WP B: Regionalization of Decadal Sea Level projections

WP B is concerned with the establishment of a scientific basis for obtaining reliable local projections of sea level trends, thereby improving the scientific basis for providing quantitative and detailed (high-resolution and high-end) estimates of future coastal sea level changes in the two focus regions. WP B will investigate the interaction of the large-scale ocean circulation with shelf seas and ice sheets on regional and local scale thereby downscaling climate related sea level information to coastal locations. It will provide new insight into small-scale processes related to the downscaling of sea level to coastal regions. The information will be merged with local geophysical processes controlling vertical motion of the solid earth and changes in coastline morphology. WP B thus involves regional studies to investigate specific geophysical processes relevant for coastal sea level changes, e.g. the relationship between ocean circulation and sea level changes in shelf regions, processes relating to interaction of ocean circulation and ice sheets, and sea level changes in specific coastal regions due to land movement (subsidence) and lateral inundation (morphodynamics) in interaction with regional sea level changes. New insight gained by WP B in terms of sea level change will feed into WP C by providing improved coastal sea level change scenarios for the selected pilot regions. Massive adaption measures (such as massive embankments) might contribute to changes in extreme sea levels. This in turn requires intensive work on explaining past changes in local sea level including those driven by human interventions.

Challenges that need to be addressed:

To connect regional sea level variability, studied in WP A, with coastal impact assessment investigations, considered in WP C, major advancements are required in downscaling sea level projections from the basin scale to any coastal location under consideration, thereby also down-scaling uncertainty information resulting from various remote processes, e.g. in form of ice sheet probability density information. It also requires major advancements in understanding the interplay of resulting local sea level changes with extreme sea level events and with coastal morphodynamics. To this end, advancements in modeling on regional to coastal scale are prerequisite to dynamical linkages between the open ocean and coastal regions, thereby considering locally heterogeneous dynamics and nonsteric effects of changing sea level over shelf areas (Landerer et al., 2007). We expect that in shelf seas, sea level should be strongly affected by the local circulation dynamics itself. Moreover, along coastlines, extreme events such as storm surges can exacerbate the effect of sea level rise. However, both processes are not quantified, making it difficult to date to connect basin-scale and regional sea level projections to coastlines.

Downscaling approaches can also involve statistical methods, which might be easier to implement in our case study regions. However, these approaches typically require an extended database to derive necessary statistical relations. Similar to the open ocean, we therefore also need to improve the observational coastal records to better assess shelf sea dynamics and coastal sea level changes, involving tide gauge and altimetric records (1) by extending the altimetric data base (e.g., Fenoglio-Marc et al., 2012) to the coast and (2) by improving the understanding of natural and anthropogenic induced land motion in tide gauge stations (Trisirisatayawong et al., 2011; Wahl et al., 2013). Of fundamental interest for a reduced uncertainty of sea level changes in our coastal study regions is the local interaction of ocean currents and sea level with ice sheets, which has a strong impact on sea level projections of remote coastal regions. For this purpose we need to substantially advance our understanding of the physical mechanisms of ice sheet changes driven by ice-ocean interactions by regional or local observational and modeling analyses (Straneo et al., 2013). Moreover, modeling the contribution of ice sheet and glacier mass loss to coastal sea level requires an adequate representation of ice sheet dynamics and of changing grounding lines in models (Pattyn et al., 2013), calling for the development of full Stokes models with a high-resolution at the grounding line. Coupling ice sheets to the solid Earth’s loading response and the related changes of regional sea level are a further factor influencing the dynamics of the grounding line and ice shelf (Konrad et al., 2013).

All coastal sedimentary systems are strongly controlled by feedbacks between water circulation and sediment transport through morphological features, such as subaqueous dunes or channels, coastal systems often show complex morphological responses to changes in sea level and ocean circulation as outcome of counteracting processes of sediment deposition versus erosion (Amos et al., 2010). Sea level change and extreme events (see below) therefore often induce morphodynamic changes, especially in deltaic regions where deterioration can be observed first in the submarine delta front (Plater and Kirby, 2012, Unverricht et al., 2013). However, on decadal and longer timescales morphodynamic models often suffer from clear limitations in reproducing complex morphological evolution (e.g. Chu et al., 2013 and refs. therein). To advance our understanding of the coastal evolution of our study regions, it is required to quantify and understand the sediment transport, erosion, deposition and preservation processes (that control coastal sedimentation from decadal to event timescales) as a function of changing sea level; at the same time we urgently need to improve morphodynamic models for those regions to allow quantitative predictions of those changes.

Finally, to advance the coastal management in our study regions it is necessary to improve projections of future sea level extremes, affected by extra-tropical storms (Woodworth et al., 2007) and by the increase in local mean sea level (Dangendorf et al., 2013). For many coastal regions with smooth topography a rising mean sea level will represent an increase in the frequency of storm surges, which have the potential to cause large economic and ecological damage when hitting insufficiently protected coasts (Hallegatte et al., 2011a, b). The estimation of changes in frequency and intensity of storm surges is, however, not straightforward, since they depend on many local parameters, sea level being only one. For coastal adaptation measures, knowing extremes of sea level change is crucial, as secular trends in sea level will most directly affect peaks in storm surges. However, to what extent secular sea level changes can be added linearly to surge changes (Woodworth et al., 2007) has to be investigated for various scenarios in our case study regions (Mudersbach et al., 2013). Previous model results point to a significant increase of storm surge elevations for the continental North Sea coast of between 15 to almost 25 cm (Woth, 2005). However, it has to be investigated whether this also holds in the North and Baltic Seas and for the Indonesian Archipelago.

Approach:

WP B will combine observations and modeling to downscale regional sea level changes to shelf-sea and coastal regions, both dynamically and statistically. Respective work aims at improving our understanding of sea level dynamics in the two case study regions to improve our ability to predict or project coastal sea level changes there. Work will involve regional simulations with model systems incorporating high-resolution coupled atmosphere-ocean-wave models, including tide. Altimetric databases will be extended to the coast, to consistently link it with tide gauge/GNSS observations and solid Earth response models, thus bridging the spatial scales from regional to local, and merging historical long-term records with current sea level monitoring and future predictions. This task will be facilitated by new observation technologies of modern satellite altimetry sensors. At the same time, work will target detailed ocean-ice sheet interaction studies leading to an improved process understanding of ice sheet mass loss which is demanded by WP A. Concerning observations of ice sheet changes, three fundamentally complementary approaches exist:

  1. the geometric approach, based primarily on satellite altimetry
  2. the input-output approach, which estimates the budget of surface mass balance and ice discharge using InSAR space techniques
  3. the gravimetric approach using the GRACE and the prepared GRACE follow on missions.

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

Research projects that fall within both the Research Areas A and B (WP A/B) are:

In addition, research projects that fall within both the Research Areas B and C (WP B/C) are:

Approach

The work program is 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

Coastal sea level projections

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Topic II

Feedback with extreme events and morphodynamics

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Topic III

Sea level - ice sheet interaction

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Topic IV

Coastal sea level information

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