Matthew R. Mazloff

We investigate the accuracy of barotropic tides and phase-locked baroclinic (internal) tides in forward (non-data-assimilative) simulations of four different high-resolution global ocean models: the HYbrid Coordinate Ocean Model (HYCOM), the Modular Ocean Model Version 6 (MOM6), the Nucleus for European Modelling of the Ocean (NEMO), and the Massachusetts Institute of Technology general circulation model (MITgcm). Consistent with a previous study that employed only HYCOM, we find that the phase-locked internal tides in our HYCOM, MOM6 and MITgcm simulations are large relative to altimeter observations unless an explicit parameterized topographic internal wave drag, which damps barotropic and baroclinic tides, is included. We also find that harmonic analysis of long records of model output results in estimates of phase-locked internal tides that compare more favorably with observed baroclinic surface elevations from along-track satellite-altimetry. Increasing the resolution within the same dynamical core reduces amplitudes of phase-locked internal tides, possibly due to a more efficient energy cascade to higher-frequency internal wave continuum motions. We also document details of the astronomical tidal forcing in the MITgcm runs, which have been widely used by the wideswath altimetry community, that adversely affect the accuracy of modeled barotropic and baroclinic tides. We verify the impacts of the erroneous tidal forcing with results of less expensive shallow-water simulations that also employ the erroneous forcing.

We investigate the accuracy of barotropic tides and phase-locked baroclinic (internal) tides in forward (non-data-assimilative) simulations of four different high-resolution global ocean models: the HYbrid Coordinate Ocean Model (HYCOM), the Modular Ocean Model Version 6 (MOM6), the Nucleus for European Modelling of the Ocean (NEMO), and the Massachusetts Institute of Technology general circulation model (MITgcm). Consistent with a previous study that employed only HYCOM, we find that the phase-locked internal tides in our HYCOM, MOM6 and MITgcm simulations are large relative to altimeter observations unless an explicit parameterized topographic internal wave drag, which damps barotropic and baroclinic tides, is included. We also find that harmonic analysis of long records of model output results in estimates of phase-locked internal tides that compare more favorably with observed baroclinic surface elevations from along-track satellite-altimetry. Increasing the resolution within the same dynamical core reduces amplitudes of phase-locked internal tides, possibly due to a more efficient energy cascade to higher-frequency internal wave continuum motions. We also document details of the astronomical tidal forcing in the MITgcm runs, which have been widely used by the wideswath altimetry community, that adversely affect the accuracy of modeled barotropic and baroclinic tides. We verify the impacts of the erroneous tidal forcing with results of less expensive shallow-water simulations that also employ the erroneous forcing.

We investigate the accuracy of barotropic tides and phase-locked baroclinic (internal) tides in forward (non-data-assimilative) simulations of four different high-resolution global ocean models: the HYbrid Coordinate Ocean Model (HYCOM), the Modular Ocean Model Version 6 (MOM6), the Nucleus for European Modelling of the Ocean (NEMO), and the Massachusetts Institute of Technology general circulation model (MITgcm). Consistent with a previous study that employed only HYCOM, we find that the phase-locked internal tides in our HYCOM, MOM6 and MITgcm simulations are large relative to altimeter observations unless an explicit parameterized topographic internal wave drag, which damps barotropic and baroclinic tides, is included. We also find that harmonic analysis of long records of model output results in estimates of phase-locked internal tides that compare more favorably with observed baroclinic surface elevations from along-track satellite-altimetry. Increasing the resolution within the same dynamical core reduces amplitudes of phase-locked internal tides, possibly due to a more efficient energy cascade to higher-frequency internal wave continuum motions. We also document details of the astronomical tidal forcing in the MITgcm runs, which have been widely used by the wideswath altimetry community, that adversely affect the accuracy of modeled barotropic and baroclinic tides. We verify the impacts of the erroneous tidal forcing with results of less expensive shallow-water simulations that also employ the erroneous forcing.

The Southern Ocean is a major sink for atmospheric carbon dioxide and critical to the current and future carbon cycle. This net annual flux reflects the balance of strong seasonal variability characterized by opposing periods of winter outgassing and summer uptake. Using a simple framework, we evaluate how model biases in both the amplitude and timing of dissolved inorganic carbon (DIC) and total alkalinity (TA) and the amplitude of sea surface temperature (SST) impact simulated p . We examine seasonal fluxes and p south of the Subantarctic Front in 42 Earth System Model and three state estimate simulations. Only 11 of the 45 simulations have a seasonal p cycle correlation of 0.7 to observed p , while 25 have a correlation of p seasonality in the evaluated models. These findings highlight the difficulty and importance of capturing the seasonal processes influencing the carbonate system to correctly model and predict the Southern Ocean carbon sink and its response to a changing climate.

We investigate the accuracy of barotropic tides and phase-locked baroclinic (internal) tides in forward (non-data-assimilative) simulations of four different high-resolution global ocean models: the HYbrid Coordinate Ocean Model (HYCOM), the Modular Ocean Model Version 6 (MOM6), the Nucleus for European Modelling of the Ocean (NEMO), and the Massachusetts Institute of Technology general circulation model (MITgcm). Consistent with a previous study that employed only HYCOM, we find that the phase-locked internal tides in our HYCOM, MOM6 and MITgcm simulations are large relative to altimeter observations unless an explicit parameterized topographic internal wave drag, which damps barotropic and baroclinic tides, is included. We also find that harmonic analysis of long records of model output results in estimates of phase-locked internal tides that compare more favorably with observed baroclinic surface elevations from along-track satellite-altimetry. Increasing the resolution within the same dynamical core reduces amplitudes of phase-locked internal tides, possibly due to a more efficient energy cascade to higher-frequency internal wave continuum motions. We also document details of the astronomical tidal forcing in the MITgcm runs, which have been widely used by the wideswath altimetry community, that adversely affect the accuracy of modeled barotropic and baroclinic tides. We verify the impacts of the erroneous tidal forcing with results of less expensive shallow-water simulations that also employ the erroneous forcing.

East Antarctica (EA) experienced an unprecedented atmospheric river (AR)-driven heatwave in March 2022. Using high-resolution model simulations, this study investigates how atmospheric moisture (Q) and sea surface temperature (SST) influence AR development and associated surface impacts through latent heat exchange and circulation evolutions. A 20% increase in Q and an ~1.3°C increase in SST over the target region enhanced area-averaged precipitation over EA by up to 10.25% and 17.23%, respectively, while amplifying Dome C warming by up to 5.38°C and 3.95°C. However, the responses are nonlinear, with AR strength and precipitation responding more strongly to reduced Q and enhanced SST. Increased Q accelerates blocking development and AR intensification, shifting the AR eastward and enhancing oceanic precipitation. In contrast, SST warming enhances evaporation and moisture retention, producing a delayed but stronger precipitation response. Temperature changes show strong spatial variability among sensitivity experiments, likely due to uncertainties in cloud radiative effects.

The record‐breaking Arctic cyclone of January 2022 caused an unprecedented sea ice loss in the Barents Sea near Svalbard, which was severely underestimated by both the ECMWF operational forecast and the CESM1 global climate model. To investigate this discrepancy and further explore the mechanisms that led to sea ice loss, we utilize a regional ocean–sea ice MITgcm model forced with ERA5 atmospheric reanalysis, validated against satellite‐derived ice concentrations and thickness, and Argo observations. We tested model sensitivity to initial conditions by choosing two different ocean reconstructions and tested sensitivity to the air‐sea‐ice drag coefficient. The model shows that cyclone‐driven winds induced strong upper‐ocean mixing of cold, fresh polar waters with subsurface warm, salty Atlantic Water, bringing heat to the surface to rapidly melt thin seasonal ice north of Svalbard. This process was evident in Argo float profile changes but not in the operational forecast and global climate model. In contrast, wind‐driven mechanical divergence dominated sea ice loss south of Svalbard, where the direction of sea ice retreat followed the direction of the prevailing winds. Increasing the air‐ice drag coefficient substantially improved the agreement between our modeled and observed ice area evolution. The balance between thermodynamic and mechanical effects on the sea ice loss was highly sensitive to drag parameterization. Our results highlight the need to improve the representation of upper ocean stratification, vertical mixing, and air‐sea‐ice momentum fluxes to reproduce cyclone‐triggered rapid sea ice loss events in the Arctic in winter.

Antarctic sea ice extent began declining in 2015, reaching its minimum in the post-1970s observational era in 2023. To diagnose the drivers of this decline, we analyze an observationally constrained sea ice–ocean model spanning 2013–2023 and identify three distinct phases of sea ice retreat. First, intensifying westerlies preconditioned the Southern Ocean via increased upwelling of warm, saline circumpolar deep water (CDW). Second, strong winds in 2015–2016 enhanced the mixing of CDW into the upper ocean, thereby initiating sea ice loss, particularly in East Antarctica. Third, sustained mixing of CDW into the surface layer, combined with reduced equatorward sea ice–derived freshwater export, maintained an unprecedentedly low sea ice state. East Antarctic sea ice loss was primarily subsurface driven via enhanced upward CDW flux, whereas West Antarctic sea ice loss was also forced by longwave radiative flux anomalies. Our findings suggest that persistent upwelling-favorable conditions under anthropogenic forcing may push the Southern Ocean into a prolonged low sea ice state.

The impact of coupling an atmosphere model to a dynamical ocean model, rather than using persistent SST anomalies, is assessed for wintertime medium-range forecasts over the North Pacific and North Atlantic. This assessment is based on 20 years (1998–2017) of hindcasts produced by the Global Ensemble Prediction System (GEPS) of Environment and Climate Change Canada (ECCC). We compare an uncoupled atmospheric model (versions 5, GEPS5) with an atmosphere–ocean coupled model (version 6, GEPS6) alongside European Centre for Medium-Range Weather Forecasts Reanalysis v5 (ERA5) as the verification dataset. We find that by the third pentad, or days 11–15, coupling to a dynamic ocean model weakens the Aleutian Low, the Icelandic Low, and the Atlantic Subtropical High. This produces less integrated vapor transport (IVT) over the Pacific and Atlantic Oceans, whose spatial patterns are modulated by phases of Madden–Julian Oscillation (MJO). Coupling also results in colder sea surface temperature (SST) over the Kuroshio Current Extension region and produces a weaker Aleutian Low due to less upward latent heat fluxes. The weaker Aleutian Low further reinforces its weakening through a positive feedback loop. Lastly, the coupling to a dynamical ocean reduces the latent heat flux bias variance by 10 %–20 %, thus improving the IVT.

Using a 1/48 global MITgcm simulation, this study quantifies meridional internal wave energy fluxes in the Southern Ocean across 35S, 45S, 55S, and 65S from 29 May to 15 November 2012. Fluxes are separated into three bands: semidiurnal, near‐inertial, and continuum. Zonally integrated total fluxes are poleward at O(10) GW (15.3, 7.0, and 15.9 GW across 35S, 45S, and 55S, respectively), dropping to 0.3 GW at 65S. The semidiurnal tidal band accounts for over 80% of the total flux, contributing 13.4, 5.7, and 14.8 GW at 35–55S, before dropping to 0.15 GW at 65S. This sharp decrease represents a convergence of ∼15 GW between 55S and 65S, corresponding to a background diapycnal diffusivity of . Equatorward semidiurnal fluxes are spatially intermittent and limited to a few topographic features, including the Macquarie Ridge and mid‐ocean ridges. In contrast, poleward fluxes are more broadly distributed in the Pacific but dominated by a few major hotspots in the Atlantic and Indian sectors, most notably the Drake Passage. Continuum band fluxes decrease from 2.3 GW at 35S to 0.2 GW at 65S, with a consistent approximately 0.7 GW convergence across each 10 latitude band. In the near‐inertial band, from 35S to 55S, zonally integrated flux accounts for just 1%–3% of the total internal wave flux and is mostly equatorward, acting against the net flux. Despite 17.4 GW of near‐inertial wind work between 35S and 65S, the meridional flux divergence remains weak, exporting only 1%–4% of the wind input.