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Introduction to Ocean Color and Sea Surface Temperature Data from Satellites

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 introduces ocean color data and sea surface temperature (SST) data, and some basic terms associated with these data types. The use of Giovanni for data analysis will be demonstrated, including selection of a spatial region of interest and temporal period of interest. The famous Gulf Stream is the first example. Images of the chosen region are important in this module, because they allow comparison of ocean color and SST data, as well as illustrating changes in the ocean over a month and a year's time. This module should provide an initial understanding of the concepts of ocean color, SST, the characteristics of the selected region, and useful terms.

  B. Summary of Topic

There are several basic terms required to better understand our demonstrations of how ocean color and SST data can be used for research. These terms include:

chlorophyll The pigment in plants that allows photosynthesis to take place.
circulation The movement of water in a body of water.
current The directed movement of a volume of water.
light absorption The process by which light energy is retained to some extent in a substance. Absorption can result in modification of the substance.
light reflection The directed return of unmodified light energy from a substance or boundary between substances.
light scattering The process by which light interacts with, and is redirected by, molecules of the atmosphere, ocean, or land surface
ocean color A phrase that summarizes the interaction of seawater, and substances and particles contained in seawater, with sunlight, causing modification of sunlight observable by the human eye and remote-sensing instruments.
photosynthesis The process by which plants (phytoplankton in the open ocean) use light to synthesize organic carbon from carbon dioxide and water.
phytoplankton Free-floating marine organisms that are capable of photosynthesis.
primary productivity The rate at which organic carbon is formed by plants via photosynthesis.
sea surface temperature The temperature of the layer of seawater nearest to the atmosphere, approximately 0.5m deep.
solar radiation Incoming light (including ultraviolet and infrared energy) from the Sun.

  C.  Research Setting

The research setting for this module is the well-known Gulf Stream. The Gulf Stream originates in the Caribbean Sea and terminates in the northern North Atlantic. It is the western boundary current of the North Atlantic subtropical gyre. It begins upstream of Cape Hatteras, where the Florida Current cease to follow the continental shelf.

The Gulf Stream is chosen because it provides clear patterns in both ocean color and SST data that can be easily comprehended. The Gulf Stream system is one of the world's most intensely studied current systems. It has been intensely studied because it plays an important role in the poleward transfer of heat and salt (part of the oceans' thermohaline circulation system) and serves to warm the European subcontinent. The Gulf Stream transports significant volumes of warm water (heat) poleward. The Gulf Stream System is so powerful that it was visible even in the earliest satellite altimetry studies (which measure sea surface height, SSH).

The Gulf Stream has three major "sections". The Loop Current in the Gulf of Mexico transforms into the Gulf Stream as it flows through the Florida Straits. The characteristics of the Gulf Stream as it flows northward along the coasts of Florida, South Carolina, and North Carolina are a narrow, linear, directed flow. There are few eddies generated in this section of the Gulf Stream, and what eddies do exist tend to be permanent features, most notably the gyre generated by the Charleston Bump.  

As the Gulf Stream turns into the Atlantic Ocean - the "transition zone" section - the transport of the Gulf Stream almost doubles downstream of Cape Hatteras. The primary reason for the downstream increase in transport at or near Cape Hatteras is mostly due to increased velocities in the deep waters of the Gulf Stream. This increase in velocity is associated with deep recirculation cells in the north and south of the current. According to Geosat altimetry results, the current transports a maximum amount of water in the fall and a minimum in the spring.

The third section of the Gulf Stream is the "open ocean" section. In this section, the energy of the current dissipates due to drag from the surrounding waters of the Sargasso Sea and the northern Atlantic Ocean. In this section, the flow of the Gulf Stream begins to meander, forming characteristic loops and swirls. Occasionally, a loop can be "cut off", creating a ring system. If the loop extends to the north, it captures warmer Sargasso Sea water in its interior, creating a warm core ring system. If the loop extends southward, it captures colder north Atlantic water, creating a cold core ring system. While warm core ring systems can be detected for months in remote-sensing imagery, the cold core of the ring will sink and be covered over by warm water, so it is more difficult to detect the signature of cold core rings in SST data.

D. Data Description

For this data, we utilized SeaWiFS monthly global 9km products using chlorophyll-a concentration and MODIS SST data. Both of these data types are available in Giovanni. Giovanni is the GES DISC Online Visualization and ANalysis Infrastructure, which allows rapid access to global data products and the creation of graphical output.  

E. Stepwise Investigational Instructions

 1.   Sea Surface Temperature of the Gulf Stream    2.Sea Surface Temperature of the Gulf Stream

3.a.Sea Surface Temperatureof a portion of the Gulf Stream b.Sea Surface Temperature of a portion of the Gulf Stream  c.Sea Surface Temperature of a portion of the Gulf Stream

Image 1                                                                       Image 2

  1. Go to Giovanni  -- choose MODIS-Aqua data          1.  Return to region selection
  2. Box in the desired region                                            2.  Put in the exact coordinates for the region
  3. Click on sea surface temperature                             3.  Customize the color
  4. Put in desired years and months                              4.  Click generated plot
  5. Click "Pre-Defined" color option                                 5.  Save the image                               
  6. Click "Generate Plot"

Image 3 (a-c)

Repeat the steps for each image as done for Image 2. In this case, the three Gulf Stream "sections" discussed earlier were defined. Image 3a shows the Gulf Stream as it flows along the southeastern coast of the United States. Image 3b shows the transition zone as the Gulf Stream turns to the east, into the Atlantic Ocean. Image 3c shows the open ocean section of the Gulf Stream, characterized by a wave-like appearance.

 

F. Presentation of Data Analyses

Sea Surface Temperature of coastal current for the Gulf StreamSea Surface Temperture of transition for the Gulf StreamSea Surface Temperature of open water for the Gulf Stream

Here are views of the same regions and time periods in ocean color data from SeaWiFS:

     Coastal Current

Gulf Stream, coastal, June-August 2004

     Transition Zone

Gulf Stream chlorophyll, transitional zone, June-August 2004I

     Open Ocean

Gulf Stream, open ocean, chlorophyll, June-August 2004

 

G. Interpretation of Data Analyses

The top SST image shows the starting point of the Gulf Stream. In the image, the current runs parallel to the coastline. In the middle image, the transition from coastal current to open ocean is illustrated. During the transition, the water velocity begins to slow down as it moves further away from the coast. The bottom image shows the open water nature of the Gulf Stream. You can see how the open water is ridging (dark blue 41Nto 43N going across) and spreading out once the transition is done.  

A useful analogy for further explanation of the behavior of the Gulf Stream what these images is that of a river flowing from the mountains to the coast. In the mountains, the river flows very rapidly -- imagine rapids and waterfalls.  As the slope of the mountains decreases, the flow of the river, even thought it may be carrying more water, slows. In the coastal plain, the river will twist and meander, occasionally forming oxbow lakes when the river erodes a passage through a meandering loop. The formation of oxbow lakes by rivers is analogous to the formation of warm- and cold-core ring systems in the Gulf Stream.

IThe color scale for the chlorophyll images was adjusted from 0.1 to 0.3 milligrams per cubic meter to highlight the characteristic range of values along the Gulf Stream boundary. Along the coast, this color scale highlights the eddies caused by seafloor topography along the U.S. East Coast. Near-shore chlorophyll concentrations are generally higher, and somewhat less accurate, due to the influence of the bottom and substances and particles in the water.

  H. Discussion and Statement of Conclusions

This module presents an initial look at the SST and ocean color data available in Giovanni. There are many ideas for research topics that could be pursued. Some topic examples: seasonal variability, especially during the spring; focusing only on the variability of one part of the Gulf Stream (either coastal, transition zone, or open water); examining annual averages and yearly anomalies to determine if there are any persistent trends in the data; and evaluating the best ways to visualize the data to detect ring systems and eddies in the open ocean portion of the Gulf Stream. Many more ideas could be conceived!

Links for Further Information

CZCS Classic Scenes: Gulf Stream Rings

The Charleston Bump

 

 

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Last updated: Apr 08, 2016 10:52 AM ET
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