Origin and Pathway of Equatorial 13°C Water in the Pacific Identified by a Simulated Passive Tracer and Its Adjoint*

Abstract

The origin and pathway of the thermostad water in the eastern equatorial Pacific Ocean, often referred to as the equatorial 138C Water, are investigated using a simulated passive tracer and its adjoint, based on circulation estimates of a global general circulation model. Results demonstrate that the source region of the 138C Water lies well outside the tropics. In the South Pacific, some 138C Water is formed northeast of New Zealand, confirming an earlier hypothesis on the water's origin. The South Pacific origin of the 138C Water is also related to the formation of the Eastern Subtropical Mode Water (ESTMW) and the Sub-Antarctic Mode Water (SAMW). The portion of the ESTMW and SAMW that eventually enters the density range of the 138C Water (25.8 , s u , 26.6 kg m 23 ) does so largely by mixing. Water formed in the subtropics enters the equatorial region predominantly through the western boundary, while its interior transport is relatively small. The fresher North Pacific ESTMW and Central Mode Water (CMW) are also important sources of the 138C Water. The ratio of the southern versus the northern origins of the water mass is about 2 to 1 and tends to increase with time elapsed from its origin. Of the total volume of initially tracer-tagged water in the eastern equatorial Pacific, approximately 47.5% originates from depths above s u 5 25.8 kg m 23 and 34.6% from depths below s u 5 26.6 kg m 23 , indicative of a dramatic impact of mixing on the route of subtropical water to becoming the 138C Water. Still only a small portion of the water formed in the subtropics reaches the equatorial region, because most of the water is trapped and recirculates in the subtropical gyre.

Southern Ocean contributions to the Eastern Equatorial Pacific heat content during the Holocene

“…In the Pacific basin, once SAMW subducts it travels northwestward with some SAMW reaching the Solomon Sea near Papua New Guinea (Herraiz-Borreguero and Rintoul, 2011) and then it likely enters the New Guinea Coastal Undercurrent. This is also supported by tracer model experiments which show that the thermostad originates in SAMW formation regions but enters the equatorial region through the Western Boundary Currents (Qu et al, 2009). The water then travels eastward across the equatorial pacific to reach the EEP.…”

“…The thermostad is a depth interval of relatively uniform temperature below the Pacific Equatorial Undercurrent (EUC) (Tsuchiya, 1981) and arguably originates from SAMW, although other possible origins include the southeast Pacific and northeast of New Zealand (Qu et al, 2009;Spero and Lea, 2002;Toggweiler et al, 1991). Assuming SAMW is a major contributor to the thermostad water, the pathway from the Southern Ocean to the EEP is likely via the New Guinea Coastal Undercurrent.…”

“…The water then travels eastward across the equatorial pacific to reach the EEP. A tracer experiment study suggests that the thermostad water eventually upwells to the surface in the Peru cold tongue possibly communicating signals from outside the equatorial region to the surface (Qu et al, 2010), however, whether or not the subsurface temperature from the thermostad influences SST has not been proven. The equatorial Pacific thermostad extends between approximately 5 • N to 5 • S and is characterized by a temperature range of 11-14 • C and density of σ θ = 26.2-26.65 kg/m 3 (Tsuchiya, 1981).…”

“…SAMW forms in the Southern Ocean north of the Subantarctic Front (Hartin et al, 2011) during the winter when mixed layers can exceed 400 m and transports heat, salinity, and nutrients from the southern high latitudes equatorward (Herraiz-Borreguero and Rintoul, 2011;Spero and Lea, 2002). A model tracer experiment suggests that SAMW is likely a major contributor to the equatorial Pacific thermostad (∼150-300 m) (Qu et al, 2009), also referred to as the 13 • C Water (Tsuchiya, 1981). Additionally, radiocarbon ( 14 C) data from the Peru margin and Holocene δ 13 C reconstructions from the eastern equatorial Pacific (EEP), suggest that SAMW reaches the equator (Spero and Lea, 2002;Toggweiler et al, 1991).…”

Investigating bomb radiocarbon transport in the southern Pacific Ocean with otolith radiocarbon

“…and orange roughy otolith 14 C data were calculated by subtracting the atmospheric 14 C age corresponding to the mean year of the otolith 14 C sample from the otolith 14 C age. Atmospheric 14 C values were determined from INTCAL13 (Reimer et al, 2013). Otolith 14 C ages were unavailable for Australasian snapper and nannygai in the published literature.…”