Outline  TOPEX/Poseidon –Measurement approach –Data set examples  Jason-1 –Near-term launch planned  Jason-2 –Wide-swath ocean topography  Argo –A global array of profiling floats Ocean Topography from Space Modified from M. D. King’s Lecture

The most effective measurement of ocean currents from space is ocean topography, the height of the sea surface above a surface of uniform gravity, the geoid Ocean Topography: Circulation and Heat Storage in the Ocean geoid sea surface ocean topography

q Ocean topography is the height of sea surface relative to the geoid, a surface of uniform gravity

Ocean Topography  Determines the speed and direction of ocean surface currents –Water flows around the highs and lows of ocean topography, just as wind blows around the high and low pressure centers in the atmosphere  The ocean volume expands when heated, causing higher surface elevation, and contracts when cooled, causing lower surface elevation –Therefore, ocean topography also reveals the heat storage of the water column  Ocean topography is the only observable from space that reveals the temperature and currents of the ocean at depths –This is a unique link between sea surface and deep ocean characteristics warm water cold water air Water flows out of the page Water flows into the page

Ocean Topography from Satellite Altimetry  A radar altimeter measures the altitude of a spacecraft above the sea surface  Precision orbit determination measures the altitude of the spacecraft above a reference surface of the Earth  The height of sea surface relative to the same reference surface is the difference of the two altitudes q Ocean topography is the height of sea surface relative to the geoid, a surface of uniform gravity

TOPEX/Poseidon Launched August 10, 1992

TOPEX/Poseidon Measurement System

Required Measurement Accuracy  Ocean topography must be measured with high accuracy –A mere 1 cm error over a few km or longer gives an error in water transport of 5 megatons/sec »25 times the discharge rate of the Amazon »15% of the transport through the Florida Straits  The amount of heat carried by 5 megatons/sec water is about 200 trillion watts –~ 20% of the northward oceanic transport of heat in the North Atlantic Ocean responsible for the relatively mild winters of northern Europe  It is therefore critical to make ocean topography measurement within a few cm –Compared to the altitude of the spacecraft from Earth, this accuracy requirement is one in 200 million

Challenges of Ocean Topography from Satellite Altimetry  The largest variability of sea surface height is due to the ocean tides with a global rms amplitude of about 32 cm –The signals of tides must be removed before using the data for studying ocean circulation  Due to the relatively long repeat periods of a satellite (often longer than a few days), the short period tides are often aliased to much longer time periods –A sunsyncronous orbit would alias solar tides into a period of infinity and make them become indistinguishable from the ocean topography of the mean circulation –TOPEX/Poseidon is in a 66° inclination orbit with an altitude of 1336 km to avoid aliasing tides into the ocean topography signal  The best ocean tide models are accurate derived from TOPEX/Poseidon data and are accurate with an rms error of 2-3 cm –This knowledge allows the removal of this signal from ocean altimeter measurements

180°W 120°W60°W0°60°E120°E180°E Location Map of Ocean Currents

Mean Sea Surface Topography of the Ocean

Standard Deviation of Sea Surface Height

TOPEX/Poseidon Performance and Results  The TOPEX/Poseidon mission has been measuring the height of the sea surface with 4 cm accuracy, more than 10 times better than previous missions q After averaging over space and time scales relevant to climate, the accuracy approaches 2 cm These remarkable achievements represent a decade-long effort involving oceanographers, geodesists, electrical and aerospace engineers

Calibration of Radar Altimeters  At the instant the satellite passes overhead, geocentric sea surface height is observed independently by the altimeter and in situ measurement systems q Assuming that the in situ systems are properly calibrated, the difference represents the ‘altimeter bias’

Generally from the atmosphere to the oceans Either direction (from the warmer to the cooler medium). Either direction (with some chemicals being transferred predominantly in one direction of the other). Non-El Niño equatorial conditions Schematic Vertical Slice Along the Equatorial Pacific

Generally from the atmosphere to the oceans Either direction (from the warmer to the cooler medium). Either direction (with some chemicals being transferred predominantly in one direction of the other). El Niño conditions Schematic Vertical Slice Along the Equatorial Pacific

Sea Surface Height Anomaly during El Niño  Early November 1997 was marked by a large increase in the areal extent of above- average sea level,especially off the west coast of North America (shown as red and white) –At this time, sea level “peaked” at about 35 cm above average in the eastern Pacific, near the Galapagos Islands  Another such sea level peak (about 25 cm above average) occurred at this tide gauge in July '97

El Niño / La Niña  The early detection of the El Niño was a great success of the mission  The loss of lives and property from the strongest El Niño on record was kept to a minimum owing to the early warnings  The data have been routinely used by NOAA to improve the forecast of El Niño and other climatic events  TOPEX/Poseidon ocean topography of the Pacific Ocean during El Niño and La Niña –Red and orange represent highs –Purple and blue represent lows

Sea Surface Height, Sea Surface Temperature, and Wind Anomalies Temperature Scale (°C) -5+5  TOPEX/Poseidon –Sea surface height anomalies  AVHRR –SST anomalies  SeaWinds –Wind anomalies

El Niño/La Niña Ocean Topography, Winds, and Sea Surface Temperature

Argo: A Global Array of Profiling Floats  Argo is an international program to deploy 3,000 profiling floats to collect observations of the temperature and salinity structure of the upper ocean –globally and in real-time  Subsurface observations from Argo—combined with surface temperature, topography, and winds observed by existing satellites—will enable advances both in research programs

Argo Float Operation  Argo floats can be deployed by C- 130 aircraft or ships of opportunity  Air deployments include secure packaging and parachute  Upon impact, the float is released to sink  Argo floats are programmed to sink to 2,000 meters  They drift at depth for 10 days  Then they rise to the surface measuring temperature and salinity  Data and position are transmitted ashore via satellite

Argo Float Operation  The mixed layer is defined as the maximum depth at which the water is no colder than 1°C of the surface temperature  In winter, the ‘deep blue’ indicates deep vertical mixing extending to 1,000 meters or more –This corresponds to locations where surface water has been cooled sufficiently to sink and mix, forming ‘Labrador Sea Water’  In summer, the ‘red’ indicates a warm, shallow mixed layer of ~10 m depth