A wind proxy based on migrating dunes at the Baltic Coast: statistical analysis of the link between wind conditions and sand movement

We statistically analyse the relationship between the structure of migrating dunes in the Southern Baltic and the driving wind conditions over the past 26 years, with the long-term aim of using migrating dunes as proxy for past wind conditions at interannual resolution. Dunes as wind proxies are not a totally new idea to the scientific community, but existing studies have so far analysed the link of dune structure and wind only on temporal resolutions of decades or millennia. The present analysis is based on the dune record derived from geo-radar measurements by Ludwig et al. (2016). The dune system is located at the Baltic Sea coast of Poland and is migrating from west to east along the coast. Ludwig et al. (2016) suggested that the analysed dunes show an alternation in the sediment composition that can be used to determine the annual migration velocity which can be seen as a wind proxy. Here, we present a detailed statistical analysis of this record and calibrate it as a wind proxy. To our knowledge there are no adequate, homogeneous meteorological station data for this area available to validate this proxy. Therefore we based our analysis on a gridded regional meteorological reanalysis data set (coastDat2) over the recent decades. We include precipitation and temperature into our analysis, in addition to wind, to learn more about the dependency between these three atmospheric factors and their common influence on the dune system. We set up a statistical linear model based on the correlation between the number of days with west and south-west wind directions above a pre-defined wind speed threshold and the dune migration velocities. To some extent, the dune intervals can be seen analogous to a tree ring widths, and hence we used a proxy-validation method usually applied in dendrochronology when the available meteorological record is short, namely the cross-validation with the leave-one-out-method. This revealed correlations between the wind record from the reanalysis and the reconstructed wind record derived from the dune structure in the range of 0.28 and 0.63. Thus, our study verifies that this type of dunes can be validated with dendrochronological methods and derive acceptable validation values as a wind proxy. The identified link between the dune annual layers and wind conditions from the meteorological reanalysis was additionally supported by the co-variability between dune layers and sea-level variations in the Southern Baltic Sea. Baltic Sea level variability in winter time is known to be strongly driven by westerly winds over this region. These results, therefore, provide an independent support, solely based on observations, of the link between annual dune layers and prevailing wind conditions.