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OVERTURNING in the SUBPOLAR NORTH ATLANTIC PROGRAM a New International Ocean Observing System

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OVERTURNING IN THE SUBPOLAR NORTH ATLANTIC PROGRAM A New International Ocean Observing System M. SUSAN LOZIER, SHELDON BACON, AMY S. BOWER, STUART A. CUNNINGHAM, M. FEMKE DE JONG, LAURA DE STEUR, BRAD DEYOUNG, JÜRGEN FISCHER, STEFAN F. GARY, BLAIR J. W. GREENAN, PATRICK HEImbACH, NAOMI P. HOllIDAY, LOÏC HOUPERT, MARK E. INAll, WIllIAM E. JOHNS, HELEN L. JOHNSON, JOHANNES KARSTENSEN, FEILI LI, XIAOPEI LIN, NEIll MACKAY, DAVID P. MARSHAll, HERLÉ MERCIER, PAUL G. MYERS, ROBERT S. PICKART, HELEN R. PIllAR, FIAmmETTA STRANEO, VIRGINIE THIERRY, ROBERT A. WEllER, RICHARD G. WIllIAms, CHRIS WIlsON, JIAYAN YANG, JIAN ZHAO, AND JAN D. ZIKA A new ocean observing system has been launched in the North Atlantic in order to under- stand the linkage between the meridional overturning circulation and deep-water formation. he ocean’s meridional overturning circulation over Europe and North America (Knight et al. 2006; (MOC) is a key component of the global climate Zhang and Delworth 2006; Sutton and Hodson 2005; T system (IPCC 2013). The MOC, characterized Smith et al. 2010). Finally, variability of the inflow in the Atlantic (the AMOC) by a northward flux of warm Atlantic waters into high latitudes has been of warm upper-ocean waters and a compensating linked to the decline of Arctic sea ice (Serreze et al. southward flux of cool deep waters, plays a funda- 2007) and mass loss from the Greenland Ice Sheet mental role in establishing the mean climate state (Rignot and Kanagaratnam 2006; Holland et al. 2008; and its variability on interannual to longer time Straneo et al. 2010), both of which have profound scales (Buckley and Marshall 2016; Jackson et al. consequences for climate variability. 2015). Coupled with the winter release of locally Though less studied than its impact on climate, stored heat, the heat advected northward as part of the AMOC’s role in the ocean carbon cycle has the upper AMOC limb (Rhines et al. 2008) keeps emerged as a recent concern. The North Atlantic is the Northern Hemisphere generally, and western a strong sink for atmospheric CO2 (Takahashi et al. Europe in particular, warmer than they would be 2009; Khatiwala et al. 2013), accounting for ~40% otherwise. Variations in AMOC strength are believed of the annual mean global air–sea CO2 flux, with to influence North Atlantic sea surface temperatures nearly half of that flux occurring north of 50°N. (Knight et al. 2005; Delworth et al. 2007; Robson Furthermore, modeling (Halloran et al. 2015; Li et al. 2012; Yeager et al. 2012), leading to impacts on et al. 2016) and observational (Sabine et al. 2004) rainfall over the African Sahel, India, and Brazil; studies show that the North Atlantic plays a crucial Atlantic hurricane activity; and summer climate role in the uptake of anthropogenic carbon. The AMERICAN METEOROLOGICAL SOCIETY APRIL 2017 | 737 AMOC is believed to play a strong role in creating Linkage between convection and AMOC variability: this carbon sink (Pérez et al. 2013): in addition to Climate models. Current Intergovernmental Panel transporting anthropogenic carbon northward from on Climate Change (IPCC) projections of AMOC the subtropical gyre (Rosón et al. 2003), as these slowdown in the twenty-first century based on an northward-flowing surface waters cool, they absorb ensemble of climate models (see Figs. 12–35 in IPCC additional CO2 that is carried to depth when deep 2013) are widely attributed to the inhibition of deep waters form (Steinfeldt et al. 2009). The carbon flux convection at high latitudes in the North Atlantic. in the subpolar North Atlantic is also driven by a Similarly, simulations using twentieth-century strong annual cycle of net community production coupled ocean–sea ice models also find that AMOC (Kortzinger et al. 2008). AMOC variability can im- intensification is connected to increased deep-water pact this productivity if there is a disruption to the formation in the subpolar North Atlantic (Danabaso- northward flow of nutrients (Palter and Lozier 2008) glu et al. 2016). This link between AMOC strength or to the supply of nutrients to the surface by convec- and North Atlantic water mass production was made tion and mixing. Thus, AMOC variability, through explicit in a study of climate models where a freshwa- its direct impact on CO2 uptake via transport and ter anomaly was spread uniformly over the subpolar overturning and indirectly through its effect on domain (Stouffer et al. 2006). These “hosing” experi- ocean primary productivity, has the potential to ments yielded AMOC decreases, with concomitant alter the ocean’s role as a major sink for carbon in decreases in surface air and water temperatures in the the subpolar North Atlantic. high-latitude North Atlantic. However, the adequacy With such a profound array of implications, it is no of coarse-resolution models to simulate the ocean’s surprise that a mechanistic understanding of AMOC dynamical response to freshwater sources has been variability is a high priority for the climate science called into question in the past few years. For example, community. Hypotheses concerning what drives the Condron and Winsor (2011) argue that the climatic overturning fall into two categories (Visbeck 2007; response to anomalous freshwater input needs to be Kuhlbrodt et al. 2007): is the AMOC “pushed” by studied with models that resolve the dynamics of nar- buoyancy forcing at high latitudes, or is it “pulled” row coastal flows into and around the North Atlantic by vertical mixing supported by wind and tidal forc- basin. Similarly, although a growing number of model ing? While both mechanisms contribute to the long- simulations suggest that present-day and projected term equilibrium state of the AMOC, it is generally ice loss from the Greenland Ice Sheet may affect the believed that overturning variability on interannual AMOC, the nature and magnitude of the prescribed to millennial time scales is linked to changes in buoy- freshwater fluxes may not appropriately describe how ancy forcing and to the associated changes in the and where Greenland meltwater enters the ocean formation of dense water masses at high latitudes in (Straneo and Heimbach 2013). Clearly, observational the North Atlantic. Below, we provide a brief review studies are needed to guide and constrain modeling of that linkage in the modeling and observational efforts aimed at understanding the mechanistic link context. between convective activity and AMOC variability. AFFILIATIONS: LOZIER AND LI—Duke University, Durham, North Kingdom; LIN—Ocean University of China/Qingdao National Labora- Carolina; BACON AND HOllIDAY—National Oceanography Centre, tory for Marine Science and Technology, Qingdao, China; MACKAY Southampton, United Kingdom; BOWER, PICKART, STRANEO, WEllER, AND WIlsON—National Oceanography Centre, Liverpool, United YANG, AND ZHAO—Woods Hole Oceanographic Institution, Woods Kingdom; MERCIER AND THIERRY—CNRS, Laboratory of Ocean Physics Hole, Massachusetts; CUNNINGHAM, GARY, HOUPERT, AND INAll— and Satellite Oceanography, Ifremer centre de Bretagne, Plouzané, Scottish Association for Marine Science, Oban, United Kingdom; France; MYERS—University of Alberta, Edmonton, Alberta, Canada; PIllAR—Niels Bohr Institute, University of Copenhagen, Copenhagen, DE JONG—Duke University, Durham, North Carolina, and Royal Netherlands Institute for Sea Research, Texel, and Utrecht Univer- Denmark; WIllIAms—University of Liverpool, Liverpool, United King- dom; ZIKA—Imperial College London, London, United Kingdom sity, Utrecht, Netherlands; DE STEUR—Royal Netherlands Institute for CORRESPONDING AUTHOR E-MAIL: M. Susan Lozier, Sea Research, Texel, and Utrecht University, Utrecht, Netherlands; [email protected] DEYOUNG—Memorial University, St. John’s, Newfoundland, Canada; FISCHER AND KARSTENSEN—GEOMAR Helmholtz Centre for Ocean Re- The abstract for this article can be found in this issue, following the table search, Kiel, Germany; GREENAN—Bedford Institute of Oceanography, of contents. Dartmouth, Nova Scotia, Canada; HEImbACH—The University of Texas DOI:10.1175/BAMS-D-16-0057.1 at Austin, Austin, Texas; JOHNS—University of Miami, Miami, Florida; In final form 19 August 2016 JOHNSON AND MARSHAll—University of Oxford, Oxford, United ©2017 American Meteorological Society 738 | APRIL 2017 Linkage between convection and AMOC variability: there has been no conclusive observational evidence Observations. Dense water formation in the Nor- linking the formation of Nordic seas’ overflow wa- dic seas and in the North Atlantic subpolar gyre ters with AMOC variability (Jochumsen et al. 2012; (NASPG) produces the water masses in the AMOC Hansen and Østerhus 2007). lower limb (Fig. 1). The deepest constituents of the One possible reason for the lack for a clear con- lower limb originate as dense intermediate waters nection between convection and AMOC variability formed in the Nordic seas. These waters, referred to is that not all of the export pathways of dense waters collectively as overflow waters (OW), flow over the have been monitored. The DWBC has traditionally shallow sills of the Greenland–Scotland Ridge (GSR) been considered the sole conduit for the lower limb into the North Atlantic: to the east of Iceland is the of the AMOC. However, this assumption has been Iceland–Scotland Overflow (ISO) Water (ISOW), challenged by observational and modeling studies which has traditionally been thought to follow the to- that reveal the importance of interior, and boundary, pography around the Reykjanes Ridge to the Irminger pathways (e.g., Bower et
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