Sensitivity of the Ocean's Meridional Overturning Circulation to Surface Conditions in the Paleogene

Deep circulations in the ocean affect the distribution of physical, chemical, and biological properties, and are intimately entwined with the planetary-scale climate. Numerous proxies, including neodymium (Nd) in fossil fish teeth, point to a source region in the South Pacific for much of the low-latitude deep-water during the early Paleogene. We use the MIT general circulation model (MITgcm) to test the sensitivity of deep-water formation to uncertainty in surface boundary conditions in a number of numerical modeling experiments with realistic and idealized bathymetries. Finally, the MITgcm is run with a passive tracer, ENd , for some of the experiments mentioned above and to multiple initial surface distributions of ENd. In our experiments that use idealized basin shapes, appropriate for the early Cenozoic, the formation of North Pacific deep-water occurred in all of our experiments in which we vary the magnitude of the surface density gradient. While the rate of deep-water formation is sensitive to the strength of the surface density gradient, the location of the source regions was not. For our experiments that use realistic bathymetry, the formation of South Pacific deep-water occurred in a majority of our experiments. Here the Southern Ocean has the greatest poleward latitudinal extent, and therefore preference for deep-water formation. When salinity is added into the equation of state we find that this causes an increase in the extent of Southern Ocean deep-water. Lastly, we explore simulations using ENd as a passive tracer. Throughout most of the realistic simulations explored, the densest water occurs in the Southern Ocean. There is a strong sensitivity to where in the Southern Ocean the densest water occurs though, either in the South Pacific or Atlantic. With ENd values different in these regions, various simulations produced different tracer distributions. We found this variability in the sinking region to be very sensitive to runoff and seasonality. The sensitivity to the spatial distribution of surface and interior ENd values was found to have little affect on the final ENd distribution, given that the ENd value in the sinking regions was kept constant

A Kinetic Model for Semidilute Bacterial Suspensions

Suspensions of self-propelled microscopic particles, such as swimming bacteria, exhibit collective motion leading to remarkable experimentally observable macroscopic properties. Rigorous mathematical analysis of this emergent behavior can provide significant insight into the mechanisms behind these experimental observations; however, there are many theoretical questions remaining unanswered. In this paper, we study a coupled PDE/ODE system first introduced in the physics literature and used to investigate numerically the effective viscosity of a bacterial suspension. We then examine the kinetic theory associated with the coupled system, which is designed to capture the long-time behavior of a Stokesian suspension of point force dipoles (infinitesimal spheroids representing self-propelled particles) with Lennard-Jones--type repulsion. A planar shear background flow is imposed on the suspension through the novel use of Lees--Edwards quasi-periodic boundary conditions applied to a representative volume. We show the existence and uniqueness of solutions for all time to the equations of motion for particle configurations---dipole orientations and relative positions. This result follows from first establishing the regularity of the solution to the fluid equations. The existence and uniqueness result allows us to define the Liouville equation for the probability density of configurations. We show that this probability density defines the average bulk stress in the suspension underlying the definition of many macroscopic quantities of interest, in particular the effective viscosity. These effective properties are determined by microscopic interactions highlighting the multiscale nature of this work

A Kinetic Model for Semidilute Bacterial Suspensions

Suspensions of self-propelled microscopic particles, such as swimming bacteria, exhibit collective motion leading to remarkable experimentally observable macroscopic properties. Rigorous mathematical analysis of this emergent behavior can provide significant insight into the mechanisms behind these experimental observations; however, there are many theoretical questions remaining unanswered. In this paper, we study a coupled PDE/ODE system first introduced in the physics literature and used to investigate numerically the effective viscosity of a bacterial suspension. We then examine the kinetic theory associated with the coupled system, which is designed to capture the long-time behavior of a Stokesian suspension of point force dipoles (infinitesimal spheroids representing self-propelled particles) with Lennard-Jones--type repulsion. A planar shear background flow is imposed on the suspension through the novel use of Lees--Edwards quasi-periodic boundary conditions applied to a representative volume. We show the existence and uniqueness of solutions for all time to the equations of motion for particle configurations---dipole orientations and relative positions. This result follows from first establishing the regularity of the solution to the fluid equations. The existence and uniqueness result allows us to define the Liouville equation for the probability density of configurations. We show that this probability density defines the average bulk stress in the suspension underlying the definition of many macroscopic quantities of interest, in particular the effective viscosity. These effective properties are determined by microscopic interactions highlighting the multiscale nature of this work

Nd Isotopic Structure of the Pacific Ocean 70-30 and Numerical Evidence for Vigorous Ocean Circulation and Ocean Heat Transport in a Greenhouse World

The oceanic meridional overturning circulation (MOC) is a crucial component of the climate system, impacting heat and nutrient transport, and global carbon cycling. Past greenhouse climate intervals present a paradox because their weak equator-to-pole temperature gradients imply a weaker MOC, yet increased poleward oceanic heat transport appears to be required to maintain these weak gradients. To investigate the mode of MOC that operated during the early Cenozoic, we compare new Nd isotope data with Nd tracer-enabled numerical ocean circulation and coupled climate model simulations. Assimilation of new Nd isotope data from South Pacific Deep Sea Drilling Project and Ocean Drilling Program Sites 323, 463, 596, 865, and 869 with previously published data confirm the hypothesized MOC characterized by vigorous sinking in the South and North Pacific ~70 to 30 Ma. Compilation of all Pacific Nd isotope data indicates vigorous, distinct, and separate overturning circulations in each basin until ~40 Ma. Simulations consistently reproduce South Pacific and North Pacific deep convection over a broad range of conditions, but cases using strong deep ocean vertical mixing produced the best data-model match. Strong mixing, potentially resulting from enhanced abyssal tidal dissipation, greater interaction of wind-driven internal wave activity with submarine plateaus, or higher than modern values of the geothermal heat flux enable models to achieve enhanced MOC circulation rates with resulting Nd isotope distributions consistent with the proxy data. The consequent poleward heat transport may resolve the paradox of warmer worlds with reduced temperature gradients

Operationalizing resilience for conservation objectives: the 4S’s

Although resilience thinking is increasingly popular and attractive among restoration practitioners, it carries an abstract quality that hinders effective application. Because resilience and its components are defined differently in social and ecological contexts, individual managers or stakeholders may disagree on the definition of a system’s state, occurrence of a state change, preferred state characteristics, and appropriate methods to achieve success. Nevertheless, incentives and mandates often force managers to demonstrate how their work enhances resilience. Unclear or conflicting definitions can lead to ineffective or even detrimental decision-making in the name of resilience; essentially, any convenient action can be touted as resilience-enhancing in this case. We contend that any successful resilience management project must clearly identify up-front the stressors of concern, state traits, scales of appropriate management, and success indicators (the 4S’s) relevant to the management targets. We propose a deliberate process for determining these components in advance of resilience management for conservation. Our recommendations were inspired and informed by two case studies wherein different definitions of stressors, state, scales, and success would result in very different management choices, with potentially serious consequences for biodiversity targets

Viscosity of Bacterial Suspensions: Hydrodynamic Interactions and Self-induced Noise

The viscosity of a suspension of swimming bacteria is investigated analytically and numerically. We propose a simple model that allows for efficient computation for a large number of bacteria. Our calculations show that long-range hydrodynamic interactions, intrinsic to self-locomoting objects in a viscous fluid, result in a dramatic reduction of the effective viscosity. In agreement with experiments on suspensions of Bacillus subtilis, we show that the viscosity reduction is related to the onset of large-scale collective motion due to interactions between the swimmers. The simulations reveal that the viscosity reduction occurs only for relatively low concentrations of swimmers: Further increases of the concentration yield an increase of the viscosity. We derive an explicit asymptotic formula for the effective viscosity in terms of known physical parameters and show that hydrodynamic interactions are manifested as self-induced noise in the absence of any explicit stochasticity in the system

Viscosity of Bacterial Suspensions: Hydrodynamic Interactions and Self-induced Noise

The viscosity of a suspension of swimming bacteria is investigated analytically and numerically. We propose a simple model that allows for efficient computation for a large number of bacteria. Our calculations show that long-range hydrodynamic interactions, intrinsic to self-locomoting objects in a viscous fluid, result in a dramatic reduction of the effective viscosity. In agreement with experiments on suspensions of Bacillus subtilis, we show that the viscosity reduction is related to the onset of large-scale collective motion due to interactions between the swimmers. The simulations reveal that the viscosity reduction occurs only for relatively low concentrations of swimmers: Further increases of the concentration yield an increase of the viscosity. We derive an explicit asymptotic formula for the effective viscosity in terms of known physical parameters and show that hydrodynamic interactions are manifested as self-induced noise in the absence of any explicit stochasticity in the system

Navigating inferior vena cava filters in invasive cardiology procedures: A systematic review

BACKGROUND: Transfemoral venous access (TFV) is the cornerstone of minimally invasive cardiac procedures. Although the presence of inferior vena cava filters (IVCFs) was considered a relative contraindication to TFV procedures, small experiences have suggested safety. We conducted a systematic review of the available literature on cardiac procedural success of TFV with IVCF in-situ. METHODS: Two independent reviewers searched PubMed, EMBASE, SCOPUS, and Google Scholar from inception to October 2020 for studies that reported outcomes in patients with IVCFs undergoing TFV for invasive cardiac procedures. We investigated a primary outcome of acute procedural success and reviewed the pooled data for patient demographics, procedural complications, types of IVCF, IVCF dwell time, and procedural specifics. RESULTS: Of the 120 studies initially screened, 8 studies were used in the final analysis with a total of 100 patients who underwent 110 procedures. The most common IVCF was the Greenfield Filter (36%), 60% of patients were males and the mean age was 67.8 years. The overall pooled incidence of acute procedural success was 95.45% (95% confidence interval 89.54. - 98.1) with no heterogeneity (I2 = 0%, p = 1) and there were no reported filter related complications. CONCLUSION: This systematic review is the largest study of its kind to demonstrate the safety and feasibility of TFV access in a variety of cardiac procedures in the presence of IVCF