Variability of the meridional overturning circulation at the Greenland–Portugal OVIDE section from 1993 to 2010 (original) (raw)
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Journal Of Geophysical Research: Oceans, 2021
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Deep-sea Research Part I-oceanographic Research Papers, 2010
The horizontal circulation of the subpolar gyre and the Meridional Overturning Circulation (MOC) are investigated here by comparing two snapshots of the North Atlantic as delivered by two hydrographic sections between Greenland and Portugal. The corresponding cruises were carried out in June-July 2002 and June-July 2004 on R/V Thalassa within the framework of the Ovide project. The absolute transports in June 2004 are described in detail, and then compared with transports in June 2002. The MOC (in density coordinates), driven by the volume balance between the northward North Atlantic Current (NAC) and the net southward export of dense water from the subpolar gyre, did not change: (1 Sv=10 6 m 3 s −1). Its upper limb, above σ 1 =32.1, is decomposed into two main branches, the Eastern NAC (ENAC) and the Western NAC (WNAC), that transport about 8 Sv each. In the lower limb of the MOC, we find a 4-5 Sv increase in the cyclonic circulation of the subpolar gyre between June 2002 and 2004, affecting mainly the intermediate water without changing the MOC σ amplitude. Accordingly, the 14±2 Sv transport over Reykjanes Ridge in June 2004 (between 58 50′N and Iceland) is estimated to have been 4-5 Sv stronger than in June 2002. Sustaining this observation, a relatively warm and salty anomaly coming from the Iceland Basin was found in the East Greenland-Irminger Current (EGIC) in June 2004, along with a modified vertical structure of the transport that shows a 4-5 Sv intensification of the net southward flow in the corresponding layer. Overall, in June 2004, the EGIC (from the surface to σ 0 =27.8) is found at 23.7±1.4 Sv in June 2004, and the Deep Western Boundary Current (DWBC) below sums up to 11.2±1.7 Sv, so that the western boundary current is stronger than in June 2002.
Transports across the 2002 Greenland-Portugal Ovide section and comparison with 1997
Journal of Geophysical Research, 2007
The first Ovide cruise occurred in June-July 2002 on R/V Thalassa between Greenland and Portugal. The absolute transports across the Ovide line are estimated using a box inverse model constrained by direct acoustic Doppler current profiler velocity measurements and by an overall mass balance (±3 Sv, where 1 Sv = 10 6 m 3 s −1 ) across the section. Main currents are studied and compared to the results of the similar Fourex section performed in August 1997 and revisited here. The meridional overturning cell (MOC) is estimated in two different ways, both leading to a significantly lower value in June 2002 than in August 1997, consistent with the relative strength of the main components of the MOC (North Atlantic Current and deep western boundary current). It has been found that the MOC calculated on density levels is more robust and meaningful than when calculated on depth levels, and it is found to be 16.9 ± 1.0 Sv in 2002 versus 19.2 ± 0.9 Sv in 1997. The 2002 heat transport of 0.44 ± 0.04 × 10 15 W is also significantly different from the 0.66 ± 0.05 × 10 15 W found in 1997, but it is consistent with the much weaker integrated warm water transport across the section than in 1997.
Biogeosciences, 2014
The interannual to decadal variability in the transport of anthropogenic CO 2 (Cant) across the subpolar North Atlantic (SPNA) is investigated, using summer data of the FOUREX and OVIDE high-resolution transoceanic sections, from Greenland to Portugal, occupied six times from 1997 to 2010. The transport of Cant across this section, T cant hereafter, is northward, with a mean value of 254 ± 29 kmol s −1 over the 1997-2010 period. We find that T cant undergoes interannual variability, masking any trend different from 0 for this period. In order to understand the mechanisms controlling the variability of T cant across the SPNA, we propose a new method that quantifies the transport of Cant caused by the diapycnal and isopycnal circulation. The diapycnal component yields a large northward transport of Cant (400 ± 29 kmol s −1 ) that is partially compensated by a southward transport of Cant caused by the isopycnal component (−171 ± 11 kmol s −1 ), mainly localized in the Irminger Sea. Most importantly, the diapycnal component is found to be the main driver of the variability of T cant across the SPNA. Both the Meridional Overturning Circulation (computed in density coordinates, MOC σ ) and the Cant increase in the water column have an important effect on the variability of the diapycnal component and of T cant itself. Based on this analysis, we propose a simplified estimator for the variability of T cant based on the intensity of the MOC σ and on the difference of Cant between the upper and lower limb of the MOC σ ( Cant). This estimator shows a good consistency with the diapycnal component of T cant , and help to disentangle the effect of the variability of both the circulation and the Cant increase on the T cant variability. We find that Cant keeps increasing over the past decade, and it is very likely that the continuous Cant increase in the water masses will cause an increase in T cant across the SPNA at long timescale. Nevertheless, at the timescale analyzed here (1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010), the MOC σ controls the T cant variability, blurring any T cant trend. Extrapolating the observed Cant increase rate and considering the predicted slow-down of 25 % of the MOC σ , T cant across the SPNA is expected to increase by 430 kmol s −1 during the 21st century. Consequently, an increase in the storage rate of Cant in the SPNA could be envisaged.
2012
The variability of the Atlantic meridional overturning circulation (AMOC) is investigated in several climate simulations with the ECHO-G atmosphere-ocean general circulation model, including two forced integrations of the last millennium, one millennial-long control run, and two future scenario simulations of the twenty-first century. This constitutes a new framework in which the AMOC response to future climate change conditions is addressed in the context of both its past evolution and its natural variability. The main mechanisms responsible for the AMOC variability at interannual and multidecadal time scales are described. At high frequencies, the AMOC is directly responding to local changes in the Ekman transport, associated with three modes of climate variability: El Niño-Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the East Atlantic (EA) pattern. At low frequencies, the AMOC is largely controlled by convection activity south of Greenland. Again, the atmosphere is found to play a leading role in these variations. Positive anomalies of convection are preceded in 1 year by intensified zonal winds, associated in the forced runs to a positive NAO-like pattern. Finally, the sensitivity of the AMOC to three different forcing factors is investigated. The major impact is associated with increasing greenhouse gases, given their strong and persistent radiative forcing. Starting in the Industrial Era and continuing in the future scenarios, the AMOC experiences a final decrease of up to 40% with respect to the preindustrial average. Also, a weak but significant AMOC strengthening is found in response to the major volcanic eruptions, which produce colder and saltier surface conditions over the main convection regions. In contrast, no meaningful impact of the solar forcing on the AMOC is observed. Indeed, solar irradiance only affects convection in the Nordic Seas, with a marginal contribution to the AMOC variability in the ECHO-G runs.
Constraints on oceanic meridional heat transport from combined measurements of oxygen and carbon
Climate Dynamics, 2016
Major advances in our understanding of the large scale ocean can be related to the World Ocean Circulation Experiment (WOCE), which was a multinational ship-based program of unprecedented scale in the 1990s to measure ocean circulation and related transports of heat, salt, carbon, and nutrients. Though great strides were made on many fronts, WOCE did not provide a complete picture of the air-sea fluxes and large scale ocean transports. These needs are still unfilled to this day, despite the advent of the Argo float program and other improvements in sensor oceanography and satellites. The root difficulty is that many elements of the ocean circulation are still too uncertain to infer reliable fluxes and transports. The ocean gains heat in the tropics and carries it poleward. The ocean heat transport is not symmetric about the equator, however, as the Atlantic overturning circulation transports heat from deep within the Southern Hemisphere to the Northern Hemisphere (Crowley 1992; Marshall et al. 2014; Schneider et al. 2014). This asymmetry in ocean heat transport is a key driver of the climate mean state and variability through its influence on sea surface temperatures. Warmer sea surface temperatures in the north are associated with larger oceanic heat loss to the atmosphere and a major factor causing the displacement of the inter-tropical convergence zone north of the equator (Philander et al. 1996; Fuckar et al. 2013; Marshall et al. 2014; Schneider et al. 2014). The northward meridional heat transport also controls climate variability on decadal to millennial timescales by regulating the sea ice cover in the Arctic and its positive feedback on natural and anthropogenic warming (Crowley 1992; Mahlstein and Knutti 2011). Northward heat transport is also invoked as a major driver of the variability associated with the North Atlantic Oscillation (Bryden et al. 2014).