Mario Hoppema

The reinvigoration of the Southern Ocean carbon sink

Uptake uptick Has global warming slowed the uptake of atmospheric CO2 by the Southern Ocean? Landschützer et al. say no (see the Perspective by Fletcher). Previous work suggested that the strength of the Southern Ocean carbon sink fell during the 1990s. This raised concerns that such a decline would exacerbate the rise of atmospheric CO2 and thereby increase global surface air temperatures and ocean acidity. The newer data show that the Southern Ocean carbon sink strengthened again over the past decade, which illustrates the dynamic nature of the process and alleviates some of the anxiety about its earlier weakening trend. Science, this issue p. 1221; see also p. 1165 Carbon uptake by the Southern Ocean has increased again after its slowdown in the 1990s. [Also see Perspective by Fletcher] Several studies have suggested that the carbon sink in the Southern Ocean—the ocean’s strongest region for the uptake of anthropogenic CO2 —has weakened in recent decades. We demonstrated, on the basis of multidecadal analyses of surface ocean CO2 observations, that this weakening trend stopped around 2002, and by 2012, the Southern Ocean had regained its expected strength based on the growth of atmospheric CO2. All three Southern Ocean sectors have contributed to this reinvigoration of the carbon sink, yet differences in the processes between sectors exist, related to a tendency toward a zonally more asymmetric atmospheric circulation. The large decadal variations in the Southern Ocean carbon sink suggest a rather dynamic ocean carbon cycle that varies more in time than previously recognized.

Winter-summer differences of carbon dioxide and oxygen in the Weddell Sea surface layer

Abstract Mid-winter total inorganic carbon (TCO 2 ) and oxygen measurements are presented for the central fully ice-covered Weddell Sea. Lateral variations of these properties in the surface layer of the central Weddell Sea were small but significant. These variations were caused by vertical transport of Warm Deep Water into the surface layer and air-sea exchange before the ice cover. Oxygen saturation in the surface layer of the central Weddell Sea was near 82%, whereas in the eastern shelf area this was 89%. Surprisingly, p CO 2 , as calculated under the assumption of (reported) conservativeness of alkalinity, was also found to be below saturation (86–93%). This was not expected since ongoing Warm Deep Water entrainment into the surface layer tends to increase the p CO 2 . Rapid cooling and subsequent ice formation during the previous autumn, however, might have brought about a sufficiently low undersaturation of CO 2 , that as to the point of sampling had not yet been replenished through Warm Deep Water entrainment. In the ensuing early summer the measurements were repeated. In the shelf area and the central Weddell Sea, where the ice-cover had almost disappeared, photosynthesis had caused a decrease of p CO 2 and an increase of oxygen compared to the previous winter. In between these two regions there was an area with significant ice-cover where essentially winter conditions prevailed. Based on the summer-winter difference a (late-winter) entrainment rate of Warm Deep Water into the surface layer of 4–5 m/month was calculated. A complete surface water balance, including entrainment, biological activity and air-sea exchange, showed that between the winter and summer cruises CO 2 and oxygen had both been absorbed from the atmosphere. The TCO 2 increase due to entrainment of Warm Deep Water was partly countered by (autumn) cooling, and partly through biological drawdown. Part of the CO 2 removed through biological activity sinks down the water column as organic material and is remineralised at depth. It is well-known that bottom water formation constitutes a sink for atmospheric CO 2 . However, whether the Weddell Sea as a whole is a sink for CO 2 depends on the ratio of two counteracting processes i.e. entrainment, which increases CO 2 in the surface and the biological pump, which decreases it. As deep water is not only entrained into the surface, but also conveyed out of the Weddell Sea, the relative importances of these (CO 2 -enriched) deep water transports are important as well.

Seasonally different carbon flux changes in the Southern Ocean in response to the southern annular mode

Stratospheric ozone depletion and emission of greenhouse gases lead to a trend of the southern annular mode (SAM) toward its high-index polarity. The positive phase of the SAM is characterized by stronger than usual westerly winds that induce changes in the physical carbon transport. Changes in the natural carbon budget of the upper 100 m of the Southern Ocean in response to a positive SAM phase are explored with a coupled ecosystem-general circulation model and regression analysis. Previously overlooked processes that are important for the upper ocean carbon budget during a positive SAM period are identified, namely, export production and downward transport of carbon north of the polar front (PF) as large as the upwelling in the south. The limiting micronutrient iron is brought into the surface layer by upwelling and stimulates phytoplankton growth and export production but only in summer. This leads to a drawdown of carbon and less summertime outgassing (or more uptake) of natural CO2. In winter, biological mechanisms are inactive, and the surface ocean equilibrates with the atmosphere by releasing CO2. In the annual mean, the upper ocean region south of the PF loses more carbon by additional export production than by the release of CO2 into the atmosphere, highlighting the role of the biological carbon pump in response to a positive SAM event.

Annual uptake of atmospheric CO2 by the Weddell Sea derived from a surface layer balance, including estimations of entrainment and new production

Data from two cruises, one in April/May 1996 and one in December/January 1993, covering the same wide area in the offshore Weddell Sea, were used to derive the annual extent of entrainment and the capacity of the biological pump. The former property was obtained with the help of dissolved oxygen data, whereas the latter was approximated with nutrients. Especially the data from April/May, representing the initial state of the winter surface layer, were crucial to assess the annual extent of these processes. The results were applied to our carbon dioxide data. The annual increase of the Total CO2 (TCO2) concentration in the surface layer due to vertical transport amounts to 16.3 μmol kg−1. An entrainment rate of deep water in the surface layer amounting to 35±10 m yr−1 was deduced. The compensating, biologically mediated TCO2 reduction was calculated to be larger than the TCO2 increase due to vertical transport. Since the balance of these two processes determines whether the Weddell Sea is a source or a sink of CO2, this indicates that the Weddell Sea, albeit upwelling area, is definitely a sink for atmospheric CO2 on an annual basis. This conclusion is further supported by contemplations that the biological drawdown of CO2 in the Weddell Sea as a whole is probably underestimated by our calculations. The new production for the Weddell Sea on a per unit area basis was found to be much higher than that for the Antarctic Ocean, when the latter value is being obtained by traditional biological methods. On the other hand, the CO2 uptake by the Weddell Sea on a per unit area basis is somewhat smaller than the CO2 uptake by the world ocean.

Annual export production in the interior Weddell Gyre estimated from a chemical mass balance of nutrients

Nitrate, phosphate and silicate data are presented from 1992 austral winter and 1998 austral autumn cruises with “FS Polarstern” in the Weddell Gyre. Because in the Weddell Gyre, away from the boundary current, the surface layer is eventually formed from upwelled deep water, the difference in nutrient concentrations between these layers can be used to compute net nutrient consumptions (identical with the export production). This method renders a value for the export production that is based on observed annual changes. The results are consistent for two years and two regions within the central gyre. The calculated net nitrate and phosphate consumptions were scaled to net carbon consumptions using canonical Redfield ratios, yielding 16–17 μmol C kg−1 yr−1. This equals 21±4 g C m−2 yr−1 as a robust estimate for the marginal ice zone. The net annual silicate consumption in the surface layer, which equals the export of biogenic silica, amounts to 15–18 μmol kg−1 yr−1. There is a tendency for higher values in the eastern Weddell Gyre. The estimated silicate consumption of about 1.8 mol Si m−2 yr−1 is relatively high compared to earlier estimations of biogenic silica export. The silicate to carbon consumption ratio of about 1 is very high, and documents the dominance of diatoms in the export of organic material.

CO2 in the Weddell Gyre and Antarctic Circumpolar Current: austral autumn and early winter

Abstract Quasi-continuous fugacity of CO 2 (fCO 2 ) data were collected in the eastern Weddell Gyre and southern Antarctic Circumpolar Current (ACC) of the Southern Ocean during austral autumn 1996. Full depth Total CO 2 (TCO 2 ) sections are presented for austral autumn and winter (1992) cruises. Pronounced fCO 2 gradients were observed at the Southern Ocean fronts. In the Weddell Gyre, fCO 2 regimes appeared to coincide with surface and subsurface hydrographic regimes. The southern ACC was supersaturated with respect to CO 2 , as was part of the northern Weddell Gyre. The southern Weddell Gyre was markedly undersaturated. The great potential of autumn cooling for generating undersaturation and CO 2 uptake from the atmosphere was demonstrated. In the northeastern Weddell Gyre, upwelling of CO 2 - and salt-rich deep water was shown to play a role as the horizontal fCO 2 distribution closely resembled that of the surface salinity. The total uptake of atmospheric CO 2 by the Weddell Gyre in autumn (45 days) was calculated to be 7·10 12 g C. The deep TCO 2 distribution noticeably reflected the different water masses in the region. A new deep TCO 2 maximum was detected in the ACC, which apparently characterizes the boundary between the equatorward flowing Antarctic Bottom Water (AABW) and the Circumpolar Deep Water (CDW). East of the Weddell Gyre, the AABW stratum is much thicker (>2000 m) than more to the west, on the prime meridian (

The contribution of the Weddell Gyre to the lower limb of the Global Overturning Circulation

The horizontal and vertical circulation of the Weddell Gyre is diagnosed using a box inverse model constructed with recent hydrographic sections and including mobile sea ice and eddy transports. The gyre is found to convey 42 ± 8 Sv (1 Sv = 106 m3 s-1) across the central Weddell Sea and to intensify to 54±15 Sv further offshore. This circulation injects 36±13 TW of heat from the Antarctic Circumpolar Current to the gyre, and exports 51 ± 23 mSv of freshwater, including 13 ± 1 mSv as sea ice to the mid-latitude Southern Ocean. The gyres overturning circulation has an asymmetric double-cell structure, in which 13 ± 4 Sv of Circumpolar Deep Water (CDW) and relatively light Antarctic Bottom Water (AABW) are transformed into upper-ocean water masses by mid-gyre upwelling (at a rate of 2 ± 2 Sv) and into denser AABW by downwelling focussed at the western boundary (8 ± 2 Sv). The gyre circulation exhibits a substantial throughflow component, by which CDW and AABW enter the gyre from the Indian sector, undergo ventilation and densification within the gyre, and are exported to the South Atlantic across the gyres northern rim. The relatively modest net production of AABW in the Weddell Gyre (6±2 Sv) suggests that the gyres prominence in the closure of the lower limb of global oceanic overturning stems largely from the recycling and equatorward export of Indian-sourced AABW.

Interannual variations of the Antarctic Ocean CO2 uptake from 1986 to 1994

The interannual variations of CO2 sources and sinks in the surface waters of the Antarctic Ocean (south of 50°S) were studied between 1986 and 1994. An existing, slightly modified one-dimensional model describing the mixed-layer carbon cycle was used for this study and forced by available satellite-derived and climatological data. Between 1986 and 1994, the mean Antarctic Ocean CO2 uptake was 0.53 Pg C year−1 with an interannual variability of 0.15 Pg C year−1. Interannual variation of the Antarctic Ocean CO2 uptake is related to the Antarctic Circumpolar Wave (ACW), which affects sea surface temperature (SST), wind-speed and sea-ice extent. The CO2 uptake in the Antarctic Ocean has increased from 1986 to 1994 by 0.32 Pg C. It was found that over the 9 years, the surface ocean carbon dioxide fugacity (fCO2) increase was half that of the atmospheric CO2 increase inducing an increase of the air–sea fCO2 gradient. This effect is responsible for 60% of the Antarctic Ocean CO2 uptake increase between 1986 and 1994, as the ACW effect cancels out over the 9 years investigated.

Increase of carbon dioxide in the bottom water of the Weddell Sea, Antarctica

High precision Total CO2 (TCO2) data are presented from the NW Weddell Sea obtained during two cruises which were 3 years apart. A TCO2 increase from 1993 to 1996 was observed in the newly formed bottom water, whereas no TCO2 increase was found in the surrounding water masses. Accompanying this TCO2 increase in the bottom water was an oxygen decrease. Obviously, bottom water with variable characteristics is produced along the margins of the Weddell Sea. Examination of possible causes leads to the conclusion that the bottom water variability is largely due to varying amounts of Warm Deep Water intruding onto the shelves of the Weddell Sea, thus changing the shelf water end-member of bottom water formation. Analysis of the data, using the observed differences of oxygen to perform a correction, is suggesting that some part of the TCO2 increase of the bottom water is due to the increased level of anthropogenic CO2. The TCO2 increase of the bottom water is commensurate to a tentative annual increase of about 1 µmol kg-1 in the surface water source of this bottom water. This would agree fairly well with the increase of the partial pressure of CO2 in the atmosphere.

Intense nutrient removal in the remote area off larsen ice shelf (Weddell Sea)

Abstract Using Weddell Sea data collected during a cruise with “FS Polarstern” in austral summer 1992/1993, depletions of nutrients and TCO2 in the summer surface layer were calculated. The analogous depletion-like properties for temperature (Heat Storage) and salinity were also computed. The latter properties are useful to describe the physical conditions over the time period pertinent to the depletions. For different areas a strong correlation exists of Heat Storage and nutrient/TCO2 depletions, which is caused by a common factor – the period of light availability. Offshore of the Larsen shelf, an area usually inaccessible due to perennial ice cover, high nutrients/TCO2 depletions are achieved over a short period of time, pointing to a rapidly producing biological system. Primary productivity, calculated from the TCO2 depletion, amounts to about 100 mg C m−2 day−1 for the central Weddell Sea, but 570–1140 mg C m−2 day−1 for the offshore Larsen region. These values agree fairly well with the open-ocean Antarctic and other coastal areas, respectively.