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 or low pressure during the summer or winter. Follow the North America example.
Location
July January
North America low high
South America
Asia
Africa
North Pacific
South Pacific
North Atlantic
South Atlantic
Indian
West Equatorial Pacific
East Equatorial Pacific
Why do you think the southern hemisphere oceans show less seasonal variation in terms of atmospheric pressure than do the northern hemisphere oceans?
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 here. Press “back” to get back here.
Which is the more predominant system that has influenced the eastern Pacific over the last few days, the Aleutian Low or the Pacific High?
What is it that you see on the satellite image that makes you say this?
Does your assessment agree with what you would predict as "normal" for the current season? Why or why not?
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 all the examples of the Coriolis effect that you can see. This current atmospheric pressure map may help you identify the location of the high and low pressure systems.
Ocean Circulation
-On the map on the next page, draw the major surface currents of the worlds oceans. Use the figure in your book if you need help.
-Label the following currents and gyres on your map:
|
Currents |
Gyres |
|
The California Current |
North Pacific |
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The Kuroshio Current |
South Pacific |
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The Gulf Stream |
North Atlantic |
|
The Canary Current |
South Atlantic |
|
West Wind Drift |
Indian |
|
The Peru Current |
(Note: All of these are also called subtropical gyres) |
|
The North Equatorial Current |
|
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The South Equatorial Current |
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 are also not in the center of the ocean basins. Instead, because of the eastward rotation of the Earth, they are displaced westwards (the earth rotates out from under them, so to speak).
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 below to help, if you like.
Salinity Variations
Back to Salinity variations. The difference between annual precipitation and evaporation can explain the longitudinal (N_-S) variation in salinity, but explains less of the latitudinal (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” 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?
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?
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?
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?
To investigate some of the causes and effects of El Niño, it is useful to get a 2-dimensional view of the oceans by looking at temperature-depth profiles in the equatorial Pacific.
These two top figures show the development of the 1997-98 El Niño event.
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What are the axes on the upper diagrams?
What are the axes on the lower diagrams?
What is the difference between the upper and lower diagrams?
Where
within the oceans is the biggest temperature anomalies during October of this
particular El Niño event? Is it on the surface?
What would you call the region with the biggest anomaly? We have given it a name in this class.
The bottom set of diagrams show the situation in 1999, a more “normal” year.
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Are there any major anomalies anywhere?
What do these results imply about upwelling during El Niño events?
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.
During what time of year does upwelling occur in Monterey Bay? Why is it not the same time as in Peru?
Does upwelling bring higher salinity or lower salinity water to the surface?
Can you find the 1997-98 El Niño event on the Monterey Bay record?
Can you find the 1991-92 El Niño event on the Monterey Bay record?
During what time of year is biologic productivity likely to be highest in the Monterey Bay area?
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.
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
