29 research outputs found
Impact of sea ice transport on Beaufort Gyre liquid freshwater content
The Arctic Oceanâs Beaufort Gyre (BG) is a wind-driven reservoir of relatively fresh seawater, situated beneath time-mean anticyclonic atmospheric circulation, and is covered by mobile pack ice for most of the year. Liquid freshwater accumulation in and expulsion from this gyre is of critical interest due to its potential to affect the Atlantic meridional overturning circulation and due to the importance of freshwater in modulating vertical fluxes of heat, nutrients and carbon in the ocean, and exchanges of heat and moisture with the atmosphere. Here, we investigate the hypothesis that wind-driven sea ice transport into/from the BG region influences the freshwater content of the gyre and its variability. To test this hypothesis, we use the results of a coordinated climate response function experiment with four ice-ocean models, in combination with targeted experiments using a regional setup of the MITgcm, in which we rotate the surface wind forcing vectors (thereby changing the ageostrophic component of these winds). Our results show that, via an effect on the net thermodynamic growth rate, anomalies in sea ice transport into the BG affect liquid freshwater adjustment. Specifically, increased ice import increases freshwater retention in the gyre, whereas ice export decreases freshwater in the gyre. Our results demonstrate that uncertainty in the ageostrophic component of surface winds, and in the dynamic sea ice response to these winds, has important implications for ice thermodynamics and freshwater. This sensitivity may explain some of the observed inter-model spread in simulations of Beaufort Gyre freshwater and its adjustment in response to wind forcing
Binary black hole shadows, chaotic scattering and the Cantor set
We investigate the qualitative features of binary black hole shadows using the model of two
extremally charged black holes in static equilibrium (a MajumdarâPapapetrou solution). Our
perspective is that binary spacetimes are natural exemplars of chaotic scattering, because they
admit more than one fundamental null orbit, and thus an uncountably infinite set of perpetual null
orbits which generate scattering singularities in initial data. Inspired by the three-disc model, we
develop an appropriate symbolic dynamics to describe planar null geodesics on the double black
hole spacetime. We show that a one-dimensional (1D) black hole shadow may constructed through
an iterative procedure akin to the construction of the Cantor set; thus the 1D shadow is self-similar.
Next, we study non-planar rays, to understand how angular momentum affects the existence and
properties of the fundamental null orbits. Taking slices through 2D shadows, we observe three
types of 1D shadow: regular, Cantor-like, and highly chaotic. The switch from Cantor-like to
regular occurs where outer fundamental orbits are forbidden by angular momentum. The highly
chaotic part is associated with an unexpected feature: stable and bounded null orbits, which exist
around two black holes of equal mass M separated by a1 < a < â
2a1, where a1 = 4M/â
27. To
show how this possibility arises, we define a certain potential function and classify its stationary
points. We conjecture that the highly chaotic parts of the 2D shadow possess the Wada property.
Finally, we consider the possibility of following null geodesics through event horizons, and chaos in
the maximally extended spacetime
Searching for stochastic gravitational waves using data from the two colocated LIGO Hanford detectors
Searches for a stochastic gravitational-wave background (SGWB) using terrestrial detectors typically involve cross-correlating data from pairs of detectors. The sensitivity of such cross-correlation analyses depends, among other things, on the separation between the two detectors: the smaller the separation, the better the sensitivity. Hence, a colocated detector pair is more sensitive to a gravitational-wave background than a noncolocated detector pair. However, colocated detectors are also expected to suffer from correlated noise from instrumental and environmental effects that could contaminate the measurement of the background. Hence, methods to identify and mitigate the effects of correlated noise are necessary to achieve the potential increase in sensitivity of colocated detectors. Here we report on the first SGWB analysis using the two LIGO Hanford detectors and address the complications arising from correlated environmental noise. We apply correlated noise identification and mitigation techniques to data taken by the two LIGO Hanford detectors, H1 and H2, during LIGOâs fifth science run. At low frequencies, 40â460 Hz, we are unable to sufficiently mitigate the correlated noise to a level where we may confidently measure or bound the stochastic gravitational-wave signal. However, at high frequencies, 460â1000 Hz, these techniques are sufficient to set a 95% confidence level upper limit on the gravitational-wave energy density of Ω(f) < 7.7 Ă 10[superscript -4](f/900ââHz)[superscript 3], which improves on the previous upper limit by a factor of ~180. In doing so, we demonstrate techniques that will be useful for future searches using advanced detectors, where correlated noise (e.g., from global magnetic fields) may affect even widely separated detectors.National Science Foundation (U.S.)United States. National Aeronautics and Space AdministrationCarnegie TrustDavid & Lucile Packard FoundationAlfred P. Sloan Foundatio
Search for eccentric black hole coalescences during the third observing run of LIGO and Virgo
Despite the growing number of confident binary black hole coalescences observed through gravitational waves so far, the astrophysical origin of these binaries remains uncertain. Orbital eccentricity is one of the clearest tracers of binary formation channels. Identifying binary eccentricity, however, remains challenging due to the limited availability of gravitational waveforms that include effects of eccentricity. Here, we present observational results for a waveform-independent search sensitive to eccentric black hole coalescences, covering the third observing run (O3) of the LIGO and Virgo detectors. We identified no new high-significance candidates beyond those that were already identified with searches focusing on quasi-circular binaries. We determine the sensitivity of our search to high-mass (total mass M>70 Mâ) binaries covering eccentricities up to 0.3 at 15 Hz orbital frequency, and use this to compare model predictions to search results. Assuming all detections are indeed quasi-circular, for our fiducial population model, we place an upper limit for the merger rate density of high-mass binaries with eccentricities 0<eâ€0.3 at 0.33 Gpcâ3 yrâ1 at 90\% confidence level
Ultralight vector dark matter search using data from the KAGRA O3GK run
Among the various candidates for dark matter (DM), ultralight vector DM can be probed by laser interferometric gravitational wave detectors through the measurement of oscillating length changes in the arm cavities. In this context, KAGRA has a unique feature due to differing compositions of its mirrors, enhancing the signal of vector DM in the length change in the auxiliary channels. Here we present the result of a search for U(1)BâL gauge boson DM using the KAGRA data from auxiliary length channels during the first joint observation run together with GEO600. By applying our search pipeline, which takes into account the stochastic nature of ultralight DM, upper bounds on the coupling strength between the U(1)BâL gauge boson and ordinary matter are obtained for a range of DM masses. While our constraints are less stringent than those derived from previous experiments, this study demonstrates the applicability of our method to the lower-mass vector DM search, which is made difficult in this measurement by the short observation time compared to the auto-correlation time scale of DM
Response of Arctic Freshwater to the Arctic Oscillation in Coupled Climate Models
The freshwater content (FWC) of the Arctic Ocean is intimately linked to the stratificationâa physical characteristic of the Arctic Ocean with wide relevance for climate and biology. Here, we explore the relationship between atmospheric circulation and Arctic FWC across 12 different Coupled Model Intercomparison Project Phase 5 control run simulations. Using multiple lagged regression, we seek to isolate the linear response of Arctic FWC to a step change in the strength of the Arctic Oscillation (AO), as well as the second and third orthogonal modes of SLP variability over the Arctic domain. There is broad agreement amongst models that a step change to a more anticyclonic AO leads to an increase in Arctic FWC, with an e-folding timescale of five to ten years. However, models differ widely in the degree to which a linear response to SLP variability can explain FWC changes. While the mean states, timescales and magnitudes of FWC variability may be broadly similar, the physical origins of variability are highly inconsistent between models. We perform a robustness test that incorporates a Monte Carlo approach, to determine which response functions are most likely to represent causal, physical relationships within the models, and which are artefacts of regression. Convolution with SLP reanalysis data shows that the four most robust response functions have some skill at reproducing observed accumulation of FWC during the late 1990s and 2000s, consistent with the idea that this change was largely wind-driven
Arctic Ocean response to Greenland Sea wind anomalies in a suite of model simulations
Multiâmodel Arctic Ocean ``Climate Response Functionâ (CRF) experiments are analyzed in order to explore the effects of anomalous wind forcing over the Greenland Sea (GS) on poleward ocean heat transport, Atlantic Water (AW) pathways, and the extent of Arctic sea ice. Particular emphasis is placed on the sensitivity of the AW circulation to anomalously strong or weak GS winds in relation to natural variability, the latter manifested as part of the North Atlantic Oscillation (NAO). We find that anomalously strong (weak) GS wind forcing, comparable in strength to a strong positive (negative) NAO index, results in an intensification (weakening) of the poleward AW flow, extending from south of the North Atlantic Subpolar Gyre, through the Nordic Seas, and all the way into the Canadian Basin. Reconstructions made utilizing the calculated CRFs explain ~50 % of the simulated AW flow variance; this is the proportion of variability that can be explained by GS wind forcing. In the Barents and Kara Seas there is a clear relationship between the windâdriven anomalous AW inflow and the sea ice extent. Most of the anomalous AW heat is lost to the atmosphere, and loss of sea ice in the Barents Sea results in even more heat loss to the atmosphere, and thus effective ocean cooling. Release of passive tracers in a subset of the suite of models reveals differences in circulation patterns and shows that the flow of AW in the Arctic Ocean is highly dependent on the wind stress in the Nordic Seas.
Plain Language Summary
The North Atlantic Current is an extension of the Gulf Stream, which brings warm Atlantic Water northward as the current flows through the Nordic Seas. Eventually it enters the cold deep Arctic Ocean basins through the Barents Sea and Fram Strait. Nine different numerical oceanâice models have been analyzed and compared in order to investigate: (1) their ability to simulate this northward flow of Atlantic Water, (2) its dependence on wind forcing, and (3) its impact on Arctic sea ice. Consistently, in all models, stronger winds in the Greenland Sea result in a stronger northward flow of warm Atlantic Water. The response on ocean circulation occurs from the North Atlantic, through the Nordic Seas and the Barents Sea, to the deep Canadian Basin. The flow of warm Atlantic Water within the Arctic Ocean is thus highly dependent on the wind stress in the Nordic Seas. There is particularly clear response in the Barents and Kara Seas where a windâdriven anomalous warm inflow drives a smaller sea ice extent and thickness, and an increased heat transfer from the ocean to the atmosphere above. Weaker winds in the Greenland Sea produces weaker flow and hence a larger sea ice extent and thicknes
Arctic Ocean freshwater content and its decadal memory of sea-level pressure
Arctic freshwater content (FWC) has increased significantly over the last two decades, with potential future implications for the Atlantic meridional overturning circulation downstream. We investigate the relationship between Arctic FWC and atmospheric circulation in the control run of a coupled climate model. Multiple linear lagged regression is used to extract the response of total Arctic FWC to a hypothetical step increase in the principal components of seaâlevel pressure. The results demonstrate that the FWC adjusts on a decadal timescale, consistent with the idea that windâdriven ocean dynamics and eddies determine the response of Arctic Ocean circulation and properties to a change in surface forcing, as suggested by idealized models and theory. Convolving the response of FWC to a change in seaâlevel pressure with historical seaâlevel pressure variations reveals that the recent observed increase in Arctic FWC is related to natural variations in seaâlevel pressure