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1. Observed decline of the Atlantic meridional overturning circulation 2004–2012

   https://doi.org/10.5194/os-10-29-2014  

   Smeed, D. A. ; McCarthy, G. D. ; Cunningham, S. A. ; Frajka-Williams, E. ; Rayner, D. ; Johns, W. E. ; Meinen, C. S. ; Baringer, M. O. ; Moat, B. I. ; Duchez, A. ; et al ( January 2014 , Ocean Science)

   null (Ed.)

   Abstract. 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 = 106 m3 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). 

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2. Towards two decades of Atlantic Ocean mass and heat transports at 26.5° N

   https://doi.org/10.1098/rsta.2022.0188  

   Johns, William E. ; Elipot, Shane ; Smeed, David A. ; Moat, Ben ; King, Brian ; Volkov, Denis L. ; Smith, Ryan H. ( December 2023 , Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   Continuous measurements of the Atlantic meridional overturning circulation (AMOC) and meridional ocean heat transport at 26.5° N began in April 2004 and are currently available through December 2020. Approximately 90% of the total meridional heat transport (MHT) at 26.5° N is carried by the zonally averaged overturning circulation, and an even larger fraction of the heat transport variability (approx. 95%) is explained by the variability of the zonally averaged overturning. A physically based separation of the heat transport into large-scale AMOC, gyre and shallow wind-driven overturning components remains challenging and requires new investigations and approaches. We review the major interannual changes in the AMOC and MHT that have occurred over the nearly two decades of available observations and their documented impacts on North Atlantic heat content. Changes in the flow-weighted temperature of the Florida Current (Gulf Stream) over the past two decades are now taken into account in the estimates of MHT, and have led to an increased heat transport relative to the AMOC strength in recent years. Estimates of the MHT at 26.5° N from coupled models and various surface flux datasets still tend to show low biases relative to the observations, but indirect estimates based on residual methods (top of atmosphere net radiative flux minus atmospheric energy divergence) have shown recent promise in reproducing the heat transport and its interannual variability.This article is part of a discussion meeting issue ‘Atlantic overturning: new observations and challenges’. 

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3. Arctic Mediterranean exchanges: a consistent volume budget and trends in transports from two decades of observations

   https://doi.org/10.5194/os-15-379-2019  

   Østerhus, Svein ; Woodgate, Rebecca ; Valdimarsson, Héðinn ; Turrell, Bill ; de Steur, Laura ; Quadfasel, Detlef ; Olsen, Steffen M. ; Moritz, Martin ; Lee, Craig M. ; Larsen, Karin Margretha ; et al ( January 2019 , Ocean Science)

   Abstract. The Arctic Mediterranean (AM) is the collective name forthe Arctic Ocean, the Nordic Seas, and their adjacent shelf seas. Water enters into thisregion through the Bering Strait (Pacific inflow) and through the passages across theGreenland–Scotland Ridge (Atlantic inflow) and is modified within the AM. The modifiedwaters leave the AM in several flow branches which are grouped into two differentcategories: (1) overflow of dense water through the deep passages across theGreenland–Scotland Ridge, and (2) outflow of light water – here termed surface outflow– on both sides of Greenland. These exchanges transport heat and salt into and out ofthe AM and are important for conditions in the AM. They are also part of the global oceancirculation and climate system. Attempts to quantify the transports by various methodshave been made for many years, but only recently the observational coverage has becomesufficiently complete to allow an integrated assessment of the AM exchanges based solelyon observations. In this study, we focus on the transport of water and have collecteddata on volume transport for as many AM-exchange branches as possible between 1993 and2015. The total AM import (oceanic inflows plusfreshwater) is found to be 9.1 Sv (sverdrup,1 Sv =10⁶ m³ s⁻¹) with an estimated uncertainty of 0.7 Sv and hasthe amplitude of the seasonal variation close to 1 Sv and maximum import in October.Roughly one-third of the imported water leaves the AM as surface outflow with theremaining two-thirds leaving as overflow. The overflow water is mainly produced frommodified Atlantic inflow and around 70 % of the total Atlantic inflow is convertedinto overflow, indicating a strong coupling between these two exchanges. The surfaceoutflow is fed from the Pacific inflow and freshwater (runoff and precipitation), but isstill approximately two-thirds of modified Atlantic water. For the inflowbranches and the two main overflow branches (Denmark Strait and Faroe Bank Channel),systematic monitoring of volume transport has been established since the mid-1990s, andthis enables us to estimate trends for the AM exchanges as a whole. At the 95 %confidence level, only the inflow of Pacific water through the Bering Strait showed astatistically significant trend, which was positive. Both the total AM inflow and thecombined transport of the two main overflow branches also showed trends consistent withstrengthening, but they were not statistically significant. They do suggest, however,that any significant weakening of these flows during the last two decades is unlikely andthe overall message is that the AM exchanges remained remarkably stable in the periodfrom the mid-1990s to the mid-2010s. The overflows are the densest source water for thedeep limb of the North Atlantic part of the meridional overturning circulation (AMOC),and this conclusion argues that the reported weakening of the AMOC was not due tooverflow weakening or reduced overturning in the AM. Although the combined data set hasmade it possible to establish a consistent budget for the AM exchanges, the observationalcoverage for some of the branches is limited, which introduces considerable uncertainty.This lack of coverage is especially extreme for the surface outflow through the DenmarkStrait, the overflow across the Iceland–Faroe Ridge, and the inflow over the Scottishshelf. We recommend that more effort is put into observing these flows as well asmaintaining the monitoring systems established for the other exchange branches.

    

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4. 100 Years of Progress in Understanding the Dynamics of Coupled Atmosphere–Ocean Variability

   https://doi.org/10.1175/AMSMONOGRAPHS-D-18-0025.1  

   Battisti, David S. ; Vimont, Daniel J. ; Kirtman, Benjamin P. ( October 2019 , Meteorological Monographs)

   In situ observation networks and reanalyses products of the state of the atmosphere and upper ocean show well-defined, large-scale patterns of coupled climate variability on time scales ranging from seasons to several decades. We summarize these phenomena and their physics, which have been revealed by analysis of observations, by experimentation with uncoupled and coupled atmosphere and ocean models with a hierarchy of complexity, and by theoretical developments. We start with a discussion of the seasonal cycle in the equatorial tropical Pacific and Atlantic Oceans, which are clearly affected by coupling between the atmosphere and the ocean. We then discuss the tropical phenomena that only exist because of the coupling between the atmosphere and the ocean: the Pacific and Atlantic meridional modes, the El Niño–Southern Oscillation (ENSO) in the Pacific, and a phenomenon analogous to ENSO in the Atlantic. For ENSO, we further discuss the sources of irregularity and asymmetry between warm and cold phases of ENSO, and the response of ENSO to forcing. Fundamental to variability on all time scales in the midlatitudes of the Northern Hemisphere are preferred patterns of uncoupled atmospheric variability that exist independent of any changes in the state of the ocean, land, or distribution of sea ice. These patterns include the North Atlantic Oscillation (NAO), the North Pacific Oscillation (NPO), and the Pacific–North American (PNA) pattern; they are most active in wintertime, with a temporal spectrum that is nearly white. Stochastic variability in the NPO, PNA, and NAO force the ocean on days to interannual times scales by way of turbulent heat exchange and Ekman transport, and on decadal and longer time scales by way of wind stress forcing. The PNA is partially responsible for the Pacific decadal oscillation; the NAO is responsible for an analogous phenomenon in the North Atlantic subpolar gyre. In models, stochastic forcing by the NAO also gives rise to variability in the strength of the Atlantic meridional overturning circulation (AMOC) that is partially responsible for multidecadal anomalies in the North Atlantic climate known as the Atlantic multidecadal oscillation (AMO); observations do not yet exist to adequately determine the physics of the AMO. We review the progress that has been made in the past 50 years in understanding each of these phenomena and the implications for short-term (seasonal-to-interannual) climate forecasts. We end with a brief discussion of advances of things that are on the horizon, under the rug, and over the rainbow.

    

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5. Expedition 361 Scientific Prospectus: South African Climates (Agulhas LGM Density Profile)

   https://doi.org/10.14379/iodp.sp.361.2015  

   Hall, I.R. ; Hemming, S.R. ; LeVay, L.J. ( August 2015 , Scientific prospectus)

   null (Ed.)

   The Agulhas Current is the strongest western boundary current in the Southern Hemisphere, transporting some 70 Sv of warm and saline surface waters from the tropical Indian Ocean along the East African margin to the tip of Africa. Exchanges of heat and moisture with the atmosphere influence southern African climates, including individual weather systems such as extratropical cyclone formation in the region and rainfall patterns. Recent ocean models and paleoceanographic data further point at a potential role of the Agulhas Current in controlling the strength and mode of the Atlantic Meridional Overturning Circulation (AMOC) during the Late Pleistocene. Spillage of saline Agulhas water into the South Atlantic stimulates buoyancy anomalies that act as a control mechanism on the basin-wide AMOC, with implications for convective activity in the North Atlantic and Northern Hemisphere climate. International Ocean Discovery Program (IODP) Expedition 361 aims to extend this work to periods of major ocean and climate restructuring during the Pliocene/Pleistocene to assess the role that the Agulhas Current and ensuing (interocean) marine heat and salt transports have played in shaping the regional- and global-scale ocean and climate development. This expedition will core six sites on the southeast African margin and Indian–Atlantic ocean gateway. The primary sites are located between 416 and 3040 m water depths. The specific scientific objectives are • To assess the sensitivity of the Agulhas Current to changing climates of the Pliocene/Pleistocene, in association with transient to long-term changes of high-latitude climates, tropical heat budgets, and the monsoon system; • To reconstruct the dynamics of the Indian–Atlantic gateway circulation during such climate changes, in association with changing wind fields and migrating ocean fronts; • To examine the connection between Agulhas leakage and ensuing buoyancy transfer and shifts of the AMOC during major ocean and climate reorganizations during at least the last 5 My; and • To address the impact of Agulhas variability on southern Africa terrestrial climates and, notably, rainfall patterns and river runoff. Additionally, Expedition 361 will complete an intensive interstitial fluids program at four of the sites aimed at constraining the temperature, salinity, and density structure of the Last Glacial Maximum (LGM) deep ocean, from the bottom of the ocean to the base of the main thermocline, to address the processes that could fill the LGM ocean and control its circulation. Expedition 361 will seek to recover ~5200 m of sediment in total. The coring strategy will include the triple advanced piston corer system along with the extended core barrel coring system where required to reach target depths. Given the significant transit time required during the expedition (15.5 days), the coring schedule is tight and will require detailed operational planning and flexibility from the scientific party. The final operations plan, including the number of sites to be cored and/or logged, is contingent upon the R/V JOIDES Resolution operations schedule, operational risks, and the outcome of requests for territorial permission to occupy particular sites. All relevant IODP sampling and data policies will be adhered to during the expedition. Beyond the interstitial fluids program, shipboard sampling will be restricted to acquiring ephemeral data and to limited low-resolution sampling of parameters that may be critically affected by short-term core storage. Most sampling will be deferred to a postcruise sampling party that will take place at the Gulf Coast Repository in College Station, Texas (USA). A substantial onshore X-ray fluorescence scanning plan is anticipated and will be further developed in consultation with scientific participants. 

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