Abstract
Primarily due to the constraints of observation technologies (both field and satellite measurements), our understanding of ocean salinity is much less mature compared to ocean temperature. As a result, the characterizations of the two most important properties of the ocean are unfortunately out of step: the former is one generation behind the latter in terms of data availability and applicability. This situation has been substantially changed with the advent of the Argo floats which measure the two variables simultaneously on a global scale since early this century. The first decade of Argo-acquired salinity data are analyzed here in the context of climatology and seasonality, yielding the following main findings for the global upper oceans. First, the six well-defined “salty pools” observed around ±20° in each hemisphere of the Pacific, Atlantic and Indian Oceans are found to tilt westward vertically from the sea surface to about 600 m depth, forming six saline cores within the subsurface oceans. Second, while potential temperature climatology decreases monotonically to the bottom in most places of the ocean, the vertical distribution of salinity can be classified into two categories: A double-halocline type forming immediately above and below the local salinity maximum around 100–150 m depths in the tropical and subtropical oceans, and a single halocline type existing at about 100 m depth in the extratropical oceans. Third, in contrast to the midlatitude dominance for temperature, seasonal variability of salinity in the oceanic mixed layer has a clear tropical dominance. Meanwhile, it is found that a two-mode structure with annual and semiannual periodicities can effectively penetrate through the upper ocean into a depth of ~2000 m. Fourth, signature of Rossby waves is identified in the annual phase map of ocean salinity within 200–600 m depths in the tropical oceans, revealing a strongly co-varying nature of ocean temperature and salinity at specific depths. These results serve as significant contributions to improving our knowledge on the haline aspect of the ocean climate.
Highlights
Along with temperature and pressure, salinity is one of the three key variables for determining the seawater density
These and our results suggest that the climatological ocean circulations may act as background “salt conveyors”, salt parcels can be discretely entrained by numerous migrating eddies and swirls, the combined effects of which lead to the formation of six well-defined “salty pools” in the middle and western subtropical oceans, as described by Schmitt and Blair (2015) as “a river of salt”
For the twelve zonal bands, a dividing latitude of ~ 45° appears to separate the vertical distributions into two regimes for both hemispheres: the upper-saltier regime within 0–45°, and the upper-fresher regime beyond 45°, which remain largely true for the 0–500 m layers
Summary
Along with temperature and pressure, salinity is one of the three key variables for determining the seawater density. Roemmich and Gilson (2009) compared the annual cycles in salinity of Argo data to the World Ocean Atlas, and to the National Oceanography Center air–sea flux climatology, the Reynolds sea surface temperature (SST) product, and AVISO satellite altimetric height These products are consistent with Argo data on hemispheric and global scales, but show considerable regional differences that may either point to systematic errors in the datasets or their syntheses, to physical processes, or to temporal variabilities. Based on multiple sources of salinity data from Argo, ship, mooring, CTD and satellite observations as well as numerical models, the mixed layer salinity budget in the tropical Pacific Ocean was assessed (Hasson et al 2013) They conclude that the mean spatial distribution of mixed layer salinity results from a subtle balance between surface forcing (evaporation minus precipitation), horizontal advection (at low and high frequencies) and subsurface forcing (entrainment and mixing), all terms being of analogous importance. Note that the explained variance and significance test of Argo derived annual and semiannual temperatures, which are basically applicable to their counterparts in salinity and closely related to the sensitivity or robustness of the present results, can be found in Appendix B of Chen and Wang (2016)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
