The field data collected on monthly polygonal hydrographic surveys off the West Coast of Mexico in 1996--98 are analyzed. Monthly measured temperature and salinity profiles have been computed from the experimental data. The profiles for the 1997 summer-fall and 1998 winter periods differ essentially from profiles computed for the same months in the year 1996. The inferred cause of the difference is the ingress of large volumes of relatively warm and fresh water whose T, S-characteristics identify these water masses as the Pacific Tropical Surface Water. By January 1998, those water masses had filled the entire upper 90-m layer. The heat storage of a 150-m column increased to 15.3 GJ/m2, as compared with its January 1996 value of 10.1 GJ/m2. The process which caused sea surface temperature anomalies in region B of the Pacific, occurred one month before the rise in temperature and drop down in salinity were observed on the site. ¿ 2000 American Geophysical Union