A “mirror layer” of temperature and salinity in the ocean

Abstract

Using the newly available 13-year Argo float data for the period 2004-2016, a three-dimensional temperature-salinity (T-S) correlation field is constructed for the global upper ocean. It is revealed that the layer-averaged correlation between T and S time series has a significant peak (~ 0.7) at a depth of approximately 300 m, suggesting that the density-compensated thermohaline covariation may lead to a maximum T-S coupling in the vicinity of the pycnocline. This argument is supported by subsequent findings that the spatial distributions of temperature and salinity have excellent consistencies in terms of both climatology and seasonality around this depth, which we call a "T-S mirror layer". Since mass, heat and salt are mainly transported by dynamic events and processes, dominant currents and prevailing eddies in the pycnocline zone are found to determine the fundamental patterns of the global T/S climatologies which are characterized by a unified pattern of six well-defined warm/salty pools with collocated centroids and a clear west preference. Our results also suggest that the climatological ocean circulations transport heat/salt as a conveyor belt, meanwhile heat and salt parcels are discretely transported by numerous migrating mesoscale eddies, the combined effects of which lead to the formation of six well-defined "warm/ salty pools" in the western subtropical oceans with a core depth of ~ 300 m at the "mirror layer".

Mirror patterns of physical variables in the ocean

“…Figure 2 gives the temperature and salinity patterns in the T-S mirror layer in spring (similar to the results in the other seasons). In all the four datasets, the mirror patterns are featured with six warm and salty "tongues" extending eastward in the subtropics, as have likewise been noticed by Chen and Geng (2019) in the α-T-S mirror layer. The T-S mirror layers in EN4 and Ishii are shallower than that in the other two (WOA13 and Argo).…”

“…This work will derive another kind of "T-S mirror layer" via spatial correlation between temperature and salinity patterns. The T-S mirror layer derived in this way possesses the same physical implications as that introduced by Chen and Geng (2019). However, the depths of these two kinds of mirror layers are not the same.…”

“…Argo in this work is not the version used by Chen and Geng (2019). It is an analyzed 3D grid product produced by the China Argo Real-time Data Centre in 2013 (https :// www.argo.org.cn).…”

“…In monthly Argo (Array for Real-time Geostrophic Oceanography) data, Chen and Geng (2019) found that the Pearson correlation between layer-averaged temperature and salinity (hereafter referred to as "T-S") time series peaks at approximately 300 m depth. Furthermore, the T-S patterns at that depth, both in climatology and seasonality, are consistent.…”

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“…Normally, a mesoscale eddy is a rotational water column that extends to hundreds (even exceeding one thousand) of meters in the upper ocean through the so-called Ekman spiral, mixed layer, mirror layer (Chen and Geng, 2018), and pycnocline (thermocline, halocline) layer, as well as acoustic waveguide in hydrodynamics and geophysics, and the photic zone and the compensation layer in biological oceanography. An ideal strategy for eddy observation is to simultaneously capture its surface signature along with its interior structure.…”