1 Kelvin Waves El Nio October 22 2 Oceans response to changing winds Oceans response to changing winds external (or surface)waves external (or surface)waves Sea surface pycnocline CpCp Barotropic Wave 3 CpCp Sea Surface pycnocline Baroclinic Wave 4 Coastal Kelvin waves get trapped against horizontal boundary Coastal Kelvin waves get trapped against horizontal boundary Side View Lo Hi pgf Cf pgf is balanced by Cf Hi Lo divergence convergence p increasing p decreasing C v=0 Top View 5 Wave Phase Speed: for baroclinic waves ~0.5 to 3 m/s for barotropc (external) waves L: measure of how far a water parcel can travel before it is affected by Coriolis: L: measure of how far a water parcel can travel before it is affected by Coriolis: f is smallest near the equator and largest near the poles, L increases toward the equator, coastal waves are not trapped at the equator 6 Equatorial Kelvin Waves Equator acts like a coast, with water on either side Equator acts like a coast, with water on either side Lo Hi pgf C DivergenceConvergence Equator Equatorial Kelvin Waves move only to the East L e ~250 km, C~3 m/s 7 Figure in Stewart. Left: Horizontal currents associated with equatorially trapped waves generated by a bell-shaped displacement of the thermocline. Right: Displacement of the thermocline due to the waves. The figures show that after 20 days, the initial disturbance has separated into an westward propagating Rossby wave (left) and an eastward propagating Kelvin wave (right). From Philander et al. (1984). 8 Figure 14.4 in Stewart. Cross section of the Equatorial Undercurrent in the Pacific calculated from Modular Ocean Model with assimilated surface data (See 14.5). The section an average from 160E to 170E from January 1965 to December Stippled areas are westward flowing. From Nevin S. Fuckar. 9 El Nio (x) 80 m m Eq. under current pycnocline surface z=0 180 Normal state Trade Winds pile up water in west 10 El Nio (x) 200 m Eq. under current pycnocline surface z=0 180 El Nino state - Winds relax or even reverse in western and central Pacific pycnocline flattens out, undercurrent stops 11 equator Change in Trade Winds generate Kelvin wave that propagates eastward, depressing the pycnocline as it goes 12 pyc. depressed upwelling shuts off pyc. uplifted In 2-3 months Kelvin Wave hits the coast of South America, part of it reflects as a RossbyWave, part moves poleward as a coastal Kelvin wave 13 Rossby Wave California Current CKW Humbolt Current Coastal Kelvin Waves (CKW) go all the way to Canada and Chile, altering Eastern Boundary Currents and shutting down upwelling Also influence ACC 14 15 16 17 Figure in Stewart. Tropical Atmosphere Ocean tao array of moored buoys operated by the NOAA Pacific Marine Environmental Laboratory with help from Japan, Korea, Taiwan, and France. Figure from NOAA Pacific Marine Environmental Laboratory. 18 19 20 21 TOPEX Animation Animation of SSH form the TOPEX Altimeter Animation of SSH form the TOPEX Altimeterhttp://topex-www.jpl.nasa.gov/gallery/videos.html 22 Figure 14.6 in Stewart. Correlation coefficient of annual-mean sea-level pressure with pressure at Darwin. The Southern Oscillation Index is sea-level pressure at Tahiti minus sea-level pressure at Darwin. From Trenberth and Shea (1987). Southern Oscillation 23 Figure 14.7 in Stewart. Normalized Southern Oscillation Index from 1951 to The normalized index is sea-level pressure anomaly at Tahiti divided by its standard deviation minus sea-level pressure anomaly at Darwin divided by its standard deviation. Then the difference is divided by the standard deviation of the difference. The means are calculated from 1951 to Monthly values of the index have been smoothed with a 5-month running mean. Strong El Nio events occurred in 195758, 196566, 197273, 198283, 199798. Data from NOAA. 24 Figure 14.8 in Stewart. Anomalies of sea-surface temperature (in C) during a typical El Nio obtained by averaging data from El Nios between 1950 and Months are after the onset of the event. From Rasmusson and Carpenter (1982). 25 Figure in Stewart. Sketch of regions receiving enhanced rain (dashed lines) or drought (solid lines) during an El Nio event. (0) indicates that rain changed during the year in which El Nio began, (+) indicates that rain changed during the year after El Nio began. From Ropelewski and Halpert (1987). 26 Figure in Stewart. Changing patterns of convection in the equatorial Pacific during an El Nio, set up a pattern of pressure anomalies in the atmosphere (solid lines) which influence the extratropical atmosphere. From Rasmusson and Wallace (1983). 27 Figure in Stewart. Correlation of yearly averaged rainfall averaged over all Texas each year plotted as a function of the Southern Oscillation Index averaged for the year. From Stewart (1994). 28