The
diagrams to the left shows air temperatures, which are proxies for average sea surface temperatures (SST) for August and
January. The diagrams are also contoured with lines of equal temperature. Atmospheric and Ocean Circulation Lab name__________________________________
Objectives:
The main goal of this lab is to learn about atmospheric and oceanic circulation and how these two processes are strongly inter-dependent and strongly influence each other.
Format:
We will use the website http://www.mpcfaculty.net/alfred_hochstaedter/Earth%20Science/circlab.htm to access large color images for this lab, since the larger color images makes the information so much easier to see. All images on the websites are thumbnails, so that clicking on them will bring you to larger versions. The diagrams are reproduced here on these pages so that you can use them to write notes and remember the pertinent points of this lab.
The
diagrams to the left shows air temperatures, which are proxies for average sea surface temperatures (SST) for August and
January. The diagrams are also contoured with lines of equal temperature.
At what northern latitude is the 20ºC contour in the August?
At what northern latitude is the 20ºC contour in the January?
Does
the ocean temperature change radically with the seasons?
How does the high heat capacity of water influence this seasonal temperature change?
See current SSTs at: http://www.ssec.wisc.edu/data/sst/latest_sst.gif
The
diagram at left is data from the NOAA satellite TIROS, which measures long-wave
radiation emitted from the Earth’s surface and atmosphere. This kind of
radiation is related to the surface temperatures on Earth. Green and blue colors
represent temperatures below 0ºC. The darkest purple represent temperatures near
-30ºC.
How does the seasonal temperature variation of the continents compare with that of the oceans?
Is it greater or smaller?
How does the low heat capacity of rock influence the seasonal temperature variation of the continents?
How will the differences between the seasonal temperature variations in the oceans and continents affect the locations of high and low pressure systems in the atmosphere? Another way of asking this question is: How will the differences between the seasonal temperature variations in the oceans and continents affect atmospheric circulation?
Below are average atmospheric pressure maps for summer and winter. Contour lines of equal atmospheric pressure (isobars) are also shown. In a very broad sense, low pressure systems are most often located near the equator and near 60º, whereas high pressure systems are most often located near 30º.
Focus on the differences between summer and winter. Complete the following table by showing whether the region has high, low, or moderate pressure during the summer and winter. Follow the North America example.
Location July January
North America low high
South America (~30ºS)
Asia
Africa (~20ºS)
North Pacific (~45ºN)
South Pacific (~30ºS)
North Atlantic (~50ºN)
South Atlantic (~25ºS)
Indian (~30ºS)
Do the southern hemisphere oceans show more or less seasonal variation of atmospheric pressure than do the northern hemisphere oceans?
Why do you think this phenomena occurs?
Which way do the winds blow? To answer this question we need to understand how the Pressure Gradient Force (PGF) and the Coriolis Effect interact to create circular wind patterns around high and low pressure systems. The PGF is the force that
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pushes air away from high pressure systems and pulls it towards low pressure systems. The Coriolis effect deflects this moving air, to the right in the northern hemisphere and to the left in the southern hemisphere. The result is that winds blow counterclockwise around low pressure systems and clockwise around high pressure systems in the northern hemisphere.
Draw on the big map (last page of handout) the wind directions that you would predict from the location of high and low pressure systems.
You can check your answers by clicking to see the August wind stress and January wind stress maps, or the North America maps.
Based on your predictions, complete the table for North America for wind directions that you would feel if you were standing on the west or east coast…
East Coast West Coast
July: Winds from (N or S)
Air temp (warm/cool)
January: Winds from (N or S)
Air temp (warm/cool)
Now, which has a more moderate climate, the east coast or west coast of North America?
Do you think this is true for other continents? Why or Why not?
Air masses attain their characteristics from the ground or water over which they flow. When would you predict the rainy season is in India and the rest of continental Southeast Asia?
When is the dry season in India and the rest of continental Southeast Asia? What is your rationale?
The Aleutian Low –
Pacific High Today.
Look at today's satellite images by following the link below. Press “back” to get back here.
http://virga.sfsu.edu/scripts/gwir_big.html
Draw a generalized picture of today's high and low pressure systems in the Pacific in the space below. On your picture draw in and label any examples of the Coriolis effect that you can see.
What is it that you see on the satellite image that informs your description?
Does your assessment agree with what you would predict as "normal" for the current season? Why or why not?
How well does your sketch compare with the atmospheric pressure map located here?:
http://virga.sfsu.edu/gif/sathts_pac_snd_00.gif
Ocean Circulation
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-On the map below, draw the followingcurrents and gyres. Use the figure in your book if you need help. |
Currents |
Gyres |
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The California Current |
North Pacific |
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The Kuroshio Current |
South Pacific |
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The Gulf Stream |
North Atlantic |
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The Canary Current |
South Atlantic |
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West Wind Drift |
Indian |
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The Peru Current |
(Note: All of these are also called subtropical gyres) |
The Sub-Tropical Gyres
The Coriolis effect causes water to pile up in the middle of these gyres. The “hills of water” are small, only 2-3 meters high, so you’d never see or feel them. Nevertheless, they do influence oceanic circulation. The center of the gyres is also not in the center of the ocean basins, but rather displaced westwards.
So, given a westward displaced gyre, which currents flow
fastest and strongest, the eastern boundary currents or the western boundary
currents?
Explain your answer. You can use the image at left to help, if you like.
Salinity Variations
Back to Salinity variations. The difference between annual precipitation and evaporation can explain the latitudinal (N_-S) variation in salinity, but explains less of the longitudinal (E-W) variations. How might surface water circulation patterns influence surface salinity values as shown to the right? Specifically, why isn’t the surface water off the coast of California as salty as in the middle of the north Pacific at 30ºN?
Upwelling and Downwelling
These diagrams show the various ways that upwelling and downwelling can be created. Note that the situation in “A” on the left could also be produced by winds moving towards you out of the paper, as we discussed in class. These images will form the basis for the answers to the following questions.
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The diagram below shows a map on the left and a cross section on the right. Assuming a north wind as shown on the map, draw in the direction of surface currents and deeper water currents on the cross section.
Does your diagram show evidence for upwelling here? Would you expect the upwelled water to be warm or cold? Why?
The
diagram to the left shows SSTs for the Monterey Bay Region. Note that upwelling,
as denoted by the cold regions, occurs at specific areas along the coast. Note,
the upwelling does not coincide with the location of canyons. Based on the shape
of the coast line, why does upwelling principally occur along these certain
stretches of coastline?
Ocean Circulation and Biologic Productivity.
Organisms need nutrients to live. Thus the availability of nutrients determines the location and abundance of organisms, especially those at the base of the food web such as plankton. The location of plankton blooms in the oceans can be identified by satellite because the plankton contains chlorophyll and appear as different colors to sensitive satellite instruments such as the Sea-viewing Wide Field-of-View Sensor (SeaWiFS). In general, high concentrations of nutrients, and thus plankton, correspond to areas of upwelling and high productivity.
Why do high concentrations of nutrients and plankton at the surface correspond to zones of upwelling?
Examine the map of global chlorophyll concentrations (which
corresponds to plankton concentrations and thus nutrient concentrations and
biologic productivity). As usual, reds and yellows are high values, whereas
blues and purples are low values. The polar regions generally have high plankton
concentrations because of the 24-hour light during summer months in high
latitudes.
Examine the coasts of the continents. In general, which coasts of continents have higher biologic productivity: the east coast or west coast?
Why? What cases this difference in biologic productivity along east and west coasts of continents?
The centers of the subtropical gyres have the lowest biologic productivity values. Why?
Why do the equatorial regions have productivity values that are higher than the centers of the gyres?
The data in this section comes from SeaWiFS, a satellite project that records the color of the ocean from space. Their home page is at
http://oceancolor.gsfc.nasa.gov/SeaWiFS/
Check out this animation of changing chlorophyll values: http://oceancolor.gsfc.nasa.gov/SeaWiFS/HTML/SeaWiFS.BiosphereAnimation.70W.html
El Niño is defined as the warming of the eastern equatorial Pacific. Under typical conditions, the waters off the west coast of equatorial South America is relatively cool (fig to left), high pressure dominates in the eastern equatorial Pacific, low pressure dominates in the western equatorial Pacific (see figs at beginning of exercise), and the trade winds blow strongly to the west.
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During El Niño conditions (fig to right) the eastern equatorial Pacific is abnormally warm, the low pressure system moves eastward, and the trade winds weaken.
Current Equatorial Pacific SST and SST anomalies are at
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/
An animation of recent SST anomalies is at:
http://www.cdc.noaa.gov/map/clim/sst_olr/sst_anim.shtml
Are we currently in an El Nino, La Nina, or La Nada?
Why did you make this
interpretation?
Everything you could ever want to know about El Nino/La Nina is here:
http://www.elnino.noaa.gov/lanina.html
Whereas the previous figures showed actual sea surface temperatures (SST), these figures show sea-level height anomalies, from which SST anomalies may be inferred. Anomalies are the difference between the observed sea-level height or temperature and the normal (average) sea-level height or temperature. The white areas indicate sea level 13-30 cm (5-10 inches) above normal. These figures show the development of the 1997-98 El Niño, which is one of the strongest on record.
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Why would warmer water produce a sea-level height anomaly?
Considering the above figures, are there any signs of an El Niño event in the sea-level height anomalies of March 1997?
As the El Niño event developed in October of 1997, what happened to the sea-level height anomalies in the western Equatorial Pacific?
Historical Records in Monterey Bay
The diagram below right is for Monterey Bay. It shows temperature and salinity profiles contoured against time from a mid- Monterey Bay location. The lowermost diagram is a measure of density, which is calculated from temperature and salinity.
Draw arrows that show upwelling events on these diagrams.
Does upwelling bring higher salinity or lower salinity water to the surface? How do you know?
Please label the 1997-98 El Niño event on the Monterey Bay record? How do you recognize it?
Please label the 1991-92 El Niño event on the Monterey Bay record?
Origin of Water Masses in Monterey Bay
Why is deep Monterey Bay water saltier than surface Monterey Bay water? There are at least two possibilities:
1) Evaporation during the summer creates salty water at the surface that then sinks.
2) The southerly flowing California Current introduces low salinity over the higher salinity water.
Which one of these possibilities is more likely to be correct? Please explain why, using the hypothesis testing method of the scientific method.
Variation in Nutrients and Biologic Productivity in Monterey Bay
What time of year would you predict the nutrient levels, such as phosphate and nitrate to be the greatest in Monterey Bay?
Why?
Please click here to check your answer:
http://www.mbari.org/bog/projects/centralcal/summary/images/m1_nuts_ts_contour.jpg
Was your prediction right? Why or why not?
What time of year would you predict the biologic productivity--as measured by Chlorophyll levels--to be the greatest?
Why?
Please click here to check your answer:
http://www.mbari.org/bog/Projects/CentralCal/summary/images/maj_bio_ts_contour.jpg
Was your prediction right? Why or why not?
All this Monterey Bay data comes from Francisco Chavez’ Biological Ocean Group at MBARI:
He has compiled lots of time series data of Monterey Bay chemistry:
http://www.mbari.org/bog/projects/centralcal/summary/ts_summary.htm
A location map of his permanently moored CTD stations is at:
http://www.mbari.org/bog/roadmap/major_stations.htm
