Oceans and Climate

Oxygen in the ocean

Oxygen is in a state of super-saturation in surface waters, mostly because of entrainment of bubbles from breaking waves (see 2.9.2). As these bubbles of air are carried below the surface, the increase in pressure forces gas into solution. Hence not only oxygen but most atmospheric gases are slightly supersaturated at the ocean surface. There is also biological production of oxygen in parts of the water column where there is sufficient light for photosynthesis Fig. 3.8. Geographical...

The thermohaline circulation

Railway Map Derbyshire

The main outline of the thermohaline circulation was seen in Chapter 1 1.3.2, Figs. 1.14 and 1.16 . In regions where surface water is made denser through evaporative salinification, winter cooling, salt rejection during sea-ice formation or sub-ice shelf freezing, sufficiently extreme conditions can result in convection occurring to considerable depths, even to the bottom of the ocean. Along continental shelves and under ice shelves these processes may be widespread. In the open ocean deep...

Solubility of gases

Solubility Atmospheric Gases

The atmosphere is almost entirely a mixture of gases. These gases can enter the ocean across the air-sea interface. If no molecules of a particular gas were in solution in the upper ocean then this transfer would act as a drain on the atmospheric store of the gas in question (see 3.2). Once sufficient molecules from the atmosphere accumulate in the water for as many to leave the sea as enter in a given time, equilibrium is reached and the water is said to be saturated with respect to the gas....

The transfer of particles

The ocean surface is subject to a continual barrage of particles from the atmosphere. Some of these owe their origin to previous expulsion from the ocean surface, as discussed in 2.9.2. Others derive from chemical and physical processes within the atmosphere causing coagulation of smaller particles or gas molecules. Yet more have been swept, or thrown, up from a land surface and carried perhaps thousands of kilometres in the atmospheric circulation. The cascade of these particles into the sea...

The carbon cycle

Geosecs Atlantic

Table 3.2 showed that the ocean is a net sink for carbon dioxide, while it is a net source for most of the other greenhouse gases. In this section we shall explore the reason for this behaviour, but first we will examine the role of this oceanic sink in the full carbon cycle. Carbon cycles between the Earth, biosphere, ocean and atmosphere, as shown schematically in Fig. 3.4. Carbon dioxide is mixed through the troposphere within a few months. It takes much longer, probably hundreds of years,...

Timescales and feedbacks

The climate system is complex not just because within it a large number of processes and exchanges occur, but because different components act on widely differing timescales (Fig. 1.22) and interact in highly non-linear ways. The atmosphere, particularly the lower troposphere, has a large diurnal cycle. It reacts to changes in the basic heating source very rapidly. Within a few days transport and mixing can occur over continental scales, and within a few weeks, globally. In contrast, the...

Oceanic forcing by airsea exchange of moisture and heat

We have repeatedly seen the climatic importance of latent heat, transferred to the atmosphere through evaporation. Evaporation and precipitation also help to drive the ocean circulation by creating horizontal density gradients. Evaporation, by removing water and concentrating the dissolved salts, increases salinity, and hence density. Precipitation, by adding water, reduces the salinity, and therefore density. Density gradients produced in this way are less important to the regional ocean...

Temperature salinity and density

Salinity Temperature And Density

The sea surface temperature is altered through the heat loss or gain at the ocean surface and by mixing with water from below the surface or from advective horizontal mixing with water from another location. We have already encountered these processes. The high specific heat of water (Table 1.3) means that to change the temperature of 1 kg of water by 1 C requires over 4000 J of energy. Recalling that the global ocean absorbs 30 Wm-2 on average a typical summer mixed layer of 20 m depth would...

Winddriven circulation of the ocean

Vorticity Open University 1989

In the last three sections we have seen how the wind acts directly on the ocean and the local effect of ocean surface drag on the atmosphere. The momentum transferred to the water, plus the heat and fresh water, creates pressure gradients in the ocean leading to motion on a larger scale - the thermohaline circulation. This will be considered in 2.12. The larger scale impact of the ocean on the atmosphere is also through the creation of pressure gradients, but via surface heating or cooling. The...

The atmosphere

The atmosphere is a largely homogeneous mixture of gases, both horizontally and vertically, over the height range important for climate namely the troposphere and stratosphere Fig. 1.3 . The composition of this apparently stable mixture, air, is shown in Table 1.1. The balance of the dominant constituents of air is thought to have evolved considerably over the lifetime of the planet for instance, oxygen is likely to have been a product, rather than a necessity, of life the Gaia hypothesis . The...

Basic forces within the atmosphere and ocean

There are five basic forces underlying the circulation of the atmosphere and the ocean (i) gravity (ii) pressure gradients generated by density or mass differences (iii) drag, or the momentum gain or loss across the air sea, air land or land sea interface (iv) the Coriolis force due to the intrinsic rotation of the Earth and (v) tidal motion due to the astronomical influence of the Moon and the Sun. Gravity and the pressure gradient force provide the basic balance in the vertical but horizontal...

Heat exchange through latent and sensible heat

Sensible Heat Loss And Latent Hqeat Loss

Radiation dominates the exchange of heat between the atmosphere and ocean. Other physical mechanisms do, however, also contribute to the net heat flux. The most important of these is latent heat transfer. When water is evaporated from the ocean surface energy is supplied to the molecules to free them from the strong inter-molecular bonds within liquid water. This process was discussed in 1.3.1. When the water molecules condense to form water droplets, usually in clouds, this energy is released...

The Ekman spiral and Langmuir circulation

Wind Ekman Spiral

The orbital motion of water induced by surface waves penetrates some metres beneath the sea surface. The direct effect of the wind, however, penetrates to a depth of scores of metres. This is because of a larger scale physical force balance than the microscale pressure anomalies causing wave motion. The resulting flow varies with depth in the Ekman spiral. Small-scale circulation cells driven by this spiral are known as Langmuir circulation cells. In the winter of 1893 4 Fridtjof Nansen's...

The oceanic heat balance

The exchange of heat between the ocean and atmosphere principally consists of the four terms discussed above. Solar radiation provides the input to the ocean, and, in most situations, the net long-wave radiation, latent heat and sensible heat result in transfer of energy to the atmosphere. The net heat exchange, Q (in Wm-2), at a location is therefore where QI is the incident solar radiation that is absorbed by the ocean and positive Q indicates addition of energy to the ocean. This expression...

DMS and climate

Marine Atmosphere Boundary Layer

Dimethyl sulphide, or DMS, was first observed to be present in the ocean in considerable quantity in the early 1970s. Since the mid-1980s there has been Table 4.1. Gaseous sulphur emission rates Units are Tg S yr-1. Emission source Northern Hemisphere Southern Hemisphere Global Marine DMS and H2S 11 13 24 Terrestrial biogenic 0.6 0.4 1.0 Anthropogenic Source After Table 5.2 of Houghton et al., 2001. considerable interest in this gas as a major source for oceanic sulphate aerosols, which are now...

Chemical interaction of the atmosphere and ocean

Both the ocean and the atmosphere are composed of a mixture of chemical compounds, some inert but many chemically very reactive. The two fluids therefore conceal an almost infinitely varied chemistry, which the presence of living organisms complicates even further. We are largely unaware of the molecular-scale world, where these processes occur, as we go about our everyday lives. Nonetheless, its invisible activity contributes fundamentally to the climate system, as we saw in Chapter 1. The...

Solar radiation

The interior of the Sun, where the nuclear reactions occur that ultimately lead to life on Earth, is incredibly hot, at a temperature of several million degrees Celsius. However, the electromagnetic radiation (see Appendix A) that provides the energy for the climate system is derived from the outer layers of the Sun. The greatest amount of radiation comes from the photosphere, a layer some 300 km thick in the solar atmosphere. This varies in temperature from 10000K1 at the bottom to 5000K at...

Momentum transfer and drag

The main driving force of the ocean surface circulation is the wind. Blowing over the surface, the wind exerts a stress on the ocean, transferring momentum Fig. 2.23. The balance of forces in the atmospheric boundary layer. The surface wind is deflected in the direction of the pressure gradient force (PGF) due to the retarding effect of drag, which also reduces the size of the Coriolis force, as this is proportional to speed. Note that the Coriolis force must always be at right angles to the...

Waves the production of aerosols and condensation nuclei

The moving atmosphere produces a stress on the sea surface. The ocean is a fluid and exertion of a stress upon it will result in motion within the fluid. This takes many forms. To the atmospheric observer the most obvious of these is the production of surface waves. The type of waves, or the sea state, is a clear function of the wind speed light zephyrs give tiny ripples, great storms produce towering mountains of foaming water. The sea state is of vital importance for the air-sea exchange of...

Gas exchange across the airsea interface

The solubility of a gas determines the relative ease with which it can be absorbed into the ocean if there were no other gases present in the atmosphere, and if an equilibrium state was achieved. The troposphere, however, is a mixture of many gases and the basic driving force for exchange of gas across the air-sea interface is the difference in gas concentration, or partial pressure, between the two media. The flux, F, of a gas across the interface into the ocean is often written as Fig. 3.2....

Physical interaction between the ocean and atmosphere

The ocean and the atmosphere share a common boundary the air-sea interface. This direct physical contact enables the two fluids1 to exchange energy and matter. In this chapter we will examine these exchanges from a physical perspective, leaving a discussion of the chemical controls on matter, particularly gas, transfer to Chapter 3. Physical interaction between air and sea takes a number of forms. We are all familiar with the production of waves on water, due to wind blowing across the surface....