Remote Sensing & Satellite Oceanography and El Niño-La Niña NASA ECOL 2011 Biological Oceanography Lecture 12 - 26 October 2004

Ship-Based Oceanography Traditionally, collecting oceanographic data involved ship sampling - going out to sea in ships and recording measurements from various instruments. E.g.: CTD with cassette of water bottles – ‘fancier’ version! Water samples  water bottles. Current velocity and direction  current meters. Temperature, salinity, dissolved oxygen, chlorophyll-a  CTDs.

Samples of organisms may also be collected using various gears, such as: plankton nets, fishing nets, benthic grabs, benthic sleds. Mocness plankton net Now, also have data loggers to record e.g. water temperature over a set period. Data can be downloaded on retrieval of the data logger, or the data can be transmitted via radio or satellite to a receiver (i.e. telemetry).

U.K. Research Vessel Discovery Conventional ship sampling has major disadvantages: Only a small part of the ocean can be covered at any one time, i.e. limited coverage. Surveying is time consuming and conditions at sea can change rapidly. Ship time is expensive! U.K. Research Vessel Discovery

Satellite Remote Sensing Increasingly, remote sensing techniques are being used to study the oceans from space  bringing new insights into biological oceanography. Remote sensing is the science of collecting information about the Earth’s surface without being in contact with it, includes aerial photography and satellite remote sensing. Satellite remote sensing uses various sensors and photographic equipment onboard spacecrafts or satellites orbiting >700 km above Earth’s surface.

Satellite Oceanography The launch of Seasat in 1978 - first satellite specifically designed for ocean surveillance. Satellites enable spatially detailed measurements almost instantaneously over wide areas, thus providing a global view. Resolution: hundreds of km to 1 km pixels. Seasat satellite Photo: JPL

Also, good for following seasonal changes. Satellite remote sensing  provides synoptic data – i.e. can cover a large region at the same time or within a short space of time. Also, good for following seasonal changes. NASA Daily coverage by the SeaWiFs (Sea-viewing Wide Field of View Sensor) satellite (at 705 km height, 15 orbits per day, 1.1 km resolution, swath width 2801 km)

What Can We Measure From Space? Despite the variety of instruments available, remote sensing is able to measure just 4 basic properties, based on detecting electromagnetic radiation through the atmosphere: colour of near-surface waters sea surface temperatures (SSTs) surface roughness and wave height height and slope of the ocean surface From these basic measurements, a range of other properties can be derived.

Different Sensors on Satellites class type Primary measurement Derived properties Visible waveband sensors Infra-red sensors Microwave Radar instruments Multi-spectral scanners Spectrometers imaging radiometers Scanning microwave radiometers Scatterometer Imaging radar Altimeter Ocean colour Surface height & slope Surface roughness Sea surface temperature Mixed layer temperature Skin temperature PASSIVE SENSORS ACTIVE Chlorophyll-a Suspended particulates Bathymetry Surface winds Wave height Internal waves Geostrophic currents Sea-floor bathymetry

Limitations of Remote Sensing Many aspects of the oceans cannot be measured by remote sensing with present technology, e.g. salinity. Remote sensing cannot penetrate far below the sea surface – limited to surface or near-surface information only. Some sensors cannot penetrate cloud cover. Can only directly study one specific aspect of ocean biology – the phytoplankton.

Ocean Colour Phytoplankton (as in many plants) contain pigments that are necessary for photosynthesis, primarily chlorophyll-a. Presence of Chl-a in surface waters changes the colour of the water from blue to green. By measuring the exact colour of the sea, we can determine phytoplankton concentrations – assuming: [Chl-a]  phytoplankton abundance, hence primary productivity (g C m-2 yr-1)

Global Primary Productivity in terms of g C m-2 yr-1 NASA False colour composite satellite image showing levels of chlorophyll-a concentrations worldwide (land areas are masked off and  appears black).

Why Study Phytoplankton? Phytoplankton are widespread and form the basic element of the oceanic food web. Total phytoplankton biomass > biomass of all other marine animals together. Phytoplankton also play a significant role in the world’s climate system: Presence in water causes light to be scattered and absorbed  warms upper layers of oceans. Chemical compounds produced and released to atmosphere  help in cloud formation. Major role in the global carbon cycle.

Sea Surface Temperatures (SSTs) Infra-red sensors measure emitted radiation from the sea surface, which increases as sea surface temperature (SST) increases. SST data are used: to help us predict weather patterns, to track ocean currents, and to monitor El Niño and La Niña. E.g. SST data are collected by the AVHRR (Advanced Very High Resolution Radiometer) on the NOAA-9 satellite.

SSTs compiled on 24 October 2004 Latest Global SSTs Temperatures in °F SSTs compiled on 24 October 2004

Application of Satellite Oceanography: El Niño - La Niña Much attention in recent years on the El Niño phenomenon. El Niño is Spanish for “The Boy Child” - i.e. the Christ Child. At first, the term was originally used by fishermen along the coasts of Ecuador and Peru to refer to a warm ocean current, which occurred around Christmas time and disrupted fishing.

Coastal Upwelling off Peru (Normal Conditions) N S W E Southerly Wind Southern Hemisphere PERU Deflection to the left by the Earth’s rotation Offshore surface current High productivity Upwelling of cold, nutrient-rich water

In Normal Years Southerly wind blow along coast of Peru, generating the northward flowing Peru Current (also called Humboldt Current). Earth’s rotation and Coriolis Effect deflects resulting surface currents towards the west (in Southern Hemisphere), away from coast. Colder, nutrient-rich waters is upwelled to replace offshore current. Supports high plankton abundance, also enormous populations of seabirds and fishes  high productivity of coastal waters.

In El Niño Years A warm, nutrient-poor water mass pools against the coast of Peru. SSTs are 6-10°C higher than normal. Peru Current interrupted, and upwelling stops. Plankton abundance falls and fish populations die or move. Conditions may last 6-18 months - disastrous for Peru’s economy (depends heavily on pelagic fisheries).

SSTs and El Niño Water current direction and SSTs under normal conditions and during El Niño.

El Niño - Southern Oscillation We now know that El Niño is not only oceanographic, but is also linked to atmospheric conditions. The atmospheric component, with pressure changes and wind currents, is called the Southern Oscillation. Hence sometimes referred to together as the El Niño/Southern Oscillation (ENSO). ENSO has worldwide effects on weather and oceanography.

Normal Conditions High Low Pressure Pressure Convective loop Rain in the East Dry in the West Strong trade winds blow westwards Low Pressure High Pressure L H Equator Warm waters pushed westwards South America Australia Cold Peru Current Normally high pressure system over the East Pacific, and low pressure over West Pacific. Strong trade winds blow from east to west due to differences in atmospheric pressure. Low pressure in west leads to increased upward convection, more water vapour into atmosphere, more rain in the west. Surface ocean currents move westwards along equator, transporting warm water to western Pacific. Upwelling along Peruvian coast so thermocline is shallow. (Thermocline - sharp temp gradient marking two water masses). Warm Water Thermocline Cold Water

- increased convection W E El Niño Conditions Drought – Australia and S.E. Asia Rainstorms – Central America, Peru Trade winds weaken or reverse - increased convection H L Warm water flow eastwards South America Warm water pools off S. America Australia A dramatic change occurs in the atmosphere over the Pacific Ocean. Now, high pressure over the west and low pressure over the east. This causes a change in the wind direction and convection loops. Trade winds decrease or reverse, blowing eastwards. Equatorial surface current slows or stops. Subsurface eastwards-flowing current strengthens, pushing warm water to coast of S. America. This increases convection and rainfall follows the warm water eastwards. Associated flooding in Peru, but drought in Australia and Indonesia. Warm water layer off Peru deepens so thermocline deeper. Upwelling stops and productivity falls dramatically. Large changes in global atmospheric circulation in turn force weather changes right around the world. Warm water deepens to east thermocline deepens Thermocline Cold Water

Global Effects of El Niño High thunderstorm clouds can disrupt the jet stream and cause extreme changes in weather worldwide: East: severe drought in Indonesia, Australia, India, Africa - leading to forest fires, crop failure, famine. West: heavy rainfall, severe storms and flooding in Peru, Central America, southern USA. The 1982-83 El Niño was a severe event that caught scientists by surprise.

Measuring El Niño Development of an El Niño can be followed, e.g. using altimeter (height) data from the joint US/French TOPEX/Poseidon satellite. The satellite records sea surface height relative to normal ocean conditions. Discovered the link with atmospheric component and important in helping to predict the next big El Niño, which occurred in 1997-98. Sea surface height is the most modern and powerful tool for taking the “pulse” of the oceans.

1997-1998 El Niño 23 October 1997 – El Niño’s warm water has spread all along N. America to Alaska. In the west, sea level drops 18cm below normal (purple) as sea temp becomes colder. The surface area of warm water mass is ~1.5 times size of US. 1 December 1997 – an entire winter of El Niño persists. 14 March 1999 – the warm pool has vastly reduced. Sea surface height along central equatorial Pacific near normal (green). Remnants of warm water still to N and S of equator. A typical mature El Niño condition – transition back to normal. 11 July 1998 – shows decaying remnants of the El Niño warm pool north of equator. Purple area is a pulse of cold water moving across the equator - associated with El Niño’s little sister – La Niña (when cold water dominates the tropical Pacific). At the height of the El Niño episode with a huge mass of warm water pooling along Peru and into the eastern equatorial Pacific. This was also one of the biggest El Niños to have occurred this century. Images from JPL/NASA

La Niña La Niña (“the girl child”), sometimes follows El Niño, as in 1999. Characteristics: Abnormal cooling in the eastern Pacific. Easterly trade winds stronger than usual (blow east to west). Drives more warm water westward. More than normal deep, cold water rises to surface. Produces a “cold tongue” eastwards along the equatorial Pacific.

El Niños Through History Southern Oscillation Indices - biggest El Niños were recorded in 1982-83 and 1997-98, and happening more frequently?

El Niños - the Normal Trend? El Ninos and La Ninas are natural events - have been occurring for thousands of years. But, could human activity be influencing El Niño events? Is this associated with global warming? Some climate models project El Niño-type weather will be the norm if global warming continues. Or is it just a random clustering of natural events?

TOPEX and Jason-1 JPL Jason-1 and TOPEX calibration, with Jason-1 approx. one minute (~370 km) behind its sister satellite Jason-1 is a follow-on to the highly successful TOPEX/Poseidon mission that helped predict the 1997-78 El Niño and improve understanding of ocean circulation and its effects on global climate. For the latest TOPEX/Poseidon images and El Niño-La Niña updates, check: http://topex-www.jpl.nasa.gov/science/jason1-quick-look/ http://topex-www.jpl.nasa.gov/science/el-nino.html http://www.pmel.noaa.gov/tao/elnino/1997.html#press