Observing Oceans from Space

Why study the ocean from Space (I)

Historically, studying the ocean has been done by - People in boats, placing nets or instruments over the side to collect materials and data Drawbacks to this approach: - Boats are slow, relative to the size of the ocean - The ocean and conditions are so variable that it is difficult to characterize it from single collection points

ICESat and ICESat-2

ICESat (Ice, cloud, Elevation Satellite) - NASA's first Earth Observing System mission for measuring ice sheet mass and balance, cloud and aerosol heights, as well as land topography and vegetation characteristics - Operated from 2003 to 2009 Uses a laser altimeter - Pulses of visible light sent to earth - The round time for the laser pulse between the instrument and earth surface is used to measure ice sheet elevations and monitor those elevations over time

The Coastal Zone Colour Scanner (CZCS)

- A multi-channel scanning radiometer aboard the Nimbus 7 satellite - In service from 1978 to 1986. - First satellite instrument devoted to the measurement of ocean colour. - Had six spectral bands, four of which were used primarily for ocean colour. - CZCS attempted to: - Determine the quantity of material - Distinguish between organic and inorganic materials, and between various organic particulates

History of Measuring SST

- Before the 1980's, measuring sea surface temperatures was limited to instruments on shorelines, ships and buoys. - The first automated method of gathering sst was by measuring water flowing through the input ports of ships. - However,the depth of the input ports was not consistent from ship to ship resulting in varying temperatures.

What about Below the surface

- Below surface measurements are primarily taken from moorings and drifters. - Moorings are good for measuring time series through the depths of the water column - Most deeper ocean temperature data are measured from drifters - Data from moorings and drifters are also used to validate satellite instruments.

Sea Ice

- Frozen sea water that is often covered in snow - Sea ice forms, grows and melts in the ocean - Glaciers, icebergs and ice shelves may be in the ocean but they did not originate there so are not considered sea ice.

ICESat-2

- ICESat-2 is the 2nd-generation of the laser altimeter ICESat mission - Scheduled for launch in 2017. - The sole instrument will be the Advanced Topographic Laser Altimeter System (ATLAS). - The ATLAS laser will emit visible, green laser pulses at a wavelength of 532 nm.

First Mapping of Global Reefs

- Imagers on LandSat 1 through 5 were usually turned off while flying over oceans to conserve power and data storage - Finally, instruments on The Enhanced Thematic Mapper Plus on LandSat 7 were turned on over coastal waters and, occasionally, open ocean. - From 2000 to 2003, scientists involves with the Millennium Global Reef Mapping Project used 1,724 LandSat 7 images to develop a global map of coral reefs

Why is sea ice important?

- It keeps polar regions cool and helps moderate global climate - It has a high albedo and reflects about 80% of incoming solar radiation - As sea ice melts, sea water is exposed. Because of its dark colour, it has a high albedo and absorbs 90% of incoming solar radiation.

MABEL

- Multiple Altimeter Beam Experimental Lidar (MABEL) - MABEL is not flown in space but rather high in the atmosphere - Provides the foundation for future ICESat- 2 science by testing the laser altimetry used - Uses individual photons to measure height - Laser pulses sent 5,000 times a second - Able to detect changes as small as centimeters

What now?

- NASA continues to make ocean color measurements with its MODIS instruments. - Since the launch of SeaWiFS, the Indian Space Research Organization has launched its Oceansat satellites and the European Space Agency launched Envisat, which measures ocean color with its MERIS (Medium-resolution Imaging Spectrometer) instrument

Ocean Surface Topography

- Ocean surface topography (sea-level) is measured through the use of satellites - Ocean and land topography are defined the same way and consider height above the geoid. - Accurate ocean topography helps us better understand ocean tides, currents, heat distribution, global weather and climate etc. - Sea surface height is caused by gravity and ocean circulation

Arctic Sea Ice: Current Conditions

- On feb 2015, Arctic Sea Ice reached its maximum extent for the year (14.54 million square kilometres) - This is the lowest in the satellite record - Maximum was 15.64 million square kilometres

Phytoplankton, Chlorophyll and Coloyr

- Phytoplankton make up approximately 50% of the photosynthesis on earth - Chlorophyll concentrations are derived from images of the ocean's colour. - Chlorophyll a absorbs more blue and red light than green, with the resulting reflected light changing from blue to green as the amount of chlorophyll in the water increases - The greener the water, the more phytoplankton are present in the water, and the higher the chlorophyll concentrations.

5. Sea Level

- Rising sea levels are from two primary factors - Thermal expansion of water from increased temperatures (water expands as it warms) - Increased melting of land- based ice (glaciers and polar ice caps).

2. Sea Surface Temperature (SST)

- Sea surface temperature is a measure of the energy due to the motion of molecules at the top layer of the ocean. - Satellite measurements of temperatures are taken from approximately 10 μm below the surface (infrared bands) to 1mm (microwave bands) depths using radiometers

Sea-Viewing Wide Field-of-view Sensor (SeaWIFS)

- Sensor designed to collect global ocean biological data. - Its primary mission was to quantify chlorophyll produced by marine phytoplankton. - In operation from 1997 to 2010, far exceeding its designed operating period of 5 years. - The sensor resolution is 1.1 - Was able to tilt up to 20 degrees to avoid sunlight from the sea surface. Useful at equatorial latitudes where the sun's reflection can obscure watercolour.

Where are we now?

- Since the 1980's most of the information about global SST has come from satellite observations. - MODIS has been a primary source of data - These can collect more SST data in 3 months than all other combined SST measurements taken before using satellites

3. Sea Life Phytoplankton

- Single-celled algae and other plant-like organisms - The base of the ocean food web - Phytoplankton use chlorophyll and other pigments to photosynthesize; this absorbs atmospheric carbon dioxide to produce sugars for fuel. - Chlorophyll in the water changes the way it reflects and absorbs sunlight, - Making mapping their presence and abundance possible - Phytoplankton can act as an indicator of ocean health be useful in the study of the ocean carbon cycle Crucigenia. Credit: US Federal Government

4. Coral Reefs

- The ocean absorbs ~1/3 of excess atmospheric carbon dioxide thereby increasing ocean acidification - Coral reefs produce limestone to grow and must do this at a rate faster than they are being eroded. Ocean acidification slows this process. - Changing water temperatures stresses coral polyps and they lose the algae that gives them their colour (bleaching). - Bleached coral is not dead but highly stressed and more susceptible to diseases and other stressors.

Studying the Ocean from Space: An Example

- The ocean around Tasmania consists of various opposing currents. This interaction increases mixing, which results in complex patterns of phytoplankton distributions. - Would take 10 years for a ship to cover the area. - For a satellite just one minute

How can satellites measure SST

- The ocean emits radiation in the infrared and the microwave wavelengths which can be measured from space. - These wavelengths vary with the temperature of the ocean - Infrared radiation of the ocean comes from the top 10 microns of the surface. - Microwave radiation is from from the VERY top 1-millimeter layer.

LandSat 8

- The sensors have higher sensitivity to brightness and colour than Landsat 7 - The satellite can observe Earth in wavelengths, which enables distortions caused by the atmosphere near the coast to be adjusted for, (ultra-blue band at 0.43- 0.45 micrometers supplemented the blue band at 0.45-0.51 micrometers). -Extra sensitivity made it easier to detect coral reefs and to quantify their area and depth, allowing for reef health to be monitored.

Sea Level: Ocean Surface Topography Missions

- Topex/Poseidon Mission - 1992 to 2006 - Joint mission between CNES (the French space agency) and NASA - The first time scientists were able to map ocean topography with accuracy Credit: NASA - Jason Series - Jason 1 2001-2013 - Jason 2, 2008 launch; still in orbit - Jason 3: scheduled to launch in 2016

5 Ocean Biophysical Attributes:

1. Sea Ice 2. Sea Surface temperature (sst) 3. Sea life: phytoplankton 4. Coral reefs 5. Sea levels

What did the ocean surface missions do?

Measure ocean surface topography to an accuracy of less than 5 cm - Help to validate models of ocean circulation - Useful in predicting and understanding El Niño, La Niña, and the Pacific Decadal Oscillation - Useful for the mapping of: - Basin-wide current variations - Heat stored in the upper ocean - Global tides

Why study the ocean from Space?

Satellites offer many improvements to historical methods - Ability to collect information for a large area in a short period of time and in all weather and sea conditions 0 Easy to have continuous data over extended periods of time - Allows for a large area to be viewed at once rather than piercing together a patchwork of data A combination of approaches may be required depending on what is being studied