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The Southern Ocean

Table of Contents

  •   A.  Statement of Topic
  •   B.  Summary of Topic
  •   C.  Research Setting (Spatial Region and Temporal Period)
  •   D.  Data Description
  •   E.  Stepwise Investigational Instructions
  •   F.  Presentation of Data Analyses
  •   G.  Interpretation of Data
  •   H.  Discussion and Statement of Conclusions
  A.  Statement of Topic

This module examinesclimate patterns in the Southern Ocean, the body of water which encircles the continent of Antarctica.   The Southern Ocean is a unique body of water;   the primary circulation is in one direction, and the seasonal patterns are quite distinct, due to the large differences in the amount of sunlight the area receives between winter and summer, and partly due to the movement of the sea ice boundary. Selection of a particular spatial regionwhich shows contrasting patterns in a single year will be demonstrated. This module will provide an introduction to the Southern Ocean and climate patterns in the high llatitudes..

  B.  Summary of Topic

The Southern Ocean is a complex system with several different patterns of variability.   Distinct "fronts" (areas in which variables change rapidly) can be discerned in both SST and chlorophyll data.The Southern Ocean is a large circumpolar body of water encircling the continent of Antarctica, and it includes several sub-regions, such as the Amundsen Sea, Weddell Sea, Bellingshausen Sea, the Ross Sea, the Drake Passage, and the Scotia Sea.

  C.  Research Setting

Due to the pattern of circulation, the Southern Ocean is somewhat isolated from the waters of the Atlantic, Pacific, and Indian Oceans -- at the same time, it acts as a connecting link in the circulation of the world ocean.  In terms of area, the Southern Ocean is the fourth largest ocean after the Pacific Ocean, Atlantic Ocean, and Indian Ocean, but before the Artic Ocean.

In the Southern Ocean, SST varies from about ten degrees Celsius to about two degrees Celsius.  Cyclonic storms travel eastward around the continent and frequently are intense due to the temperature contrast between ice and open ocean.  The ocean area to the Antarctic Circle has, on average, the strongest winds found anywhere on Earth. In the winter the ocean freezes outward in the Pacific sector and in the Atlantic sector. Also, the surface temperatures go below zero degrees Celsius, but at some coastal points intense persistentwinds from the interior of Antarctica keep the shoreline ice-free throughout  the winter.

One important feature of the Southern Ocean is the Antarctic Circumpolar Current. This current is the world's biggest ocean current, flowing eastward. This current flows roughly a thousand times more than the flow of the Amazon River.

D.   Data Description

For this data,SeaWiFS monthly global 9km products using chlorophyll a concentration and MODIS SST data were utilized, which are available in Giovanni.   This module will examine patterns and changes over a year in one section of the Southern Ocean.

  E.  Stepwise Investigational Instructions

In this investigation, the spatial and temporal coordinates for the study area will be defined using Giovanni.  Because Giovannn only uses a standard global map projection, the full Southern Ocean is very long and narrow, so initially the area to examine will be the section of the Southern Ocean just south of the Atlantic Ocean.

Southern Ocean, sea surface temperature, 2003

For chlorophyll concentration data, the same area is chosen, but a different year (due to some very interesting patterns in the data that year.  In both of these images, the location of South Georgia Island has been labeled, as the patterns of interest are located in that area.

Southern Ocean, chlorophyll data, 2000

 

F.  Presentation of Data Analyses

     The next step is to "zoom" into the area of interest, in this case the oceanic region around South Georgia Island.   Each of the images below can be clicked to show a larger version of the image.   For each of the chlorophyll images, the color palette corresponds to the color bar below, with units of milligrams chlorophyll per cubic meter of seawater.   The units for the SST color bar, which appears in each image, are degrees Celsius.

chlorophyll concentration color bar

     Annual SST and chlorophyll a concentration

Southern Ocean, South Georgia Island, sea surface temperature, 2003     Southern Ocean, South Georgial Island, chlorophyll data, 2000

 

     January SST and chlorophyll a concentration

     January is the height of the austral (Southern Hemisphere) summer.   Though SST is only slightly warmer in January than the average frigid temperature at the latitude of South Georgia Island, the long summer days provide plenty of sunlight for phytoplankton growth.  In the chlorophyll image, all three of the Southern Ocean Circumpolar Current fronts -- the Subtropical Front, the SubAntarctic Front, and the Antarctic Polar Front -- can be clearly seen.

Southern Ocean, January 2003, sea surface temperatureSouthern Ocean, South Georgia, chlorophyll data, January 2000

 

     April SST and chlorophyll a concentration

     April is autumn in the Southern Hemisphere, and the growth of phytoplankton is slower as the sea gets stormier.   Phytoplankton activity around South Georgia Island is still noteworthy, and the patterns of turbulent mixing along the Subtropical Front are very clear.

Southern Ocean, South Georgia, sea surface temperature, April 2003Southern Ocean, South Georgia, chlorophyll data, April 2000

 

     September SST and chlorophyll a concentration

    July and August are the depths of the Antarctic winter, and there is very little data for these months -- clouds and ice cover the water, and there is too little light for remote sensing of chlorophyll.   So September provides a view of early spring in the Southern Ocean.    As expected, the phytoplankton have not responded to the availability of light, so biological activity in this area is subdued compared to the summer months.

Southern Ocean, South Georgia, sea surface temperature, September 2003Southern Ocean, South Georgia, chlorophyll data, September 2000

 

G.  Interpretation of Data Analyses

     The images shown above illustrate the importance of the two main seasons in the high latitudes -- summer and winter.   In the polar regions, spring and fall are mainly extensions of the two seasonal extremes.    Slight variations in SST are augmented by large differences in the amount of available sunlight, providing optimum conditions for phytoplankton growth in summer and an opposite set of conditions in the winter.

    Much of the biological activity in the Southern Ocean occurs along the major fronts:  the Subtropical, the SubAntarctic, and the Antarctic Polar Front.    The first two fronts appear to be associated with fairly well-defined temperature changes, but the Antarctic Polar Front is harder to detect in SST data.

    Because it lies directly in the path of the Circumpolar Current, South Georgia Island disrupts the flow of the current.   "Downstream" of the island, the eddies created in the wake of the island appear to stimulate phytoplankton growth. 

  H.  Discussion and Statement of Conclusions

     The uniqueness of the Southern Ocean makes it a vital region for the biology of the oceans, from the smallest phytoplankton to the abundant krill to the penguins and whales that inhabit this region.    The images shown here demonstrate how the normal seasonal patterns of the Antarctic affect the biology of the oceans.  Further investigation can address how much the productivity in a given region varies from year to year, and how the fronts in the Circumpolar Currren are related to temperature.    Another area of investigation can be where eddies form in the Circumpolar Current and how these eddies affect the distribution of temperature and chlorophyll.

     Links

    SOIREE:   A Phytoplankton Party in the Southern Ocean

     South Georgia:   A View Through the Clouds

     The Low Zone

     Polynyas, CO2, and Diatoms in the Southern Ocean

 

 

 

 

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Last updated: Apr 08, 2016 11:02 AM ET
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