Towards downscaling changes of oceanic dynamics Hans von Storch and Zhang Meng ( 张萌 ) Institute for Coastal Research Helmholtz Zentrum Geesthacht, Germany and 物理海洋教育部重点实验室 ( 中国海洋大学 ) Physical Oceanography Laboratory, Ocean University of China 27 June 2015, 26th IUGG General Assembly, Prague

The concept of „(statistical) downscaling“ was introduced into climate sciences in the late 1980s/early 1990s when the need for regional and local information about climate change emerged. A number of different methods have evolved and matured since then. "Downscaling" is based on the view that regional climate is conditioned by climate on larger, for instance continental or even planetary, scales. Information is cascaded "down" from larger to smaller scales. The regional climate is the result of interplay of the overall atmospheric, or oceanic, circulation and of regional specifics, such as topography, land- sea distribution and land-use. As such, empirical/statistical downscaling seeks to derive the local scale information from the larger scale through inference from the cross-scale relationships, using a function F such that: local climate response = F (external, large scale forcing) Concept of Statistical Downscaling

When using downscaling for assessing regional climate change, three implicit assumptions are made: (1) The predictors are variables of relevance and are realistically modeled by the AOGCM and described in global re-analyses. (2) The transfer function is valid also under altered climatic conditions. This is an assumption that in principle can not be proven in advance. The observational record should cover a wide range of variations in the past; ideally, all expected future realizations of the predictors should be contained in the observational record. (3) The employed predictors fully represent the climate change signal.

崔茂常 (Cui M.), H. von Storch, and E. Zorita, 1995: Coastal sea level and the large-scale climate state: A downscaling exercise for the Japanese Islands. - Tellus 47 A, Case 1

崔茂常 (Cui M.), H. von Storch, and E. Zorita, 1995: Coastal sea level and the large-scale climate state: A downscaling exercise for the Japanese Islands. - Tellus 47 A, Link Japanese sea level and North Pacific air pressure Case 1

崔茂常 (Cui M.), H. von Storch, and E. Zorita, 1995: Coastal sea level and the large-scale climate state: A downscaling exercise for the Japanese Islands. - Tellus 47 A, Link Japanese sea level and North Pacific SST Case 1

Percentiles as Predictands R = percentiles of high tide water levels at a number of tide gauges along the North Sea coast. Each 3- months winter has about 180 high tides; from the distribution of these 180 values, percentiles are derived and related to L = large-scale winter mean SLP. Case 2 Langenberg, H., A. Pfizenmayer, H. von Storch and J. Sündermann, 1999: Storm related sea level variations along the North Sea coast: natural variability and anthropogenic change.- Cont. Shelf Res. 19:

Predictand: Intra-annual percentiles of water levels Winter mean Winter percentiles after subtraction of winter mean of high tides water levels. Case 2

Scenario “1% CO 2 increase” at the end of the 21 st century Thermal expansion not taken into account! North Sea Case 2

Outlook – regional variability in the South China Sea Defining a test case OCGM output, coarse grid Advantage: easy access, test of logistics easier Future: High resolution (10 km grid) simulations and global re-analyses; logistically demanding because of very large data volume. South China Sea Is this variability caused only by atmospheric variability or also conditioned by large- scale oceanic dynamics? Barotropic mass-streamfunction after removal of mean annual cycle

Summary Empirical downscaling methods have matured in the past decades. They are mostly used for estimating the state and statistics of a wide range of meteorological variables Applications to oceanographic and ecological variables are rare (or?) Empirical downscaling is a method which will remain to play an important role for assessing ongoing change and projecting possible future changes. 由物理海洋教育部重点实验室 ( 中国海洋大学 ) Supported by POL Visiting Fellowship Program