Page 81 - (DK) Ocean - The Definitive Visual Guide

TIDES 79
Sun
Monthly Cycle Tidal Currents
SPRING
new Moon TIDES
In addition to the daily cycle of high and The vertical variation in sea level that occurs locally with tides can
low tides, there is a second, monthly, cycle. happen only through horizontal flows of water, called tidal currents.
high In this case, the Sun and Moon combine Over each daily tidal cycle, the currents typically (but not invariably) run
tide low
low tide to drive the cycle. As with the Moon, the fastest about half-way between high and low tide at that location – at
tide interaction between the Earth and Sun intermediate times they slow (“slack water”) and then reverse direction.
causes bulges in the Earth’s oceans, though The shape of a coast can have a crucial influence on current strength.
high these are smaller than those caused by the Bottlenecks to water flow, such as narrow channels and promontories,
tide
Moon. Twice a month, at the times of new are often associated with very powerful currents, called tidal races, that
and full Moon, the Sun, Moon, and Earth develop twice or four times a day. Where the flowing water meets
are aligned, and the two sets of tidal bulges underwater obstructions, phenomena such as whirlpools or vortices
full
Moon reinforce each other. The result is spring (spiralling, funnel-shaped disturbances), eddies (larger, flatter, circular
tides – high tides that are exceptionally currents), and standing
high, and low tides that are exceptionally waves may develop.
Sun
low. By contrast, at the times of first and Other tide-related
last quarter Moon, the effects of the Sun phenomena include tide
NEAP
and Moon partly cancel out, bringing rips – turbulence caused by
TIDES
first quarter
Moon tides with a smaller range, called neaps. converging currents – and
low overfalls, defined as a tidal
tide
high ALTERNATING SPRINGS AND NEAPS Wellington current flowing opposite
tide
Twice a month (top), the alignment of the Sun, to the wind direction.
Moon, and Earth creates spring tides. At other
times (left), when the Sun and Moon lie at right
high last angles, it creates neap tides. The alternation
tide low quarter between springs and neaps can be seen in the TIME OF
tide Moon LOW WATER,
28-day tidal graph shown below. WELLINGTON
COOK STRAIT CURRENTS
NEW MOON FIRST QUARTER FULL MOON LAST QUARTER NEW MOON
These maps show the pattern of
strong tidal currents in the Cook
Strait, between the North and South Wellington
islands of New Zealand, which occur
twice a day, just over six hours apart.
TIDE HEIGHT channel in the Strait. TIME OF
Water must funnel through a narrow
HIGH WATER,
WELLINGTON
SPRING NEAP SPRING NEAP SPRING

LOW TIDE AT BAMBURGH BEACH
Due to tides, large swathes of coast INTRODUCTION
around the world are alternately covered
and uncovered by the sea. This intertidal
sandflat in Northumberland, England, has
a tidal range averaging about 4m (13ft).