Transition from balanced to unbalanced dynamics in the eastern tropical Pacific from in situ, numerical simulation and altimetry data

Saulo Soares (SIO UCSD, United States)

CoAuthors

Sarah Gille (SIO UCSD, United States); Teresa Chereskin (SIO UCSD, United States); Eric Firing (Dept. of Oceanography, SOEST, UH, United States); Julia Hummon (Dept. of Oceanography, SOEST, UH, United States); Cesar Rocha (WHOI, United States)

Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)

Session: Science III: Mesoscale and sub-mesoscale oceanography

Presentation type: Type Forum

Inertia-gravity waves (IGWs) or other ageostrophic/unbalanced phenomena contribute significantly to ocean variability at spatial scales smaller than 20—100 km, often overwhelming the variability due to balanced, geostrophic motions. Understanding the regional and seasonal dependencies of the scale at which variability transitions from dominantly balanced to significantly unbalanced is important for interpreting high-resolution altimetric data.
In a recent analysis of a high-resolution model, this transition occurs at longer length scales in the southeast tropical Pacific than at similar latitudes in the northeast tropical Pacific. In the northern hemisphere, the transition scale experiences a large seasonal difference, while in the southern, the seasonality is very weak.
We use recently recovered, previously unprocessed, shipboard Acoustic Doppler Current Profiler (ADCP) transits from the eastern Pacific to investigate the transition scale and the mechanisms responsible for its regional and seasonal variations. To that end, we compute one-dimensional wavenumber spectra and decompose them into rotational and divergent components, and subsequently into their vortex and wave components. Results are then compared with the high-resolution model output and with high-resolution along-track nadir altimetry.
While the ADCP observations, in general, support the regional and seasonal patterns seen in the model simulation, details differ between the two, particularly in the physics behind the patterns. For instance, the observed seasonal amplitude in the northeast Pacific is smaller than in the model and driven entirely by balanced dynamics. In fact, we do not see robust evidence of seasonality in the IGW component. Furthermore, the inferred IGW wavenumber spectrum has characteristics of being dominated by waves with near-inertial frequencies, while internal tides strongly influence the model wavenumber spectrum.
Sea surface height (SSH) measurements from altimeters only provide estimates of geostrophically balanced motions. The estimates’ usefulness and robustness depend on the degree to which the balanced component dominates. From these in situ-based transition scales, we expect robust and useful altimeter-derived currents to be limited to scales of around 200 km in the eastern tropical Pacific. The implications of the inferred frequency content of the IGW field for SSH spectra are discussed.
 
Saulo Soares
SIO UCSD
United States
saulo@ucsd.edu