ACCESS-OM2 v1.0: a global ocean-sea ice model at three resolutions

We introduce ACCESS-OM2, a new version of the ocean–sea ice model of the Australian Community Climate and Earth System Simulator. ACCESS-OM2 is driven by a prescribed atmosphere (JRA55-do) but has been designed to form the ocean–sea ice component of the fully coupled (atmosphere–land–ocean–sea ice) ACCESS-CM2 model. Importantly, the model is available at three different horizontal resolutions: a coarse resolution (nominally 1∘ horizontal grid spacing), an eddy-permitting resolution (nominally 0.25∘), and an eddy-rich resolution (0.1∘ with 75 vertical levels); the eddy-rich model is designed to be incorporated into the Bluelink operational ocean prediction and reanalysis system. The different resolutions have been developed simultaneously, both to allow for testing at lower resolutions and to permit comparison across resolutions. In this paper, the model is introduced and the individual components are documented. The model performance is evaluated across the three different resolutions, highlighting the relative advantages and disadvantages of running ocean–sea ice models at higher resolution. We find that higher resolution is an advantage in resolving flow through small straits, the structure of western boundary currents, and the abyssal overturning cell but that there is scope for improvements in sub-grid-scale parameterizations at the highest resolution

Sensitivity of Marine Heatwave Metrics to Ocean Model Resolution

Sustained extreme temperature events in the ocean, referred to as marine heatwaves (MHWs), generate substantial ecological, social, and economic impacts. Ocean models provide insights to the drivers, persistence, and dissipation of MHWs. However, the sensitivity of MHW metrics to ocean model resolution is unknown. Here, we analyze global MHW metrics in three configurations of a global ocean-sea ice model at coarse (1◦), eddy-permitting (0.25◦), and eddy-rich (0.1◦) resolutions. We show that all configurations qualitatively represent broad-scale global patterns of MHWs. These simulated MHWs are, however, weaker, longer-lasting, and less frequent than in observations. The 0.1◦ configuration, despite local biases, performs best both globally and regionally. Based on these results, model projections of future MHW metrics using coarse-resolution models are expected to be biased toward weaker and less frequent MHWs, when compared with results using an eddy-rich model.The authors thank the Consortium for Ocean-Sea Ice Modeling in Australia (COSIMA; www.cosima.org.au), funded by the Australian Research Council through its Linkage Program (LP160100073), for making the ACCESS-OM2 suite of models available in https://doi.org/10.5281/zenodo

Estimating Argo Float Trajectories Under Ice

Abstract Since the Argo program began, 568 floats returned almost 31,000 profiles, at high‐southern latitudes, with no measured position. These data are either disseminated with positions linearly interpolated between known positions, or with no geographic positions. Here, we present a simple method for estimating unknown Argo float trajectories. We try to identify trajectories that approximately follow contours along three different properties: potential vorticity (f/H), sea‐level, and density at 1,000 m. No single property‐constraint can be used to estimate trajectories for all position‐gaps. Each constraint fails for 9%–18% of gaps, where no continuous contour between the end‐points exists. But all constraints fail for the same position‐gap, for fewer than 1% of cases. For a given position‐gap, when a trajectory is identified using two or three different constraints, we select the shortest trajectory to be used to “fill the gap”. This selection process could be performed better by an Expert Operator, inspecting each estimated trajectory, and selecting the trajectory that is most consistent with a priori knowledge of the circulation in the vicinity of the position‐gap. Nonetheless, using the objective metric for selection, we find that 41.2% of position‐gaps use the f/H‐constraint, 32.1% use density, and 25.8% use sea‐level. We assess the estimated trajectories for consistency, by comparing bottom depths beneath trajectories to the deepest measurements in each profile. We find inconsistencies for 11.6% of position‐gaps using our method, compared to 28.0% using linearly‐interpolated trajectories. Adoption of the estimated trajectories for measurements under ice may yield benefits to many applications

A search for the Tasman Front

The traditional view of the circulation in the Tasman Sea includes a coherent, quasi-zonal, eastward flow towards the northern tip of New Zealand that is widely referred to as the Tasman Front. This flow was first suggested in the 1960s by oceanographers reasoning that the source of volume transport “feeding” the East Auckland Current, around northern New Zealand and extending down the east coast, must be off eastern Australia. The first reported in situ measurements of the Tasman Front included hand-drawn temperature sections that highlighted features that are not supported by observations. Here, we objectively map the original data, showing that the published reports of strong, sub-surface, north-south temperature gradients were unjustified. In the absence of additional observations, we can’t be sure of the context of those original measurements - so we have objectively “searched” fields in a recent 25-year ocean reanalysis to identify scenarios that are consistent with the observations. We suggest that the most likely interpretation of the original data, is that they measured an eddy field - or a series of discontinuous streams across the central Tasman Sea. We also analyse data from surface drifting buoys to show that water traversing the Tasman Sea may take several different paths. We conclude that a continuous, zonal, eastward flow across the Tasman Sea is less common than widely-accepted conceptual models imply. Instead, we suggest that the eastward flow between Australia and northern New Zealand is perhaps better described as an “eastern extension of the EAC” - since this doesn’t imply the presence of a front, and doesn’t preclude a broad flow. For clarity, we also suggest that the southern branch of the EAC, known as the EAC extension, should be referred to as the “southern extension of the EAC”.Data was sourced from the Integrated Marine Observing System (IMOS, www.aodn.org.au). IMOS is a national collaborative research infrastructure, supported by the Australian Government. Satellite altimeter data is provided by NASA, ESA, ISRO, NOAA, and CNES. SST observations are provided by NOAA (www.nodc.noaa.gov) and Remote Sensing Systems (www.remss.com). Model data used in this study are from the Bluelink suite of global models (http://dapds00.nci.org.au/ thredds/catalog/gb6/BRAN/catalog.html) and from the Consortium for Ocean-Sea Ice Modelling in Australia (COSIMA; http://cosima.org.au). COSIMA is supported by the Australian Research Council Linkage grant LP160100073. Model runs used resources and services from the National Computational Infrastructure (NCI), which is supported by the Australian Government. We obtained the SCOW dataset from http:// cioss.coas.oregonstate.edu/scow/. We acknowledge support from the Australian Research Council Centre of Excellence for Climate Extremes (CE170100023

Influência de vórtices na concentração de clorofila da confluência Brasil-Malvinas: Mecanismos inferidos por sensoriamento remoto

This study aims to (a) identify the spatial patterns in the surface phytoplankton abundance, here indexed as satellite chlorophyll-a concentration ([chla]), associated with mesoscale eddies of the Brazil-Malvinas Confluence (BMC) and (b) infer which are the physical mechanisms responsible for such patterns. In this work, the 57 largest eddies in the broad BMC region were selected from a global eddy dataset based on merged satellite altimetry data. Using those eddies, the main mechanisms which may eddies affect the [chla] were tested: (a) the eddy-pumping, (b) the eddy Ekman pumping and (c) the eddy advection of meridional [chla] gradients. Mean spatial chlorophyll and chlorophyll patterns associated with the selected eddies, as well as correlations between variables that describe the [chla], and its spatial anomalies (Δ[chla]), distribution and the eddies’ physical characteristics, were calculated. Positive and negative anomalies associated with cyclonic and anticyclonic eddies’ interiors, respectively, and significant inverse correlations between eddies’ amplitudes and the difference between Δ[chla] inside and outside the eddies, were found as evidence of eddy-pumping mechanism. The [chla] spatial pattern associated to the eddy advection of meridional gradients was evident in the eastern side of the features, as low (high) [chla] being advected from south (north) in anticyclonic (cyclonic) eddies. The magnitude of the difference between Δ[chla] in the east and west eddies’ hemispheres presented itself more dependent on the [chla] meridional gradient intensity than on the eddy rotational velocity. Furthermore, the non-linear parameter of the studied eddies suggest that BCM eddies are capable to transport the water parcels trapped inside them, helping in the propagation of the eddy-pumping signature

Search for intermediate-mass black hole binaries in the third observing run of Advanced LIGO and Advanced Virgo

International audienceIntermediate-mass black holes (IMBHs) span the approximate mass range 100−105 M⊙, between black holes (BHs) that formed by stellar collapse and the supermassive BHs at the centers of galaxies. Mergers of IMBH binaries are the most energetic gravitational-wave sources accessible by the terrestrial detector network. Searches of the first two observing runs of Advanced LIGO and Advanced Virgo did not yield any significant IMBH binary signals. In the third observing run (O3), the increased network sensitivity enabled the detection of GW190521, a signal consistent with a binary merger of mass ∼150 M⊙ providing direct evidence of IMBH formation. Here, we report on a dedicated search of O3 data for further IMBH binary mergers, combining both modeled (matched filter) and model-independent search methods. We find some marginal candidates, but none are sufficiently significant to indicate detection of further IMBH mergers. We quantify the sensitivity of the individual search methods and of the combined search using a suite of IMBH binary signals obtained via numerical relativity, including the effects of spins misaligned with the binary orbital axis, and present the resulting upper limits on astrophysical merger rates. Our most stringent limit is for equal mass and aligned spin BH binary of total mass 200 M⊙ and effective aligned spin 0.8 at 0.056 Gpc−3 yr−1 (90% confidence), a factor of 3.5 more constraining than previous LIGO-Virgo limits. We also update the estimated rate of mergers similar to GW190521 to 0.08 Gpc−3 yr−1.Key words: gravitational waves / stars: black holes / black hole physicsCorresponding author: W. Del Pozzo, e-mail: [email protected]† Deceased, August 2020