Seasonal variability of water characteristics in the Challenger Deep observed by four cruises

Abstract

Thirty conductivity-temperature-depth profiler casts in the Challenger Deep were conducted during four cruises from December 2015 to February 2017. Two cruises took place in the summer, and two in the winter. The results demonstrated that water characteristics varied seasonally. The temperature minimum values were the same between the four cruises, but its depth was noticeably shallower in the winter than that in the summer. The θ-S diagram indicated that deep water is more saline in the summer than that in winter at the same potential temperature. Mixing is more intense between 5000 and 6800 m in the summer than that in the winter. The dissipation rate and eddy diffusivity vertically averaged between 5000 and 6800 m in the summer were εT = 3.277 × 10−8 m2s−3 and KzT = 2.58 × 10−2 m2s−1, respectively. The geostrophic flows below the reference level of 3000 dbar were cyclonic in the summer, travelling westwards in the northern and eastwards in the southern areas of the Challenger Deep.

Water characteristics of abyssal and hadal zones in the southern Yap Trench observed with the submersible Jiaolong

“…This suggests that the temperature over the central trench is depressed more than that at the fl ank. This is similar to the phenomena in the Challenger Deep of the Mariana Trench (Huang et al, 2018). At lower depths, temperature increases to the almost same in the three stations.…”

“…In this paper, the reference level is 3 000 m according to previous abyssal studies and adjacent trenches and basins (Johnson and Toole, 1993;Taira et al, 2005;Huang et al, 2018;Liu et al, 2018). At this weak circulation level, the meridional geostrophic velocity across transects Section-C and Section-D is set to be zero, hence we treated the calculated fl ows relative to the level as the geostrophic fl ows at abyssal and hadal depths.…”

“…Usually the water exceeding 3 000 m is defi ned as abyssal water (Macdonald et al, 2009;Hautala, 2018). The abyssal water in Mariana-Caroline regions is from LCPW, with the infl ow of LCPW into the East Mariana Basin (EMB) and a partial outfl ow to the West Mariana Basin (Uehara and Taira, 1990;Johnson and Toole, 1993;Yanagimoto and Kawabe, 2007;Huang et al, 2018). Siedler et al (2004) found that the channels at the Yap-Mariana Junction and Caroline Ridge were essential for the exchange of deep water such as water fl owing to East Caroline Basin (ECB) from EMB.…”

Seasonal Variation of the Deep Limb of the Pacific Meridional Overturning Circulation at Yap‐Mariana Junction

“…The mechanism for the seasonal LCPW and UCPW intrusions into the YMJ channel may be related to the pressure gradient force between two adjacent basins but is not investigated in this study. The pressure gradient can be formed regionally or globally through a number of physical processes, such as the deep ocean mixing (Zhou et al, 2014), the route shift of deep inflows (Huang et al, 2018), or seasonal watermass formation at high latitudes. The seasonal formation of bottom water near the Antarctic shelf has been documented (e.g., Evans et al, 2014;Fukamachi et al, 2000;Koshlyakov & Tarakanov, 2003;Williams et al, 2008) and may factor in the YMJ channel since the PMOC is supplied only from the south.…”

“…After passing through the Samoan Passage (Rudnick, 1997;Voet et al, 2015) or around the Manihiki Plateau (Pratt et al, 2019), it enters the Central Pacific Basin and bifurcates into the eastern and western branches (Johnson & Toole, 1993;Kawabe et al, 2003Kawabe et al, , 2006. The western branch passes through the Melanesian and East Mariana Basins and finally flows into the western Pacific through the deep channel at the Yap-Mariana Junction (YMJ; solid black arrow in Figure 1a, Huang et al, 2018;Kawabe & Fujio, 2010;Siedler et al, 2004). The U-PMOC flows northward in the South Pacific, detouring around the subtropical gyres (Reid, 1997) and entering the equatorial Pacific after passing east of Solomon Rise (Kawabe et al, 2006;Kawabe & Fujio, 2010).…”