The regime shift in the Baltic Sea area — Caused by the change of the NAO sign?

Abstract

In the Baltic Sea area during the late 1980s air and sea surface temperature increased. A longer growing season and increases in phytoplankton biomass as well as changes in the zooplankton and fish communities accompanied this. These changes are supposed to represent a regime shift in the ecology of the central Baltic Sea that could have been caused by a related sign change in the North Atlantic Oscillation (NAO) index. But the NAO has changed sign more frequently and most of the time no corresponding regime shift did occur therefore we should check this hypothesis more thorough. We investigated additional to the NAO a broad range of variables including air temperature, sea surface temperature, phytoplankton and zooplankton biomass, fish abundance and oxygen. The time series data are analyzed with respect to autocorrelation, linear trends and the occurrence of breakpoints whereby always tests for statistical significance are conducted. Tests for structural breakpoints in these time series reveal for some of the investigated variables the existence of such breakpoints in the 70–80 ties of the last century. But surprisingly in many physical and biological variables and in the most common climate indices no clear breakpoints can be identified. Specifically the change of the NAO sign around 1987, which is proposed to be the reason for an ecological regime shift in the Baltic Sea is not statistically significant. The coincidence of the sign change and an ecological regime shift could just be pure random, but even the evidence for a clear ecological regime shift is missing. In summary we strongly advocate to apply sound statistical procedures for detecting regime shifts instead of eye fitting and qualitative descriptions. A strong hypothesis like the postulation of a regime shift 1987 caused by the change in the NAO sign does require strong evidence, but it seems we might not have that.