CoAuthors

Teresa Chereskin (Scripps Institution of Oceanography UCSD, United States); Sarah Gille (Scripps Institution of Oceanography UCSD, United States)

Recent 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. Results from a high-resolution model suggest that the length scale of the transition from IGWs to geostrophic flow varies regionally and seasonally. Shipboard Acoustic Doppler Current Profiler (ADCP) lines can be useful for evaluating this transition but have been geographically limited. A concentrated effort to recover previously unprocessed ADCP transits from the eastern Pacific has expanded the volume and geographic range of available data records. From these expanded ADCP records, we compute one-dimensional wavenumber spectra and decompose them into rotational and divergent components (roughly proportional to vortex and wave components). We compare results with high-resolution model output from the MITgcm and with high-resolution along-track nadir altimetry. Our analysis focuses primarily on the eastern tropical Pacific upper thermocline. Over similar latitudes in opposing hemispheres, the model predicts that the transition from predominantly rotational to divergent spectra occurs at longer length scales in the southeast tropical Pacific than in the northeast. The ADCP observations cannot confirm this pattern and differ from the model in a number of details. Generally, transition scales in the observations show depth dependency and are shorter nearer the surface. The transition scales, when averaged in the upper ocean and between different sonars, are longer in the southeast than in the northeast tropical Pacific. These scales, ranging from 40 to 200 km, and the inferred dynamics contrast to the ~70 km transition scales at midlatitudes that we studied previously in the southern California Current.