Frequencies and pathways forcing major inflows to the Baltic Sea

Transcription

1 12th Polish German Seminar The Baltic Sea at the middle of the 21st Century , Sopot, Poland Frequencies and pathways forcing major inflows to the Baltic Sea Andreas Lehmann, Piia Post*, Katharina Höflich, Kai Myrberg** and Klaus Getzlaff GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany * Tartu University, Estonia ** SYKE Finnish Environmental Institute/Marine Research Centre, Helsinki, Finland

2 Motivation This work is a contribution to Baltic Earth Grand Challenge 'Salinity dynamics in the Baltic Sea'

3 Map of the Baltic Sea ICES SD 25 Bornholm Basin Motivation: Major Baltic Inflows Occurrence of MBIs (Matthäus 2006)

4 Motivation: Major Baltic Inflows Landsort sea surface elevation during MBI Occurrence of MBIs (Matthäus, 2006)

5 Motivation: Major Baltic Inflows Sea level, Salinity and Runoff SSE Landsort (black), SD 25 deep salinity (red), runoff (blue), MBI (green)

6 Motivation: Large Scale atmospheric variability First EOF DJFM-averaged SLPanomalies, NCEP-NCAR reanalysis Data Total number of (DJFM) deep cyclones < 980 hpa, NCEP-NCAR reanalysis Data Lehmann et al. 2011

7 Motivation: Major Baltic Inflows Long-term salinity change Occurrence of MBIs ICES SD 28 Salinity Gotland Basin Mean salinity & accumulated runoff Decreasing number of MBIs since 1983: Relation to changes in large scale atmospheric patterns (AP)? Relation to changes in P-E + R? Relation to large volume changes (LVC)? Relation changes in regional atmospheric patterns (pathways/frequency)?

8 Sea level Landsort Large Volume Change SSE Landsort (blue), filtered TS(red; Pasanen et al. 2013), minima (yellow), maxima (green); LVC (cyan) defined as volume change of at least 0.5% of the total volume of the Baltic Sea

9 Sea level Landsort Large Volume Change SSE Landsort (blue), filtered TS(red), minima (yellow/cyan), maxima (green)

10 LVC Large Volume Change Large volume changes (LVCs, 77 events), threshold 29 cm, MBIs (red)

11 LVC Sea level Landsort SSE Landsort (blue), filtered TS(red), minima (yellow/cyan), maxima (green)

12 Cyclone tracking Composite of MBI: 1951, 1965, 1969, 1973, 1993 LVC: 1983, 1989, 1990, 2005, 2007 Preliminary result 77 events will be analysed soon Cyclone frequency, composite of 10 major inflows

13 Atmospheric circulation types/classes 10 Jenkinson-Collison classes/types or so called Lamb automatic types of atmospheric circulation based on 6 different flow indices, that quantify the zonal and meridional airflow and vorticity

14 Atmospheric circulation types/classes Sequences of JCC circulation classes for 74 events 121-days long periods. Day 0 is the maximum in sea surface elevation

15 Atmospheric circulation types/classes days before max >38 >40 >42 AC >36 C >34 SW >32 S >30 SE >28 E >26 NE N NW W >24 >44 Histogram of 10 JCC circulation classes with varying threshold cm

16 Atmospheric circulation types/classes 30-1 days before max >38 >40 >42 AC >36 C >34 SW >32 S >30 SE >28 E >26 NE N NW W >24 >44 Histogram of 10 JCC circulation classes with varying threshold cm

17 Atmospheric circulation types/classes 1-30 days after max >38 >40 >42 AC >36 C >34 SW >32 S >30 SE >28 E >26 NE N NW W >24 >44 Histogram of 10 JCC circulation classes with varying threshold cm

18 Conclusions Major inflows or LVCs are related to specific deep cyclone tracks and atmospheric types(jcc) There are two main routes, one cyclone track approaching from the west at about 62 N, passing Oslo, Norway and Gotland island, and a second one approaching from the west at about 66 N, crossing Scandinavia south-eastwards passing the Bay of Bothnia and entering Finland With respect to deep cyclone tracks and JCCs there is no obvious difference between major salt water inflows (MBI) and Inflows without strong salt water influx (LVC) Changes in the large-scale atmospheric patterns could not explain the decreasing number of MBIs since 1983 Changes in freshwater input (mostly river runoff) determines the average salinity of the Baltic Sea However, on shorter time scales there is a direct impact of increased/decreased freshwater input on the salt transport into the Baltic Sea during major inflows

19 12th Polish German Seminar The Baltic Sea at the middle of the 21st Century , Sopot, Poland A simple approach to determine the Stokes Drift for the entire Baltic Sea Andreas Lehmann, Irina Didenkulova*, Katharina Höflich and Klaus Getzlaff GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany * Tallinn University, Estonia ** SYKE Finnish Environmental Institute/Marine Research Centre, Helsinki, Finland

20 Motivation and approach SAR TANDEM-X measures ocean surface velocity Ocean surface velocity is combination of Ekman surface flow + baroclinic flow components + Stokes drift Surface Ekman and baroclinic flow field can be calculated by numerical models Stokes drift needs an additional approach Stokes velocity is a function of signifcant wave height and period Stokes drift velocity can not be validated by direct measurements Wave height and period can be calculated by wave models such as WAM Alternative formulas of sea state parameters are available (e.g Schmager 1985) These formulas are a function of wind speed and fetch Formulas can be validated by wave measurements Atmospheric forcing and fetch have been calculated on BSIOM model grid Wave climate based on Schmager has been calculated for the period (with seasonal ice coverage considered) Wave climate has been validated by wave measurements Stokes drift..., 13 October 2014

21 Mean significant wave height Stokes drift..., 13 October 2014

22 Seasonal means of significant wave height Stokes drift..., 13 October 2014

23 Change of seasonal significant wave height

24 Validation of signifcant wave period Stokes drift..., 13 October 2014

25 Validation of signifcant wave period Stokes drift..., 13 October 2014