Search

To mitigate temporal aliasing effects in monthly mean global gravity fields from the GRACE and GRACE‐FO satellite tandem missions, both tidal and non‐tidal background models describing high‐frequency mass variability in atmosphere and oceans are needed. To quantify tides in the atmosphere, we exploit the higher spatial (31 km) and temporal (1 hr) resolution provided by the latest atmospheric ECMWF reanalysis, ERA5. The oceanic response to atmospheric tides is subsequently modeled with the general ocean circulation model MPIOM (in a recently revised TP10L40 configuration that includes the feedback of self‐attraction and loading to the momentum equations and has an improved bathymetry around Antarctica) as well as the shallow water model TiME (employing a much higher spatial resolution and more elaborate tidal dissipation than MPIOM). Both ocean models consider jointly the effects of atmospheric pressure variations and surface wind stress. We present the characteristics of 16 waves beating at frequencies in the 1–6 cpd band and find that TiME typically outperforms the corresponding results from MPIOM and also FES2014b as measured from comparisons with tide gauge data. Moreover, we note improvements in GRACE‐FO laser ranging interferometer range‐acceleration pre‐fit residuals when employing the ocean tide solutions from TiME, in particular, for the S1 spectral line with most notable improvements around Australia, India, and the northern part of South America. Plain Language Summary: In addition to many rather slow processes such as the melting of glaciers, rapid mass redistribution related to the weather also measurably affect the Earth's gravity field. The ability of monitoring liquid freshwater changes within the Earth system from the satellite gravity missions GRACE (2002–2017) and GRACE‐FO (since 2018) relies on accurate background models of mass variability in atmosphere and oceans for both tidal and non‐tidal processes. Atmospheric tides are primarily excited in the middle atmosphere by solar energy absorption at periods of 24 hr and its overtones. We find additional tidal signatures in the atmosphere excited by periodic deformations of both crust and sea‐surface of the Earth. We thus introduce here a new data set for the atmospheric tides and their corresponding oceanic response that features both more waves and higher accuracy than other background models previously used for the processing of GRACE and GRACE‐FO satellite gravimetry data. Key Points: Sixteen relevant tidal lines identified in hourly data from ERA5 atmospheric reanalysisDedicated simulations with a high‐resolution global hydrodynamic model to simulate ocean tides with atmospheric influenceNew tidal models reduce pre‐fit residuals in GRACE‐FO Laser Ranging Interferometer data [ABSTRACT FROM AUTHOR]

The response of the Antarctic circumpolar circulation to wind stress variability is studied in a simple model of the Southern Ocean. The model consists of a zonally reentrant channel with mid-ocean barrier, forced by surface heat flux and stochastic wind stress. The MITgcm code is used to solve for temperature, sea level, and the velocity field.The channel transport responds actively to the stochastic wind forcing through the excitation of several eigenmodes. The fundamental basin mode is excited, which has a period of about 20 days. However, the response is dominated by a topographic mode with a period of 3 days. This barotropic mode is related primarily to topographic Rossby waves propagating on the submarine sill, and Kelvin waves propagating westward on the southern boundary. The mode shares characteristics with numerical predictions of normal modes in the Southern Ocean, as well as with recently observed modes of variability around Antarctica, with periods between 1 and 2 days. These results show that both topographic normal modes and Rossby basin modes can play a role in the adjustment of the ACC to high-frequency wind stress variability.The presence of the resonance in the system leads to interesting phase behavior between the wind stress forcing and the ocean transport: for a considerable frequency range, the transport seems to lead the wind stress variability. This shows that the classical notion of transport passively lagging wind forcing is inadequate when eigenmodes are excited. [Copyright &y& Elsevier]

We examine the upper ocean thermohaline structure in the southeast Indian Ocean and its temporal variability based on XBT/XCTD observations from four cruises across the Southern Ocean from Fremantle, Australia to Prydz Bay, Antarctica. The transects were occupied in March 1998, November 1998, March 2000 and March 2002. Three major fronts—the subtropical front, subantarctic front and polar front—are clearly identified from our surveys and compared with earlier studies. Particularly, two polar fronts, separated by a few degrees of latitude, appear southeast the Kerguelen Plateau. The primary polar front is characterized by a strong horizontal thermal gradient extending deep in the water column, while the secondary polar front is identified by the 2°C isotherm at Tmin depth and has a relatively shallow frontal expression. Dynamic height across the Antarctic Circumpolar Current (ACC) was calculated for the transects in 2000 and 2002. With a negligible yearly variation in the total transport across the ACC, the higher sample density in 2002 reveals more detailed structure of the ACC: two jets associated with the SAF and primary polar front are embedded within the broad ACC.We find a strong temporal variation in the upper ocean thermal structure in the polar ocean southeast the Kerguelen Plateau. The depths of the mixed layer and Tmin layer increase over time. The mixed layer temperature decreases while the Tmin temperature increases during the same period. In addition, we see an increase from 1998 to 2002 in the length of the ice-free period prior to each XBT/XCTD sampling and surface wind forcing, calculated as the friction velocity cubed during the ice-free period. Our analysis suggests that the longer the ocean is exposed to the atmosphere and the stronger the wind stirring, the more enhanced is the turbulent mixing. This results in a greater mixed layer depth and more entrainment of colder water from the Tmin layer to the mixed layer. This surface forcing also enhances internal diffusive processes that mix the Tmin water with the warmer waters above and below the Tmin layer. The surface forcing is apparently dominant in determining the upper ocean thermal structure in this polar region. [Copyright &y& Elsevier]