1. Introduction Ocean mesoscale eddies are ubiquitous in the global ocean. They are remarkably vigorous in regions such as the Kuroshio and Oyashio and their extensions, the Gulf Stream and its extension, the Agulhas Return Current, and the Antarctic Circumpolar Current ( Chelton et al. 2011 ). Besides dominating oceanic kinetic energy ( Ferrari and Wunsch 2009 ), mesoscale eddies are key contributors in transporting heat and materials, leaving profound impacts on climate and biogeochemical
1. Introduction Ocean mesoscale eddies are ubiquitous in the global ocean. They are remarkably vigorous in regions such as the Kuroshio and Oyashio and their extensions, the Gulf Stream and its extension, the Agulhas Return Current, and the Antarctic Circumpolar Current ( Chelton et al. 2011 ). Besides dominating oceanic kinetic energy ( Ferrari and Wunsch 2009 ), mesoscale eddies are key contributors in transporting heat and materials, leaving profound impacts on climate and biogeochemical
1. Introduction Recent work has demonstrated the important role of the Indian Ocean in modulating global climate variability ( SanchezGomez et al. 2008 ; Schott et al. 2009 ; Luo et al. 2012 ) and regional rainfall ( Ashok et al. 2001 ; Ummenhofer et al. 2009 ). In particular, the role of upper-ocean heat content (OHC) in the Indian Ocean has been highlighted in recent discussions of the so-called global warming hiatus ( Lee et al. 2015 ; Nieves et al. 2015 ). Several studies have linked
1. Introduction Recent work has demonstrated the important role of the Indian Ocean in modulating global climate variability ( SanchezGomez et al. 2008 ; Schott et al. 2009 ; Luo et al. 2012 ) and regional rainfall ( Ashok et al. 2001 ; Ummenhofer et al. 2009 ). In particular, the role of upper-ocean heat content (OHC) in the Indian Ocean has been highlighted in recent discussions of the so-called global warming hiatus ( Lee et al. 2015 ; Nieves et al. 2015 ). Several studies have linked
1. Introduction Recent work has demonstrated the importance of eastern Indian Ocean variability for regional rainfall and drought for Australia ( Ummenhofer et al. 2008 , 2009b ), Indonesia ( Hendon 2003 ), and more widely across Southeast Asia (e.g., Sinha et al. 2011 ). Given the slower evolution of anomalies in the ocean, as opposed to the higher-frequency variability of the atmosphere and the associated benefits for seasonal predictions, an improved understanding of the drivers of eastern
1. Introduction Recent work has demonstrated the importance of eastern Indian Ocean variability for regional rainfall and drought for Australia ( Ummenhofer et al. 2008 , 2009b ), Indonesia ( Hendon 2003 ), and more widely across Southeast Asia (e.g., Sinha et al. 2011 ). Given the slower evolution of anomalies in the ocean, as opposed to the higher-frequency variability of the atmosphere and the associated benefits for seasonal predictions, an improved understanding of the drivers of eastern
1. Introduction The Atlantic multidecadal variability (AMV) is a mode of basinwide sea surface temperature (SST) variability over the North Atlantic Ocean with pronounced signals at decadal-to-multidecadal time scales ( Schlesinger and Ramankutty 1994 ; Kerr 2000 ). The AMV significantly affects global and regional climate [see review by Zhang et al. (2019) ] through its impact on the global-mean temperature ( Ting et al. 2009 ), the position of the Atlantic intertropical convergence zone
1. Introduction The Atlantic multidecadal variability (AMV) is a mode of basinwide sea surface temperature (SST) variability over the North Atlantic Ocean with pronounced signals at decadal-to-multidecadal time scales ( Schlesinger and Ramankutty 1994 ; Kerr 2000 ). The AMV significantly affects global and regional climate [see review by Zhang et al. (2019) ] through its impact on the global-mean temperature ( Ting et al. 2009 ), the position of the Atlantic intertropical convergence zone
; Zhang and Zhang 2001 ; Serreze et al. 2007 ; Årthun and Schrum 2010 ; Smedsrud et al. 2010 ; Årthun et al. 2011 ). The strongest heat flux is about 500 W m −2 near the marginal ice zone in winter, which can cool the warm saline Atlantic water all the way to the ocean bottom ( Hakkinen and Cavalieri 1989 ). Moreover, the warm Atlantic water inflow keeps the southern Barents Sea largely ice free and increases air–sea interactions ( Helland-Hansen and Nansen 1909 ; Sandø et al. 2010
; Zhang and Zhang 2001 ; Serreze et al. 2007 ; Årthun and Schrum 2010 ; Smedsrud et al. 2010 ; Årthun et al. 2011 ). The strongest heat flux is about 500 W m −2 near the marginal ice zone in winter, which can cool the warm saline Atlantic water all the way to the ocean bottom ( Hakkinen and Cavalieri 1989 ). Moreover, the warm Atlantic water inflow keeps the southern Barents Sea largely ice free and increases air–sea interactions ( Helland-Hansen and Nansen 1909 ; Sandø et al. 2010
1. Introduction Oceanic salinity plays an important role in the climate system due to its significant influence on oceanic stratification and barrier layers ( Sprintall and Tomczak 1992 ; Thompson et al. 2006 ; Balaguru et al. 2016 ) and ocean circulation ( Gordon et al. 2003 ; Feng et al. 2015 ; Hu and Sprintall 2016 , 2017a , b ), and has a close link to the global hydrological cycle ( Durack and Wijffels 2010 ; Durack et al. 2012 ). Investigation of ocean salinity variability and
1. Introduction Oceanic salinity plays an important role in the climate system due to its significant influence on oceanic stratification and barrier layers ( Sprintall and Tomczak 1992 ; Thompson et al. 2006 ; Balaguru et al. 2016 ) and ocean circulation ( Gordon et al. 2003 ; Feng et al. 2015 ; Hu and Sprintall 2016 , 2017a , b ), and has a close link to the global hydrological cycle ( Durack and Wijffels 2010 ; Durack et al. 2012 ). Investigation of ocean salinity variability and
1. Introduction The Indian Ocean receives heat and mass from the Pacific at a low latitude via the Indonesian throughflow (ITF; see Godfrey 1996 for a review). A potential consequence is that variations in Indian Ocean temperature may not be only a result of atmospheric forcing over the Indian Ocean, but also may be influenced by changes in the ITF. An important question, and the focus of this study, is to what degree low-frequency changes in upper-ocean temperatures in the Indian Ocean are
1. Introduction The Indian Ocean receives heat and mass from the Pacific at a low latitude via the Indonesian throughflow (ITF; see Godfrey 1996 for a review). A potential consequence is that variations in Indian Ocean temperature may not be only a result of atmospheric forcing over the Indian Ocean, but also may be influenced by changes in the ITF. An important question, and the focus of this study, is to what degree low-frequency changes in upper-ocean temperatures in the Indian Ocean are
1. Introduction Is the ocean circulation thermally or mechanically forced? Already a century ago, Sandström (1908) concluded from his laboratory experiments that the heating and cooling at the ocean surface by itself would not be able to excite a circulation in the interior of the ocean. His arguments were elaborated by Jeffreys (1925) and Defant (1961) , who concluded that the circulation must be mechanically forced. Nevertheless, for a long time, a widespread view among oceanographers
1. Introduction Is the ocean circulation thermally or mechanically forced? Already a century ago, Sandström (1908) concluded from his laboratory experiments that the heating and cooling at the ocean surface by itself would not be able to excite a circulation in the interior of the ocean. His arguments were elaborated by Jeffreys (1925) and Defant (1961) , who concluded that the circulation must be mechanically forced. Nevertheless, for a long time, a widespread view among oceanographers
important to understand upper-ocean processes that maintain SST under the stratocumulus cloud deck for global simulation and climate prediction. However, until recently, the upper ocean in this region has been sparsely observed, which limits our ability to better understand and simulate the behavior of the atmosphere and ocean globally. In fact, most atmosphere–ocean coupled general circulation models (CGCMs) have systematic errors in the southeast Pacific, including too warm SSTs and too little cloud
important to understand upper-ocean processes that maintain SST under the stratocumulus cloud deck for global simulation and climate prediction. However, until recently, the upper ocean in this region has been sparsely observed, which limits our ability to better understand and simulate the behavior of the atmosphere and ocean globally. In fact, most atmosphere–ocean coupled general circulation models (CGCMs) have systematic errors in the southeast Pacific, including too warm SSTs and too little cloud
1. Introduction The Southern Ocean (SO) surface water masses are known to sink to the bottom of the sea in two ways: 1) near-boundary convection, also called “continental shelf slope convection,” and 2) open-ocean deep convection ( Killworth 1983 ; Gordon 2014 ). In the Southern Hemisphere (SH), near-boundary convection is closely linked to the coastal polynya, also called “latent heat polynya” ( Curry and Webster 1999 ; Wadhams 2000 ), while open-ocean deep convection is closely linked to
1. Introduction The Southern Ocean (SO) surface water masses are known to sink to the bottom of the sea in two ways: 1) near-boundary convection, also called “continental shelf slope convection,” and 2) open-ocean deep convection ( Killworth 1983 ; Gordon 2014 ). In the Southern Hemisphere (SH), near-boundary convection is closely linked to the coastal polynya, also called “latent heat polynya” ( Curry and Webster 1999 ; Wadhams 2000 ), while open-ocean deep convection is closely linked to
