a similar way as Ting and Held, for a tropical SST anomaly. The control climate of the GCM is zonally symmetric; this symmetric climate is then perturbed by a monopole SST anomaly centered at 40-degrees-N. The database was updated today. |