Effect of environment dependence of convergence zones in the tropical and subtropical sea areas of Western Pacific

Abstract

The convergence zone (CZ) is a special acoustic phenomenon in deep sea, which is strongly correlated to the profile features of the water column. This paper describes the different attributes of a typical CZ in the tropical and subtropical sea areas of the Western Pacific. A Gaussian beam model is used to compute the transmission loss (TL), and the typical sound field of tropical and subtropical sea areas of the Western Pacific is obtained on the basis of Argo data. It is noted that CZ properties reflected by the sound pressure field are evidently influenced by a regional difference of sound speed profile(SSP) structure, and the CZ in the summer Pacific could be classified into 3 types (tropics, south subtropics and north subtropics) which are closely related to the hydrological environment. There is a strong and thin thermocline in the tropics, with a sharp negative gradient layer in SSP. In the south subtropics, the main thermocline is divided into two parts by an homogeneous layer. The pressure field analysis results show that there is a general surface duct in the tropics when source locates in subsurface layer, and an unusual subsurface duct in the northern area of subtropics. The TL difference inside the duct and outside the duct can reach 10∼20 dB (sound source frequency is 1 kHz). When a source is located at 20m, CZ distance reaches maximum in the tropics, where the first CZ is about 60–70 km; and, from tropics to subtropics, the CZ range decreases about 10 km. When a source is located at 200m, CZ range in the tropics suddenly decreases: the maximum appears in the northern area of subtropics, and decreases from subtropics to tropics. The range of the first CZ is about 60 km in the northern area of subtropics, and about 50 km in tropics. The conclusion which is drawn is that the properties of the CZ in the tropical and subtropical sea area of the western Pacific have a clear difference. It is also concuded that a special phenomenon of subsurface duct caused by the peculiar hydrological environment of the northwestern Pacific in the summer is worthy of further study.