Meridional oceanic heat transport in the Atlantic Ocean (In: State of the Climate in 2013) (original) (raw)
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Journal Of Geophysical Research: Oceans, 2021
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2012
1] A mean state of the full-depth summer circulation in the Atlantic Ocean in the region in between Cape Farewell (Greenland), Scotland and the Greenland-Scotland Ridge (GSR) is assessed by combining 2002-2008 yearly hydrographic measurements at 59.5°N, mean dynamic topography, satellite altimetry data and available estimates of the Atlantic-Nordic Seas exchange. The mean absolute transports by the upper-ocean, mid-depth and deep currents and the Meridional Overturning Circulation (MOCs = 16.5 AE 2.2 Sv, at s 0 = 27.55) at 59.5°N are quantified in the density space. Inter-basin and diapycnal volume fluxes in between the 59.5°N section and the GSR are then estimated from a box model. The dominant components of the meridional exchange across 59.5°N are the North Atlantic Current (NAC, 15.5 AE 0.8 Sv, s 0 < 27.55) east of the Reykjanes Ridge, the northward Irminger Current (IC, 12.0 AE 3.0 Sv) and southward Western Boundary Current (WBC, 32.1 AE 5.9 Sv) in the Irminger Sea and the deep water export from the northern Iceland Basin (3.7 AE 0.8 Sv, s 0 > 27.80). About 60% (12.7 AE 1.4 Sv) of waters carried in the MOCs upper limb (s 0 < 27.55) by the NAC/IC across 59.5°N (21.1 AE 1.0 Sv) recirculates westward south of the GSR and feeds the WBC. 80% (10.2 AE 1.7 Sv) of the recirculating NAC/IC-derived upper-ocean waters gains density of s 0 > 27.55 and contributes to the MOCs lower limb.
Timely detection of changes in the meridional overturning circulation at 26 N in the Atlantic
Journal of …, 2007
It is investigated how changes in the North Atlantic meridional overturning circulation (MOC) might be reliably detected within a few decades, using the observations provided by the RAPID-MOC 26°N array. Previously, detectability of MOC changes had been investigated with a univariate MOC time series exhibiting strong internal variability, which would prohibit the detection of MOC changes within a few decades. Here, a modification of K. Hasselmann's fingerprint technique is used: (simulated) observations are projected onto a time-independent spatial pattern of natural variability to derive a time-dependent detection variable. The fixed spatial pattern of natural variability is derived by regressing the zonal density gradient along 26°N against the strength of the MOC at 26°N within the coupled ECHAM5/Max Planck Institute Ocean Model's (MPI-OM) control climate simulation. This pattern is confirmed against the observed anomalies found between the 1957 and the 2004 hydrographic occupations of the section. Onto this fixed spatial pattern of natural variability, both the existing hydrographic observations and simulated observations mimicking the RAPID-MOC 26°N array in three realizations of the Intergovernmental Panel on Climate Change (IPCC) scenario A1B are projected. For a random observation error of 0.01 kg m Ϫ3 , and only using zonal density gradients between 1700-and 3100-m depth, statistically significant detection occurs with 95% reliability after about 30 yr, in the model and climate change scenario analyzed here. Compared to using a single MOC time series as the detection variable, continuous observations of zonal density gradients reduce the detection time by 50%. For the five hydrographic occupations of the 26°N transect, none of the analyzed depth ranges shows a significant trend between 1957 and 2004, implying that there was no MOC trend over the past 50 yr.
The Upper, Deep, Abyssal and Overturning Circulation in the Atlantic Ocean at 30°S in 2003 and 2011
Progress in Oceanography
Mass transports for the thermocline, intermediate, deep and abyssal layers in the 45 Atlantic Ocean, at 30S and for 2003 and 2011, have been estimated using data from 46 GO-SHIP hydrographic transoceanic sections and applying three inverse models with 47 different constraints. The uppermost layers comprise South Atlantic Central Water 48 (SACW) and Antarctic Intermediate Water (AAIW), with a net northward transport in 49 the range of 12.1-14.7 Sv in 2003 and 11.7-17.7 Sv in 2011, which can be considered as 50 the northward returning limb of the Meridional Overturning Circulation (MOC). The 51 western boundary Brazil Current transports twice as much SACW in 2003 (-20.20.7 52 Sv) than in 2011 (-9.70.7 Sv). A poleward current consisting of AAIW and Upper 53 Circumpolar Deep Water (UCDW) flows beneath the Brazil Current. The eastern 54 boundary Benguela Current, characterized by a high mesoscale eddy activity, 55 transports 15.60.9 Sv in 2003 and 11.20.8 Sv in 2011, east of the Walvis Ridge. In the 56 ocean interior, the northward flow is mainly located east of the Mid Atlantic Ridge 57 (MAR) where Agulhas Rings (ARs), observed in both 2003 and 2011, transport warm 58 and salty water from the Indian to the Atlantic Ocean. For the deep layers, the 59 southward transport of North Atlantic Deep Water (NADW) occurs as the Deep 60 Western Boundary Current and also in the eastern basin. The western and eastern 61 basins transport similar amounts of NADW to the south during both years, although 62 the eastern pathway changes substantially between both years. The total NADW
Measuring the Atlantic Meridional Overturning Circulation at 26°N
Progress in Oceanography, 2015
The Atlantic Meridional Overturning Circulation (AMOC) plays a key role in the global climate system through its redistribution of heat. Changes in the AMOC have been associated with large fluctuations in the earth's climate in the past and projections of AMOC decline in the future due to climate change motivate the continuous monitoring of the circulation. Since 2004, the RAPID monitoring array has been providing continuous estimates of the AMOC and associated heat transport at 26°N in the North Atlantic. We describe how these measurements are made including the sampling strategy, the accuracies of parameters measured and the calculation of the AMOC. The strength of the AMOC and meridional heat transport are estimated as 17.2 Sv and 1.25 PW respectively from April 2004 to October 2012. The accuracy of ten day (annual) transports is 1.5 Sv (0.9 Sv). Improvements to the estimation of the transport above the shallowest instruments and deepest transports (including Antarctic Bottom Water), and the use of the new equation of state for seawater have reduced the estimated strength of the AMOC by 0.6 Sv relative to previous publications. As new basinwide AMOC monitoring projects begin in the South Atlantic and sub-polar North Atlantic, we present this thorough review of the methods and measurements of the original AMOC monitoring array.
Observed decline of the Atlantic meridional overturning circulation 2004–2012
Ocean Science, 2014
The Atlantic meridional overturning circulation (AMOC) has been observed continuously at 26 • N since April 2004. The AMOC and its component parts are monitored by combining a transatlantic array of moored instruments with submarine-cable-based measurements of the Gulf Stream and satellite derived Ekman transport. The time series has recently been extended to October 2012 and the results show a downward trend since 2004. From April 2008 to March 2012, the AMOC was an average of 2.7 Sv (1 Sv = 10 6 m 3 s −1 ) weaker than in the first four years of observation (95 % confidence that the reduction is 0.3 Sv or more). Ekman transport reduced by about 0.2 Sv and the Gulf Stream by 0.5 Sv but most of the change (2.0 Sv) is due to the mid-ocean geostrophic flow. The change of the mid-ocean geostrophic flow represents a strengthening of the southward flow above the thermocline. The increased southward flow of warm waters is balanced by a decrease in the southward flow of lower North Atlantic deep water below 3000 m. The transport of lower North Atlantic deep water slowed by 7 % per year (95 % confidence that the rate of slowing is greater than 2.5 % per year).