Enhanced Ocean Boundary Layer Observations on NDBC TAO Moorings
PIs: Karen Grissom (NOAA/NWS/NDBC), William Kessler (NOAA/PMEL), Meghan Cronin (NOAA/PMEL), and Jessica Masich (NRC, NOAA/PMEL)
NOAA’s Pacific Marine Environmental Laboratory and National Data Buoy Center installed Acoustic Doppler Current Profilers on Tropical Atmospheric Ocean moorings at nine sites across the tropical Pacific. These sensors measured velocity in the near-surface ocean between seven meters and 65 meters depth, typically a the ‘blind spot’ in the water column for observations of ocean currents.
These pilot stations spanned the eastern and western equatorial Pacific and the Inter Tropical and South Pacific Convergence Zones. Combining the velocity measurements with concurrent meteorological and shallow temperature and salinity observations, this project sought to characterize the near-surface velocity field and its role in mediating the atmospheric and oceanic boundary layers. In particular, the project sought to explore: the structure and physics of the surface-forced diurnal cycle across all of the sites; the role of barrier layers at the Eastern Edge of the Warm Pool; frontal dynamics and their effects on the upper ocean Ekman response; and the structure of Ekman divergence from the equator.
This project was funded by the NOAA’s Global Ocean Monitoring and Observing Program (GOMO) as one of its six technology development projects in support NOAA’s contribution to TPOS 2020.
The efficient deployment and success of the real-time data return system was a marked success by Karen Grissom and the team at NDBC. Velocity data was returned in real time at most of the sites, and the original five-site scope of the project was expanded to nine separate sites via efficient use of instrumentation and shiptime. Analysis of the velocity data was underway as early as 2018, when most of the instruments were still in the water.
This analysis has revealed a wide geographic scope for systematic deep diurnal jets, which directly connect wind forcing to the interior ocean below the mixed layer, and appear to deepen with stronger wind forcing (Masich et al., 2021). Robust, high-resolution velocity records at each site provide ample data for exploring the effect of fronts on the Ekman response and for describing the near-surface Ekman divergence off the equator, both of which are underway. We have also identified opportunities to explore barrier layer dynamics, the near-surface response to Westerly Wind Bursts, and the reversing jet phenomenon via this and possible future deployments of this type of instrumentation.
1. This type of deployment yielded useable real-time data for the majority of the deployment duration at nearly all of the sites across the tropical Pacific, and could thus be used to resolve a number of important near-surface phenomena of interest to TPOS stakeholders in real time.
2. To resolve diurnal and subdiurnal processes, a less conservative sampling scheme must be employed than the current 2 to 6 mins of recording per hour.
3. To estimate the influence of wind on tropical Pacific dynamics via systematic deep diurnal jets, deployments of this type should focus on the eastern equatorial Pacific.
Masich, J., W.S. Kessler, M.F. Cronin, K. R. Grissom. Diurnal cycles of near-surface currents across the tropical Pacific. JGR-Oceans. 2021.
Velocity data can be found at: https://www.pmel.noaa.gov/ocs/ndbc-tpos-mooring-enhancement-pilot-project
All other data (temperature, salinity, meteorological) can be found at: https://www.pmel.noaa.gov/tao/drupal/disdel/
Example data from 2S, 140W from January and February 2017: a) wind velocities; b) shortwave radiation; c) temperature; d) zonal current; and e) meridional current. From Masich et al., 2021.
Diurnal anomalies in temperature and ocean current at 0, 155W. Daytime heating (red contours) coincides with daytime currents (grey arrows; shown with respect to the 6:30 am profile) that strengthen and turn towards the direction of the wind (purple arrows). These diurnal anomalies propagate downwards into the evening.