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

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 ε_T = 3.277 × 10^-8 m²s^-3 and K_zT = 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.