with contributions from Jan Aure, Roald Sætre and Didrik Danielssen Decadal hydrographic variability in the Skagerrak and along the Norwegian coast Santander, Spain, 10-12 May 2011 Jon Albretsen with contributions from Jan Aure, Roald Sætre and Didrik Danielssen ICES/NAFO Symposium on the Variability of the North Atlantic and its Marine Ecosystems during 2000-2009

Content Decadal hydrographic variability in the Skagerrak and along the Norwegian coast Analysis is primarily based on measurements Focus on the hydrographic properties of the Norwegian Atlantic Drift and implications to the North Sea and Skagerrak and the Norwegian coast from the south to the Barents Sea. Have omitted the hydrographic properties in the surface layer due to more complex dynamics and a more chaotic energy exchange with the atmosphere.

Norwegian waters This map is a schematic picture of the main current pattern for the Norwegian Atlantic Drift denoted in red, and the Norwegian Coastal Current shown as a blue dashed line. A small portion of the northward Atlantic drift peels off north of Shetland and enters the North Sea. The main portion of the Atlantic flow is found at 100-200m depth along the slope of the Norwegian Trench, and the hydrographic properties and trends at 200m depth are investigated. Note that Atlantic water will also enter the North Sea in the surface layer, but these waters will not be presented here.

Data used The fixed stations along the Norwegian coast 1940’s – 2010 (IMR) The Skagerrak transect (Torungen – Hirtshals) 1952 – 2010 (IMR) The Shetland polygon 1940’s – 2010 (NISE and ICES) This map points out the different data that are used in our analysis. The fixed stations are distributed along the Norwegian cost, and along with the Skagerrak transect, these data accounts for extreme valuable hydrographic time series, not only for climate analysis, but also within model validation, water transport analysis etc. The upstream location of most Atlantic water entering the Nordic Seas is between the Faroe Islands and Shetland, and hydrographic measurements are collected within a polygon west of Shetland at 200m depth. Source is the NISE and ICES database, for data before and after 2004 respectively. The winter inflow to the North Sea between the Orkneys and Norway is analyzed from ocean model results, and the time series correspond well to the winter NAO index. Winter inflow to the North Sea 1955 – 2008 (IMR, NORWECOM)

From real values to normalized values Want to compare hydrographic properties at areas with different level of variability – normalize all time series according to the 1961-90 state. Data are retrieved from the Sognesjøen data serie Interpretation: the value normalized value is in terms of the number of standard deviations the value is above or below the mean Data from the winter season (JFM) is applied in our analysis as we believe it better reflects the long-term climatic signals.

Hydrographic decadal changes Hirtshals Torungen Lista Utsira Eggum Ingøy These panels show the normalized decadal means from the 1950’s for Skagerrak) and the 1940’s for the fixed stations. The stations are picked out as representative for the Norwegian coast. All stations representing the Skagerrak and the Norwegian coast from the South to the Barents Sea indicate an enhanced warming the last decade. Some stations indicate a raise in salinity as well, but this signal is less clear. Depth 200m, representing Atlantic water masses

Hydrographic changes in Atlantic water masses for the last decade from south to north This graph focus on the 2000-09 increase in temperature and salinity from the south to the north, and normalized values are shown. Shows a tremendous increase in temperature the last decade compared with the 1961-90 normal period, in particular from the Norwegian west coast and northward. The salinity has increased as well, more evenly along the coast. Skagerrak → Nordic Sea → Barents Sea

Modelled winter inflow to the North Sea between Norway (Utsira) and the Orkneys The figure ranges from 1955 to 2008. The thin line indicate the annual winter values, and the thick line is the 5 year running average. High inflow the last two decades, but closer to normal after the extreme years 1989, 1990 and 1993, and to some extent 1997 and 2000. These values corresponds well with the evolvement in the winter NAO-index (North Atlantic Oscillation). Main message: After increased inflow of Atlantic water masses in the 1990’s, the volume transport has been more normal during the last decade. Despite the lowering of inflow of Atlantic water the last decade, the temperature has increased.

Comparison of decadal changes in temperature with the salinity change West of Shetland Skagerrak and the Norwegian coast Compare the Atlantic inflow between Shetland and the Faroe Islands with corresponding water masses in the Skagerrak and along the Norwegian coast, still represented by values at 200m depth. Data series have been separated into before (red) and after (blue) 1990 and plotted in TS-diagrams. Shows clearly for both the West-Shetland sites and the Skagerrak/Norwegian coastal sites that the temperature have increased for a fixed salinity value during the last two decades, thus indicating a warming of the Atlantic water masses in Norwegian waters. These diagrams also show the close link between the properties of Atlantic waters in the Faroe Island-Shetland channel and the Skagerrak and Norwegian coast. Normalized values at 200m depth from 1940-2009 (sources: NISE and ICES) Normalized decadal means at 200m depth from 1940-2009 from 9 stations in Skagerrak and along the Norwegian coast

Quantifying the temperature increase Decadal anomalies for 1990-99 and 2000-09 (JFM) We quantify the temperature increase in the Atlantic water masses for the last decade compared with the 1961-90 normal period to approximately 1 degree in the Skagerrak and along the Norwegian coast, corresponding well with estimates from other publications on North Atlantic Drift. The temperature increase in the 1990’s is estimated to account for less than half of this. Some of this warming is controlled by the intensity of the cyclonic Subpolar Gyre. Along with low NAO index, as observed in the last decade, the Subpolar Gyre tends to be displaced towards the west, opening for greater inflows of saltier and warmer water from the eastern North Atlantic, the Subtropical Gyre. Hátun et al. (2005) South → North

Reflecting the global temperature increase ≈ 0.5 oC The same data points from the Norwegian coast are shown as decadal anomalies to quantify the warming. The observed temperature increase of 0.5 degrees reflects the NOAA global temperature increase well. Decadal anomalies at 200m depth from 1940-2009 from 9 stations in Skagerrak and along the Norwegian coast Red: Before 1990 Blue: After 1990

Hydrographic conditions in the Skagerrak bottom water – decadal variability Sill depth 270m In contrast to the heating of the Atlantic water in the Nordic Seas and the North Sea, I will end this presentation by showing some analyzed observations from the Skagerrak basin, also known as the Norwegian Trench. With a maximum depth of more than 700m and a sill depth about 270m, the bottom water is changed only occasionally, every 1 to 4 years. The hydrographic properties in the Skagerrak bottom waters have changed the last two decades toward a warmer and saltier climate. The last two decades, the bottom water exchange mechanism has mainly been due to Atlantic inflow. Before 1990, both Atlantic inflow into the basin and convection of winter cooled North Sea water renewed the stagnating bottom water. But during the winter 2010, the intense cooling of the North Sea shelf waters cascaded into the deep water. Max. depth 700m

Hydrographic conditions in the Skagerrak bottom water – time series from the 1960’s and the last decade 1960-69 2000-2010 T S D Note the dramatic decrease in bottom temperature in 2010 by almost 2 degrees. The temperature drop is comparable to the convection of cooled North Sea waters in 1963 and 1966. This type of physical process may have implications to the marine life, e.g. the shrimp stock. Publications from the 1960’s and 70’s presented theories that cooling of the Skagerrak basin lead to dramatic decrease in shrimp catches and an increase in density of jelly fishes. Fishermen met similar conditions in 2010, but I will emphasize that the reduction in shrimp catches is probably also influenced by other factors than just water temperature. O2 600m depth

Summary Measurements of the hydrographic properties of the Atlantic water in the Shetland-Faroe Island channel, the Skagerrak and along the Norwegian coast indicate an enhanced warming the last decade. After increased inflow of Atlantic water masses into the North Sea in the 1990’s, the volume transport has normalized (i.e., been reduced) during the last decade. Despite the lower inflow of Atlantic water during the last decade, the temperature has increased. The warming is quantified to about 1 oC where half of this is accounted for in the decadal fluctuating strength of the Subpolar Gyre. The remaining increase in temperature (0.5 oC) reflects the monitored NOAA global temperature increase well. Cold winters in the North Sea area (as in 2010) may induce a cooling of the Skagerrak bottom waters, and this type of physical process may have implications to the marine life, e.g. the shrimp stock.