ASL 720 Project

Ocean Color Remote Sensing

Submitted by:­ Akanksha Jain 2010cs50273 Ankur Garg 2010cs10208 Mukesh Kumar 2010cs50288 Saurabh Agarwal 2010cs50293 Saurabh Anand 2010cs50294 Vivek Mittal 2010cs50301 Prince Chandan 2010me20788

1 Contents

1. Introduction 3 2. Applications of Ocean Color Remote Sensing 3 3. Season Variance 4 4. Upwelling / Downwelling 7 5. Chlorophyll concentration comparison between seaWIFS and 13 MODIS­aqua 6. Chlorophyll and Sea Surface Temperature Correlation 17 7. Work Division 21 8. References 22

2 Introduction

The "color" of the ocean is determined by the interactions of incident light with substances or particles present in the water

Ocean Remote Sensing is concerning the study of the interaction between the visible electromagnetic radiation coming from the sun and aquatic environments

Information on the abundance of phytoplankton and dissolved particulate matter can be traced by the values obtained at different locations via the images

Applications of Ocean Color Remote Sensing

The following are the applications of ocean color remote sensing : ● Surface Temperature ● Mapping of chlorophyll concentrations ● Measurement of inherent optical properties such as absorption and backscatter ● Determination of phytoplankton physiology, phenology, and functional groups ● Studies of ocean carbon fixation and cycling ● Monitoring of ecosystem changes resulting from climate change ● Fisheries management ● Mapping of coral reefs, sea­grass beds, and kelp forests ● Mapping of shallow­water bathymetry and bottom type for military operations ● Detection of harmful algal blooms and pollution events

3 Seasonal Variance

We wanted to study how the chlorophyll concentration changes with respect to the seasons. We took the chlorophyll a concentration data from the SeaWiFS satellite at 9km for different months of 2010. Here are the images :

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As we can see, during the summers the concentration of chlorophyll decreases significantly and is very high during the colder seasons. This can be attributed to the significant increase in the phytoplankton population during the different seasons. Phytoplankton are microscopic plants that are the principal photosynthetic organisms in the ocean and form the base of ocean food webs. Chlorophyll a is the most important pigment involved in phytoplankton photosynthesis. Seasonal phytoplankton variability is related to stratification, destratification and incident solar irradiance which all essentially affect the nutrient availability and hence the population of the phytoplanktons change during the seasons.

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Seasonal warming leads to a stratified water column which helps retain phytoplankton in well­lit and nutrient­rich surface waters, causing seasonal biomass peaks (blooms). However, strong stratification during summer at low latitudes (like in the Arabian Sea) and midlatitudes cuts off the supply of new nutrients to upper layers and often leads to low rates of photosynthesis and biomass. The absorption rate of carbon dioxide in water also changes due to the temperature changes in the different seasons. All the above reasons contribute to the variance.

6 Upwelling and Downwelling

1. Introduction

Upwelling and downwelling describe mass movements of the ocean, which affect both surface and deep currents. These movements are essential in stirring the ocean, delivering oxygen to depth, distributing heat, and bringing nutrients to the surface. Upwelling is the movement of cold, deep, often nutrient­rich water to the surface mixed layer; and downwelling is the movement of surface water to deeper depths. Downwelling occurs when surface waters converge (come together), pushing the surface water downwards. Regions of downwelling have low productivity because of the nutrients get used up and are not continuously resupplied by the cold, nutrient­rich water from below the surface. Upwelling occurs when surface waters diverge (move apart), enabling upward movement of water. Upwelling brings water to the surface that is enriched with nutrients important for primary productivity (algal growth) that in turn supports richly productive marine ecosystems. Upwelling regions are often measured by their productivity due to the influx of nutrients to the surface mixed layer and euphotic zone (sunlit layer) by upwelling currents. This drives photosynthesis of phytoplankton (tiny alga), which form the base of the ocean food web.

2. Areas with Upwelling

Whether any region will have upwelling or not, depends on many factors including wind direction, place of the region on earth etc. Based on such factors, various regions may or may not show upwelling. We are mainly interested in upwelling along the coastal areas. The following map shows the regions of upwelling along the coasts of different continents.

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3. Regions of Chlorophyll­a concentration

We make use of the upwelling/downwelling concepts to explain the differences between the Chlorophyll­a concentrations between the different coastal areas around the globe.

African Coast: The following two maps show chlorophyll­a concentrations along the coast of southern Africa from two different sensors. This map is from the sensor SeaWIFS

8 This map is from the sensor MODIS­Aqua

Using the maps above from both sensors, we see that the western coast of African southern continent has more Chlorophyll­a concentration as compared to the Eastern coast.

Bay of Bengal Coast We also observed data from the Indian coasts – Bay of Bengal Coast and Arabian Sea coast. The following two maps are from SeaWIFS and Modis Aqua sensors for Chlorophyll­a concentration in East coast of India

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Arabian Sea Coast The following two maps are from SeaWIFS and Modis Aqua sensors for Chlorophyll­a concentration in West coast of India.

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Data from both east and west coast show us that Chlorophyll­a concentration is more in the coastal areas of Arabian Sea as compared to the Chlorophyll­a concentration in the coastal areas of Bay of Bengal.

4. Relation between Chlorophyll concentration and Upwelling

We analyzed maps of chlorophyll concentration and the map of regions with upwelling. On the following map of regions with upwelling, we see that the in southern African continent, the west coast show quite high upwelling as compared to the east coast.

Also from the chlorophyll concentration plots, we find that chlorophyll concentration is high along the west coast of Africa.

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Similarly, along the Indian coast,

The upwelling is high along the coast of Arabian Sea as compared to the coast of Bay of Bengal. Also, we have seen from the plots of chlorophyll concentration, that it is higher in the coastal areas of Arabia Sea as compared to the coastal areas of Bay of Bengal.

From the above two cases, we find a direct correlation between the chlorophyll­a concentration and upwelling. Higher the upwelling in an area, it is more probable to have higher concentrations of chlorophyll­a. Due to it being nutrient rich, the phytoplankton growth is supported and vegetation around that coastal area increases. This way we find that there is more chlorophyll­a concentration in areas with high upwelling as compared to those which don’t. We also see this correlation when we compare Sea temperature and Chlorophyll concentration. The regions with lower temperature have higher chlorophyll concentration. A small reason for cooler sea surface temperatures is upwelling, because it brings cold water from deep sea to the surface.

12 Comparison between MODIS­aqua & seaWIFS

To comment on the usability of two satellite,we chose MODIS­aqua at 9km and seaWIFS at 9km. We took the data of chlorophyll concentration from these satellites and compare it.

● We get the data from the a website of NASA which receives the most recently and preprocessed data. Link of website: Ocean Color Radiometry Online Visualization and Analysis http://gdata1.sci.gsfc.nasa.gov/daac­bin/G3/gui.cgi?instance_id=ocean_month

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● We downloaded the data and find out the correlation for the two sensors. Data was in .hdf format and can’t open in normal editors, so we used HDF viewer to read data. Then we used matlab to find the correlation between two data. Given below is the matlab code which we have used to find out correlation.

fileinfo1=hdfinfo('MAMO_Chlo_9km.hdf'); sdsinfo1=fileinfo1.SDS(1); data1=hdfread(sdsinfo1); fileinfo2=hdfinfo('SWFMO_Chlo.hdf'); sdsinfo2=fileinfo2.SDS(1); data2=hdfread(sdsinfo2); correlation=corrcoef(data1,data2);

14 The correlation variable in above code gives the correlation between the data of two sensors. After the execution of the above code in matlab, we find out value of correlation. Correlation matrix 1.0 0.7997

0.7997 1.0

Diagonal of this matrix corresponds to correlation of with itself hence they are all equal to 1. Off­Diagonal of this matrix corresponds to sample correlation coefficients between the two random variables. In our case these variables are the two satellites whose data we are analysing. Correlation coefficient ranges between [­1 1]. 1 denotes a perfect correlation and 0 denotes no correlation. The positive and negative signs denotes positive and negative correlation between the random variables. abs(correlation)>0.7 indicates strong correlation.

● Result:We are getting the value of the correlation between two satellites = 0.7997, which indicate that the data of two sensors are strongly positively correlated.

● Conclusion: Since the data coming from two satellites are strongly correlated, we can use any sensor for remote sensing because the results of both sensors are almost similar.

● Further Observations: ○ The precision of Modis is higher at high chlorophyll values, while of SeaWIFS is higher at low chlorophyll values as seen from the maps, but there is not much theoretical support to this analysis from the resources searched. ■ These sensors can be used accordingly if there is more practical evidence to the analysis ○ It has been proven in many works that the net error by the two sensors is almost equivalent, which is almost true here too (if judging the differences by averaging)

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● Reasons of similarity ○ The orbital characteristics and spatial resolution of the MODIS instruments are similar to those of SeaWiFS ○ Cross­track swath width of both sensors are about 2300 km. ○ The MODIS data processing algorithms, after taking account of the sensor specifications (e.g., polarization response, spectral response, etc), are nearly identical to those used for SeaWiFS.

16 Chlorophyll & Sea Surface Temperature

At the base of the ocean food web are single­celled algae and other plant­like organisms known as phytoplankton. Like plants on land, phytoplankton use chlorophyll and other light­harvesting pigments to carry out photosynthesis. Where phytoplankton grow depends on available sunlight, temperature, and nutrient levels. Cold waters tend to have more nutrients than warm waters, phytoplankton tend to be more plentiful where waters were cold.

● Plot for Chlorophyll Concentration (Dec, 2003 ­ Dec, 2004) (in mg/m3)

● Plot for Ocean Surface Temperature (Dec, 2003 ­ Dec, 2004) (in 0C)

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● Plot for Chlorophyll Concentration (Dec, 2012 ­ Dec, 2013) (in mg/m3)

● Plot for Ocean Surface Temperature (Dec, 2012 ­ Dec, 2013) (in 0C)

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The chlorophyll maps show milligrams of chlorophyll per cubic meter of seawater each month. Places where chlorophyll amounts were very low, indicating very low numbers of phytoplankton are purple. Places where chlorophyll concentrations were high, meaning many phytoplankton were growing, are red.

The observations come from the MODIS­Aqua 9km (for chlorophyll conc.) and MODIS­Terra 9km (for temperature) sensors of NASA Satellite. These satellites measure the temperature of the top millimeter of the ocean surface. In this map, the coolest waters appear in purple (approximately ­2 degrees Celsius), and the warmest temperatures appear in red (45 degrees Celsius).

The highest chlorophyll concentrations are in cold polar waters or in places where ocean currents bring cold water to the surface and where tiny surface­dwelling ocean plants are thriving, such as around the equator and along the shores of continents. When surface waters are cold, it is easier for deeper water to rise to the surface, bringing nutrients to sunlit areas where phytoplankton can use them. When surface water is warm, cooler, nutrient­rich water is trapped below. Because the vertical layers of the ocean aren’t mixing, nutrients that have built up in deep waters can’t reach the surface. In places where ocean currents cause upwelling, sea surface temperatures are often cooler than nearby waters, and chlorophyll concentrations are higher. This connection is evident in multiple places. A band of cool, plant­rich waters circles the globe at the Equator, with the strongest signal in the Atlantic Ocean and the open waters of the Pacific Ocean.

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In many coastal areas, the rising slope of the sea floor pushes cold water from the lowest layers of the ocean to the surface. The rising, or upwelling water carries iron and other nutrients from the ocean floor. Cold coastal upwelling and subsequent phytoplankton growth are most evident along the west coasts of North and South America and southern Africa.

20 Work Division

Seasonal Variance: Saurabh Anand, Ankur Garg, Vivek Mittal

Upwelling / Downwelling: Saurabh Anand, Ankur Garg, Vivek Mittal

Chlorophyll concentration comparison between seaWIFS and MODIS­aqua: Mukesh Kumar, Akanksha Jain

Chlorophyll and Sea Surface Temperature Correlation: Prince Chandan, Saurabh Agarwal

21 References

1. NASA Earth Observatory. http://earthobservatory.nasa.gov/GlobalMaps/view.php

2. Satellite Remote Sensing of Ocean Color and Temperature http://oceancolor.gsfc.nasa.gov/DOCS/Presentations/uqtalk_franz.pdf

3. Seasonal and ENSO variability in global ocean phytoplankton chlorophyll http://www.po.gso.uri.edu/color/publications/2002GB001942.pdf

4. Seasonal and interannual variability of ocean color http://hal.upmc.fr/docs/00/15/50/32/PDF/yd_swfs.pdf

5. Seasonal variation of chlorophyll and primary productivity http://www.ias.ac.in/jarch/epsci/106/00000037.pdf

6. NOAA’s Ocean Service Education http://oceanservice.noaa.gov/education/kits/currents/03coastal4.html

7. Wind driven surface currents: Upwelling and Downwelling

http://oceanmotion.org/html/background/upwelling­and­downwelling.htm

8. Ocean Color Radiometry Online Visualization and Analysis http://gdata1.sci.gsfc.nasa.gov/daac­bin/G3/gui.cgi

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