The 1997-1998 oceanic El Nino signal along the southeast and northeast Pacific boundaries - an altimetric view

Changes in the sea surface heights (SSH) and geostrophic currents along the eastern boundaries of the Pacific (North, Central and South America) are examined during the 1997-1998 El Nino using altimeter data and proxy winds. These show that 'symmetric' SSH signals left the equator and propagated into both Hemispheres in two episodes, with primary periods of high equatorial SSH during May-July and October-December 1997. These are the 'distant signals' from the mid-latitude perspective. As the signals spread poleward in each Hemisphere, their loss of symmetry demonstrates the degree to which they were altered by topographic features, local winds, and/or local currents. The first four EOFs are calculated for 2-D SSH fields in 10degrees wide strips along the eastern margins (60degreesN-60degreesS) and extending out along the equator from the coast to 110degreesW. These account for approximately 40% of the overall variability and represent the main features of the seasonal cycles and El Nino interannual variability. Snapshots of the 2-D SSH fields depict the structure of the El Nino signal at different phases of its evolution. (C) 2002 Elsevier Science Ltd. All rights reserved.