WP A: Origin of regional sea level changes at annual to multi-decadal scale

WP A will analyze the mechanisms of sea level variability with primary focus on processes leading to spatially highly inhomogeneous pattern of regional sea level change on time scales of up to 50 years. The WP will provide a comprehensive understanding of the natural and anthropogenic factors governing regional sea level variability. Respective work will include oceanographic, geodetic and cryospheric research on regional sea level pattern and will also address terrestrial hydrological contributions. We will determine the origin of regional sea level changes at decadal to centennial time scales and identify the mechanisms causing coastal sea level changes on decadal time scale. To reach these goals the WP has to address also basin-scale to global data quality issues of in situ and satellite data. Results from WP A will provide knowledge of processes leading to sea level changes in the past, today and in the future and will provide uncertainties in projected components. Such information is required as a boundary condition for coastal systems and their change on decadal to centennial time scales. Output from WP A will feed directly into WP B where an improved knowledge base will be created for coastal sea level change information with focus on the selected study regions. Output from WP A will also feed directly into WP C by providing regional climate information.

Challenges that need to be addressed:

To provide an improved detailed understanding of the dynamics and forcing functions of past and contemporary regional sea level changes, a challenge exists to identify all processes involved. Therefore, we need to understand the key processes leading to past, present and future regional sea level variability.
Moreover, processes affecting regional changes are geographically diverse: while the tropical Indo-Pacific is governed by steric changes associated with wind-driven ocean circulation dynamics and related to modes of climate variability such as ENSO, PDO and IOD, trends at the North American and European coasts will also be affected by thermohaline processes. In addition, land motion and mass redistribution effects relevant to the tropical Indo-Pacific (local land motion due to volcanic activity; groundwater or hydrocarbon extraction; sediment compaction; ocean-shelf ice interactions governing the Antarctic ice sheets) are different from those affecting the North American and European coasts (post-glacial vertical motion; melting of the Greenland ice sheet). Furthermore, we need a comprehensive understanding for anthropogenic factors superimposed to natural variability before coastal change can be simulated and predicted on decadal to centennial time scales since both govern regional sea level variability as a boundary condition to coastal systems. This includes an improved reconstruction of the spatial patterns of regional sea level change, with particular emphasis on the decadal variability and trends during the last century.

Detailed uncertainty measures are essential for coastal zone coastal sea level change impacts, but are missing for regional sea level projections. Progress in this regard clearly necessitates an integration of hitherto rather disparate strands of research. As an example, large uncertainties in solid-Earth and gravity models have been identified recently as a major problem for determining the Earth’s mantle viscosity structure, and thus also for quantifying the solid Earth‘ response to the last de-glaciation (GIA models); as a result, reconstructed paleo-sea levels in coastal regions can deviate from the eustatic value by more than 20 m due to continental levering or hydroisostatic correction (e.g., Lambeck et al., 2002;) with substantial variability due to the regional loading response, whereas this mechanism is negligible for many island sites (e.g. Jevrejeva et al., 2014). The contribution of present and future terrestrial hydrology has also been identified as major contribution to sea level  projections over the next 50 years and beyond.

In recent years, satellite measurements in combination with the global ARGO profiling network have revolutionized the observational capabilities in this regard (Kusche et al., 2012). Nevertheless, many important data quality issues need to be addressed that are essential for improving our understanding of sea level changes and their representation in model simulations. This is essential for understanding ongoing and future sea level projections. The GRACE (since 2002) and GOCE (2009-2013) missions enabled the computation of high-resolution geoid models, and began to allow deriving changes in ocean mass. The full potential of those novel measurements for investigations of  regional to coastal sea level changes has still not been reached. In combination with data from the new German-US GRACE-follow on mission (anticipated starting data 2017), a new view on satellite gravity and altimetry data is required for any sea level effort.

Approach:

WP A will provide an integrated approach investigating changes in sea level due to changes in the ocean circulation and due to ice-sheet – sea level – solid Earth interactions. WP A will utilize a combination of analyses of historic and contemporary ocean observations, sea level reconstructions from geological archives, and global ocean models with enhanced process dynamics in relevant regions. The work program will thus involve elements with specific focus on these regional phenomena, but will also address aspects of a more global or generic nature.

Research projects within the SPP Research Area A (WP A):

Approach

The work program will be structured in four basic topics; work within each topic is expected to be addressed by several working groups as part of the SPP.

Topic I

Origin of regional sea level changes at decadal-to-centennial time scale

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Topic II

Historic sea level estimates (paleo time scale)

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Topic III

Sea level projections

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Topic IV

Observations and calibrated reconstructions

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