Seasonal Variability

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 will consist of examining three major areas which demonstrate recurrent patterns of seasonal variability.  Each major area will be illustrated with SST and ocean color data.   Seasonal changes are the most common cause of regular patterns of variability in the oceans. 

B.  Summary of Topic

Seasonal variabilty for the three major areas to be examined is caused by changes in the amount of sunlight each area receives, changes in temperature, and changes in wind speed.  Each of these factors is present in all three areas, but the factors are not equally important in all three areas.    However, in each case, the proper combination of factors usually produces a phytoplankton bloom.  A"bloom" occurs when phytoplankton respond to favorable environmental factors by initiating heightened photosynthetic activity, leading to an explosive growth of the phytoplankton population.

C.  Research Setting

The research settings for this module are the North Atlantic Bloom, the Arabian Sea Monsoon, and Central American winter wind-mixed productivity events. The North Atlantic Bloom is the largest seasonal occurring biological  event in the oceans, driven by light and temperature changes. The Arabian Sea monsoon is the archetypal example of wind-driven upwelling in a major ocean basin. The Central American wind-mixed productivity events are caused by powerful and brief wind jets which emanate from the Central American cordillera. All of these events provide recognizable patterns in both SST and ocean color.

The spring phytoplankton bloom is one of the most widespread changes in the oceanic biosphere that occurs every spring in the North Atlantic.  The bloom begins as days in the northern Hemisphere begin to lengthen in the spring, starting in March. The explosion of phytoplankton growth sweeps from the Sargasso Sea northward, reaching the far north Atlantic in June as the seasonal sea ice recedes.

The monsoons of the Arabian Sea are extremely strong and steady in comparison to storms that dominate mid-latitude loations. There are two monsoons that occur each year, one in the winter and the other in the summer.  The winter monsoon is weaker and originates in the northeast Arabian Sea.  The summer monsoon is stronger and originates in the southwest Arabian Sea, hence it is called the "Southwest Monsoon" (SW Monsoon) . As the SW Monsoon strengthens in the summer, the wind changes direction in a counterclockwise direction. The SW monsoon is at its strongest in July.

There are three low-elevation gaps in the Sierra Madre mountains of Central America, which create unique conditions that result in intense wind jets that blow offshore over the eastern tropical Pacific at irregular time intervals.  The three wind jets, which result from a pressure difference caused by winter high-pressure systems over the Gulf of Mexico and Caribbean Sea, have a profound effect on the upper-ocean temperature and the distributions of nutrients and phytoplankton off the Pacific coast of Central America.

D.   Data Description

For this module, SeaWiFS monthly global 9km products (chlorophyll a concentration) and MODIS SST data which are available in Giovanni, were utilized. 

E.  Stepwise Investigational Instructions

For each study region, the methods used to generate images and analysis results were the same.After going Tthe exact coordinates for each study region were entered into Giovanni.   SST or chlorophyll a concentration were the parameter choices, and area plot of time averaged parameter was the initial plot type.  Hovmoller plots were also utilized for data visualization. 

F.  Presentation of Data Analyses

 THE NORTH ATLANTIC BLOOM

   North Atlantic Bloom - February

   North Atlantic Bloom - March

   North Atlantic Bloom -  April

   North Atlantic Bloom - May

   North Atlantic Bloom -- June

   North Atlantic Bloom - July

   Hovmoller plot of the North Atlantic Bloom in 2003:   
            Note that the productivity of the North Atlantic Bloom is stronger on the western side of the basin than on the eastern side.

   THE ARABIAN SEA MONSOON

   (All of the images in this section are linked to larger versions - click the image to view.)  

  April 2004 - conditions prior to the SouthWest Monsoon

    May 2004 - wind-driven upwelling develops off the coast of Oman;  chlorophyll concentrations increase in upwelling areas

    June 2004 - upwelling intensifies, and cloud cover increases;  coastal upwelling jets evident in both SST and chlorophyll data

    July 2004 - upwelling along the coast of Oman is at peak strength;  elevated chlorophyll concentrations extend into the Arabian Sea;   monsoon cloud cover is extensive

    August 2004 - the monsoon begins to diminish;   coastal upwelling declines, but upwelled waters move into the central Arabian Sea;  "glimpses" through the clouds show high chlorophyll concentrations

    September 2004 - the clouds are gone;  cool upwelled waters persist near the Oman coast and northward;  high chlorophyll concentrations cover a substantial area of the Arabian Sea

CENTRAL AMERICAN WIND-MIXED PRODUCTIVITY EVENTS

   Although the high wind events that cause mixing in the surface layers of the Pacific Ocean off of Central America are episodic and short-lived (2-3 days), they occur every winter.   The appearance of a wind-mixing event is lower sea surface temperature in a narrow zone extending away from the coast.   The lower SST may only persist for a few days, but the nutrients brought to the surface initiate and sustain phytoplankton blooms that last longer, and which can be carried away from the wind-mixing zone by surface currents.    The images below illustrate the three mixing zones in SST and chlorophyll concentration data.

    Sea Surface Temperature data - color scale is in degrees Centigrade.  Arrows indicate zones of reduced SST corresponding to wind jet direction through the cordillera.

    Chlorophyll concentration data - color scale is in milligrams chlorophyll per cubic meter.

G.  Interpretation of Data Analyses

   The Giovanni images illustrate how each of these seasonal phenomena appears in SST and chlorophyll concentration data.    Due to their seasonal nature, each of these phenomena has a distinct "signature" that can be readily visualized in these data types.    Of interest to scientists are the factors that cause variability in the intensity and spatial extent of each of these phenomena.  

H.  Discussion and Statement of Conclusions

    In this module, the use of Giovanni to demonstrate seasonally-occurring processes in the oceans was shown.   SST and chlorophll concentration data allow direct observation of how the interaction of sunlight, ocean temperature, and nutrient availability affects the growth and productivity of phytoplankton.   Investigation of variability in regularly-occurring seasonal oceanic processes is a primary research topic for which Giovanni can  be utilized.

   Links

   The North Atlantic Bloom

   The Papagayo Wind

   The Ras Al Hadd Jet - Stirring the Arabian Sea