Hydrography and Oceanography
Hong Kong's waters are characterised by the interaction of oceanic and
fresh water masses which vary in relative effect throughout the year. The
variable freshwater discharge from the Pearl River has a marked influence
on Hong Kong waters. In addition, three different ocean currents affect
Hong Kong during the year.
During the summer, an oceanic flow from the south-west to the
north-east brings the warm, high-salinity water of the Hainan Current into
Hong Kong waters. This interacts with fresh water from the Pearl River
and divides Hong Kong into three distinct sections. In the west, where the
fresh water influence is greatest, the environment is estuarine and the
water is brackish. In the east, the water is mainly oceanic with relatively
minor dilution from direct monsoonal rainfall and runoff from small
streams. The limits of the central transitional zone vary depending upon
the relative influence of Pearl River water and marine currents.
During the winter, the Kuroshio oceanic current brings warm water of
high salinity from the Pacific through the Luzon Strait. The fresh water of
the Pearl River has a lesser influence than in summer and water salinity is
more uniform across Hong Kong. Besides the Kuroshio current, the
coastal Taiwan current brings cold water down the South China coast,
which affects inshore waters, making the surface water cold during the
winter.
The normal tidal range in Hong Kong waters is between one and two
metres, depending on the relative influence of the moon and sun. The tidal
pattern is complex due to the relative effects of the diurnal and
semi-diurnal components. The basic pattern during flood tides is for
oceanic water to flow north into Mirs Bay, west through Victoria Harbour
and north through Kap Shui Mun and the Ma Wan Channel. Maximum
tidal currents generally range from 0.5 to 1.5 knots and locally up to 5
knots at narrow channels. This flow is reversed during the ebb tide,
though currents in some parts may be stronger due to the influence of the
Pearl River.
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