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1. Introduction Salinity is one of the fundamental ocean variables that are routinely measured, and variations in salinity have been used extensively in climate studies. First, ocean salinity is strongly impacted by air–sea freshwater exchange, land freshwater discharges, sea ice formation and melting, and ocean dynamics ( Rao and Sivakumar 2003 ; Foltz et al. 2004 ; Dong et al. 2014 ; Haumann et al. 2016 ; Liu et al. 2019 ). Since salinity is easier to measure than the air–sea freshwater
1. Introduction Salinity is one of the fundamental ocean variables that are routinely measured, and variations in salinity have been used extensively in climate studies. First, ocean salinity is strongly impacted by air–sea freshwater exchange, land freshwater discharges, sea ice formation and melting, and ocean dynamics ( Rao and Sivakumar 2003 ; Foltz et al. 2004 ; Dong et al. 2014 ; Haumann et al. 2016 ; Liu et al. 2019 ). Since salinity is easier to measure than the air–sea freshwater
97% of the global water inventory, with 80% of the surface water flux occurring over the oceans ( Schmitt 1995 ). The global ocean’s salinity field reflects the large-scale long-term balance between the surface freshwater flux [evaporation minus precipitation (EP) and terrestrial runoff minus the total surface freshwater flux ( F W )] and the ocean’s advective and mixing processes. Any change in the hydrological cycle, therefore, will be reflected in the ocean salinity field. Large and coherent
97% of the global water inventory, with 80% of the surface water flux occurring over the oceans ( Schmitt 1995 ). The global ocean’s salinity field reflects the large-scale long-term balance between the surface freshwater flux [evaporation minus precipitation (EP) and terrestrial runoff minus the total surface freshwater flux ( F W )] and the ocean’s advective and mixing processes. Any change in the hydrological cycle, therefore, will be reflected in the ocean salinity field. Large and coherent
global surface freshwater flux ( Durack 2015 ). This flux has a distinct pattern with large-scale regions, such as in the subtropics, having a net negative freshwater flux ( E > P ), and large-scale regions in the higher latitudes, having a net positive freshwater flux ( E < P ). This pattern is well reflected in the ocean’s salinity distribution, making salinity a powerful “rain gauge.” This concept can be traced to Wust (1936) and Sverdrup et al. (1942 , 124–127), who noted first the broad
global surface freshwater flux ( Durack 2015 ). This flux has a distinct pattern with large-scale regions, such as in the subtropics, having a net negative freshwater flux ( E > P ), and large-scale regions in the higher latitudes, having a net positive freshwater flux ( E < P ). This pattern is well reflected in the ocean’s salinity distribution, making salinity a powerful “rain gauge.” This concept can be traced to Wust (1936) and Sverdrup et al. (1942 , 124–127), who noted first the broad
local precipitation ( Kataoka et al. 2014 ; Tozuka et al. 2014 ; Doi et al. 2015 ; Marshall et al. 2015 ) and thereby leads to ocean salinity anomalies ( Feng et al. 2015b ; N. Zhang et al. 2016 ). Yet, ocean salinity changes associated with Ningaloo Niño have never been addressed in a systematical fashion; as a result, our understanding of salinity-related ocean processes remains fragmental. Ocean salinity may exert feedback effects on the Ningaloo Niño warming through affecting the oceanic
local precipitation ( Kataoka et al. 2014 ; Tozuka et al. 2014 ; Doi et al. 2015 ; Marshall et al. 2015 ) and thereby leads to ocean salinity anomalies ( Feng et al. 2015b ; N. Zhang et al. 2016 ). Yet, ocean salinity changes associated with Ningaloo Niño have never been addressed in a systematical fashion; as a result, our understanding of salinity-related ocean processes remains fragmental. Ocean salinity may exert feedback effects on the Ningaloo Niño warming through affecting the oceanic
range of mixed layer depth, large fraction of mixed layer water permanently subducted in the ocean interior, strong oceanic heat loss, and, for the North Atlantic Madeira mode water ( Sugimoto and Hanawa 2005 ), vigorous salt-finger mixing. The autumn and winter mixed layer deepening should lead to a corresponding reemergence of sea surface salinity (SSS) anomalies, but because of sparse observations there have been no observational studies of SSS reemergence. Alexander et al. (2001) considered a
range of mixed layer depth, large fraction of mixed layer water permanently subducted in the ocean interior, strong oceanic heat loss, and, for the North Atlantic Madeira mode water ( Sugimoto and Hanawa 2005 ), vigorous salt-finger mixing. The autumn and winter mixed layer deepening should lead to a corresponding reemergence of sea surface salinity (SSS) anomalies, but because of sparse observations there have been no observational studies of SSS reemergence. Alexander et al. (2001) considered a
1. Introduction Salinity, along with temperature, is known as a key parameter in physical oceanography. Since salinity controls the density of the seawater, understanding its spatiotemporal distribution is of great importance for an accurate description of dynamics and thermodynamics of the ocean. However, because of technical difficulty in salinity observation, its variability has not been understood compared to that of temperature. While temperature directly affects the atmosphere as a heat
1. Introduction Salinity, along with temperature, is known as a key parameter in physical oceanography. Since salinity controls the density of the seawater, understanding its spatiotemporal distribution is of great importance for an accurate description of dynamics and thermodynamics of the ocean. However, because of technical difficulty in salinity observation, its variability has not been understood compared to that of temperature. While temperature directly affects the atmosphere as a heat
variability from reanalysis products, which often violate basic physical constraints and are inconsistent with observational estimates ( Trenberth et al. 2011 ). With the ocean receiving over 80% of the total global rainfall ( Schanze et al. 2010 ), oceanic observations of salinity offer a unique opportunity in terms of measuring the integrated effect of changes in the hydrological cycle ( Trenberth et al. 2007 ). Only recently, however, has the observational network expanded to the point where the mean
variability from reanalysis products, which often violate basic physical constraints and are inconsistent with observational estimates ( Trenberth et al. 2011 ). With the ocean receiving over 80% of the total global rainfall ( Schanze et al. 2010 ), oceanic observations of salinity offer a unique opportunity in terms of measuring the integrated effect of changes in the hydrological cycle ( Trenberth et al. 2007 ). Only recently, however, has the observational network expanded to the point where the mean
, the regional sea level trend shows complex geographical distributions (e.g., Stammer and Hüttemann 2008 ; Cazenave and Llovel 2010 ; Han et al. 2010 , 2014 ; Merrifield et al. 2012 ; Hamlington et al. 2020 ) and tends to agree with the steric sea level (SSL) trend associated with ocean temperature and salinity changes (e.g., Qiu and Chen 2012 ; Han et al. 2014 ; Llovel and Lee 2015 ; Frederikse et al. 2020 ). The SSL estimate–based on in situ observations of ocean subsurface
, the regional sea level trend shows complex geographical distributions (e.g., Stammer and Hüttemann 2008 ; Cazenave and Llovel 2010 ; Han et al. 2010 , 2014 ; Merrifield et al. 2012 ; Hamlington et al. 2020 ) and tends to agree with the steric sea level (SSL) trend associated with ocean temperature and salinity changes (e.g., Qiu and Chen 2012 ; Han et al. 2014 ; Llovel and Lee 2015 ; Frederikse et al. 2020 ). The SSL estimate–based on in situ observations of ocean subsurface
1. Introduction Salinity declined along the Antarctic continental margin during the late 20th century, more so in the Pacific sector where much of the underlying data were obtained on the Ross Sea continental shelf ( Fig. 1 ; Jacobs and Giulivi 1998 ; Boyer et al. 2005 ). Measurements there typically show shelf water generated during winter sea ice formation, with temperatures near the sea surface freezing point and salinity gradually increasing with depth. Summer observations from 1963
1. Introduction Salinity declined along the Antarctic continental margin during the late 20th century, more so in the Pacific sector where much of the underlying data were obtained on the Ross Sea continental shelf ( Fig. 1 ; Jacobs and Giulivi 1998 ; Boyer et al. 2005 ). Measurements there typically show shelf water generated during winter sea ice formation, with temperatures near the sea surface freezing point and salinity gradually increasing with depth. Summer observations from 1963
1. Introduction In the ocean, the temperature is strongly controlled by heat exchange with the atmosphere and the physical properties of seawater. For seawater, the freezing point is slightly below 0°C, setting the lower temperature limit. The warmest waters are encountered in the tropics, where strong negative feedbacks presumably have kept sea surface temperature close to 30°C throughout a considerable part of the earth’s history (cf. Pierrehumbert 1995 , 2002 ). The oceanic salinity field
1. Introduction In the ocean, the temperature is strongly controlled by heat exchange with the atmosphere and the physical properties of seawater. For seawater, the freezing point is slightly below 0°C, setting the lower temperature limit. The warmest waters are encountered in the tropics, where strong negative feedbacks presumably have kept sea surface temperature close to 30°C throughout a considerable part of the earth’s history (cf. Pierrehumbert 1995 , 2002 ). The oceanic salinity field
