Introduction into Physical Oceanography

Abstract

The fundamental basis to understand the distribution and variability of abiotic variables within the oceans such as e.g. temperature and salinity are the underlying physical dynamics. These dynamics depend on the setting of the ocean basins and external forcing mechanisms. In this chapter water mass characteristics and their formation processes are described as well as fundamental principles, which set the oceans into motion. These fundamentals are the premise to understand possible future climate changes, the distribution and evolution of marine ecosystems and related economic interests and conflicts.

Glossary

Upwelling

Describes the wind-driven upward motion of cool and usually nutrient-rich water towards the surface, where it replaces warmer and usually nutrient-depleted surface water

Convection

Is a density driven water mass formation process. At high latitudes ice formation within the ocean as well as strong heat loss from the ocean to the atmosphere can increase the density of surface waters until they start to sink (convect) and get disconnected from the surface forming a new water mass

Subduction

Is a wind driven water mass formation process. If the wind field e.g. produces a convergent flow in the surface layer, waters tend to pile up and get “pumped” towards the ocean interior. Hence, when this surface waters are pumped into the interior and disconnected from the surface oceanographers refer to these waters as subducted waters

El Nino

Is a climate phenomenon of ocean-atmosphere interactions. The term “El Nino” refers to the warm phase of this phenomenon, when sea surface temperatures in the tropical eastern Pacific are unusually warm compared to the average state. This does not only have local impacts on climate and the economy, but is related to climate variability all over the globe

The Coriolis Force (Coriolis Effect)

Is a fictitious force, which acts on objects, which are in motion relative to an already rotating reference frame, e.g. air or water moving on the rotating earth or a ball rolling on a rotating plate. The effect (Coriolis effect) of this force on air or water parcels moving on the earth is that they get deflected at a right angle to the right (left) in the Northern (Southern) Hemisphere

Geostrophy

Describes a balance of forces from the simplified equations of motion, in this case the balance of the pressure gradient force and the Coriolis force. The invoked geostrophic flow is directed along isobars and has the high pressure to its right (left) in the Northern (Southern) Hemisphere

Potential temperature/density

Is the temperature/density corrected for the pressure effect. A water parcel with a certain temperature/density will have a higher temperature/density if it is exposed to a higher pressure. The pressure within the ocean increases with increasing depth due to the overlying water body. Hence, when temperatures/densities of different depth layers are compared to each other, one wants to get rid of this pressure effect and defines the potential temperature (θ) and the potential density (σ _θ ) as the temperature/density a water parcel would have, if it would be adiabatically brought to a standard reference pressure e.g. at the surface. Adiabatically means without a transfer of heat or matter with the surroundings

Kelvin waves

Are a special kind of waves, which cannot freely propagate at the oceans surface, but instead can only propagate within a so-called wave guide, which means along topographic boundaries or the equator. Kelvin waves are geostrophically balanced waves and can be excited by any kind of pressure gradients, which then get balanced by the Coriolis force, e.g. in the Northern Hemisphere they are aligned with the coast to the right in the alongshore propagation direction. Kelvin waves play an important role for the adjustment of the circulation towards changes in the forcing. Note that also the tides propagate in form of coastal Kelvin waves

Rossby Waves/planetary waves

Are waves, where the restoring mechanism is the conservation of potential vorticity. Without going into too much detail about the concept of potential vorticity, the principle is that motions with changing latitude create a gradient in the potential vorticity as the Coriolis parameter is dependent on latitude. This then leads to a restoring mechanism towards the original potential vorticity and hence, a disturbance with latitude can start to oscillate and travel in form of a planetary wave. These planetary waves are important for the signal propagation within the ocean, communicating a temporal change in the forcing to e.g. a geostrophically balanced flow, leading to an adjustment of the balance

The Ekman balance (Ekman transport/pumping)

Describes a balance of forces in this case between frictional forces (at the surface or bottom of the ocean) and the Coriolis force. If the balance is vertically integrated over the extent of the surface Ekman layer, the net Ekman transport within this layer is directed at an 90° angle to the right/left of the wind in the Northern/Southern hemisphere. This Ekman transports can lead to divergent/convergent flows in the surface layer, which then can cause water to be lifted up (Ekman suction) or pushed into the ocean interior (Ekman pumping)