Tomographic and hydrographic inversions of the sound speed field of the northeast Pacific Ocean

Bathythermometric observations from standard National Oceanographic Data Center data sets are used to show evidence of Rossby waves in the northeast Pacific Ocean following several El Niño-Southern Oscillation events between 1968 and 1991. Observations of a westward propagating warm anomaly following the 1982–1983 El Niño are consistent with evidence from ocean models and satellite observations that coastally trapped Kelvin waves of tropical origin excite Rossby waves in the northeast Pacific. Observed Rossby wave phase speeds are about 1.6 cm s−1 higher than those predicted by linear shallow-water theory. At midlatitudes, observed waves are more than 1.5 times faster than theory. Phase speeds of Rossby waves in a ⅛° resolution nonlinear primitive equation model exceed standard linear theory, but only by about 0.1 cm s−1, despite the fact that the model should replicate mechanisms that have recently been proposed to explain the observed anomalously fast phase speeds.; Statistical methods of sequential estimation are used to combine acoustic and hydrographic observations of the sound speed field of the Northeast Pacific Ocean in 1987. Results are averaged within a megameter-scale area and show that two layers of the upper ocean are cooler at a two standard-deviation level than a time-varying reference ocean constructed from the Levitus [1982] climatology. Hydrographic observations alone do a surprisingly good job of estimating the three-dimensional sound speed field, and account for between 85% and 95% of the a priori error variance. Conversely, tomographic inversions using acoustic travel time data are less powerful than expected at this task, accounting for between 56% and 94% of the a priori error variance. Results from inversions using hydrographic data only are consistent with those using acoustic data only and with inversions using both types of data. This finding is strong evidence that the acoustic data are being correctly interpreted.