Wang, He, Robert Hallberg, A Wallcraft, Brian K Arbic, and Eric P Chassignet, April 2024: Improving global barotropic tides with sub-grid scale topography. Journal of Advances in Modeling Earth Systems, 16(4), DOI:10.1029/2023MS004056. Abstract
In recent years, efforts have been made to include tides in both operational ocean models as well as climate and earth system models. The accuracy of the barotropic tides is often limited by the model topography, which is in turn limited by model horizontal resolution. In this work, we explore the reduction of barotropic tidal errors in an ocean general circulation model (Modular Ocean Model version 6; MOM6) using sub-grid scale topography representation. We follow the methodology from Adcroft (2013, https://doi.org/10.1016/j.ocemod.2013.03.002), which utilizes statistics from finer resolution topographic data sets to represent sub-grid scale features with a light computational cost in a structured finite volume formulation. The geometric effect from sub-grid scale topography can be introduced to the model with only a few parameters at each grid cell. The porous barriers, which are implemented at the walls of the grid cells, are used to modify transport between grid cells. Our results show that the globally averaged tidal error in lower-resolution simulations is significantly reduced with the use of porous barriers. We argue this method is a potentially useful tool to improve simulations of tides (and other flows) in low-resolution simulations.
Range, Molly M., Brian K Arbic, Brandon C Johnson, Theodore C Moore, Vasily V Titov, Alistair Adcroft, Joseph K Ansong, Christopher J Hollis, Jeroen Ritsema, Christopher R Scotese, and He Wang, October 2022: The Chicxulub impact produced a powerful global tsunami. AGU Advances, 3(5), DOI:10.1029/2021AV000627. Abstract
The Chicxulub crater is the site of an asteroid impact linked with the Cretaceous-Paleogene (K-Pg) mass extinction at ∼66 Ma. This asteroid struck in shallow water and caused a large tsunami. Here we present the first global simulation of the Chicxulub impact tsunami from initial contact of the projectile to global propagation. We use a hydrocode to model the displacement of water, sediment, and crust over the first 10 min, and a shallow-water ocean model from that point onwards. The impact tsunami was up to 30,000 times more energetic than the 26 December 2004 Indian Ocean tsunami, one of the largest tsunamis in the modern record. Flow velocities exceeded 20 cm/s along shorelines worldwide, as well as in open-ocean regions in the North Atlantic, equatorial South Atlantic, southern Pacific and the Central American Seaway, and therefore likely scoured the seafloor and disturbed sediments over 10,000 km from the impact origin. The distribution of erosion and hiatuses in the uppermost Cretaceous marine sediments are consistent with model results.
This study examines the relative roles of the Arctic freshwater exported via different pathways on deep convection in the North Atlantic and the Atlantic Meridional Overturning Circulation (AMOC). Deep water feeding the lower branch of the AMOC is formed in several North Atlantic marginal seas, including the Labrador Sea, Irminger Sea and the Nordic Seas, where deep convection can potentially be inhibited by surface freshwater exported from the Arctic. The sensitivity of the AMOC and North Atlantic to two major freshwater pathways on either side of Greenland is studied using numerical experiments. Freshwater export is rerouted in global coupled climate models by blocking and expanding the channels along the two routes. The sensitivity experiments are performed in two sets of models (CM2G and CM2M) with different control simulation climatology for comparison. Freshwater via the route east of Greenland is found to have a larger direct impact on Labrador Sea convection. In response to the changes of freshwater route, North Atlantic convection outside of the Labrador Sea changes in the opposite sense to the Labrador Sea. The response of the AMOC is found to be sensitive to both the model formulation and mean state climate.
The sensitivity of large scale ocean circulation and climate to overflow representation is studied using coupled climate models, motivated by the differences between two models differing only in their ocean components: CM2G (which uses an isopycnal–coordinate ocean model) and CM2M (which uses a z-coordinate ocean model). Analysis of the control simulations of the two models shows that the Atlantic Meridional Overturning Circulation (AMOC) and the North Atlantic climate have some differences, which may be related to the representation of overflow processes. Firstly, in CM2G, as in the real world, overflows have two branches flowing out of the Nordic Seas, to the east and west of Iceland, respectively, while only the western branch is present in CM2M. This difference in overflow location results in different horizontal circulation in the North Atlantic. Secondly, the diapycnal mixing in the overflow downstream region is much larger in CM2M than in CM2G, which affects the entrainment and product water properties. Two sensitivity experiments are conducted in CM2G to isolate the effect of these two model differences: in the first experiment, the outlet of the eastern branch of the overflow is blocked, and the North Atlantic horizontal circulation is modified due to the absence of the eastern branch of the overflow, although the AMOC has little change; in the second experiment, the diapycnal mixing downstream of the overflow is enhanced, resulting in changes in the structure and magnitude of the AMOC.
