Camargo, S. J. and H. Tasso, 1992: Renormalization-Group in Magnetohydrodynamic Turbulence. Physics of Fluids B-Plasma Physics, 4(5): 1199-1212.
The renormalization group (RNG) theory is applied to magnetohydrodynamic (MHD) equations written in Elsasser variables, as done by Yakhot and Orszag for Navier-Stokes equations. As a result, a system of coupled nonlinear differential equations for the "effective" or turbulent "viscosities" is obtained. Without solving this system, it is possible to prove their exponential behavior at the "fixed point" and also determine the effective viscosity and resistivity. Strictly speaking, the results do not allow negative effective viscosity or resistivity, but in certain cases the effective resistivity can be continued to negative values, but not the effective viscosity. In other cases, the system tends to zero effective viscosity or resistivity. The range of possible values of the turbulent Prandtl number is also determined; the system tends to different values of this number, depending on the initial values of the viscosity and resistivity and the way the system is excited.
Clement, B. M. and D. G. Martinson, 1992: A Quantitative Comparison of 2 Paleomagnetic Records of the Cobb Mountain Subchron from North-Atlantic Deep-Sea Sediments. Journal of Geophysical Research-Solid Earth, 97(B2): 1735-1752.
We present a new paleomagnetic record of the Cobb Mountain Subchron obtained from deep-sea sediments cored at Ocean Drilling Program site 647 in the southern Labrador Sea. The details of the transitional field behavior documented by this record appear to be very similar to those recorded in a previously published record of this subchron obtained at Deep-Sea Drilling Project (DSDP) site 609 in the North Atlantic (Clement and Kent, 1987). We used a quantitative correlation technique (Martinson et al., 1982) to establish statistically the degree of similarity between these two records and thereby constrain the spatial variability in these transitional fields. The error in the alignments is reduced significantly by aligning records of virtual geomagnetic pole positions rather than directions, indicating that these records document such large scale changes in the fields that we can not distinguish them from dipolar changes, given the proximity of these two sites. These replicate records of the Cobb Mountain Subchron provide evidence that deep-sea sediments are capable of providing high resolution records of geomagnetic field behavior. A reexamination of the sequence of polarity transitions recorded at DSDP site 609 in light of these results suggests the presence of two preferred transitional field configurations. The field appears to change from one configuration to the other for several reversals and then back to the original configuration, suggesting that a geographical influence on the reversal process persists through this sequence. The variability in these reversal records provides insights into the response of the geodynamo to this geographical influence.
Ffield, A. and A. L. Gordon, 1992: Vertical Mixing in the Indonesian Thermocline. Journal of Physical Oceanography, 22(2): 184-195.
Western Pacific central and tropical waters characterized by a subsurface salinity maximum spread.into the Indonesian seas as part of the Indonesian throughflow. Within the Indonesian seas this salinity maximum is attenuated and, in some places, completely removed. A simple advection-diffusion model verifies the importance of vertical mixing in the transformation of Western Pacific waters to Indonesian thermocline water. The profiles indicate a predominant North Pacific presence in most of the seas, although some South Pacific water is present in the eastern seas of Halmahera, Seram, and Banda. The main interocean route is through the western seas of Sulawesi, Makassar, and Flores, while the flow pathway in the eastern seas is less certain. The Banda Sea can be renewed from either the northern passages (Halmahera and Maluku) or from the south via the Flores Sea. Using representative basin property profiles derived from the archived data allows determination of a range of vertical diffusivities and residence times that best reproduce the transformation of Pacific waters into Indonesian water. In the Makassar thermocline a lower limit of 1 x 10(-4) m2 s-1 for vertical diffusivity is inferred from the model results with reasonable throughflow and precipitation values. This estimate is roughly an order of magnitude greater than those deduced for the interior oceanic thermocline in an environment not conducive to salt fingers. In the Banda Sea a K(z) of 1 x 10(-4) m2 s-1 implies a predominant North Pacific source. If K(z) is higher, then a larger South Pacific presence is possible.
Gordon, A. L., R. F. Weiss, W. M. Smethie and M. J. Warner, 1992: Thermocline and Intermediate Water Communication between the South-Atlantic and Indian Oceans. Journal of Geophysical Research-Oceans, 97(C5): 7223-7240.
A conductivity-temperature-depth and tracer chemistry section in the southeast South Atlantic in December 1989 and January 1990 presents strong evidence that there is a significant interocean exchange of thermocline and intermediate water between the South Atlantic and Indian oceans. Eastward flowing water at 10-degrees-W composed of South Atlantic Central (thermocline) Water is too enriched with chlorofluoromethanes 11 and 12 and oxygen to be the sole source of similar theta-S water within the northward flowing Benguela Current. About two thirds of the Benguela Current thermocline transport is drawn from the Indian Ocean; the rest is South Atlantic water that has folded into the Benguela Current in association with the Agulhas eddy-shedding process. South Atlantic Central water passes in the Indian Ocean by a route to the south of the Agulhas Return Current. The South Atlantic water loops back to the Atlantic within the Indian Ocean, perhaps mostly within the Agulhas recirculation cell of the southwest Indian Ocean. Linkage of Atlantic and Indian Ocean water diminishes with increasing depth; it extends through the lower thermocline into the Antarctic Intermediate Water (AAIW) (about 50% is derived from the Indian Ocean) but not into the deep water. While much of the interocean exchange remains on an approximate horizontal "isopycnal" plane, as much as 10 x 10(6) m3 s-1 of Indian Ocean water within the 25 x 10(6) m3 s-1 Benguela Current, mostly derived from the lower thermocline and AAIW, may balance deeper Atlantic export of North Atlantic Deep Water (NADW). The addition of salt water from the evaporative Indian Ocean into the South Atlantic Ocean thermocline and AAIW levels may precondition the Atlantic for NADW formation. While AAIW seems to be the chief feed for NADW, the bulk of it enters the subtropical South Atlantic, spiked with Indian Ocean salt, within the Benguela Current rather than along the western boundary of the South Atlantic.
Hoerling, M. P., M. L. Blackmon and M. F. Ting, 1992: Simulating the Atmospheric Response to the 1985-87 El Niño Cycle. Journal of Climate, 5(7): 669-682.
The atmospheric response to the evolution of global sea surface temperatures (SSTs) from 1985 to 1987 is studied using the NCAR Community Climate Model (CCM1). Five separate 2-year integrations are performed, and results are presented for the ensemble-averaged response during the pre-El Nino 1985/86 winter and the mature El Nino 1986/87 winter.
Houghton, R. W. and Y. M. Tourre, 1992: Characteristics of Low-Frequency Sea-Surface Temperature-Fluctuations in the Tropical Atlantic. Journal of Climate, 5(7): 765-771.
Sea surface temperature anomalies in the tropical Atlantic Ocean are reexamined to investigate an apparent low-frequency oscillation that has been described as a fluctuating dipole structure with poles north and south of the equator and a node near the ITCZ. Using principal components rotated by the varimax method and simple correlations of area-averaged temperatures, we show that during the 1964-88 interval SST anomalies north and south of the ITCZ are not significantly correlated. Therefore, the low-frequency variation, with an apparent decadal period observed in the SST gradient across the ITCZ during 1964-88, does not arise from temporally coherent and out-of-phase fluctuations in each hemisphere and cannot be characterized as a dipole.
Kushnir, Y. and N. C. Lau, 1992: The General-Circulation Model Response to a North Pacific SST Anomaly - Dependence on Time Scale and Pattern Polarity. Journal of Climate, 5(4): 271-283.
A general circulation model was integrated with perpetual January conditions and prescribed sea surface temperature (SST) anomalies in the North Pacific. A characteristic pattern with a warm region centered northeast of Hawaii and a cold region along the western seaboard of North America was alternately added to and subtracted from the climatological SST field. Long 1350-day runs, as well as short 180-day runs, each starting from different initial conditions, were performed. The results were compared to a control integration with climatological SSTs.
Martinson, D. G. and J. R. Hopper, 1992: Nonlinear Seismic Trace Interpolation. Geophysics, 57(1): 136-145.
The nonlinear correlation technique has been used to guide a seismic trace interpolant to fill gaps in seismic surveys, replace noisy traces, and produce evenly spaced arrays. Given an initial alignment (NMO correction for prestack data and manually inserted correlation lines for post-stack data), the correlation aligns corresponding features between adjacent seismic traces and quantifies the traveltime difference between the traces on a point-for-point basis. This information is used to construct synthetic (interpolated) traces, at any arbitrary distance between the correlated traces, which preserve dip and amplitude changes of the individual reflectors, assuming that such dip and amplitude changes occur linearly (or some other specified functional form) between the correlated traces.
Muench, R. D., B. A. Huber, J. T. Gunn, D. M. Husby and D. G. Mountain, 1992: The Weddell Scotia Marginal Ice-Zone - Physical Oceanographic Conditions, Geographical and Seasonal Variability. Journal of Marine Systems, 3(1-2): 169-182.
Physical oceanographic conditions were measured in the Weddell and Scotia Sea marginal ice zones (MIZ's) during 1983, 1986 and 1988. The field work encompassed spring, autumn and mid-winter periods and included retreating, advancing and steady-state ice edges. Observed upper ocean structures, which typify MIZ's and reflect input of low salinity water from melting ice, included low salinity upper layers, lenses and fronts. An upper mixed layer was always present and was generally more fully developed in autumn and winter than at other times of year. Conditions in the deeper waters reflected regional oceanographic processes and significant differences were present between the Weddell and Scotia seas. Weddell Sea Water is a major source of water for the southern Scotia Sea, however, the upper Scotia Sea was dominated by warmer, less saline waters from Drake Passage. The colder, denser Weddell water appeared to have mixed isopycnally with deeper water in the Scotia Sea, present there at depths exceeding 500 m.
Neelin, J. D., M. Latif, M. A. F. Allaart, M. A. Cane, U. Cubasch, W. L. Gates, P. R. Gent, M. Ghil, C. Gordon, N. C. Lau, C. R. Mechoso, G. A. Meehl, J. M. Oberhuber, S. G. H. Philander, P. S. Schopf, K. R. Sperber, A. Sterl, T. Tokioka, J. Tribbia and S. E. Zebiak, 1992: Tropical Air-Sea Interaction in General-Circulation Models. Climate Dynamics, 7(2): 73-104.
An intercomparison is undertaken of the tropical behavior of 17 coupled ocean-atmosphere models in which at least one component may be termed a general circulation model (GCM). The aim is to provide a taxonomy-a description and rough classification-of behavior across the ensemble of models, focusing on interannual variability. The temporal behavior of the sea surface temperature (SST) field along the equator is presented for each model, SST being chosen as the primary variable for intercomparison due to its crucial role in mediating the coupling and because it is a sensitive indicator of climate drift. A wide variety of possible types of behavior are noted among the models. Models with substantial interannual tropical variability may be roughly classified into cases with propagating SST anomalies and cases in which the SST anomalies develop in place. A number of the models also exhibit significant drift with respect to SST climatology. However, there is not a clear relationship between climate drift and the presence or absence of interannual oscillations. In several cases, the mode of climate drift within the tropical Pacific appears to involve coupled feedback mechanisms similar to those responsible for El Nino variability. Implications for coupled-model development and for climate prediction on seasonal to interannual time scales are discussed. Overall, the results indicate considerable sensitivity of the tropical coupled ocean-atmosphere system and suggest that the simulation of the warm-pool/cold-tongue configuration in the equatorial Pacific represents a challenging test for climate model parameterizations.
Olson, D. B., R. A. Fine and A. L. Gordon, 1992: Convective Modifications of Water Masses in the Agulhas. Deep-Sea Research Part A-Oceanographic Research Papers, 39(1A): S163-S181.
The nature of convective modifications of water masses in the Agulhas Retroflection region is considered in relation to both the temperature and salinity (T/S) properties, and in relation to the gas transfer across the air-sea interface. Data from the 1983 Agulhas Retroflection Cruise (ARC) are used along with satellite sea surface thermal data and a set of models to explore the evolution of water masses in this area. Air-sea interaction over warm pools of water isolated from the Retroflection in rings produces a cold mode water which is found extensively in the eastern South Atlantic. Quantities of a warmer South Indian Subtropical Mode Water are also formed in the Retroflection.
Tasso, H. and S. J. Camargo, 1992: On the Nonlinear Stability of Dissipative Fluids. Nuovo Cimento Della Societa Italiana Di Fisica B-General Physics Relativity Astronomy and Mathematical Physics and Methods, 107(7): 733-740.
A general sufficient condition for nonlinear stability of steady and unsteady flows in hydrodynamics and magnetohydrodynamics is derived. It leads to strong limitations in the Reynolds and magnetic Reynolds numbers. It is applied to the stability of generalized time-dependent planar Couette flows in magneto-hydrodynamics. Reynolds and magnetic Reynolds numbers have to be typically less than 2-pi-2 for stability.
Visbeck, M. and F. Schott, 1992: Analysis of Seasonal Current Variations in the Western Equatorial Indian-Ocean - Direct Measurements and Gfdl Model Comparison. Journal of Physical Oceanography, 22(10): 1112-1128.
The seasonal cycles found in moored current measurements in the equatorial Somali Current region and along the equator between 50-degrees and 60-degrees-E are compared with the multilayer Geophysical Fluid Dynamics Laboratory model for the tropical Indian Ocean. The remote forcing of Somali Current transport variations by incident long equatorial waves from the equatorial interior subthermocline region is investigated by analyzing the model velocities of annual and semiannual period. Amplitudes and phases of linear equatorial Rossby and Kelvin waves were least-squares fitted to the model velocities between 5-degrees-S and 5-degrees-N, 55-degrees and 86-degrees-E from 100-m to 1000-m depth. Two cases of wave fits are distinguished: the "free" Kelvin wave case, where the Kelvin waves were fitted independently, and the "reflected" Kelvin wave case, where they were coupled to the Rossby waves by the western boundary condition for a straight slanted (45-degrees to the north) coastline. The wave field velocities explained approximately 70% of the spatial variance in the equatorial model subregion and also compared reasonably well with observed current variations along the equator. At the western boundary, the short-wave alongshore transport due to reflected incident long waves was determined and found to be antisymmetric about the equator. The maximum transport variation for the semiannual period due to the short waves was about 5 X 10(6) m3 s-1 between 150- and 800-m depth at 3-degrees north and south of the equator. Observational evidence for the western boundary transport variations and the sensitivity to changes in the incident wave field are discussed.
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