5. Global View - Surface Ocean
The currents and the temperature and salinity patterns of the surface of the global ocean
are determined by the global wind fields and the movement of moisture and heat between
the ocean and the atmosphere. Here we will examine the surface currents of the ocean,
noting their relationship to the global wind field, and the distribution of temperature and
salinity, noting the relation to global heat and moisture fluxes. In all cases we will keep
seasonal variability in mind. This portion of the course presents the student with basic
concepts of how the atmosphere and ocean interact and will lead to future topics on the
sub-surface ocean and other, more regional, processes.
Basic Concepts
Before we proceed there are several concepts that must be introduced now but will be
considered in more detail later.
Coriolis Force, is an apparent force due to our rotating frame of reference, the earth.
Coriolis causes water in the northern hemisphere to move to the right in the downstream
direction and to the left in the southern hemisphere.
Geostrophic flow. Ocean circulation can occur for many reasons but the principal one is
caused by the balance of the Coriolis Force and the pressure gradient. Horizontal pressure
gradients occur when the ocean surface is not level with respect to the earth’s
gravitational field.
Now we will examine the surface currents of the Pacific, Indian, Atlantic Ocean,
Antarctic and Arctic Oceans. In all the oceans the largest scale flows and the most
coherent currents are the equatorial currents , the boundary currents , and the currents
around the Antarctic. The major meridional currents are the boundary currents. Boundary
currents can be conceptually divided into poleward and equatorward western boundary
currents, poleward and equatorward eastern boundary currents, and equatorward
currents.
Pacific and Southern Ocean (Figure 1)
The equatorial currents of the Pacific Ocean include the North and South Equatorial
Currents that are forced by the westward flowing trade winds. South of the North
Equatorial Current lies the North Equatorial Counter Current and a similar current, the
South Equatorial Counter Current lies south of the South Equatorial Current. Note that
the South Equatorial Current and the North Equatorial Current lie asymmetrically about
the Equator.
At higher latitudes the major zonal currents are related to the westerlies. The current in
the southern hemisphere is the Antarctic Circumpolar Current (ACC) and its north
analog is the Kuroshio Extension/North Pacific Current (NPC). Note that the ACC
circles the southern hemisphere while the NPC is blocked by North America. The ACC
is the main link in the global ocean system.
Because the ACC is the link between the oceans and the Antarctic is so important in
general to global conditions we will expand a bit more. As one moves southward in the
Pacific the Sub-Tropical Convergence is encountered at about 35S. This area is where
waters driven westward and southward by the easterlies converge with the eastward and
northward driven waters of the ACC. Further south is another convergence, the
Antarctic Polar Convergence. Both convergences are key elements of the global
circulation.
The main poleward western boundary currents, and two of the largest oceanic currents,
are the Kuroshio and the East Australian Current (EAC). The Kuroshio (Black Current)
forms as the NEC turns northward off the Phillipines, moves northward past Taiwan,
Japan and then forms the North Pacific Current. The EAC is a smaller current with
significant seasonal variability flowing along the east coast of Australia.
The main equatorward western boundary currents in the Pacific is the Oyashio. The cold
Oyashio flows equatorward out of the Bering Sea and the Sea of Okhotsk and meets the
warm Kuroshio off Honshu. The meeting of these two currents provides very important
fisheries.
The main equatorward eastern boundary currents are the California Current off
California and Baja California and the Peru-Chile (Humboldt) Current off Chile, Peru
and Ecuador. The California Current forms from the NPC as it encounters the continent
of North America. The California Current, like all eastern boundary currents, is much
weaker than the western boundary currents and much less coherent. This is seen in the
the SST of the current compared to the SST of the Kuroshio. The Peru-Chile Current is
analogous to the California Current forming up from the ACC as it encounters the South
American continent. Both the Californian and Peru-Chile currents are strongly wind
driven and, since the winds are highly seasonal and variable they are much more variable.
Poleward eastern boundary currents include the Alaska Current off British Columbia and
Alaska and an unnamed current off southern Chile. These currents are also weaker and
more variable but are very important as they sustain important fisheries and, through
air/sea interaction processes affect weather.
Indian Ocean
The Indian Ocean, because of its smaller size and the presence of the Asian landmass to
the north lacks the well defined permanent currents of the Pacific. It is, however, the site
of strong seasonal monsoons. The summer monsoon blowing towards the northeast
while the winter monsoon blows southwestward. Another notable difference between the
Indian Ocean and the other Oceans is that the ITCZ lies south of the Equator.
Because of the dominance of the monsoon winds the currents in the Indian Ocean are
more variable. (Figures 2 and 3).
Zonal currents are dominated by the permanent Antarctic Circumpolar Current and the
more variable equatorial currents. During the SW Monsoon an eastward current
dominates the central Indian Ocean about the equator. A compensating current, the South
Equatorial Current, flows westward.
Because of the arrangement of land masses and the lack of northern waters the Indian
Ocean lacks the very large boundary currents seen in the other oceans and even these
currents are seasonal because of the strong seasonal wind signal.
During the Southwest Monsoon the Somali Current is well developed with its extension
as the East Arabian Current. Farther south the Mozambique Current flows southward
forming the Agulhas Current flowing poleward and rounds South Africa merging with
the ACC and the Benguela Current in the Atlantic. On the eastern boundary the South
Java Current flows towards the equator along Malaysia. On the eastern boundary there is
a strong current off western Australia called the Leeuwin Current.
During the northeast monsoon the Indian Ocean develops a more normal equatorial
current system but the boundary currents weaken. Gyres form in the Arabian Sea and the
Bay of Bengal with the East Indian Current flowing poleward off the east coast of
India. The western boundary, that was dominated by the Somali Current, consists of small
systems including the remains of the Somali Current, the Zanzibar Current, the East
Arabian Current. The Mozambique Current continues to flow poleward and is joined
by the East Madagascar Current both of which join to form the Agulhas Current. The
South Java Current reverses. The meridional equatorial currents are well formed with a
north and south equatorial current and an equatorial counter current although all are
shifted south.
A final note concerns the retroflection of the Agulhas Current off South Africa. The
eastward flowing part of the return flow eventually splits off from the gyre returning to
the Agulhas and the ACC to form the Indian Ocean Current.
Atlantic Ocean
The Atlantic Ocean, because of its shape is divided into North and South divisions
although the flow between the two is relatively unrestricted. The main difference
between the Atlantic and Pacific is that the Atlantic has access to the Arctic Ocean, is
much narrower, and has a mid-ocean ridge down the center. However, despite the
smaller size it has some of the largest currents (Figure 4).
The major equatorial zonal currents are similar to the Pacific but less well developed
because of the asymmetric placement of South America and Africa and the narrower
width. The higher latitude zonal currents are the North Atlantic Current that is an
extension of the Gulf Stream and the West Wind Drift across the South Atlantic.
Poleward western boundary currents in the Atlantic are the Gulf Stream off North
America and the Brazil Current off South America. The Gulf Stream is a continuation
of the NEC after it flows along northeast South America as the North Brazil Current ,
and the Guyana Current , through the Caribbean Sea as the Caribbean Current, and
Gulf of Mexico as the Loop Current. Its flow doubles between the Straits of Florida and
Cape Hatteras. The Brazil Current is an extension of the SEC.
The major equatorward western boundary currents are the Labrador Current off
Newfoundland and the Malvinas Current off Argentina, Uruguay and Brazil. The
Labrador Current meets the Gulf Stream off the Grand Banks while the Brazil Current
meets the Malvinas off Uruguay and Brazil.
The equatorward eastern boundary currents in the Atlantic are the Benguela Current off
South Africa, Namibia and Angola and the Portugal Current off Spain and Portugal that
becomes the Canary Current off northwest Africa. Note the difference in characteristics
of the two currents. The source of the Benguela Current is the Agulhas Current flowing
out of the Indian Ocean and the South Atlantic Current that comes from the Brazil
Current.
The poleward eastern boundary currents in the Atlantic are the Norwegian Current .
This current is an extension of the Gulf Stream and North Atlantic Current and transports
warm, salty Atlantic water into the Arctic Ocean. Because of the complex geography of
the North Atlantic there are other currents related to land distributions. These include the
East and West Greenland Currents and the Irminger Current off Iceland. The
Greenland currents combined with the Labrador Current and coastal currents off Nova
Scotia and New England form a very large scale coastal current that will be discussed
later.
The Southern (Antarctic) Ocean
Major oceanic fronts (areas of strong horizontal gradients - usually temperature) exist in
many areas of the ocean but the fronts found in the Southern Ocean are of global
importance because they are tied to the processes that set many global temperature and
salinity characteristics. Figure 5 shows a schematic of the fronts in the Southern Ocean.
Note the Subtropical Front, the Subantarctic Front, the Polar Front, the Antarctic
Divergence and the Continental Water Boundary. Seasonal ice cover in the Antarctic is
shown in Figure 6.
The Arctic Ocean
The polar regions of Earth are considerably different. The Arctic is an ice covered ocean
surrounded by land masses with rather open connections to the Atlantic and a limited
connection to the Pacific. In contrast, the Antarctic is land covered with a surrounding
ocean covered by ice flowing off the land and sea ice.
Surface currents in the Arctic are shown in Figure 7. Note that the two main fluxes of
surface water into the Arctic are through the Bering Strait and the Norwegian Current.
Also note that surface winds are east to west around the basin. In the Arctic the dominant
current is the Trans Polar Drift that moves from the Middle and Eastern Siberian shelf to
the East Coast of Greenland forming the East Greenland Current at that point. The Trans
Polar Drift is forced partly by the effect of freshwater flowing onto the East Siberian
shelf. Ice cover in the Arctic is shown in Figure 8.
Heat Exchange
Up to now we have talked about ocean surface currents that are mainly caused by the
global wind field and modified by the land masses: the wind driven ocean circulation.
The other part of ocean circulation is the subsurface ocean circulation and that is mainly
driven by thermohaline processes. That is processes that change the temperature and
salinity of seawater. Those processes take place mostly at the sea surface and are heat
and moisture exchange. Therefore we will next discuss the concept of heat exchange (it
will be dealt with more thoroughly later).
Heat is exchanged between the ocean and the atmosphere by both radiation and
evaporation. Heat exchange is roughly proportional to air/sea temperature differences,
wind speeds, and the relative humidty of the air. The amount of heat entering the ocean
from the sun is proportional to latitude, season, and clouds. The equatorial region is an
area of net heat gain and the higher latitudes an area of net heat loss: thus the global wind
and ocean circulation field to redistribute the heat. The mean solar radiation, heat flux
and e-p are shown in Figures 9-11.
Mean Surface Temperature and Salinity Distributions
The global mean T and S charts for the global ocean are shown in Figures 12-13. The
surface currents and T/S distributions reflect the processes that redistribute heat over the
global ocean. Warm poleward currents, mostly western boundary currents, move warm
water north, while less well defined gyres and eastern boundary currents move cooler
waters equatorward providing continuity. In the next topic we will see the effect of
moving water into higher latitudes and cooling it: the thermohaline world.
GLOBAL CHANGE
Having reviewed the ITCZ it is now appropriate to briefly discuss global change. When
one thinks of global change usually that is inferred to mean the effect of green house gas
induced global warming. While there is evidence for global warming today it is
appropriate to mention El Niño/Southern Oscillation (ENSO). This is a type of change
that occurs every ten years or so and dramatically effects the ocean and atmosphere both
globally and locally. What is ENSO?
You recall the Walker Circulation. Walker realized that the pressure system in the west
Central Pacific oscillated just like the Aleutian and Icelandic Lows do in the Northern
Oceans. He also realized that this Southern Oscillation had global significance.
Looking at the global surface pressure field you noted a high pressure region over the
eastern tropical Pacific and a low over the western central Pacific. It is the weakening
and strengthening of the low and high that forms the Southern Oscillation.
To understand why the climate is varying you must understand ENSO.