Revisiting the Estimate of the North Sea Air‐Sea Flux of CO₂ in 2001/2002: The Dominant Role of Different Wind Data Products

Abstract

For the North Sea, a semienclosed shelf sea in the northeastern North Atlantic, the seasonal and annual CO2 air‐sea fluxes (ASF) had been estimated for 2001 and 2002 in earlier work. The underlying observations, ΔpCO2, salinity, and temperature had been combined with 6‐hourly wind data derived from ERA40 reanalysis. In order to assess the impact of different wind data products on the computation of CO2 ASF, we compared ERA40 wind data with coastDat data derived from the nonhydrostatic regional climate model COSMO‐CLM. From the four observational months September, November, February, and May all but the May data show higher wind speeds for coastDat than for ERA40, especially off the Norwegian, UK, and continental coasts. Largest differences occur in the northern offshore areas. The comparison with observed wind data supports this feature generally: At Helgoland, an island in the German Bight, and at the Belgium pile “Westhinder” the ERA40 data underestimate both, the coastDat data and the observations. Wind observations for two Norwegian North Sea platforms were available: At the northern station “Troll” off the Norwegian coast the coastDat data overestimate the observations in winter. At “Ekofisk” in the central North Sea the ERA40 data fit the observations well, while the coastDat data slightly overestimate the observational data in all months but in May. The corresponding CO2 ASF estimates show strongest deviations off the Norwegian coast. Using different bulk formulas for determining the net annual ASF resulted in differences due to different wind products of up to 34%.

Winter weather controls net influx of atmospheric CO2 on the north-west European shelf

“…Modelling studies for the NW European shelf are within ±50% of observation-based estimates [e.g. 1.5–2.6 mol C m −2 y −1 for the North Sea 7,35 compared to 1.8 ± 0.3 mol C m −2 y −1 for observations 25,28–30 ]. Wider disagreement is found for long-term climate projections of wind speed (e.g.…”

“…Over the last two decades, a number of studies have quantified the air to sea flux of CO 2 over the NW European shelf, regions within it or the wider European shelf 25,28–30 . When scaled to their respective areas, these estimates (including our own) fall in the range of 1.3–2.1 mol C m −2 y −1 with a mean value of 1.8 ± 0.3 mol C m −2 y −1 (Table 1).…”

“…Firstly, we combined our 2015 estimate of air to sea CO 2 flux with previous studies to obtain a mean influx term from the atmosphere 25,28–30 .…”

The northern European shelf as an increasing net sink for CO<sub>2</sub>

“…The carbonate system was calculated using the CO2SYS program (van Heuven et al, 2009) with carbonic acid dissociation constants of Mehrbach et al (1973) as refitted by Dickson and Millero (1987), KSO − 4 dissociation constants following Dickson (1990) and the boron-salinity relation following Uppström (1974). For the Baltic Sea, we did not calculate pH as the alkalinity-salinity relationship in this region is complex due to different AT-S relations in different subregions of the Baltic Sea and a non-negligible increase in AT over the last 25 years (Müller et al, 2016).…”

The northern European shelf as increasing net sink for CO<sub>2</sub>

“…North Sea Meyer et al (2018Meyer et al ( ) 2001Meyer et al ( /2002 Baltic Sea Parard et al (2017) 1998-2011 1.2…”