Sandra-Esther Brunnabend

Revisiting the Contemporary Sea Level Budget on Global and Regional Scales

Significance Understanding sea-level change is of paramount importance because it reflects climate-related factors, such as the ocean heat budget, mass changes in the cryosphere, and natural ocean/atmosphere variations. Furthermore, sea-level rise directly affects coastal areas, which has ramifications for its population and economy. From a novel combination of Gravity Recovery And Climate Experiment and radar altimetry data we find over the last 12 y: (i) a larger global steric sea-level rise as previously reported, (ii) a mass contribution to global sea level consistent with mass loss estimates from the world’s ice sheets, glaciers, and hydrological sources, and (iii) regionally resolved sea-level budget components which differ significantly from that of the global sea-level budget. Dividing the sea-level budget into contributions from ice sheets and glaciers, the water cycle, steric expansion, and crustal movement is challenging, especially on regional scales. Here, Gravity Recovery And Climate Experiment (GRACE) gravity observations and sea-level anomalies from altimetry are used in a joint inversion, ensuring a consistent decomposition of the global and regional sea-level rise budget. Over the years 2002–2014, we find a global mean steric trend of 1.38 ± 0.16 mm/y, compared with a total trend of 2.74 ± 0.58 mm/y. This is significantly larger than steric trends derived from in situ temperature/salinity profiles and models which range from 0.66 ± 0.2 to 0.94 ± 0.1 mm/y. Mass contributions from ice sheets and glaciers (1.37 ± 0.09 mm/y, accelerating with 0.03 ± 0.02 mm/y2) are offset by a negative hydrological component (−0.29 ± 0.26 mm/y). The combined mass rate (1.08 ± 0.3 mm/y) is smaller than previous GRACE estimates (up to 2 mm/y), but it is consistent with the sum of individual contributions (ice sheets, glaciers, and hydrology) found in literature. The altimetric sea-level budget is closed by coestimating a remaining component of 0.22 ± 0.26 mm/y. Well above average sea-level rise is found regionally near the Philippines (14.7 ± 4.39 mm/y) and Indonesia (8.3 ± 4.7 mm/y) which is dominated by steric components (11.2 ± 3.58 mm/y and 6.4 ± 3.18 mm/y, respectively). In contrast, in the central and Eastern part of the Pacific, negative steric trends (down to −2.8 ± 1.53 mm/y) are detected. Significant regional components are found, up to 5.3 ± 2.6 mm/y in the northwest Atlantic, which are likely due to ocean bottom pressure variations.

Changes in total ocean mass derived from GRACE, GPS, and ocean modeling with weekly resolution

[1] We derive changes in ocean bottom pressure (OBP) and ocean mass by combining modeled ocean bottom pressure, weekly GRACE-derived models of gravity change, and large-scale deformation patterns sensed by a global network of GPS stations in a joint least squares inversion. The weekly combination allows a consistent estimation of geocenter motion, loading mass harmonics up to degree 30, and a spatially uniform mass correction term, which serves as a correction for forcing of the ocean model. We provide maps and time series of ocean mass and bottom pressure variations. Furthermore, we discuss the estimated geocenter motion and the estimated model correction. Our results indicate that the total ocean mass change is predominantly annual, with a maximum amplitude corresponding to 7.4 mm in October, which is in line with earlier work. The mean ocean bottom pressure (i.e., ocean plus atmospheric mass) shows an annual amplitude of 8.7 mm and is shifted forward by about 1.5 months. In addition, the solution exhibits typical autocorrelation times of about 2 weeks. A comparison with in situ bottom pressure time series in the southern Indian Ocean shows a good agreement, with correlations of 0.7-0.8. Based on these comparisons, we see that our results monitor realistic submonthly variations, which are strongest at high latitudes. The addition of GRACE data in the inversion is found to improve these high-latitude variations and enables better separability of the geocenter motion from other unknowns. Increasing the OBP model error from 3 cm to 4.8 cm affects mainly the higher-degree coefficients.

Improving mass redistribution estimates by modeling ocean bottom pressure uncertainties

Weekly ocean bottom pressure anomalies (OBP) are modeled using the Finite Element Sea-ice Ocean Model FESOM). The models OBP error, mostly unknown so far, is assessed by comparing two model simulations, each forced by different atmospheric forcing data sets. The mean estimated error of modeled OBP is found to be 0.04 m per 1.5° × 1.5° grid cell. The error varies strongly from 0.003 m in the equatorial region to 0.31 m in the Weddell and Ross Seas. We believe that the spatial variations of the errors are an important improvement over previous error models. The new error estimates are implemented in a joint inversion of Gravity Recovery and Climate Experiment (GRACE) gravity measurements, GPS site displacements and modeled OBP, resulting in a larger overall OBP weight in the inversion, most notably in the Polar Regions. Additionally, the inversion provides a global mass correction term to adjust the ocean mass budget of the model. The estimated term is used to correct the models fresh water balance, making it consistent with GRACE and GPS on seasonal and longer timescales. All model results, weekly GRACE estimates and the inverse solutions are compared with measurements from in situ bottom pressure recorders. The newly estimated error model of the combination solution results in higher correlations than the previously used constant error model of the combination solution.

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.

Estimating Sub-Monthly Global Mass Transport Signals Using GRACE, GPS and OBP Data Sets

In an effort to learn more about the sub-monthly variations in the global mass transport processes of the Earth, a study has been performed in which the feasibility of using weekly GRACE gravity solutions, in combination with GPS displacement data and ocean bottom pressure (OBP) models, is examined. A sensitivity study was conducted in which a range of solutions using different combinations of these three data sets were compared to each other, and with the time span of each combination ranging from 1 to 4 weeks. Data sets included weekly GFZ RL04 GRACE covariance matrices, as well as IGS GPS solutions and a new FESOM global OBP model, developed at AWI. The results showed that the temporal resolution of the solutions could be increased, while still maintaining reasonable levels of accuracy, if either GPS or OBP data were included in the combination. In particular, a 2-week triple combination of GRACE, GPS and OBP data was found to have approximately the same accuracy over land as a standard monthly GRACE solution, up to degree and order 30. These results provide encouraging support for future work involving real-data combinations.