Gulf Stream

Questions for this Chapter

• As the Gulf Stream brings warm water into the northwestern Atlantic Ocean, do sea surface temperature and chlorophyll concentrations show seasonal changes over a year?
• Where, and in what ways, does this change occur?

Data Sets for this Chapter

MODIS chlorophyll (chl a)

MODIS sea surface temperature

Visualizations for this Chapter

Animation (Giovanni)

Latitude-Time Hovmoller Plot (Giovanni)

Transect (NEO)

Location Map and Coordinates

Animation (Giovanni):

Just as cold winters are followed by spring growth and warm weather on land, many areas of the ocean have seasonal patterns.  When we analyze the outflow of the Gulf Stream along the north-eastern United States, we can see these changes over time. 

First, l et’s look at an animated time series to see how chlorophyll concentrations and sea surface temperature will change over time.   The plots below are single frames from the animated sequence generated by this procedure.

Note that you can usethe keypad underneath the animation images to change to the loop options or to slow/speed the time series.

Latitude-Time Hovmöller Plots (Giovanni):

As seen through the animation, water temperatures vary over the seasons.  The best way to understand how the temperatures are changing is to analyze how the gradient differs from north to south over time.  Hovmöller diagrams are an excellent way to visualize changes occurring over time and space.

1. Select the area specified above.

2. Parameter: Sea Surface Temperature (MODIS)

3. Temporal:      Begin Date = 2005, Jan

                            End Date = 2007, Dec

4. Select Visualization: Latitude-TimeHovmöller Diagram

5. Generate Plot

This graph displays how the temperature is changing over a 3-year time span in the North-South direction.  To visually orient yourself, rotate the figure 90 degrees in a counterclockwise direction, so that the x-axis of the figure would be vertical, with 32 N at the bottom and 50 N at the top.    (If you save this figure and then open it in a picture editing program, you can actually do this.)    We've done this below.

                                                  ^October 2007                                                 ^January 2005

Transect (NEO):

At times, it is important to generate plots which analyzedata, with specific coordinates at a specific time. NEO provides us with transect plots usingsea surface temperature and chlorophyll data averaged over month-long intervals

To access NEO, go to the website: http://neo.sci.gsfc.nasa.gov/Search.html

1. Select Sea Surface Temperature (MODIS) for January 2005 and July 2005. 
2. Under Area, enter the coordinates used for the Giovanni example.
3. Click on ‘Zoom and Roam’ and then click Probe.  This will outline the coordinate systems. 
4. Click ‘Plot Transect’ 
5. Chose a small region to draw the transect.  (Make sure to record the coordinates of this line, because you may want to do this again!)  Draw various transects to visualize how the temperature differs in different seasons.

Note that thetransect line doesn’t need to be vertical or horizontal.  Try drawing transects along the shoreline as well.

Perform the analysis again using different years and months to get a better picture of how SST and chlorophyll vary. Use the same coordinates to see accurate differences over time. 

Questions

Animation:

• Does the general pattern of SST and chlorophyll in the vicinity of the Gulf Stream follow the same pattern over a year or does it vary?
• What are the months of high chlorophyll concentration and high temperature?  Low chlorophyll concentration and low temperature?

Latitude-Time Hovmoller Plots:

Imagine this situation:

You are a conservation biologist studying off the tip of Newfoundland, Canada, approximately 46° N latitude. You want to release leatherback turtles from captivity. However, leatherback turtles will not survive

in water below 6 deg °C.

•  If you are planning a trip to release the turtles at this latitude, could you make the trip in April? How about in July?  Normally, during what months would it be safe to release the turtles?

Transect:

• How does the temperature changeat various locations?  Is the slope of each transect different for the two months?  Doesit change closer to the shoreline?
• How is this analysis different from the latitude/longitude-time Hovmöller plots in Giovanni?