7 Ocean Currents
7.1 Introduction
Water is continually in motion at all depths, even when the sea appears perfectly calm and flat. In Chaps. 3-6 of this book, we discussed a specific type of
water motion, namely periodic wave motion. Waves can be as small as ripples
on the sea surface, and as large as long tidal waves with a wavelength of thousands of kilometres. Waves can travel on the sea surface, or along interfaces
dividing water masses of different densities (internal waves). However, in all
these cases the water motion is periodic, or can be represented as a sum of
periodic motion.
Both everyday observations and specific measurements provide a lot of evidence of a different type of water motion in the ocean which is not periodic.
For example, a sailing boat with sails hanging listlessly from its spars may appear to be motionless, but it is actually drifting with the slow surface currents.
This persistent water motion is responsible for slowly transporting large volumes of surface and subsurface water over vast distances. The nature of flow
associated with large-scale ocean currents depends on a few dominant driving
mechanisms: wind stress, pressure gradients, water density gradients, and the
Coriolis effect. For clarity of analysis it will be useful to divide the currents into
two fundamental groups of flow: wind-driven surface and near-surface currents,
and density-driven subsurface currents. Although the surface and near-surface
currents affect only about 10 percent of the ocean's volume, the vast part of all
oceanographic studies was and still is devoted to them. Water motion in the
ocean depths still requires more efforts from present and future generations of
physical oceanographers.
Numerical modelling of ocean circulation, and the coupling of oceanic and
atmospheric circulation modelling has grown intensively in recent years. Such
models can skilfully mimic observed oceanic features and help in understanding
and predicting the global climate system and its impact on life in the oceans.
Present models do seem to encompass the major features observed in ocean
circulation.
S. R. Massel, Fluid Mechanics for Marine Ecologists
© Springer-Verlag Berlin Heidelberg 1999
7.1 Introduction
Water is continually in motion at all depths, even when the sea appears perfectly calm and flat. In Chaps. 3-6 of this book, we discussed a specific type of
water motion, namely periodic wave motion. Waves can be as small as ripples
on the sea surface, and as large as long tidal waves with a wavelength of thousands of kilometres. Waves can travel on the sea surface, or along interfaces
dividing water masses of different densities (internal waves). However, in all
these cases the water motion is periodic, or can be represented as a sum of
periodic motion.
Both everyday observations and specific measurements provide a lot of evidence of a different type of water motion in the ocean which is not periodic.
For example, a sailing boat with sails hanging listlessly from its spars may appear to be motionless, but it is actually drifting with the slow surface currents.
This persistent water motion is responsible for slowly transporting large volumes of surface and subsurface water over vast distances. The nature of flow
associated with large-scale ocean currents depends on a few dominant driving
mechanisms: wind stress, pressure gradients, water density gradients, and the
Coriolis effect. For clarity of analysis it will be useful to divide the currents into
two fundamental groups of flow: wind-driven surface and near-surface currents,
and density-driven subsurface currents. Although the surface and near-surface
currents affect only about 10 percent of the ocean's volume, the vast part of all
oceanographic studies was and still is devoted to them. Water motion in the
ocean depths still requires more efforts from present and future generations of
physical oceanographers.
Numerical modelling of ocean circulation, and the coupling of oceanic and
atmospheric circulation modelling has grown intensively in recent years. Such
models can skilfully mimic observed oceanic features and help in understanding
and predicting the global climate system and its impact on life in the oceans.
Present models do seem to encompass the major features observed in ocean
circulation.
S. R. Massel, Fluid Mechanics for Marine Ecologists
© Springer-Verlag Berlin Heidelberg 1999