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1. Introduction Warming of the North Atlantic over the past 50 years has not been uniform (e.g., Levitus et al. 2000 , 2005a ; Lozier et al. 2008 ). For example, using data from hydrographic stations, Lozier et al. (2008) found in the North Atlantic that the tropics and subtropics have warmed but the subpolar ocean has cooled (see also Levitus et al. 2000 ). These observations suggest that, instead of a diffusive process from the surface, ocean heat content change is largely a consequence
1. Introduction Warming of the North Atlantic over the past 50 years has not been uniform (e.g., Levitus et al. 2000 , 2005a ; Lozier et al. 2008 ). For example, using data from hydrographic stations, Lozier et al. (2008) found in the North Atlantic that the tropics and subtropics have warmed but the subpolar ocean has cooled (see also Levitus et al. 2000 ). These observations suggest that, instead of a diffusive process from the surface, ocean heat content change is largely a consequence
1. Introduction a. The Nordic seas in the climate system The Nordic seas (i.e., Greenland, Iceland, and Norwegian Seas; Fig. 1 ), a transitional region between the Arctic Ocean north of Fram Strait and the North Atlantic Ocean, are a site of key climate processes. Deep convective mixing, a driver of the thermohaline circulation, takes place in the Nordic seas where wintertime air–sea heat fluxes destabilize the stratification and produce deep mixed layers ( Nilsen and Falck 2006 ); further
1. Introduction a. The Nordic seas in the climate system The Nordic seas (i.e., Greenland, Iceland, and Norwegian Seas; Fig. 1 ), a transitional region between the Arctic Ocean north of Fram Strait and the North Atlantic Ocean, are a site of key climate processes. Deep convective mixing, a driver of the thermohaline circulation, takes place in the Nordic seas where wintertime air–sea heat fluxes destabilize the stratification and produce deep mixed layers ( Nilsen and Falck 2006 ); further
and the experimental set up are presented in section 3 . The results are presented in section 4 , and a discussion follows in section 5 . Conclusions are summarized in the last section. 2. Description of the assimilation system Mercator Océan has operated a multivariate multiple data assimilation system in real time since January 2004 ( http://www.mercator-ocean.fr ). This system, called PSY1v2, provides an oceanic large-scale analysis and a 2-week forecast of the North and tropical Atlantic
and the experimental set up are presented in section 3 . The results are presented in section 4 , and a discussion follows in section 5 . Conclusions are summarized in the last section. 2. Description of the assimilation system Mercator Océan has operated a multivariate multiple data assimilation system in real time since January 2004 ( http://www.mercator-ocean.fr ). This system, called PSY1v2, provides an oceanic large-scale analysis and a 2-week forecast of the North and tropical Atlantic
western North Atlantic Ocean is the Subtropical Mode Water (STMW), a vertically homogeneous water mass between the seasonal thermocline and the permanent thermocline. The STMW is formed by deep convection just south of the Gulf Stream (GS) during winter and contains the memory of its interaction with the atmosphere. After its formation, the STMW is advected by the GS and its recirculation gyre. The net heat loss to the atmosphere has been considered an important factor for forming and sustaining the
western North Atlantic Ocean is the Subtropical Mode Water (STMW), a vertically homogeneous water mass between the seasonal thermocline and the permanent thermocline. The STMW is formed by deep convection just south of the Gulf Stream (GS) during winter and contains the memory of its interaction with the atmosphere. After its formation, the STMW is advected by the GS and its recirculation gyre. The net heat loss to the atmosphere has been considered an important factor for forming and sustaining the
play an important role in the northwestern North Atlantic Ocean, cannot be resolved. Surface fluxes, which have a poor observational coverage over the ocean, have a major role in local adjustment of water masses and the formation of mode waters ( Brambilla and Talley 2008 ). The surface waters are furthermore dependent on a combination of sea ice melt and advection. Convection plumes (∼1 km) and convection areas (∼100 km) are crucial in intermediate water mass formation ( Marshall and Schott 1999
play an important role in the northwestern North Atlantic Ocean, cannot be resolved. Surface fluxes, which have a poor observational coverage over the ocean, have a major role in local adjustment of water masses and the formation of mode waters ( Brambilla and Talley 2008 ). The surface waters are furthermore dependent on a combination of sea ice melt and advection. Convection plumes (∼1 km) and convection areas (∼100 km) are crucial in intermediate water mass formation ( Marshall and Schott 1999
project ( Griffies et al. 2009 ). A complication, however, is that the bias correction issue becomes a critically important part of the analysis. A recent report by the Climate Variability and Predictability Decadal Climate and Prediction Panel ( CLIVAR 2011 ) recommends a technique for bias correction of CMIP5 decadal prediction experiments, which we adopt and describe below in section 3 . An abrupt warming of the subpolar gyre (SPG) region of the North Atlantic Ocean in the 1990s has been
project ( Griffies et al. 2009 ). A complication, however, is that the bias correction issue becomes a critically important part of the analysis. A recent report by the Climate Variability and Predictability Decadal Climate and Prediction Panel ( CLIVAR 2011 ) recommends a technique for bias correction of CMIP5 decadal prediction experiments, which we adopt and describe below in section 3 . An abrupt warming of the subpolar gyre (SPG) region of the North Atlantic Ocean in the 1990s has been
1. Introduction During the period from the summer of 2009 to the summer of 2010, a strong warming tendency of sea surface temperature (SST) occurred in the tropical and subtropical North Atlantic Ocean ( Fig. 1a ), ending with a record-breaking SST anomaly (SSTA) for several months in the hurricane Main Development Region (MDR: 10°–20°N, 20°–85°W; see the rectangular box in Fig. 2f ). The SSTA in the MDR reached 0.94°C for the mean from June to August (JJA) 2010 and was a record value since
1. Introduction During the period from the summer of 2009 to the summer of 2010, a strong warming tendency of sea surface temperature (SST) occurred in the tropical and subtropical North Atlantic Ocean ( Fig. 1a ), ending with a record-breaking SST anomaly (SSTA) for several months in the hurricane Main Development Region (MDR: 10°–20°N, 20°–85°W; see the rectangular box in Fig. 2f ). The SSTA in the MDR reached 0.94°C for the mean from June to August (JJA) 2010 and was a record value since
1. Introduction There has been much attention in recent years on determining the state and possible changes in the meridional overturning circulation (MOC) of the North Atlantic Ocean. The MOC effectively comprises the northward flow of upper-layer warm tropical water by the Gulf Stream system and its southward return by the deep western boundary current (DWBC). Because of the marked temperature contrast between the upper and lower branches, the MOC formally represents the main agent for the
1. Introduction There has been much attention in recent years on determining the state and possible changes in the meridional overturning circulation (MOC) of the North Atlantic Ocean. The MOC effectively comprises the northward flow of upper-layer warm tropical water by the Gulf Stream system and its southward return by the deep western boundary current (DWBC). Because of the marked temperature contrast between the upper and lower branches, the MOC formally represents the main agent for the
of understanding variability for the detection and attribution of climate change, and the impact of sea surface temperatures (SST) on the evolution of regional climates. The Atlantic is one area of the global ocean that has exhibited significant multidecadal variability in sea surface temperatures ( Kushnir 1994 ; Ting et al. 2009 ). In the last part of the twentieth century, the North Atlantic Ocean warmed significantly ( Palmer and Haines 2009 ). In particular, the North Atlantic subpolar gyre
of understanding variability for the detection and attribution of climate change, and the impact of sea surface temperatures (SST) on the evolution of regional climates. The Atlantic is one area of the global ocean that has exhibited significant multidecadal variability in sea surface temperatures ( Kushnir 1994 ; Ting et al. 2009 ). In the last part of the twentieth century, the North Atlantic Ocean warmed significantly ( Palmer and Haines 2009 ). In particular, the North Atlantic subpolar gyre
1. Introduction Atlantic multidecadal variability (AMV) is the dominant mode of sea surface temperature (SST) variability in the North Atlantic on decadal time scales ( Schlesinger and Ramankutty 1994 ). Because the ocean’s heat capacity is much higher than that of the atmosphere, a better understanding of the ocean dynamics and the pathways by which temperature anomalies in the upper ocean are communicated to the atmosphere might offer a potential to improve the predictability for the North
1. Introduction Atlantic multidecadal variability (AMV) is the dominant mode of sea surface temperature (SST) variability in the North Atlantic on decadal time scales ( Schlesinger and Ramankutty 1994 ). Because the ocean’s heat capacity is much higher than that of the atmosphere, a better understanding of the ocean dynamics and the pathways by which temperature anomalies in the upper ocean are communicated to the atmosphere might offer a potential to improve the predictability for the North
