Ocean Bottom Pressure Changes Research Articles

Mechanisms of Interannual Variability of Ocean Bottom Pressure in the Southern Indian Ocean

The study of ocean bottom pressure (OBP) helps to understand the changes in the sea level budget and ocean deep circulation. In this study, the characteristics and mechanisms of interannual OBP variability in the Southern Indian Ocean are examined using Gravity Recovery and Climate Experiment (GRACE) satellite data from 2003 to 2016. Results show that there are two energetic OBP centers in the Southern Indian Ocean (50°–60°S, 40°–60°E and 45°–60°S, 80°–120°E). The OBP magnitudes at two centers have strong variability on interannual time scales, and their values are larger during austral summer (NDJF) and winter (JJAS). Atmospheric forcing plays an important role in local OBP variability. The high (low) sea level pressure (SLP) over the Southern Indian Ocean benefits positive (negative) OBP anomalies via the convergence (divergence) of Ekman transport driven by local wind. Such SLP anomalies are related to the Southern Annular Mode (SAM), Southern Oscillation (SO) and Indian Ocean dipole (IOD). SAM can influence the OBP changes in both austral summer and winter, while SO and IOD have positive correlations with OBP variability during austral summer and austral winter, respectively. These results are validated by a mass-conservation ocean model, which further confirms the importance of atmospheric forcing on the interannual OBP variations.

Synthesizing ocean bottom pressure records including seismic wave and tsunami contributions: Toward realistic tests of monitoring systems

AbstractThe present study proposes a method for synthesizing the ocean bottom pressure records during a tsunamigenic earthquake. First, a linear seismic wave simulation is conducted with a kinematic earthquake fault model as a source. Then, a nonlinear tsunami simulation is conducted using the sea bottom movement calculated in the seismic wave simulation. By using these simulation results, this method can provide realistic ocean bottom pressure change data, including both seismic and tsunami contributions. A simple theoretical consideration indicates that the dynamic pressure change caused by the sea bottom acceleration can contribute significantly until the duration of ~90 s for a depth of 4000 m in the ocean. The performance of a tsunami monitoring system was investigated using the synthesized ocean bottom pressure records. It indicates that the system based on the hydrostatic approximation could not measure the actual tsunami height when the time does not elapse enough. The dynamic pressure change and the permanent sea bottom deformation inside the source region break the condition of a simple hydrostatic approximation. A tsunami source estimation method of tFISH is also examined. Even though the synthesized records contain a large dynamic pressure change, which is not considered in the algorithm, tFISH showed a satisfactory performance 5 min after the earthquake occurrence. The pressure records synthesized in this study, including both seismic wave and tsunami contributions, are more practical for evaluating the performance of our monitoring ability, whereas most tsunami monitoring tests neglect the seismic wave contribution.

Reducing leakage error in GRACE-observed long-term ice mass change: a case study in West Antarctica

Spatial leakage is a major limitation for quantitative interpretation of satellite gravity measurements from the gravity recovery and climate experiment (GRACE). Using synthetic data to simulate ice mass changes in the Amundsen Sea Embayment and Antarctic Peninsula, we analyze quantitatively the effects of a limited range of spherical harmonics (SH) coefficients and additional filtering, which in combination can significantly attenuate signal amplitudes. We present details of a forward modeling algorithm and show that it is capable of removing these biases from GRACE estimates. Examples show how to implement the method by constraining locations of presumed mass changes, or leaving these locations unspecified within a continental region. Our analysis indicates that leakage effects from far-field mass signals (e.g., terrestrial water storage change and glacial melting over other continents) on Antarctic mass rate estimates appear to be negligible. However, leakage from long-term ocean bottom pressure change in the surrounding Antarctic Circumpolar Current regions may bias Antarctic mass rate estimates by up to 20 Gigatonne per year (Gt/year). Experiments based on proxy GRACE measurement noise indicate that the effects of GRACE spatial noise on estimated Antarctic mass rates via constrained and unconstrained forward modelings are $$\sim $$ 5 and 15 Gt/year, respectively.

Arctic Ocean Circulation Patterns Revealed by GRACE

Abstract Measurements of ocean bottom pressure (OBP) anomalies from the satellite mission Gravity Recovery and Climate Experiment (GRACE), complemented by information from two ocean models, are used to investigate the variations and distribution of the Arctic Ocean mass from 2002 through 2011. The forcing and dynamics associated with the observed OBP changes are explored. Major findings are the identification of three primary temporal–spatial modes of OBP variability at monthly-to-interannual time scales with the following characteristics. Mode 1 (50% of the variance) is a wintertime basin-coherent Arctic mass change forced by southerly winds through Fram Strait, and to a lesser extent through Bering Strait. These winds generate northward geostrophic current anomalies that increase the mass in the Arctic Ocean. Mode 2 (20%) reveals a mass change along the Siberian shelves, driven by surface Ekman transport and associated with the Arctic Oscillation. Mode 3 (10%) reveals a mass dipole, with mass decreasing in the Chukchi, East Siberian, and Laptev Seas, and mass increasing in the Barents and Kara Seas. During the summer, the mass decrease on the East Siberian shelves is due to the basin-scale anticyclonic atmospheric circulation that removes mass from the shelves via Ekman transport. During the winter, the forcing mechanisms include a large-scale cyclonic atmospheric circulation in the eastern-central Arctic that produces mass divergence into the Canada Basin and the Barents Sea. In addition, strengthening of the Beaufort high tends to remove mass from the East Siberian and Chukchi Seas. Supporting previous modeling results, the month-to-month variability in OBP associated with each mode is predominantly of barotropic character.

Regional patterns of observed sea level change: insights from a 1/4° global ocean/sea-ice hindcast

A global eddy-admitting ocean/sea-ice simulation driven over 1958–2004 by daily atmospheric forcing is used to evaluate spatial patterns of sea level change between 1993 and 2001. In the present study, no data assimilation is performed. The model is based on the Nucleus for European Models of the Ocean code at the 1/4° resolution, and the simulation was performed without data assimilation by the DRAKKAR project. We show that this simulation correctly reproduces the observed regional sea level trend patterns computed using satellite altimetry data over 1993–2001. Generally, we find that regional sea level change is best simulated in the tropical band and northern oceans, whereas the Southern Ocean is poorly simulated. We examine the respective contributions of steric and bottom pressure changes to the total regional sea level changes. For the steric component, we analyze separately the contributions of temperature and salinity changes as well as upper and lower ocean contributions. Generally, the model results show that most regional sea level changes arise from temperature changes in the upper 750 m of the ocean. However, contributions of salinity changes and deep steric changes can be locally important. We also propose a map of ocean bottom pressure changes. Finally, we assess the robustness of such a model by comparing this simulation with a second simulation performed by MERCATOR-Ocean based on the same core model, but differing by its short length of integration (1992–2001) and its surface forcing data set. The long simulation presents better performance over 1993–2001 than the short simulation, especially in the Southern Ocean where a long adjustment time seems to be needed.

Ocean bottom pressure changes lead to a decreasing length‐of‐day in a warming climate

We use a coupled climate model to evaluate ocean bottom pressure changes in the IPCC‐A1B climate scenario. Ocean warming in the 21st and 22nd centuries causes secular oceanic bottom pressure anomalies. The essential feature is a net mass transfer onto shallow shelf areas from the deeper ocean areas, which exhibit negative bottom pressure anomalies. We develop a simple mass redistribution model that explains this mechanism. Regionally, however, distinct patterns of bottom pressure anomalies emerge due to spatially inhomogeneous warming and ocean circulation changes. Most prominently, the Arctic Ocean shelves experience an above‐average bottom pressure increase. We find a net transfer of mass from the Southern to the Northern Hemisphere, and a net movement of mass closer towards Earth's axis of rotation. Thus, ocean warming and the ensuing mass redistribution change the length‐of‐day by −0.12 ms within 200 years, demonstrating that the oceans are capable of exciting nontidal length‐of‐day changes on decadal and longer timescales.

Seasonal water storage change of the Yangtze River basin detected by GRACE

US-Germany co-sponsered satellite gravimetry mission GRACE (Gravity Recovery And Climate Experiment), launched in March 2002, has been producing monthly time series of Earth gravity models up to degree and order of 120. The GRACE mission consists of two identical satellites flying on an almost polar orbit with an altitude of about 300–500 km and satelite-to-satellite ranging of about 220 km. Thanks to the payloads of space-borne GPS receivers, accelerometers and high-precision K-band satelite-to-satellite ranging mesurements, GRACE gravity models are expected to achieve more than one order of magnitude of improvement over previous models at spatial scales of a few hundred kilometers or larger. Recovery of surface mass re-distribution based on GRACE’s time-varying gravity models is applied to studies in solid Earth geophysics, oceanography, climatology and geodesy. At secular time scales, GRACE is expected to provide valuable information on global ice changes, whose variations have profound in fluences on global climate, and in particular, on sea level changes. At seasonal time scales, GRACE is expected to reveal surface water changes with an accuracy of less than 1 cm, or ocean bottom pressure changes with an accuracy of less than 1 mbar (1 mbar = 102 Pa). These surface mass redistribution measurements would improve our understanding of the global and regional mass and energy cycles that are critical to human life. Using 15 GRACE monthly gravity models covering the period from April 2002 to December 2003, this study compares seasonal water storage changes recovered from GRACE data and hydrology models at global and regional scales, with particular focus on the Yangtze River basin of China. Annual amplitude of 3.4 cm of equivalent water height change is found for the Yangtze River basin with maximum in Spring and Autumn, agreeing with two state-of-the-art hydrology models. The differences between GRACE results and model predictions are less than 1–2 cm. We conclude that satellite gravimetry has huge potentials in monitering water storage changes in large river basins such as Yangtze.

Atmospheric and oceanic excitation of the Earth's wobbles during 1980–2000

Because of the action of various geophysical excitation mechanisms, the Earth does not rotate about its figure axis, so it wobbles as it rotates. Here, the effectiveness of atmospheric and oceanic processes in exciting the Earth's wobbles during 1980–2000 is evaluated using estimates of atmospheric angular momentum from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis project and estimates of oceanic angular momentum from the Estimating the Circulation and Climate of the Ocean (ECCO) consortium's simulation of the general circulation of the oceans. On intraseasonal timescales, atmospheric surface pressure changes are found to be the single most effective process exciting the Earth's wobbles, explaining about twice as much of the observed variance as do either atmospheric wind or ocean bottom pressure changes and nearly 4 times as much of the observed variance as do oceanic currents. However, on interannual timescales, ocean bottom pressure changes are found to be the single most effective process exciting the Earth's wobbles, explaining more than 5 times as much of the observed variance as do atmospheric wind and pressure changes combined, and more than twice as much of the observed variance as do oceanic currents. Within the Chandler band it is found that during 1980–2000 atmospheric and oceanic processes have enough power to excite the Chandler wobble and are significantly coherent with it. The single most important mechanism exciting the Chandler wobble is found to be ocean bottom pressure variations. Atmospheric and oceanic processes do not appear to have enough power to excite the Earth's wobbles to their observed levels on pentadal and longer timescales, although series longer than the 21‐yearlong series used here need to be studied in order to obtain greater statistical significance of this result.

Satellite peers through the oceans from space

Satellite remote sensing has led to remarkable strides in oceanography in recent years. All measurements made so far have been of variables that are manifested directly at the sea surface, such as color, temperature, or height. There is, however, one apparent exception to the need to measure a surface property: fluctuations in the strength of the gravity field reflect changes in the distribution of mass in the ocean, atmosphere, and solid Earth, and in the storage of water, snow, and ice on land. As ocean bottom pressure represents a column integral of the weight of atmosphere plus ocean, this raises the tantalizing possibility of effectively measuring ocean bottom pressure changes from space.