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Past and ongoing changes in the North Sea and its interface regions

John Huthnance, Ralf Weisse and NOSCCA team

NOSCCATemperature, salinity, stratification; circulationSea level, surges, wavesCarbon/pH, oxygenSPM, morphology, Wadden Sea; iceSummary conclusions

Part of NOSCCA Chapter 2

Ch 2 - Past and current climate change - includesAtmosphere; North Sea; Hydrology and Vegetation

North Sea teamJohn Huthnance, Ralf Weisse;Thomas Wahl, Helmuth Thomas, Julie Pietrzak, Alex Souza,

Sytze van Heteren, Natalija Schmelzer, Justus van Beusekom;Franciscus Colijn, Ivan Haigh, Solfrid Hjøllo, Jürgen Holfort, Liz Kent,

Wilfried Kühn, Peter Loewe, Ina Lorkowski, Kjell-Arne Mork, Johannes Pätsch, Markus Quante, Lesley Salt, John Siddorn,

Tim Smyth, Andreas Sterl, Philip Woodworth

North Sea past and ongoing - Scope

OSPAR greater North Sea to 5°W, 61°N

temperature, salinity and stratification;

currents and circulation;

mean sea level, extreme sea levels (contributions from wind generated waves and storm surges);

CO2, pH and nutrients; oxygen;

suspended matter and turbidity;

sedimentation, morphology and coastal erosion

sea ice; Wadden Sea.

“past and ongoing”: 200 years up to the present.

Temperature trend

North Sea averaged SST annual anomalies, 1870-2013 c.f. 1971-2000 (Elizabeth Kent)

Annual and seasonal mean North Sea heat content / 107J/m3 1950-2007 (Meyer et al. 2011)

Temperature linear trend distributions

SST trends (°C/decade) 1983‑2012 from HadISST1 (Dye et al. 2013)

(L to R) 1985–2004 model near-bed temperature, satellite SST, 2 m ERA40 air temperature (Holt et al. 2012)

SalinityLinear trend / decade, winter bottom salinity, from IBTS Q1 1971-2012 (Dye et al. 2013)

Winter bottom salinity from IBTS at Viking Bank, Dogger Bank and German Bight, & annual mean salinity, Helgoland Roads (Holliday et al. 2010)

Stratification

Energy needed to mix water column (model, log scale, 1 Aug. 2001; Holt & Proctor 2008)

S-N section, potential temperature (°C) near 2½°E (further E around Dogger Bank) August 2010 (Queste et al. 2013)

Modelled timing of spring stratification (Ttop – Tbottom > 0.5°C for ≥ 3 days; solid line) and spring bloom (dashed line), 1974-2003 in 60 m water near 1.4°W 56.2°N (Sharples et al. 2006)

Currents / Circulation

Multiple forcings → various time & space scalesMeasurements sparse c.f. short scales→ trends less significant, hard to discern or diagnose causesReliance on modelsImprovements wanted (models & observations) (√ tides, surges)

L: transport EOF1 (75.8% of variability) from 1962-2004 simulation (Emeis et al. 2014).

R: EOF1 time series (red), NAO index (blue; Hurrell et al. 2013)

Mean and high sea levelsConsiderable inter-annual and decadal variability in waves

(Upper inset) MSL index for Inner North Sea (black) & non-linear SSA smoothed (red); (lower inset) MSL index for the Channel (black) & non-linear SSA smoothed (red); Wahl et al. (2013); (colour) Standard deviation from de-trended annual MSL time series;

(Bottom) Cuxhaven annual mean high water and linear trend (m), 1843-2012; (Top) annual 99-percentile of high-tide levels at Cuxhaven after subtracting linear trend in annual mean; 11-year running mean is shown (red). After von Storch and Reichardt (1997).

carbon, oxygen

Surface DIC, August / September 2001, 2005, 2008 (Salt et al. 2013); anomalies relative to the average (bottom right). (Helmuth Thomas)

Simulations show enhanced CO2 uptake as atmospheric pCO2 increases, acidifying the North Sea (Lorkowski et al. 2012).

August 2010 bottom 3 m oxygen saturation (%) & 70 % saturation contour (Queste et al. 2013)

-50 0 50

DIC anomaly (2001) [mmol kg-1 ]

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DIC anomaly (2005) [mmol kg-1 ]

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DIC anomaly (2008) [mmol kg-1 ]

c)

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2100 2200

average DIC (2001/5/8) [mmol kg-1 ]

d)

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Suspended Particulate Matter Varies with

SPM monthly averages from SeaWiFs, 1998 (Pietrzak et al. 2011) Inset pixel value colour bars (x-axis) & histograms (white, y-axis)

Tides

Waves

Stratified river flow season

Coastal erosion, sedimentation, morphology

Cliff-top retreat rates at Holderness calculated in three approximate-50-year periods. Areas defended in 2005 are in grey. Retreat rates vary within each zone. (Brown 2008)

Ice

German North Sea coast for 1897 to 2012: annual accumulated areal ice volume V(AΣ) = ∫(fractional area × thickness)dt (days). Update from http://www.bsh.de/de/Meeresdaten/Beobachtungen/Eis/Eiswinter2010_11.pdf

Wadden Sea

Mean autumn ammonium and nitrite: pre-industrial times, 1960-1962 (Postma 1966), 5 wettest (1986-1988, 1994, 1995) and 5 driest years (1991-1993, 1996, 1997) in 1986-1997, 2000-2006; western Dutch Wadden Sea (van Beusekom & de Jonge 2002; van Beusekom et al. 2009). Latter three with standard deviation.

Summary conclusions – no surprises! - strong evidence of warming, especially since the 1980s, despite temperature variability on all time-scales

- shorter-term variations in salinity > climate-related changes

- absolute mean sea level in the North Sea rose ~ 1.6 mm/year over the past 100-120 years

- extreme levels rose primarily because mean sea level rose

- net CO2 uptake from atmosphere

- North Sea is net nitrogen sink for the Atlantic

- suspended matter and turbidity are very variable

- coastal erosion is extensive but irregular.

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