Winds, friction, and the Coriolis effect act in concert to generate the
patterns of flow of the upper ocean. The movement of water in response
to these forces is called Ekman transport.
The first observations of Ekman-type processes were made in the Arctic
Ocean by Fridtjof Nansen, the famous Norwegian explorer-zoologist-
oceanographer, and the only oceanographer to win a Nobel Prize (for his
humanitarian efforts). Nansen observed that sea ice floating on the
surface of the ocean drifted in a direction 20 to 30 degrees to the right of
the wind. Upon his return, Nansen explained his puzzling observations to
Ekman, who then formulated the mathematics to explain them.
The Ekman Spiral
Ekman’s reasoning went something like this: 1) as the wind blows across the
surface of the ocean, it sets the water in motion, but because of the Coriolis
effect, the water moves to the right of the wind; 2) as the top layer begins to
move, it drags the layer beneath it, due to friction; 3) as each layer moves and
drags the layer beneath it, the direction of the flow is directed more towards
the right, and, because of friction, each layer moves a bit more slowly. The
result of these forces is a “spiral”, like a stack of cards, each of which moves in a
different direction at a slower speed from top to bottom. This pattern of flow is
called an Ekman spiral.
It wasn’t until 1995 with the deployment of high-resolution acoustic doppler
current profilers that oceanographers actually observed an Ekman spiral (see
An Ekman spiral. Note that this is not
an eddy or a whirlpool. The arrows
indicate speed (longer=faster) and
direction. Each layer moves like a stack
of 3 x 5 cards moving in different
directions and at a different speed, with
the fastest at the top and the slowest at
Mean Ekman transport
A view of the forces that cause geostrophic flow. Winds
indirectly supply energy that results in Ekman transport that
sets up a horizontal pressure gradient. As surface flows
respond to the horizontal pressure gradient, they are deflected
by the Coriolis effect. A steady state is reached between the
pressure gradient and the Coriolis effect resulting in
geostrophic currents and the gyre circulation observed in the
Geostrophic “Earth-Turning” Flow
The “piling up” of water Ekman transport generates a horizontal pressure gradient (see
Chapter 8). As the water moves down the pressure gradient (from high to low pressure),
it is acted upon by the Coriolis effect and, in the Northern Hemisphere, deflects to the
right. Ultimately, a steady-state balance is reached between the Coriolis effect and the
horizontal pressure gradient and the water flows perpendicular to the the direction of
the two opposing forces.
In the Northern Hemisphere, currents (gyres) move in a clockwise (anticyclonic)
direction, while in the Southern Hemisphere, currents (gyres)move in a counterclockwise
(cyclonic direction). This steady-state flow, caused by a horizontal pressure gradient and
the Coriolis effect, is known as geostrophic flow (geostrophic means Earth turning, and
so, refers to a flow under the influence of the turning Earth, i.e., the Coriolis effect. The
currents generated by geostrophic flow are called geostrophic currents. Most major
ocean currents are geostrophic.
The net transport of water is
90 degrees to the right of the
wind in the Northern
Hemisphere and 90 degrees to
the left of the wind in the
Net flow of upper ocean
from surface to Ekman
Trade winds blowing north and south of the equator causes surface waters to diverge.
Colder water flows upwards to take its place. Equatorial upwelling cools the atmosphere
Figure 9-9a and contributes to cloudiness along the equator.
Coastal upwelling occurs as northerly winds move surface waters
offshore in the Northern Hemisphere. Note that this image has been
corrected from the erroneous one in the textbook.
Figure 9-10a (upper)
Coastal downwelling occurs as southerly winds move surface waters
onshore in the Northern Hemisphere. Note that this image has been
corrected from the erroneous one in the textbook.
Figure 9-10a (lower)
Storm systems (low pressure centers) with cyclonic circulation (Northern
Hemisphere) also cause upwelling. In recent years, oceanographers have
recognized that low pressure systems and hurricanes act like a pump to bring
nutrients to the surface and stimulate phytoplankton productivity.
Figure 9-11 (left)
High pressure systems with anticyclonic circulation (Northern Hemisphere) cause
downwelling. Downwelling can limit phytoplankton productivity by shutting off the
deepwater supply of nutrients. It has been suggested that regions where high pressure
systems persist, such as Bermuda and Hawaii, are oligotrophic because of depression of the
thermocline and limitation of nutrient supply.
Figure 9-11 (right)
Oceanographers long observed that western boundary currents were
faster and narrower than eastern boundary currents, a phenomenon
known as western intensification. However, they were unable to explain
why this occurs. In 1948, Henry Stommel, a man who has been called “the
most original and important physical oceanographer of all time,” proposed
that variations in the Coriolis effect with latitude may be the reason.
Because the Coriolis effect becomes stronger at higher latitudes, he
reasoned that it might account for the narrowing of streamlines on the
western boundaries of gyres. When he put it into a mathematical model,
that’s exactly what happened.
Stommel’s model of gyre
circulation without the Coriolis
effect (upper) and with the
Coriolis effect (lower) illustrates
the importance of the Coriolis
term for explaining western
The waters that make up the deep circulation and their patterns of flow are
identified through their temperature and salinity characteristics. A parcel of
water with a distinct and narrow range of temperature and salinity is called a
water mass. Specific T and S characteristics define water types. The distinction
is important oceanographically because water types may mix to produce a given
Water masses (and water types) are typically named according to the location
where they originate. For example, a water mass that forms in the North
Atlantic includes North Atlantic in its name. Water masses are also named
according to their density and the layer that they occupy in the world ocean.
Identification of a specific water mass is most easily accomplished by noting
its “location” on graph of temperature versus salinity, otherwise known as a
T-S diagram. Introduced in 1916 by Norwegian oceanographer Bjorn
Helland-Hansen, the T-S diagram remains one of the most useful tools in
physical oceanography today.
A T-S diagram is composed of a set of CTD measurements of T and S, say,
several to hundreds of vertical profiles, that are plotted on an X-Y axis with
salinity on the X-axis and temperature on the Y-axis. Because different
combinations of T and S correspond to the same density, lines of equal
density, or isopycnals, can also be plotted. Isopycnals and inflection points
can be used to identify specific water masses.
Vertical profiles of temperature and salinity in the Gulf Stream (left) and the corresponding T-S diagram. Note
the isopycnals in this diagram, shown as sigma-t. Each line represents the range of T and S that produce the same
density. Can you identify three combinations of T and S that produce water with a sigma-t of 25.0? From The
Oceans, Their Physics, Chemistry, and General Biology, 1942.
The Deep Circulation
If the surface circulation is the speedy hare, the deep circulation is the
slow and purposeful tortoise. The deep circulation likely controls Earth’s
climate over time scales of thousands of years. Unfortunately, it is the
hardest to study and one of the least understood components of the
world ocean system.
The deep circulation extends from about 1 km (0.5 miles) deep to the
seafloor, which means that most of the world ocean is part of the deep
circulation at any one time.
Deep water masses may by 600 to 1000 years old or more. Their “age”
represents the time that has passed since their formation and return to
Figure 9-24 The Great Ocean Conveyor
A more realistic model of world ocean circulation,
emphasizing the role of the Southern Ocean.
Some of the many processes that may influence the deep circulation. Identifying the
dominant forces is a major goal of physical oceanographers.
The Coupled Surface and Deep
Physical oceanographers now emphasize the connection between the
surface and deep circulation. Changes in one affect the other. For example, a
slow-down in the Gulf Stream impacts the delivery of salts for North Atlantic
Deep Water (NADW) formation. At the same time, a slowdown in NADW
formation may alter Gulf Stream flows as some NADW recirculates into the
Gulf Stream in the North Atlantic.
Bottom line: It’s a big, complicated ocean that we are just beginning to
understand. That said, there are ample opportunities for exploration and
discovery. Perhaps one of you have a mind to figure it out...!