Summary for class: 28 February
Topic: Atmosphere and Ocean
Interplay:
Climate
I. Annoucements
- Exam I next Tuesday. Bring scantron
882 form and #2 pencil. Exam will consist of 50 multiple choice
questions. There will be some simple math problems and conversion
factors will be supplied. Topics covered will be through the
class period today. Review voyage and drill questions, course
web site preparation and summary pages for each class period,
and class notes and handouts.
- On tuesday you will also be asked to
submit a 3x5 card with your choice of field project topic. Look
at the Guidelines for Oceanography Field Project on this site
to get ideas. You should include; what you will do (the basic
question you wish to address) and where you will do it.
- Time was given for students to ask
questions about the material we have discussed during the past
5 weeks.
II Practical applications for knowing
about currents
- Faster transportation, e.g., moving cargo, sailing
- Paths of oil spills or other ocean contaminants
- Understanding climate / predicting weather
- Organisms: certain temperatures or their migrations
- Recreation: swimming, surfing, boating, vacationing
- Fishing: locating the fish
- Predicting damage, e.g., El Niño current perturbation
- Why humans have dispersed as they have (archeaology)
- Which way sunken boats or barrels of radioactive waste would
drift
- Clean the sea by moving in and out, e.g., currents in SF
Bay
III. The current gyres
- The North Pacific current gyre consists of the North Equatorial
Current, the Kurishio Current (western boundary current), the
North Pacific Current, and the California Current (eastern boundary
current).
- The North Atlantic Ocean has a similar current gyre, with
the Gulf Stream as its western boundary current.
IV. Wind and the formation of surface
currents
- Surface currents are formed primarily by wind blowing over
the ocean. Persistent wind patterns (the prevailing trade winds
and the westerlies) create large current gyres that move clockwise
in the Northern Hemisphere and counterclockwise in the Southern
Hemisphere.
- In the Northern Hemisphere, the prevailing westerlies (blow
from west to east) drive the northern parts of the current gyres.
The northeasterlies (blow from northeast to southwest) drive
the equatorward part of the gyres. Because the currents run into
continents, they are deflected and continue in a circular pattern
(thus creating the "gyre").
- In addtion to the wind, the Coriolis effect also influences
the current gyres. Because the earth is rotating, anything that
moves for a long distance appears to take a curved path. In the
Northern Hemisphere, the path curves to the right of the direction
of motion; in the Southern Hemisphere, the path curves to the
left of the direction of motion. Thus, if wind blows over water
to create currents, the water in the currents will flow somewhat
to the right of the wind direction in our hemisphere. The Coriolis
effect accentuates the circular motion of the current gyre and
water is pushed toward the center of the gyre, creating a very
small elevation increase of the ocean's surface.
- Surface current extend to depths of 100s of meters, and have
widths of 100s of kilometers. Current speeds are typically between
<1-10 km/hr (much slower than the speed of wind-driven waves)
- Surface currents have a large effect on the climate and water
conditions of a region. For example, on the west sides of oceans,
warm currents like the Gulf Stream provide warmer sea-surface
temperatures, whereas on the east sides of oceans, currents like
the California current provide colder sea-surface temperatures.
Northern Europe has a milder climate than might be expected for
its latitude because the warm waters of the Gulf Stream provides
a moderating influence.
- We looked at the tip of South Africa, where the Benguela
current brings cold water to the western side of the tip (influence
of eastern boundary current) whereas the Agulhas current brings
warm water to the eastern side of the tip (influence of western
boundary current). One could look at the sea-surface temperatures
in this region to decide where to take a surfing vacation and
not need to take a wetsuit!
- Students act out the North Pacific current gyre in the classroom,
with the equator along the front of the room, the wall as the
continent of Asia, and the windows as the continent of North
America. The northeast trade winds blew water particles (students
in the front rows) westward toward Asia (North Equatorial current),
where they were deflected northward (Kurishio current). The westerlies
blew water particles (students in the back rows) eastward toward
North America (North Pacific current), where they were deflected
southward along the California coast (California current).
V. More about measuring ocean currents
- We did the problems on the class handout about measuring
ocean currents using spilled container ship contents. Shoes that
feel overboard in the mid-Pacific took about 6 months to travel
2400 km to the coast. This is a rate of about 0.5 km/hr (0.3
mile/hr). At this rate it would take about 3 years for shoes
(or the water itself) to travel the entire curcuit of the North
Pacific current gyre.
- The Coriolis effect causes air and water to be deflected
to the right of its motion in the Northern Hemisphere, and to
the south of its motion in the Southern Hemisphere. Thus, as
the water is moving around the current gyres, there is also a
"push" into the center of the gyre. As you discovered
in Voyage 4, the amount of chlorophyll pigment (and the overall
level of biological productivity) is low in the centers of the
current gyres. We will come back to this topic later on in the
semester, when we look at how life in the ocean has adapted to
these productivity variations.
- In Voyage 4, you examined a figure that showed sea-surface
temperatures throughout the world ocean. Here you saw that the
region of warm water around the equator is widest on the western
sides of the oceans. This is because the trade winds blowing
from east to west creates the equatorial currents that also flow
from east to west and pile up the surface water on the western
sides of the oceans. We will come back to this topic later on
in the semester, when we explore El Niños. During these
times the trade winds weaken, the equatorial currents weaken,
and that warm-water pileup on the western sides of the oceans
is reduced.
- In addition to the horizontal movement of water that we have
explored in the surface currents, there are also currents in
the ocean that move water up and down. We will explore these
currents later on in the course. Upwelling currents are important
because they bring deeper water to the surface that is filled
with nutrients that promote the growth of plants and other marine
life. Thermohaline currents cause water from the surface to sink
to the bottom of the water. They carry oxygen to deep parts of
the ocean and are important components of the climate system.
VI. Climate on earth
- The atmosphere and ocean interact in complicated ways to
produce climate. We have looked at several examples of this interaction.
(1) latent heat whereby water is evaporated from the surface
ocean and the heat carried by the wind where cooling and condensation
causes the release of heat; (2) prevailing winds (trades and
westerlies) drive currents on the ocean surface that also act
to redistribute heat. Very powerful computers are required to
model the climate system and to explore how climate might change
(e.g., by human activities).
- The heat budget (see page 114 in the textbook) explains how
the amount of heat coming onto the surface of earth (from the
sun) is balanced by the amount of heat leaving the surface of
earth. If the amount of heat coming in equals the amount going
out, the earth will maintain a consistent temperature over time.
Our concern at this time is that human activities may be causing
more heat to be retained at the earth's surface, causing the
planet to warm overall.
VII. The Greenhouse Effect
- Greenhouse gases are those that trap heat, thus preventing
the heat from escaping. This situation is somewhat analogous
to a greenhouse (or the inside of your car!), where the heat
from the sun enters in and warm the interior, but the greenhouse
(or car) glass prevents the heat from escaping. The greenhouse
and your car warms up!
- Common greenhouse gases are water, carbon dioxide, and methane.
Although the amount of water vapor remains the same in the atmosphere,
the amount of carbon dioxide and methane has certainly increased
during the past 100 years. Carbon dioxide is produced when fossil
fuels (e.g., gas, oil, coal) are burned (in car exhaust, from
a power plant or factory) and the carbon in these fuels bonds
together with oxygen to form CO2. Methane is produced from the
gaseous emissions of livestock, as in the cattle lots you may
pass on Highway 5 when driving between SF and LA.
VIII. Global warming
- Two things we know for sure: (1) the amount of greenhouse
gases in the atmosphere has increased greatly during the past
100 years; (2) the planet has warmed by about 0.5 degrees C (a
little over 1 degree F) during the past 100 years.
- There is less agreement over the cause of the warming, although
it is certainly a strong coincidence if the planet has "decided"
to warm up just at the same times that we are adding more greenhouse
gases to the atmosphere. We will probably not be able to "prove"
that human actions are the culprit until the climate has warmed
more than it has now.
- In my view, the evidence is very good (if not completely
conclusive) that human activities are effecting the climate.
This is a scary experiment, because the potential to disrupt
food supplies and coastal habitations is very great. There are
plenty of ways that we can begin to plan for the probable changes
that will come, and to reduce the amount of fossil fuels we consume
(that is, by developing alternative energy and transportation
options). We would gain other benefits from reducing our dependence
on fossil fuels: (1) we would reduce pollution; (2) we would
reduce our dependence on foreign governments that supply more
than half of the petroleum we use. Finally, we are already on
the downward slope of the world's petroleum supply, which in
another 100 years will be scare enough to become more economically
difficult to produce. We might as well start getting a jump on
what is a certain future away from petroleum use!
IX. Card question: draw a diagram
of the North Pacific current gyre and label one of its currents.