Physical Oceanography
Review Material for Spring 2004
Here are Key Concepts I hope you have learned:
Dynamical Concepts:
- Geostrophic Flow
- What is it?
- What approximations are used to obtain geostrophic relations?
- Inertial Oscillations
- Ekman Boundary Layer
- What are the currents and transports in the layer?
- What approximations are used to obtain Ekman Layer?
- What are the consequences of Ekman Layers?
- How does Ekman pumping drive geostrophic currents?
- Role of Vorticity
- What are the consequences of Earth's rotation for ocean currents?
- How does Earth's rotation lead to western boundary currents?
- What is Sverdrup's relation, and how does the curl of the wind stress lead to transport of mass in the upper ocean?
- Describe how winds lead to the observed circulation of the ocean.
- Describe how winds can produce currents going upwind.
- Deep Circulation
- What dynamical constraints lead to Stommel's theory for the deep circulation?
- How fast is the deep circulation?
- Equatorial Dynamics
- What processes lead to an Equatorial Undercurrent?
- What processes lead to El Niño/La Niña in the Pacific?
- Waves
- What are phase and group velocities?
- How are they related to the dispersion relations?
- How can waves be differentiated from turbulence?
- What balance of forces lead to surface waves?
- How do non-linear processes influence waves?
- Tides
- What dynamical processes produce the tides?
- What are the fundamental tidal frequencies?
- Why do five fundamental tidal frequencies lead to hundreds of tidal frequencies in the ocean?
- What processes make prediction of ocean tides difficult?
Observations:
- Sampling Errors
- What are they?
- Why are they important?
- Data Sets
- How can you determine what sets are useful?
- What data exists for studying the different dynamical processes listed above?
- Winds, currents, waves, tides, heat fluxes?
Here are questions I have asked on open-book exams.
Because exams are open book, I do not ask questions on facts. Rather, I tend to ask questions which require you to put together facts to analyze a problem and come to a conclusion: questions to make you think. So you will not be caught by surprise, here are a few questions along those lines.
- Sampling errors: Sampling errors are important
in all experiments, in oceanography, and in meteorology.
- Give an example of a sampling error based on your own work or interests. Do not use any example from the notes.
- How can the sampling error be reduced in your example?
- Bathymetric Maps: Bathymetric maps are widely
used in physical oceanography.
- What data (measurements) are used to produce the bathymetric maps of the ocean with the best spatial resolution? What is measured, and how is it measured?
- How are the data used to produce maps you described above?
- What regions of the ocean should have the most accurate maps of the type you described above?
- Bathymetric Maps: Your company has been given a
concession for mining deep-sea cobalt-manganese nodules. The
concession is 500 km on a side in the central Pacific. You
are asked to plan a cruise to do some exploratory dredging.
To do this, you need good bathymetric charts of the area.
Whose charts will you use, and why (all in two
paragraphs)?
- In planning your cruise, you also need wind and wave conditions. Where can you get these data? Comment on their usefulness for your planning studies.
- Maps of the Seafloor. A navy captain must take
his submarine from offshore Cape Town South Africa to
Norfolk Virginia while staying submerged at 1000 m. He asks
you, his navigator, what maps, charts, or data set he should
use to avoid running aground on a seamount. He points out he
must make the journey without sonar, and that the on-board
inertial navigation system has an accuracy of 1 km in the
horizontal and 1 m in the vertical.
- Which map, data set, or chart do you recommend?
- Please document and justify your recommendation.
- Source of Winds: Explain in simple terms why
Earth has trade winds.
- In answering the question, state the original or ultimate source of energy, show the pathways followed by the energy until it drives the trade winds, and show how the energy is converted into wind kinetic energy.
- Source of Winds: Explain in a few paragraphs
the role of the ocean in the circulation of the atmosphere.
Why does the atmosphere have winds, and what is the role, if
any, of the ocean in the formation of winds?
- Wind Information: You are planning an
experiment in a remote area of the Indian Ocean several
thousand kilometers west of Australia. You need to know the
monthly averaged sea-surface wind speed and direction in the
region before you go to sea.
- What are two original sources of wind data you could use for determining monthly mean winds for the area? By source, I mean the type of instrument, observation, or calculation that produced the wind data, not the name of an archive.
- What is the accuracy of the data?
- How do you know if the data set you chose to use can be trusted for your experiment?
- Wind Information: Sebastian Junger, in his book
The Perfect Storm, described a storm in Ocober 1991 that had
Force 12 winds south of the Grand Banks and gales that
extended as far south as Florida. A buoy near Sable Island
recorded wave heights that exceeded 30 m.
You are part of a team developing a new storm-wave forecasting model. The first version of model has been developed, and the team decides to see if it can forecast the waves observed in the perfect storm. You are asked to provide winds necessary to drive the model. It requires winds every six hours on a one or two degree grid.
- Where will you get the wind information you need for October 1991?
- Why did you pick this source of wind information?
- How do you know if the wind information can be trusted?
Background for Next Three Questions: Your company is planning to design a production platform that will operate for ten years in water 2,000 m deep 20 kilometers offshore of the Namibian Coast (west coast of southern Africa). You are asked to provide environmental information necessary for designing the structure. The area is remote, and no one has deployed instruments in the area.
- Ocean Waves: You are asked to estimate average
wave height and the maximum wave height that might be
encountered by the structure during its ten-year life.
- How would you estimate the maximum wave height?
- What ocean-wave data sets might be useful for determining the average wave conditions at the site? List two possible data sets and list advantages and disadvantages of each.
- Ocean Winds: You are asked to estimate average
wind conditions and the maximum wind velocity that might be
encountered by the structure during its ten-year life.
- What wind data sets might be useful for determining wind conditions at the site? List two possible data sets and list advantages and disadvantages of each.
- Using one of these two data sets, how would estimate the maximum wind velocity for the 10-year period?
- Ocean Currents: You are asked to estimate mean
and standard deviation of surface geostrophic currents that
might be encountered by the structure. One colleague
suggests calculating currents from a numerical model;
another colleague suggests using satellite-altimeter
measurements; a third colleague suggests that the two
approaches can be combined using a model that assimilates
altimeter data.
- List the advantages and disadvantages of each approach and recommend one to be used for the design study. Please provide reasons for your recommendation.
Heat budgets: In class and while doing your lab work, you have seen global maps of net heat flux through the sea surface.- What two terms in the heat budget are most important on a global scale?
- What data (measurements) are needed to calculate the two terms for global maps? Please be explicit, list each variable or measurement and how it is obtained or calculated. List variables, equations, and constants needed for the maps.
- Heat Budgets: Describe the two largest terms
in the heat budget.
- Where in the ocean are the terms large or small?
- How do we know the numerical values of the terms in various oceanic regions? That is, how do we know that monthly averaged values for one of the terms in the south central Pacific is say +150 W/m^2?
- What is the source of information used to compute the number?
- Heat budgets: What is the primary balance of
terms in the energy budget for the tropical oceans? What is
the dominant input and loss of energy through the sea
surface.
- Heat budgets: A strong, Arctic, cold front
moves out across the Texas coast. The air behind the front
is cold, dry, and clear.
- What terms in the heat budget are important for calculating the change in water temperature near the coast, say 10 km offshore of Galveston?
- What information is needed for calculating the temperature of the water 24 hours after passage of the front? Be specific, include information for each term in the heat budget, assuming that the water is well mixed by the wind from the surface to the bottom. List variables, equations, and constants needed for the calculation.
- Ocean Atmosphere Interactions: Two questions
to test your understanding of how the ocean influences the
atmosphere and how the atmosphere influences the ocean.
- If the ocean surface were covered by a thin plastic membrane that passed heat but not water, similar to the cover to a swimming pool, what would be the implications for the atmosphere and for weather over the land? There are many "right" answers, I am interested in reasoning leading to your answer.
- If the westerlies at 40-50° north and the trade winds from 10-25° north changed direction, would the currents off Florida and California change direction? Please give reasons for your answer; don't just state that the flow would or would not reverse.
- Sea-Surface Temperatures: Your company has
been awarded a concession for mining deep-sea cobalt-
manganese nodules. The concession is 500 km on a side in the
central South Pacific near 40°S and 140°W (see
Figure 3.1 of the notes). You are asked to plan a cruise to
do some exploratory dredging. Your company also needs to
know the monthly averaged sea-surface temperature in the
concession.
- What are two original sources of temperature data you could use for determining monthly mean temperature for the area? By source, I mean the type of instrument, observation, or calculation that produced the temperature data, not the name of an archive.
- What is the accuracy of the data?
- Ship Design: You are asked to help design an
insulated ship to carry liquified natural gas between
Houston and Amsterdam. In particular, you are asked what
will be the temperature and wind speed along the route
throughout the year.
- What data set or sets would you consult in writing your report? Why did you choose those that you did?
- What errors might influence the graphs in your report? Please be explicit, and list sources of error.
- Maps of Currents from Ship Data:
Hydrographic data taken decades apart are used to
construct maps of ocean currents (such as the map in figure
10.9. The maps agree well with currents calculated by
numerical models and currents calculated from
satellite-altimeter data.
- Why can hydrographic data collected at times that differ by decades be used to make reasonable maps of the ocean's circulation?
- What evidence do you have that maps made from such data are reasonable?
- Water Masses: Is it common to find warm salty
water lying over cooler fresher water at some depths in the
Atlantic, or is the phenomenon rare? Please explain your
answer.
- Coastal Currents: The wind blows strongly from
the north (perpendicular to the shore) in the region
offshore of Galveston Texas for a few hours then dies
down.
- What type of surface current might be produced by the wind?
- How will the current change over the course of several days if the wind stays calm?
- If the wind speed was 20 m/s, if the wind blew for 3 hours, and if a slab of water 10 m thick were accelerated by the wind, what is the initial current speed and direction after the three hours?
- Coastal Currents: A 15 m/s wind blows strongly
from the south for a few days in the region offshore of
Galveston Texas.
- What type of surface current might be produced by the wind?
- Describe the speed, direction, and depth of the current.
- Coastal Currents: A 15 m/s wind blows strongly
from the south for a few days in the region offshore of
Galveston Texas.
- What might be the influence of the currents on the stability of the water column near the base of the mixed layer?
- Describe some processes that may occur? Can you be quantitative?
- Coastal Currents: Knowing the typical wind
conditions offshore of California:
- Describe the influence of the wind on the ocean circulation.
- Describe at least three consequences of the circulation on the climate of San Francisco.
- Ekman Pumping: Sketch the zonal component of
winds in the Pacific on the dateline from the equator to 65° S.
- Draw in the Ekman transports on your sketch of winds. What is the influence of these winds on the density in the upper kilometer of the water column along the dateline?
- What zonal geostrophic currents might exist along the dateline as a result of the distribution of density in the upper kilometer of the water column produced by the winds? Draw in the currents on your sketch of winds.
- Ocean Dynamics: Here are a couple of questions
to test your understanding of how the winds produce currents
in the upper kilometer of the ocean.
- If the westerlies at 40-50° N blow from east to west, and if the trade winds at 10-25° N blow from West to East, and if the winds at 25° - 40° N are calm, please sketch the topography of the sea surface in the North Atlantic that you expect would be produced by this wind system (note, it is the opposite of the true wind system).
- Knowing the topography, sketch in the direction of the currents. Would the currents off Florida change direction (change relative to currents produced by the true winds)? Please give reasons for your answer; don't just state that the flow would or would not reverse.
- General circulation: How is the basin-scale
circulation of the ocean related to pressure gradients? How
accurate is the relation, and under what circumstances does
the relation become inaccurate? Please be quantitative, and
if possible, cite the relevant equations.
- Depth of Isobaric Surfaces in the Ocean: We
saw earlier that changes in density of the ocean are small.
Now let's look at the implication for determining the depth
of an isobaric surface.
- Calculate the change in depth of the 500 decibar pressure surface if the mean density of the water column changes by 0.10 kg/m3. Give depth in meters.
- Can this change in depth be measured directly, that is, can it be determined without using a pressure measurement?
- Surface Geostrophic Currents: A satellite
altimeter measures a current of 1.5 m/s at some point along
the ground track.
- What is the surface geostrophic current at that point?
- What can be said about the current at 200 m?
- What can be said about the velocity at 3000 m?
- Geostrophic Currents: Surface geostrophic
currents can be calculated from either hydrographic data or
from satellite altimetry, or they can be measured directly.
Altimetry gives surface geostrophic currents relative to the
earth (absolute velocity).
- What difficulties might be involved in measuring hourly mean currents 5 meters below the sea surface using moored current meters in the deep ocean.
- If the current meter measured a current of 1.0 m/s to the southwest at a depth of 5 meters, and satellite altimeter data indicates that the current was 0.75 m/s toward the southwest at the same time and place. Why might the two differ? Please give several possible explanations.
- Western Boundary Currents: Western boundary
currents are found in most ocean basins.
- Explain why fast, narrow jets of current such as the Gulf Stream must exist in the western boundary of the North Atlantic and North Pacific. Please do not state they exist because Mr. Expert Oceanographer says in his famous paper that they must exist or that the equations of motion require they must exist.
- Please give a logical argument demonstrating why they must exist.
- Describe why the Gulf Stream and Kuroshio leave the coast near 40° N Latitude.
- Boundary Currents: If the westerlies at 40-50° N and the trade
winds from 10-25° N
changed direction, would the currents off Florida and
California change direction?
- Cold-Core Rings: Sketch the surface
topography, and the isopycnal and isobaric surfaces below
the surface of a cold core ring (eddy) in the southern
hemisphere. Assume there is a level of no motion deep below
the ring.
- Sketch the direction of flow if the ring is in the southern hemisphere.
- Data Sets: Your consulting company has been
asked to help design a large offshore structure to be
located in water 2,000 m deep off the northwest coast of
Australia in the Indian Ocean. As part of the design team,
you are asked to write a report which gives the wind, wave,
current, and temperature to be expected in the region.
- What data sets would you choose for your report?
- Why did you choose each particular data set?
- What errors in the data might influence you report?
- Information on Currents: Your company is
planning to operate a drilling platform in water 2,000 m
deep offshore of South Africa. You are asked to provide
information on the variability of surface geostrophic
currents in the region. This requires data on the speed and
direction of the current from week to week for several
years. The area is remote, and no one has operated current
meters in the area. Your report on the expected variability
of currents is due in two months.
- What sources of data might be useful for determining the variability of the surface geostrophic currents?
- Describe why the source of data you choose to use is more useful than other sources of data.
- Currents: You are a member of a team measuring
currents at a point in the northwest Pacific ocean. One
member of the team using satellite altimeter data at a
cross-over point of the subsatellite track in an area where
the geoid is well known (which gives the magnitude and
direction of the current), finds the current is 25 cm/s to
the southeast. Another team member, using drifters, finds
the current is 20 cm/s to the west. A third team member,
using CTD data, finds the current is 10 cm/s to the
south.
Assume all measurements or calculations of currents are correct, with no error; and that all measurements are at the same time and place.
- Explain how the three measurements of current can differ.
- Surface Currents: You are given a three-month data
tape from an acoustic current meter that was attached to an
offshore production platform sited 100 km offshore of
Galveston in water 200 m deep. The meter recorded velocity
once per second (about 5 million observations) at a depth of
two meters below mean sea level. The two month period
includes a strong spring storm, a strong cold front, a
period of steady winds from the south, and extended periods
of calm.
- What types of currents do you expect to identify in the record? By type, I mean currents resulting from different processes in the ocean. Remember, several types of currents can occur at the same time.
- Current Measurements: Describe the
difficulties that might be involved in measuring hourly mean
currents 5 meters below the sea surface using moored current
meters in the deep ocean.
- Equatorial Undercurrent: This Fall, the
equatorial undercurrent has ceased to exist in the
Pacific.
- Why should El Niño cause the undercurrent to stop flowing? Please outline the process or steps leading from El Niño to the weakened undercurrent.
- Water Masses:
- Describe the three dominant water masses below the pycnocline in the South Atlantic.
- What is the source of each water mass?
- What is the typical depth at the core of each of the water masses?
- Why do oceanographers use the concept of water masses and the core method?
- The Meridional Overturning Circulation:
- Please explain the relevance of the meridional over turning (thermohaline) circulation in the oceans for understanding global climate and ice ages.
- Which processes influence the creation of bottom water in the North Atlantic?
- The Next Ice Age: Recently a prominent
oceanographer stated that, in his opinion, increased global
CO2 emissions will plunge Earth into the next ice age.
Please state in a short paragraph the reasoning that could
lead someone to this conclusion. Hint: it involves the
meridional overturning circulation.
- Global Warming: An important question being
asked of Earth scientists is: Is Earth warming up? To help
answer the question, you are asked to determine if the
average surface temperature of the ocean has changed over
the last 20 years. You remember that the expected increase
is small, only about 0.2&endash;0.5°C over the 20-year
period.
- What data set would you choose to determine if the ocean surface temperature has changed by a small amount?
- Why did you choose the one you did?
- If you use the data set you picked above, what errors might influence your calculation of the change in temperature?
- El Niño:
Why is El Niño important
in meteorology?
- In answering the question, please describe briefly a typicalEl Niñocycle and its influence on the atmosphere in the Pacific.
- Explain why the atmospheric influence in the Pacific is so important and why the influence of El Niño extends far beyond the Pacific.
- El Niño:
Do you expect the Equatorial
Undercurrent in the Pacific to be stronger or weaker during
the peak of anEl Niño, and why?
- Numerical Models: Give three reasons why
Semtner-Chervin's model of the ocean circulation shown in
the film in class may be incorrect.
- State each reason in one or two sentences.
- Ocean Waves: Your consulting company is asked
to estimate average wave conditions and the mximum wave
height that might be encountered by an offshore structure
during its 20 yr life.
- How would you estimate the maximum wave height?
- Knowing how waves are measured, what global data sets containing wave heights might be available to determine the average wave conditions?
- How do you know if the data set used for calculating average wave conditions is accurate?
- Ocean Waves: Your consulting company is asked
to estimate the maximum wave height that might be
encountered by an offshore structure during its 20 yr
life.
- How would you estimate the maximum wave height?
- Hints:
Would you use a data base of measured heights?
If you do not use measured heights, what information do you need to calculate heights?
(This question is a little tricky: big waves are produced by rare storms like hurricanes, which may come through your area only once in 20-40 years. How do you handle this?)
- Ocean Waves: Using scaling arguments, show
which terms in the momentum equation are important for
describing waves close to the beach.
- Ocean Wave Forecasts: Discuss the usefulness
of two different wind data sets for driving wave-forecasting
models for the Pacific. The forecasts (actually hind-casts)
will use historic data, and daily values of wind on a 2-3
degree grid are needed for one year, say from Jan 1994 to
Jan 1998.
- Ocean Wave Forecasts: What might be a useful
source of wind data for driving a model of the circulation
in Galveston Bay in near real time. That is, wind data must
be no more than 12 hours old.
- Tides: Describe a method for predicting tides
at a remote, deep-ocean site offshore of South Africa.
- Tides: If there are only five fundamental
tidal frequencies, why does the tide at a particular point
have 30-300 frequencies?
(A team of students in ocen-407 was asked to provide exactly this type of information on the variability of currents off South Africa last year.)
Revised on: 5 September, 2004
