Observations of the transition scale from balanced to unbalanced motions in the eastern tropical Pacific from historical ship data

A number of studies have shown that inertia-gravity waves (IGWs) contribute significantly to the oceanic variability at spatial scales smaller than 20-100 km, while geostrophic flows govern variability at larger scales. Understanding where and when this transition in dynamics typically occurs is important to make the best use of SWOT. A recent analysis of a high-resolution model suggests a regional and seasonal variation in the length scale of the transition from flows dominantly in geostrophic balance to flows dominated by IGWs. Over similar latitudes in the eastern tropical Pacific, the model predicts that this transition occurs at longer length scales south of the equator than north of the equator. Because of the limited availability of repeated shipboard Acoustic Doppler Current Profiler (ADCP) lines, only a few previous studies were able to estimate the transition scale from in-situ observations. Independent information on the variability of the scale, particularly its seasonality, is thus also geographically limited and subject to significant uncertainty. A concentrated effort to recover previously unprocessed ADCP transits from the eastern Pacific has expanded the volume and geographic and time range of available data records. From these expanded ADCP records, we compute one-dimensional wavenumber spectra and decompose them into rotational and divergent parts, and subsequently into its vortex and wave components. Results are compared with the high-resolution model output and with high-resolution along-track nadir altimetry. The observations, in general, support the regional and seasonal patterns seen in the model simulation, but differ from the model in a number of details. Specifically, observed transition scales are typically shorter than in the model, with a smaller amplitude seasonality. ADCP spectra also demonstrate relatively less divergent (wave) energy than the model. At longer scales, the ADCP spectra are generally more energetic than model spectra. The possible mechanisms driving these results are evaluated and discussed. Interannual variability in the transition scale associated with El Niño and La Niña events is also evaluated and discussed.