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  • From joint sea surface temperature/sea level pressure (SST/SLP) EOF analyses, lowfrequency variability modes are compared. The multi-decadal oscillation (MDO) changed phases twice during the 20th century, with its north Atlantic SST patterns resembling the Atlantic multi-decadal oscillation (AMO). The quasi-decadal oscillation (QDO) SST patterns displayed a double tripole configuration over the entire Atlantic basin, leading to tropical inter-hemispheric out-of-phase relationship. From the mid-1960s onward, while ST anomalies were negative to the north (negative phases of MDO/AMO), the Sahelian drought persisted with a weaker hurricane power dissipation index (PDI). During that period, the QDO modulated the intensity of the Sahelian drought.
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  • Oscillators, Electric
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  • Frequencies of oscillating systems--Measurement... Nonlinear oscillators
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  • The physics of partially neutralized plasmas is largely unexplored, partly because of the difficulty of confining such plasmas. Plasmas are confined in a stellarator without the need for a plasma current, and regardless of the degree of neutralization. The Columbia Non-neutral Torus (CNT) is a stellarator dedicated to the study of non-neutral, and partially neutralized plasmas. This thesis describes the first systematic studies of plasmas of arbitrary neutrality. The degree of neutralization of the plasma can be parameterized through the quantity η ≡ |n_e - Z n_i|/|n_e + Z n_i|. In CNT, η can be varied continuously from pure electron (η = 1) to quasi-neutral (η ≈ 0) by adjusting the neutral pressure in the chamber, which controls the volumetric ionization rate. Pure electron plasmas are in macroscopically stable equilibria, and have strong self electric potentials dictated by the emitter filament bias voltage on the magnetic axis. As η decreases, the plasma potential decouples from the emitter, and spontaneous fluctuations begin to appear. Partially neutralized plasmas (10^-3 < η < 10^-1) generally exhibit multi-mode oscillations in CNT. However, when magnetized ions are present, the electron-rich plasma oscillates at a single dominant mode (20 - 100 kHz). As the plasma approaches quasi-neutrality (η < 10^-5), it also reverts to single mode behavior (1 - 20 kHz). A parametric characterization of the single mode fluctuations detected in plasmas of arbitrary neutrality is presented in this thesis along with measurements of the spatial structure of the oscillations. The single mode fluctuations observed for η ≈ 0.01 to 0.8 are identified as an ion resonant instability propagating close to the E × B velocity of the plasma. The experiments also show that these oscillations present a poloidal mode number m = 1, and a toroidal number n = 0, which is identical to the spatial structure of the diocotron instability in pure-toroidal traps, and implies that the ion-driven instability breaks parallel force balance and the conservation of poloidal flux in CNT. The low frequency oscillations detected in the quasi-neutral regime are a global instability convected by the E × B flow of the plasma. In this case, the mode aligns almost perfectly with the field lines, and presents a resonant m = 3 poloidal structure.
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  • Oscillators, Electric
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  • We consider a box model of the Arctic system to examine its natural variability pertaining to the decadal Arctic Oscillation (AO) and the multidecadal Low-Frequency Oscillation (LFO). We distinguish the hierarchical order of the winter over the summer open-areas with only the former perturbing the sea-level pressure to effect coupled balances. From such balances, we discern two feedback loops on the winter open-area: a positive ice-flux feedback that elevates its overall variance and a negative buoyancy feedback that suppresses its low-frequency variance to render a decadal AO peak when subjected to white atmospheric noise. This negative buoyancy feedback may also reproduce observed phasing among LFO signals forced by the AMV (Atlantic Multidecadal Variability), thus resolving some outstanding questions. For the summer open-area, its variance is induced mainly by the winter forcings and insensitive to the base state. Its decadal signal merely reflects the preconditioning winter open-area, but its LFO variance is induced additionally and in comparable measure by the winter SAT (surface air temperature) through the latter’s effect on the melt duration and the first-year ice thickness. As such, the summer open-area signal is dominantly multidecadal, which moreover is several times its winter counterpart, consistent with the observed disparity. Although the model is extremely crude, its explicit solution allows quantitative comparison with observations and the generally positive outcome suggests that the model has isolated the essential physics of the Arctic natural variability of our concern.
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  • ENSO (El Nino-Southern Oscillation) has a large influence on tropical cyclone activity. The authors examine how different environmental factors contribute to this influence, using a genesis potential index developed by Emanuel and Nolan. Four factors contribute to the genesis potential index: low-level vorticity (850hPa), relative humidity at 600hPa, the magnitude of vertical wind shear from 850 to 200hPa and potential intensity (PI). Using monthly NCEP Reanalysis data in the period of 1950-2005, we calculate the genesis potential index on a latitude strip from 60°S to 60°N. Composite anomalies of the genesis potential index are produced for El Nino and La Nina years separately. These composites qualitatively replicate the observed interannual variations of the observed frequency and location of genesis in several different basins. This justifies producing composites of modified indices in which only one of the contributing factors varies, with the others set to climatology, to determine which among the factors are most important in causing interannual variations in genesis frequency. Specific factors that have more influence than others in different regions can be identified. For example, in El Nino years, relative humidity and vertical shear are important for the reduction in genesis seen in the Atlantic basin, and relative humidity and vorticity are important for the eastward shift in the mean genesis location in the western North Pacific.
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  • Variations of the North Atlantic subtropical high (NASH) western ridge and their implication to the Southeastern United States (SE US) summer precipitation were analyzed for the years 1948-2007. The results show that the movement of the NASH western ridge regulates both moisture transport and vertical motion over the SE US, especially in the last three decades, during which the ridge moved westward towards the American continent. When the NASH western ridge is located southwest (SW) of its mean climate position, excessive summer precipitation is observed due to an enhanced moisture transport. In contrast, when the western ridge is located in the northwest (NW), a precipitation deficit prevails as downward motion dominates the region. Composite analysis indicates that SW ridging results mainly from the NASH center's intensification; whereas NW ridging is likely caused by stationary wave propagation from the eastern Pacific/US western coast. In recent decades, both the SW and NW ridge positions have been observed to increase in frequency. Our results suggest that the increase in the SW ridging consistently follows the NASH's intensification associated with anthropogenic forcing as projected by coupled climate models. However, the increased frequency of NW ridging tends to follow the positive Pacific decadal oscillation (PDO) index. Thus, the enhanced variability in the SE US summer precipitation in recent decades might be a combined result of anthropogenic forcing and internal variability of the climate system. Results suggest that, as anthropogenic forcing continues to increase, the SE US will experience more frequent wet summers and an increase in the frequency of dry summers during positive PDO phases.
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  • Through a box model of the subpolar North Atlantic, we examine the genesis and predictability of the Atlantic Multidecadal Variability (AMV), posited as a linear perturbation sustained by the stochastic atmosphere. Postulating a density-dependent thermohaline circulation (THC), the latter would strongly differentiate the thermal and saline damping, and facilitate a negative feedback between the two fields. This negative feedback preferentially suppresses the low-frequency thermal variance to render a broad multidecadal peak bounded by the thermal and saline damping time. We offer this "differential variance suppression" as an alternative paradigm of the AMV in place of the "damped oscillation"—the latter generally not allowed by the deterministic dynamics and in any event bears no relation to the thermal peak. With the validated dynamics, we then assess the AMV predictability based on the relative entropy—a difference of the forecast and climatological probability distributions, which decays through both error growth and dynamical damping. Since the stochastic forcing is mainly in the surface heat flux, the thermal noise grows rapidly and together with its climatological variance limited by the THC-aided thermal damping, they strongly curtail the thermal predictability. The latter may be prolonged if the initial thermal and saline anomalies are of the same sign, but even rare events of less than 1% chance of occurrence yield a predictable time that is well short of a decade; we contend therefore that the AMV is in effect unpredictable.
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  • Atlantic Multi-decadal Variability (AMV), also known as the Atlantic Multi-decadal Oscillation (AMO), is characterized by a sharp rise and fall of the North Atlantic basin-wide sea surface temperatures (SST) on multi-decadal time scales.Widespread consequences of these rapid temperature swings were noted in many previous studies. Among these are the drying of Sahel in the 1960-70s and change in the frequency and intensity of Atlantic hurricanes on multi-decadal time scales. Given the short instrumental data records (about a century long) the central question is whether these climate fluctuations are robustly linked with the AMV and to what extent are these connections subject to changes in a changing climate. Here we address this issue by using the CMIP3 simulations for the 20th, 21st, and pre-industrial eras with 23 IPCC models. While models tend to produce AMV of shorter time scales and less periodic than suggested by the observations, the spatial structures of the SST anomaly patterns, and their association with worldwide precipitation, are surprisingly similar between models (with differing external forcing) and observations. Our results confirm the strong link between AMV and Sahel rainfall and suggest a clear physical mechanism for the linkage in terms of meridional shifts of the Atlantic ITCZ. The results also help clarify influences that may not be robust, such as the impacts over North America, India, and Australia.
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