The Role of Oceans
|"Ocean water and currents affect climate. Because it takes
far more energy to change the temperature of water than land or air, water
warms up and cools off much more slowly than either. As a result, inland
climates are subject to more extreme temperature ranges than coastal
climates, which are insulated by nearby water.
Over half the heat that reaches the earth from the sun is absorbed by the
ocean's surface layer, so surface currents move lots of heat. Currents that
originate near the equator are warm; currents that flow from the poles are
cold." (Smithsonian Ocean Planet)
"The planetary water supply is
dominated by the oceans (see Table 8b-1). Approximately 97 % of all
the water on the Earth is in the oceans. The other 3 % is held as freshwater
in glaciers and icecaps, groundwater, lakes, soil, the atmosphere, and
within life." (Michael J. Pidwirny)
Inventory of water at the Earth's surface.
(cubic km x 10,000,000)
Percent of Total
|Ice Caps and Glaciers
|Streams and Rivers
"Water is continually cycled between its various
reservoirs. This cycling occurs through the processes of
groundwater flow. Table 8b-2 describes the approximate
residence times of water in the major reservoirs. On average water is
renewed in rivers once every 16 days. Water in the atmosphere is completely
replaced once every 8 days. Slower rates of replacement occur in large
lakes, glaciers, ocean bodies and groundwater. Replacement in these
reservoirs can take from hundreds to thousands of years. Some of these
resources (especially groundwater) are being used by humans at rates that
far exceed their renewal times. This type of resource use is making this
type of water effectively
nonrenewable." (Michael J. Pidwirny)
Approximate residence time of water found in various reserviors.
| Seasonal Snow Cover
| Soil Moisture
| Groundwater: Shallow
| Groundwater: Deep
covers 70% of the Earths surface
Volume: 97% of
all the water on the Earth
(Pure water: 1000 kg/m3)
over 90% of the ocean. Depends on temperature and salinity.
of water by evaporation
precipitation and river discharge
capacity: high compared to land
less variable than in the atmosphere
not at 0°C
because of salinity
is not level
currents, waves, atmospheric pressure differences, and variations in
main forms of circulation:
(horizontal, surface waters, fast)
(vertical, deep waters, slow)
- Well over half of the solar radiation reaching the Earths surface is
absorbed by the oceans, so that it figures prominently in the regulation
of the global climate.
- Furthermore, the mean temperature of the global ocean is only ~3.6oC
thus it represents a huge heat sink in
the global climate system.
- The primary heat source for the oceans is solar radiation entering
through the ocean surface. Almost all this insolation (incoming solar
radiation) is absorbed in the top 100 metres. Turbulent mixing in the
surface layer is promoted by the interaction of wind and waves.
- In the mixed layer of the ocean, convection and turbulence are so
effective that the temperature and salinity are almost uniform (constant)
The global average thickness
of the mixed layer is 70-100 metres.
- The base of the mixed layer is marked by a horizon where the water
properties change rapidly with depth this is called the thermocline
This is a mechanically
stable region, within which vertical motions are strongly resisted
(since water density increases with declining temperature).
Accordingly, the waters of
the deep ocean are thermally "isolated" from the atmosphere, and the
exchange of heat is very slow.
- Below the thermocline layer, the ocean is thermally stratified and
tends to a uniform temperature (since the density of sea water is greatest
at around 4oC.
The deep ocean comprises
about 80% of the total volume of the oceans.
Below the thermocline layer,
in the deep ocean, there exist slow circulations driven primarily by
density gradients. These currents are quite weak, and it is estimated that
the time required for complete
- The mixed layer, then, responds quickly to changes in the surface
climate, whereas the deep ocean layer does not. The thermal capacity of
the mixed layer implies response times to surface changes on the order of
- The thermal properties of the deep ocean constitute a time lag in the
climate system on the scale of 1000 years.
|El Nino / Southern Oscillation
|There has been a confusing
range of uses for the terms El Niņo, La Niņa and ENSO by both the scientific
community and the general public. I hope by taking this course students will
have a better understanding of their meanings.
|What is the Southern Oscillation? What is the Southern
Oscillation Index (SOI)?
|What is the Walker Circulation?
|What is El Nino? How often do El Nino events occur?
|What is La Nina?
|What is ENSO? Oceanic characteristics of ENSO and
atmospheric characteristics of ENSO. Links between atmosphere-ocean in ENSO.
|How ENSO is developed? Nature and
effects of oceanic Kelvin and Rossby waves in El Nino events.
|How does ENSO influence the world climate?
|Can ENSO be predicted?
|How ENSO is monitored? Range and nature of indices used to
indicate the state of ENSO (El Nino or La Nina).
|Typical impacts of ENSO events (temperature and
precipitation). ENSO and the climate in the USA in general, and in Texas in
|El Niņo. "El Niņo translates from Spanish as 'the
boy-child'. Peruvian anchovy fishermen traditionally used the term - a
reference to the Christ child - to describe the appearance, around
Christmas, of a warm ocean current off the South American coast, adjacent to
Ecuador and extending into Peruvian waters. El Niņo affects traditional
fisheries in Peru and Ecuador, In most years, colder nutrient-rich water
from the deeper ocean is drawn to the surface near the coast (upwelling),
producing abundant plankton, food source of the anchovy. However, when
upwelling weakens in El Niņo years, and warmer low-nutrient water spreads
along the coast, the anchovy harvest plummets. It was ruined in the four or
five most severe El Niņo events this century." (Bureau of Meteorology,
"The South American El Niņo current is caused by large-scale
interactions between the ocean and atmosphere. Nowadays, the term El Niņo
refers to a sequence of changes in circulations across the Pacific Ocean and
Indonesian archipelago when warming is particularly strong (on average every
three to eight years). Characteristic changes in the atmosphere
accompany those in the ocean, resulting in altered weather patterns across
the globe." (Bureau of Meteorology, Australia)
|The Walker Circulation. The Walker circulation is named after
Sir Gilbert Walker, a Director-General of British observatories in India
who, early this century, identified a number of relationships between
seasonal climate variations in Asia and the Pacific region.
"I cannot help believing that we shall gradually find out the
physical mechanism by which these (relationships) are maintained..."
- Sir Gilbert T. Walker, 1918
The easterly trade winds are part of the low-level component of the
Walker circulation. Typically, the trades bring warm moist air towards
the Indonesian region. Here, moving over normally very warm seas, moist
air rises to high levels of the atmosphere. The air then travels
eastward before sinking over the eastern Pacific Ocean. The rising air
is associated with a region of low air pressure, towering cumulonimbus
clouds and rain. High pressure and dry conditions accompany the sinking
|The Southern Oscillation. "By the Southern Oscillation is
implied the tendency of pressure at stations in the Pacific ... to
increase, while pressure in the region of the Indian Ocean ...
- Sir Gilbert T. Walker, 1924
This definition remains valid, but it now refers to the seesaw
pattern of reversing surface air pressure at opposite ends of the
Pacific Ocean. We now say that the Southern Oscillation occurs because
of the large changes in the Walker circulation closely linked to the
pattern of tropical Pacific sea temperatures. Because the pressure
reversals and ocean warming are more or less simultaneous, we call this
phenomenon the El Nino/Southern Oscillation or ENSO for short.
The Southern Oscillation Index. "The Southern Oscillation
Index (SOI) gives us a simple measure of the strength and phase of the
Southern Oscillation, and indicates the status of the Walker
circulation. The SOI is calculated from the monthly or seasonal
fluctuations in the air pressure difference between Tahiti and Darwin (Equation
= Tahiti Darwin). The 'typical' Walker circulation Pattern shown
in the diagram has an SOI close to zero (Southern Oscillation close to
the long-term average state). When this pattern is strong the SOI is
strongly positive (Southern Oscillation at one extreme of
its range). When the Walker circulation enters its El Niņo phase, the
SOI is strongly negative (Southern Oscillation at the
other extreme of its range). Positive values of the SOI are associated
with stronger Pacific trade winds and warmer sea temperatures to the
north of Australia. Together these give a high probability that eastern
and northern Australia will be wetter than normal. During El Niņo
episodes, the Walker circulation weakens, seas around Australia cool,
and slackened trade winds feed less moisture into the Australian/Asian
region. There is then a high probability that eastern and northern
Australia will be drier than normal." (Bureau of Meteorology, Australia)
La Niņa. SO tendencies for unusually low pressures west of the
date line and high pressures east of the date line have also been linked
to periods of anomalously cold equatorial Pacific sea surface
temperatures (SSTs) sometimes referred to as La Niņa.
Another way to say the above. High SOI
(large pressure difference) is associated with stronger than normal
trade winds and La Niņa conditions, and low SOI (smaller pressure
difference) is associated with weaker than normal trade winds and El
Sea surface temperatures. During non-El Nino and non-La Nina
conditions sea surface temperatures are approximately 6-8 degrees
Celsius warmer in the western tropical Pacific than in the eastern
tropical Pacific. These temperature disparities typically occur because
the easterly trade winds that blow across the tropical Pacific move the
warm surface water with them from east to west. Thus, you could look at
SST data to determine whether an El Nino event is occurring at present,
El Nino and US Climate
News and General
Last updated on
01/05/10 03:25 PM by Zong-Liang Yang