, North Pacific Tropical Intermediate Water at 26.8 , and Antarctic Intermediate Water at 27.2 .
Based on observations made along 137°E in the western equatorial Pacific between January 1967 and July 1991, the interannual variability of intermediate water properties is documented. The water masses examined were North Pacific Intermediate Water (NPIW), at 26.5 , North Pacific Tropical Intermediate Water at 26.8 , and Antarctic Intermediate Water at 27.2 .
At 26.5 , a sharp water mass boundary between waters of North and South Pacific origin oscillates between the equator and 30°N. These oscillations show a clear relationship to the ENSO cycle with northward movement of the boundary during the El Niño part of the cycle. A strong seasonal signal in the movement of this water mass boundary is also present, especially during the La Niña phase of the ENSO cycle. This suggests significant changes in the configuration of the equatorial water masses during El Niños, and an increased transport of South Pacific water into the Indonesian throughflow. The distribution of NPIW varies on time scales of several years. An increase in the amount of low salinity NPIW present at 137°E is seen between 1976 and 1983.
North Pacific Tropical Intermediate Water is largely absent on the 137°E line. It does show up clearly for a couple of years from 1984 until 1986, and can also be seen intermittently in the years 1971 through 1975. There is a long-term freshening of the intermediate waters near the equator during the entire period of observation.
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