Low-frequency variability in the coupled ocean-atmosphere system at midlatitudes

The winter-mean climate over the North Atlantic ocean shows a low pressure center near Iceland and a large high pressure center ranging from Florida to Spain. Between these centers, an eastward jet prevails, causing cold and dry winter conditions in Northeast America and moderate conditions in Western Europe. On this mean situation, variability shows up on various time scales. The dominant pattern of variability expresses as a weakening and strengthening of the mean pressure centers and is called the North Atlantic Oscillation (NAO). The impact of the NAO on the American and European winter climate is large and the understanding of a possible mechanism which drives the NAO from one phase into another is therefore of great importance. In the North Atlantic climate system, multiple subsystems appear to be highly related to the NAO. For instance, high correlations have been found between specific North Atlantic sea-surface temperature (SST) patterns of variability and the NAO. Therefore, it has often been suggested that the NAO is both an atmospheric and an oceanic phenomenon with possibly a crucial role for ocean-atmosphere coupling. Although the NAO fluctuates on all time scales, there seems to be a preference for low-frequency variability, ranging from a few years to a few decades. In this thesis, we focus on the NAO at the low-frequency time scale. The central question is: How do oceanic and/or atmospheric processes cause low-frequency fluctuations at midlatitudes? In order to explore this question, first, intermediate models are analyzed using a dynamical systems approach, showing the fundamental processes which lead to low-frequency variability at midlatitudes. Then, output of a complex coupled General Circulation Model (GCM) is analyzed, in order to explore whether these fundamental processes can be identified and understood in the more complex climate system. The results of chapters 2 and 3 show two fundamental mechanisms, which might generate low-frequency variability at midlatitudes: nonlinearly induced variability due to rectification processes and/or low-frequency variability arising through an internal ocean mode, the socalled gyre mode. In chapter 4, a robust decadal oscillatory signal is found, both in oceanic and atmospheric fields. In the ocean surface and subsurface fields, anomalies propagate from the northwestern Labrador Sea south-eastward along the coast and bend around Newfoundland before they enter the subtropical gyre. The pressure patterns in the atmosphere show a strong resemblance with those of the North Atlantic Oscillation. A new framework, in which the gyre mode found in chapter 3 is suggested to set the decadal time scale, is presented to explain the decadal variability. The results of chapters 2- 4 are discussed in chapter 5, leading to a general (personal) view on the NAO: the low-frequency variations of the NAO may very well be caused by low-frequency variability in the ocean

Vibrational predissociation in the HCl dimer

Contains fulltext : 13810.pdf (publisher's version ) (Open Access

Total differential cross sections for Ar–CH4 from an ab initio potential

Total differential cross sections for the Ar–CH4 scattering complex at ECM=90.1 meV were obtained from converged close-coupling calculations based on a recent ab initio potential computed by symmetry-adapted perturbation theory (SAPT). Agreement with experiment is good, which demonstrates the accuracy of the SAPT potential

Quantifying the interplay between fine structure and geometry of an individual molecule on a surface

The pathway toward the tailored synthesis of materials starts with precise characterization of the conformational properties and dynamics of individual molecules. Electron spin resonance (ESR)-based scanning tunneling microscopy can potentially address molecular structure with unprecedented resolution. Here, we determine the fine structure and geometry of an individual titanium-hydride molecule, utilizing a combination of a newly developed millikelvin ESR scanning tunneling microscope in a vector magnetic field and ab initio approaches. We demonstrate a strikingly large anisotropy of the g tensor, unusual for a spin doublet ground state, resulting from a nontrivial orbital angular momentum stemming from the molecular ground state. We quantify the relationship between the resultant fine structure, hindered rotational modes, and orbital excitations. Our model system provides avenues to determine the structure and dynamics of individual molecules. © 2021 American Physical Society.We acknowledge funding from the Dutch Research Council (NWO), and the Vidi Project “Manipulating the interplay between superconductivity and chiral magnetism at the single-atom level” with Project No. 680-47-534. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (SPINAPSE: Grant Agreement No. 818399). F.D.N. thanks the Swiss National Science Foundation for financial support under Grant No. PP00P2_176866. The work of D.I.B., A.N.R. and V.V.M. was supported by Act 211 Government of the Russian Federation Contract No. 02.A03.21.0006

Ultracold collisions of oxygen molecules

Collision cross sections and rate constants between two ground- state oxygen molecules are investigated theoretically at translational energies below $\sim 1$K and in zero magnetic field. We present calculations for elastic and spin- changing inelastic collision rates for different isotopic combinations of oxygen atoms as a prelude to understanding their collisional stability in ultracold magnetic traps. A numerical analysis has been made in the framework of a rigid- rotor model that accounts fully for the singlet, triplet, and quintet potential energy surfaces in this system. The results offer insights into the effectiveness of evaporative cooling and the properties of molecular Bose- Einstein condensates, as well as estimates of collisional lifetimes in magnetic traps. Specifically, $^{17}O_{2}$ looks like a good candidate for ultracold studies, while $^{16}O_{2}$ is unlikely to survive evaporative cooling. Since $^{17}O_{2}$ is representative of a wide class of molecules that are paramagnetic in their ground state we conclude that many molecules can be successfully magnetically trapped at ultralow temperatures.Comment: 15 pages, 9 figure