A view of the global ocean circulation shows currents swirling around the hills and valleys at the sea surface. In the Northern Hemisphere, currents flow around hills in a clockwise direction and in an anticlockwise direction (the opposite occurs in the Southern Hemisphere) around valleys. These currents form gyres on either side of the equator. Planetary waves are other large-scale phenomena that are less easy to see on an instantaneous map, but nonetheless they too have a global impact.
Changes in the extent of the Gulf Stream and the Kuroshio, as seen by Topex/Poseidon. The currents were ‘elongated’ in 1993 and 1999, and contracted in 1996 and 2001.
(Credits University of Washington)
The major ocean currents can raise sea surface height by up to a metre higher than the surrounding area. The deviation of the ocean surface elevation from the geoid is called ocean surface topography. This is used to calculate the speed and direction of ocean currents — provided that the geoid is understood independently with sufficient accuracy, which, since the CHAMP and GRACE gravimetry satellites were launched, is beginning to be the case. This understanding was enhanced further with the GOCE mission. After 4.5 years in orbit, it gathered enough data to map Earth’s gravity with unrivalled precision. The 5th gravity field model based, EGM_TIM_RL05, with a mean global accuracy of 2.4 cm in terms of geoid heights and 0.7 mGal for gravity anomalies at a spatial resolution of 100 km.
Even without this knowledge, studies of the large-scale variations have been undertaken since the beginning of altimetry. Among other things, year-to-year variations in the extent of the Gulf Stream and Kuroshio currents have been observed, that can be correlated with changes in upper ocean heat content. These have an impact on ocean-atmosphere heat exchanges, with implications for decadal climate variations.
Kelvin and Rossby waves
Though the major currents are very important, the ocean and climate are also influenced by phenomena that are more difficult to see. ‘Planetary’ waves cross the oceans along parallels and interact with general ocean circulation. These are either Rossby waves, which travel from east to west, or Kelvin waves which move in the opposite direction. They intensify currents such as the Gulf Stream or the Kuroshio. In addition, they may be reflected off the continents and return in the opposite direction, or follow coastlines. These waves and their reflections play a key role, in particular in the El Niño phenomenon.
The existence of Rossby waves had been predicted theoretically for over 50 years, but they could not be observed until the advent of high-precision altimetry satellites. Their small amplitude (a few centimetres), extent (an ocean basin) and velocity (a few kilometres per day, depending on the latitude – they take several years to cross the Pacific Ocean at the 30° latitude) made it nearly impossible to observe them using in situ measurements. Now that we know what we are looking for (they can be seen most clearly as a series of straight lines in longitude-time diagrams), we can also detect them in other kinds of satellite measurements. High-precision altimetry, however, remains the best way of detecting these waves.
Comparison of longitude-time diagrams from three different sensors (altimeter, water colour, surface temperature).
(Credits Southampton Oceanography Centre)
- Cipollini, P., P.G. Challenor, D. Cromwell, I.S. Robinson, G.D. Quartly, How satellites have improved our knowledge of planetary waves in the oceans, 15 years of progress in radar altimetry Symposium, Venice, Italy, 2006
- Fu, L.L. and D.B. Chelton, Large-scale ocean circulation, Satellite altimetry and Earth sciences, L.L. Fu and A. Cazenave Ed., Academic Press, 2001
- Brockmann, Jan Martin, Norbert Zehentner, Eduard Höck, Roland Pail, Ina Loth, Torsten Mayer‐Gürr, and Wolf‐Dieter Schuh. “EGM_TIM_RL05: An independent geoid with centimeter accuracy purely based on the GOCE mission.” Geophysical Research Letters 41, no. 22 (2014): 8089-8099.