ALBERT

Beschreibung: A recent temperature reconstruction of global annual temperature shows Early Holocene warmth followed by a cooling trend through the Middle to Late Holocene [Marcott SA, et al., 2013, Science 339(6124):1198–1201]. This global cooling is puzzling because it is opposite from the expected and simulated global warming trend due to the retreating ice sheets and rising atmospheric greenhouse gases. Our critical reexamination of this contradiction between the reconstructed cooling and the simulated warming points to potentially significant biases in both the seasonality of the proxy reconstruction and the climate sensitivity of current climate models.

Beschreibung: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1991

Beschreibung: In this thesis, I study the time-varying behavior of a ventila ted thermocline with basin scales at annual and decadal time scales. The variability is forced by three external forcings: the wind stress (chapter 3), the surface heat flux (chapter 4) and the upwelling along the eastern boundary (chapter 5). It is found that the thermocline variability is forced mainly by wind in a shadow zone while m~inly by surface buoyancy flux in a ventilated zone. A two-layer planetary geostrophic model is developed (chapter 2) to simulate a thermocline. The model includes some novel physical mechanisms. Most importantly, it captures the essential feature of subduction; it also is able to account for a time-varying surface temperature. The equation for the interface is a quasi-linear equation, which can be solved analytically by the method of characteristics. The effect of a varying Ekman pumping is investigated. In a shadow zone, it is found that the driving due to the Ekman pumping is mainly balanced by the propagation of planetary waves. However, in a ventilated zone, the cold advection of subducted water plays the essential role in opposing the Ekman pumping. The different dynamics also results in different thermocline variability between the two zones. After a change of Ekman pumping, in the shadow zone, since the baroclinic Ross by wave responds to a changing Ekman pumping slowly (in years to decades), an imbalance arises between the Rossby wave and the Ekman pumping, which then excites thermocline variability. However, in the ventilated zone, both the advection and the Ekman pumping vary rapidly after a barotropic process (about one week) to reach a new steady balance, leaving little thermocline variability. In addition, the evolution of the thermocline and circulation are also discussed. Furthermore, with a periodic Ekman pumping, it is found that linear solutions are approximate the fully nonlinear solution well, particularly for annual forcings. However, the linear disturbance is strongly affected by the basic thermocline structure and circulation. The divergent group velocity field, which is mainly caused by the divergent Sverdrup flow field, produces a decay effect on disturbances. The mean thermocline structure also strongly affects the relative importance of the local Ekman pumping and remote Rossby waves. As a result, in a shadow zone, local response dominates for a shallow interface while the remote Rossby wave dominates for a deep interface. With a strong decadal forcing, the nonlinearity becomes important in the shadow zone, particularly in the western part. The time-mean thermocline which results, becomes shallower than the steady thermocline under the mean Ekman pumping. Then, we investigate the effect on the permanent thermocline by a moving outcrop line, which simulates the effect of a varying surface heat flux. The two layer model is modified by adding an (essentially passive) mixed layer atop. The outcrop line and the mixed layer depth are specified. It is found that, opposite to a surface wind stress, a surface buoyancy flux causes strong variability in the ventilated zone through subducted water while it affects the shadow zone very little. Furthermore, two regimes of buoyancy-forced solution are found. When the outcrop line moves slowly, the solutions are non-entrainment solutions. For these solutions, the surface heat flux is mainly balanced by the horizontal advection. The mixed layer is never entrained. The time-mean thermocline is close to the steady thermocline with the time-mean outcrop line. When the outcrop line moves southward rapidly during the cooling season, the solutions become entrainment solutions. Now, deep vertical convection must occur, because the horizontal advection in the permanent thermocline is no longer strong enough to balance the surface cooling. The mixed layer penetrates rapidly such that water mass is entrained into the mixed layer through the bottom. The time-mean thermocline resembles the steady thermocline with the early spring mixed layer, as suggested by Stommel (1979). The local variability in the permanent thermocline is most efficiently produced by decadal forcings. Finally, two issues about the waves radiating from the eastern boundary are discussed. The first is the penetration of planetary waves across the southern boundary of a subtropical gyre. We find that the wave penetration across the southern boundary is substantially changed by the zonal variation of the thermocline structure. The zonal variation alters both the effective β and the wave front orientation. As a result, the wave penetration differs for interfaces at different depths. For an interface near the surface, part of the waves penetrate into the equatorial region. For middle depths, most waves will be trapped within the subtropical gyre. In contrast, for deep depths, all waves penetrate southward. The second issue of the eastern boundary waves mainly concerns with the breaking of planetary waves in the presence of an Ekman pumping and the associated two-dimensional mean flow. It is found that the breaking is affected significantly by an Ekman pumping and the associated mean flow. With an Ekman pumping, downwelling breaking is suppressed and the breaking time is delayed; upwelling breaking is enhanced and their times are shortened. The breaking times and positions are mainly determined by the maximum vertical perturbation speed while the intensity of the breaking front mainly depends on the amplitude of the perturbation. The intensity of a breaking front increases with the amplitude of the forcing, but decreases with the distance from the eastern boundary. The orientation of a breaking front is overall in northeast-southwest (x ~ -1/f2).

Beschreibung: This thesis is supported by National Science Foundation, Division of Atmospheric Sciences.

Beschreibung: The large-scale reorganization of deep ocean circulation in the Atlantic involving changes in North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) played a critical role in regulating hemispheric and global climate during the last deglaciation. However, changes in the relative contributions of NADW and AABW and their properties are poorly constrained by marine records, including δ18O of benthic foraminiferal calcite (δ18Oc). Here, we use an isotope-enabled ocean general circulation model with realistic geometry and forcing conditions to simulate the deglacial water mass and δ18O evolution. Model results suggest that, in response to North Atlantic freshwater forcing during the early phase of the last deglaciation, NADW nearly collapses, while AABW mildly weakens. Rather than reflecting changes in NADW or AABW properties caused by freshwater input as suggested previously, the observed phasing difference of deep δ18Oc likely reflects early warming of the deep northern North Atlantic by ∼1.4 °C, while deep Southern Ocean temperature remains largely unchanged. We propose a thermodynamic mechanism to explain the early warming in the North Atlantic, featuring a strong middepth warming and enhanced downward heat flux via vertical mixing. Our results emphasize that the way that ocean circulation affects heat, a dynamic tracer, is considerably different from how it affects passive tracers, like δ18O, and call for caution when inferring water mass changes from δ18Oc records while assuming uniform changes in deep temperatures.

Beschreibung: Interdecadal variability of the Atlantic Meridional Overturning Circulation (AMOC-IV) plays an important role in climate variation and has significant societal impacts. Past climate reconstruction indicates that AMOC-IV has likely undergone significant changes. Despite some previous studies, responses of AMOC-IV to global warming remain unclear, in particular regarding its amplitude and time scale. In this study, we analyze the responses of AMOC-IV under various scenarios of future global warming in multiple models and find that AMOC-IV becomes weaker and shorter with enhanced global warming. From the present climate condition to the strongest future warming scenario, on average, the major period of AMOC-IV is shortened from ∼50 y to ∼20 y, and the amplitude is reduced by ∼60%. These reductions in period and amplitude of AMOC-IV are suggested to be associated with increased oceanic stratification under global warming and, in turn, the speedup of oceanic baroclinic Rossby waves.

Beschreibung: The El Niño−Southern Oscillation (ENSO), which is tightly coupled to the equatorial thermocline in the Pacific, is the dominant source of interannual climate variability, but its long-term evolution in response to climate change remains highly uncertain. This study uses Mg/Ca in planktonic foraminiferal shells to reconstruct sea surface and thermocline water temperatures (SST and TWT) for the past 142 ky in a western equatorial Pacific (WEP) core MD01-2386. Unlike the dominant 100-ky glacial−interglacial cycle recorded by SST and δ18O, which echoes the pattern seen in other WEP sites, the upper ocean thermal gradient shows a clear half-precessional (9.4 ky or 12.7 ky) cycle as indicated by the reconstructed and simulated temperature (ΔT) and δ18O differences between the surface and thermocline waters. This phenomenon is attributed to the interplay of subtropical-to-tropical thermocline anomalies forced by the antiphased meridional insolation gradients in the two hemispheres at the precessional band. In particular, the TWT shows greater variability than SST, and dominates the ΔT changes which couple with the west−east SST difference in the equatorial Pacific at the half-precessional band, implying a decisive role of the tropical thermocline in orbital-scale climate change.

Beschreibung: Stable water isotopologues in paleoclimate archives ( δ 18 O ) have been widely used as an indicator to derive past climate variations. The modern observed spatial δ 18 O -temperature relation in the middle and high latitudes has been used to infer the paleotemperatures changes from ice core data. However, various studies have shown that the spatial slope is larger than the temporal slope at the drill site by a factor of 2. Physically, the different spatial and temporal slope has been suggested to result from the amplified local surface air temperature cooling in the polar region at Last Glacial Maximum (LGM), according to the slope ratio equation derived in our previous study. To explicitly confirm the “polar amplification” effect in understanding the differences between temporal and spatial isotope–temperature relations, here we use the same isotope-enabled atmospheric general circulation model with a moisture-tracing module embedded to quantitatively estimate the contributions of different sources to the precipitated heavy oxygen isotopes in the middle and high latitudes. Our results show that the major sources of δ 18 O in precipitation over middle and high latitudes are from oceans where the sea surface temperature cooling at Last Glacial Maximum (LGM) is less than −2   ° C , while the local moisture sources with a higher cooling can be also relevant for polar regions, such as north Greenland. Additionally, the neglect of the strengthened local inversion layer strength at LGM could be the main cause for the overestimated source temperature cooling by the slope ratio equation, especially for the polar regions in the Northern Hemisphere.

Beschreibung: Over the past two decades, the alkaline persulfate oxidation (PO) with thermal and/or ultraviolet (UV) assisted digestion method has been widely used for digestion of nitrogen containing compounds (N-compounds) in water quality routine analysis in laboratory or on-line analysis, due to its simple principle, high conversion rate, high percent recovery, low-cost. However, this digestion method still has some inevitable problems such as complex operations, high contamination potential, batch N blanks, higher reaction temperature (120–124 °C) and time-consuming (30–60 min). In this study, ozone (O3) was selected as the oxidant for digestion of N-compounds through analysis and comparison firstly. Secondly, we proposed and compared the UV and/or ultrasound (US) combined with ozone (UV/O3, US/O3 and UV/US/O3) synergistic digestion methods based on O3 with sole O3 oxidation method on digestion efficiency (digestion time and conversion rate) of standard N-compounds. Simultaneously, the influence of reaction temperature, pH of water sample, concentration of O3 and mass flow rate, UV intensity, US frequency and power on digestion efficiency were investigated, and then the optimum parameters for digestion system were obtained. Experimental results indicated that UV radiation can effectively induce and promote the decomposition and photolysis of O3 in water to generate hydroxyl radicals (•OH), while US can promote the diffusion and dissolution of O3 in water and intensify the gas-liquid mass transfer process for the reaction system. Meanwhile, results showed that the UV/US/O3 synergistic digestion method had the best digestion efficiency under the optimum conditions: water sample volume, 10 mL; pH of water sample, 11; O3 mass flow rate, 3200 mg/h; reaction temperature, 30 °C; digestion time, 25 min; UV lamp power, 18 W; distance between UV lamp and reactor, 2 cm; US frequency, 20 kHz; US power, 75 W. The conversion rate (CR) of synthetic wastewater samples varied from 99.6% to 101.4% for total dissolved nitrogen (TDN) in the range of 1.0~4.0 mg/L. The UV/US/O3 synergistic digestion method would be an effective and potential alternative for digestion of N-compounds in water quality routine analysis in laboratory or on-line analysis.

Beschreibung: Substrate clogging seriously affects the lifetime and treatment performance of subsurface flow constructed wetlands (SSF CWs), and the quantitative detection of clogging is the key challenge in the management of substrate clogging. This paper explores the feasibility of the resistivity method to detect the clogging degree of an SSF CW. The clogged substrate was found to have a high water-holding capacity, which led to low apparent resistivity in the draining phase. On the basis of the resistivity characteristics, clogging quantification was performed with a standard laboratory procedure, i.e., the Wenner method used in a Miller Soil Box. The apparent resistivity to sediment fraction (v/v) (ARSF) model was established to evaluate the degree of clogging from the apparent resistivity. The results showed that the ARSF model fit well with the actual values (linear slope = 0.986; R-squared = 0.98). The methods for in situ resistivity detection were applied in a lab-scale horizontal subsurface flow constructed wetland (HSSF CW). Combined with the ARSF model, the two-probe method demonstrated high accuracy for clogging quantification (relative error less than 9%). These results suggest that the resistivity method is a reliable and feasible technique for in situ detection of clogging in SSF CWs.