Regional sea level change in response to ice mass loss in Greenland, the West Antarctic and Alaska

Besides the warming of the ocean, sea level is mainly rising due to land ice mass loss of the major ice sheets in Greenland, the West Antarctic, and the Alaskan Glaciers. However, it is not clear yet how these land ice mass losses influence regional sea level. Here, we use the global Finite Element Sea-ice Ocean Model (FESOM) to simulate sea surface height (SSH) changes caused by these ice mass losses and combine it with the passive ocean response to varying surface loading using the sea level equation. We prescribe rates of fresh water inflow, not only around Greenland, but also around the West Antarctic Ice Sheet and the mountain glaciers in Alaska with approximately present-day amplitudes of 200, 100, and 50 Gt/yr, respectively. Perturbations in sea level and in freshwater distribution with respect to a reference simulation are computed for each source separately and in their combination. The ocean mass change shows an almost globally uniform behavior. In the North Atlantic and Arctic Ocean, mass is redistributed toward coastal regions. Steric sea level change varies locally in the order of several centimeters on advective time- scales of decades. Steric effects to local sea level differ significantly in different coastal locations, e.g., at North American coastal regions the steric effects may have the same order of magnitude as the mass driven effect, whereas at the European coast, steric effects remain small during the simulation period

A distributed computing approach to improve the performance of the Parallel Ocean Program (v2.1)

The Parallel Ocean Program (POP) is used in many strongly eddying ocean circulation simulations. Ideally it would be desirable to be able to do thousand-yearlong simulations, but the current performance of POP prohibits these types of simulations. In this work, using a new distributed computing approach, two methods to improve the performance of POP are presented. The first is a blockpartitioning scheme for the optimization of the load balancing of POP such that it can be run efficiently in a multiplatform setting. The second is the implementation of part of the POP model code on graphics processing units (GPUs). We show that the combination of both innovations also leads to a substantial performance increase when running POP simultaneously over multiple computational platforms

Changes in extreme regional sea surface height due to an abrupt weakening of the Atlantic meridional overturning circulation

As an extreme scenario of dynamical sea level changes, regional sea surface height (SSH) changes that occur in the North Atlantic due to an abrupt weakening of the Atlantic meridional overturning circulation (AMOC) are simulated. Two versions of the same ocean-only model are used to study the effect of ocean model resolution on these SSH changes: a high-resolution (HR) strongly eddying version and a low-resolution (LR) version in which the effect of eddies is parameterised. The weakening of the AMOC is induced in both model versions by applying strong freshwater perturbations around Greenland. A rapid decrease of the AMOC in the HR version induces much shorter return times of several specific regional and coastal extremes in North Atlantic SSH than in the LR version. This effect is caused by a change in main eddy pathways associated with a change in separation latitude of the Gulf Stream

Large scale surface loading signals from a GRACE, GPS and OBP combination

The movement of large masses, originating from hydrological and oceanographic variations, causes detectable variations in gravity and surface deformation. These may be detected by satellite gravimetry and a network of permanent GPS stations respectively. Alternatively, additional information on ocean bottom pressure variations may be retrieved from simulations. Within the JIGOG project (Surface mass redistribution from joint inversion of GPS site displacements, ocean bottom pressure models and GRACE global gravity models), we combine the above data sources (GRACE, GPS and OBP) in order to retrieve improved surface loading estimates. This combination has the advantage that, for example, geocenter motion can be retrieved. Furthermore, the joint inversion also allows to mitigate datagaps to a certain extent. In this study, we provide a brief overview of the methodology and results of the JIGOG project. We discuss the estimated geocenter motion and focus on the use of GPS/OBP combinations in the event of missing GRACE data. New simulation runs from the updated FESOM model are included in the discussion

Retrieval of surface loading and geocenter motion from a new combination of GRACE, modeled OBP and reprocessed GPS data

Variations in continental water storage, ocean and atmosphere cause changes in the gravitational potential of the Earth. Additionally, the solid Earth deforms noticeably under its load, which can be detected by a network of permanent GPS stations. The conservation of linear momentum implies that a shift of the center of gravity of the surface load is accompanied by a shift of the solid part of the Earth. Here, we combine GRACE gravimetry, homogeneously processed GPS site displacements, and modeled Ocean Bottom Pressure (OBP) in a least squares inversion. This approach yields weekly estimates of global surface loading, including a consistent treatment of the geocenter motion. Furthermore, we describe the updated spatially varying OBP error, used in the inversion. The inversion results are compared against a globally distributed set of in-situ bottom pressure recorders. Additionally, we propagate the weekly solutions back to the GPS site displacements and asses to what extent the measured GPS site displacements can be explained by surface loading

Changes in extreme regional sea level under global warming

An important contribution to future changes in regional sea level extremes is due to the changes in intrinsic ocean variability, in particular ocean eddies. Here, we study a scenario of future dynamic sea level (DSL) extremes using a high-resolution version of the Parallel Ocean Program and generalized extreme value theory. This model is forced with atmospheric fluxes from a coupled climate model which has been integrated under the IPCC-SRES-A1B scenario over the period 2000–2100. Changes in 10-year return time DSL extremes are very inhomogeneous over the globe and are related to changes in ocean currents and corresponding regional shifts in ocean eddy pathways. In this scenario, several regions in the North Atlantic experience an increase in mean DSL of up to 0.4 m over the period 2000–2100. DSL extremes with a 10-year return time increase up to 0.2 m with largest values in the northern and eastern Atlantic

Modeled steric and mass-driven sea level change caused by Greenland Ice Sheet melting

Meltwater from the Greenland Ice Sheet (GIS) has been a major contributor to sea level change in the recent past. Global and regional sea level variations caused by melting of the GIS are investigated with the finite element sea-ice ocean model (FESOM). We consider changes of local density (steric effects), mass inflow into the ocean, redistribution of mass, and gravitational effects. Five melting scenarios are simulated, where mass losses of 100, 200, 500, and 1000 Gt/yr are converted to a continuous volume flux that is homogeneously distributed along the coast of Greenland south of 75°N. In addition, a scenario of regional melt rates is calculated from daily ice melt characteristics. The global mean sea level modeled with FESOM increases by about 0.3 mm/yr if 100 Gt/yr of ice melts, which includes eustatic and steric sea level change. In the global mean the steric contribution is one order of magnitude smaller than the eustatic contribution. Regionally, especially in the North Atlantic, the steric contribution leads to strong deviations from the global mean sea level change. The modeled pattern mainly reflects the structure of temperature and salinity change in the upper ocean. Additionally, small steric variations occur due to local variability in the heat exchange between the atmosphere and the ocean. The mass loss has also affects on the gravitational attraction by the ice sheet, causing spatially varying sea level change mainly near the GIS, but also at greater distances. This effect is accounted for by using Green's functions