The Earth’s oceans contain nearly 97% of the planet’s water and cover nearly 70% of its surface.

The Earth’s oceans contain nearly 97% of the planet’s water and cover nearly 70% of its surface. Like the atmosphere, they play a critical role as one of the Earth’s major systems. Ocean waters have contrasting properties and are subject to complex processes that are often tied to the behavior of the atmosphere or are impacted by the same controls as those that affect the atmosphere. The text and exercises in this lab are intended to introduce you to the fundamental properties and behavior of the Earth’s oceans and
how they can be measured and interpreted

Before examining the properties and behavior of the world’s oceans, it is helpful to have a good grasp of geography. Different bodies of water have different characteristic and are subject to different controls and processes. Some bodies of water are distinct, but most of the ocean water is connected – the water of the four major ocean basins is in direct contact with numerous seas, bays, and gulfs. There can be bathymetric boundaries which separate named water bodies (basins) as well as boundaries defined by currents. The Southern Ocean, for example, is an unofficial designation for the waters off the coast of Antarctica. This “ocean” is defined by the Antarctic Circumpolar Current which separates cold Antarctic
waters from the warmer waters of the of southern Pacific, Atlantic, and Indian Oceans. For the most part, the names of water bodies reflect cultural preferences and the priority given to first discoveries. If the name becomes part of a culture’s vocabulary, it will continue, regardless of whether the name is technically accurate. (The Gulf of Mexico could easily be considered a sea in its own right. In fact, many countries argue that the Persian Gulf should be renamed the Arabian Gulf.) Regardless of what they are called, knowledge of the actual location of particular features of the world sets a baseline for important geography applicable to many classes and serves as a first entry into the world of oceanography, setting the stage for discussions of ocean currents, thermohaline circulation, and plate tectonic evolution. Because the two are so closely linked, any discussion or study of the oceans relies on first understanding how the atmosphere functions. Circulation within both the atmosphere and ocean relies on energy radiated from the Sun (Figure 1a). Because the Earth is a sphere, insolation (incoming solar radiation) does not evenly strike the planet. Around the Equator, insolation strikes at a higher angle, delivering more energy per unit area. Toward the poles, because of the curvature of the Earth, insolation strikes at a lower angle. This spreads insolation out over a larger area, resulting in less energy delivered per unit area. The difference in insolation angle results in an imbalance in heat energy between the Equator and the poles. Currents exist in both the atmosphere and the oceans to redistribute heat in an effort to
correct the energy imbalance. The heat absorbed from the Sun by the Earth’s surface drives vertical circulation within the atmosphere (Figure 1b). When air is warmed it spreads out, becoming less dense. If an airmass is less dense than the surrounding air, it will begin to rise through the atmosphere. When air rises, it creates an area of low atmospheric pressure near the ground (lifting air lowers weight). As the air mass rises, it expands because of lower atmospheric pressure aloft. The expanding air must perform work to occupy that larger volume, so it releases energy and cools. As the airmass cools, its relative humidity will increase.
Once the airmass cools to its dew point, clouds and eventually precipitation can form. In order to condense from a gas into liquid, the water vapor in the airmass must release heat energy. This can cause the airmass to cool relative to the surrounding air. When this happens, the airmass will begin to sink because it will be denser (cooler) than the surrounding air. As the airmass sinks, it is compressed due to higher pressure near the Earth’s surface. This causes the volume of air to warm. As
the sinking air warms, its relative humidity decreases, creating a region of drier, clear air. The sinking air
also creates an area of high atmospheric pressure near the ground (sinking air increases weight).
The basic circulation process described above can happen at a global scale (Figure 1b). Low-pressure
regions with rising air tend to form at the Equator (0° – Equatorial Low) and near the temperate regions
(60°n/s latitudes – Subpolar Low). Globally, these regions tend to have abundant cloud cover and rainfall.
High pressure regions with sinking air tend to form in the Subtropics (30°n/s latitudes – Subtropical
High) and at the Poles (90°n/s latitudes – Polar High). Globally, these regions tend to have clear, dry
conditions with high evaporation rates.
The pressure zones that are associated with precipitation conditions (low’s rainy; highs dry) also
generate surface winds (Figure 1c). At the Earth’s surface, winds blow high pressure zones toward low
pressure zones (known as the pressure gradient force). Because the Earth is rotating, these longdistance winds are impacted by the Coriolis effect so that they are deflected to the right of the pressure
gradient force in the northern hemisphere and to the left in the southern hemisphere. The winds blow
from the Subtropical High (30°) toward the Equatorial Low (0°) are deflected so that they consistently
travel from east to west. This belt of easterly winds that encircles the globe are known as the Trade
Winds. From the Subtropical High (30°) to the Subpolar Low (60°), the winds are deflected so that they
consistently blow to the west around the globe. This band of winds is the Westerlies. From the Polar
High (90°) to the Subpolar Low (60°), the winds are deflected so that they consistently blow to the east
around the globe. This band of winds is the Polar Easterlies. The end result of the winds trying to blow
from high to low pressure but experiencing Coriolis deflection is a pattern of globally predictable
easterly and westerly winds.
GEOL 1401 – Ocean Circulation
Ocean Water Properties
A key property of ocean water is that it is “salt water.” Salinity is the total amount of ions (in grams)
dissolved in 1 kg of seawater, written either as a
percent (%, parts per hundred) or per mille (‰,
parts per thousand). Sea salt, as the chemical
precipitate from seawater is called, is different
from regular table salt in that it is not simply
sodium chloride (NaCl). Seawater can contain a
host of different ions from a variety of sources
(Figure 2).
Sea surface salinity varies with latitude (Figure 3)
because of the impact of global atmospheric
circulation patterns. Atmospheric high pressure
zones (Subtropical and Polar Highs) are associated
with dry air, high evaporation rates, and thus high
salinity ocean water. Low pressure
zones (Equatorial and Subpolar
Lows) are associated with rising
airmasses that produce abundant
precipitation. Rainwater will dilute
ocean salinity, creating patches of
slightly lower salinity ocean water.
Changes in salinity can also be
affected locally by freshwater inputs
from large rivers or seasonally
melting sea ice. Both processes lead
to slightly less saline ocean water.
Another key property of seawater is
its temperature. Like salinity, sea
surface temperature varies with
latitude (Figure 4). However, the
reason for its variability is not tied
to atmospheric circulation, rather it
varies with the angle at which
sunlight strikes the globe. Near the
equator, sunlight strikes the Earth
at a high angle all year long. This
high angle of incidence means that
a more heat energy is transferred
to the sea surface. Closer to the
poles, the angle of incidence is
lower, meaning that less energy is
delivered per unit of sea surface
Figure 2: Seawater salinity and major chemical
Figure 3: Global sea surface salinity
Figure 4: Global sea surface temperature
GEOL 1401 – Ocean Circulation
Whole (Deep) Ocean Circulation
The most critical ocean wide current is the density driven Thermohaline Circulation (Figure 6). In regions
where surface seawater is really cold, it can become so dense that it sinks to the bottom of the world’s
oceans (North Atlantic Deep Water and Antarctic Bottom Water – Figure 5). There it moves along the
seafloor until it reaches areas of upwelling that return the deep water back to the surface. This current is
critical to life in the oceans because is carries oxygen rich surface water to the seafloor, allowing life to
thrive at depth and allowing microbes to convert organic matter to nutrients. Without this process, the
ocean food web would collapse. The current is also responsible for moving warm water poleward and
cooler water toward the Equator in an effort to even out temperature differences created by the
difference in solar insolation between the regions.
Figure 5: South to north cross section of the Atlantic Ocean
Figure 6: thermohaline circulation
GEOL 1401 – Ocean Circulation
Surface Ocean Circulation
Ocean surface currents are wind-generated “rivers” of faster-moving ocean water that circulate
throughout the oceans, providing benefits (or detriments) to navigation, as well as having huge effects
one climate and weather. Like winds in the atmosphere, ocean surface currents are large-scale features
that are affected by the rotation of the Earth – the Coriolis effect. In the Northern Hemisphere, wind
blown currents are steadily deflected to the right of their initial path of travel; in the Southern
Hemisphere, they are deflected to the left. While these gyres can be broken into named segments, they
are one continuous current (Figure 7). The end result is the formation of large circular currents or gyres
that carry water clockwise in the northern oceans and counterclockwise in the southern oceans. This is
of profound importance to global climate as circulating ocean currents can drag warm water poleward
and cooler water toward the Equator in an effort to even out temperature differences created by the
difference in solar insolation between the regions. Because they bring with them either warm or cool
water, currents that wash into an area can also have a significant impact on local climate and weather
Figure 7: Ocean surface currents

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