D6.3: Report describing the database of synoptic mixing events in the tropical Atlantic
Creators: name: Dengler, Marcus
name: Fischer, Tim
name: Stockmayer, Vera
name: Melzer, Hannah
Publisher: name: Zenodo
Year: 2024

description: About this document

The report describes a database of synoptic events of enhanced upper-ocean turbulence and co-located surface momentum and heat fluxes as well as oceanic background parameters captured in observational data sets from the tropical Atlantic observing system. 

Work package in charge

WP6

Executive summary

Sea surface temperature (SST) is a key parameter for air-sea interactions in the tropical oceans. A controlling process for tropical SST is turbulent mixing in the upper ocean, which transports heat from the mixed layer to the deeper ocean. In Task 6.4, an extensive observational database from the tropical Atlantic was used to compile average synoptic mixing events and the associated heat and momentum fluxes as well as oceanic background parameters. The focus is on the vertical distribution of turbulence during three separable physical processes that are known to enhance upper-ocean mixing and thus play a key role in controlling tropical SST



near-inertial waves

equatorial mixing driven by vertical shear of equatorial currents

Tropical Instability Waves.


The provided data can be used to evaluate the vertical mixing schemes of storm-resolving Earth system models and ocean general circulation models. The three data sets are disseminated at World Data Center PANGAEA (doi: 10.1594/PANGAEA.964320).

Conclusion & Results

Three benchmark data sets of the vertical distribution of the viscous dissipation rate of turbulent kinetic energy are provided to evaluate the vertical mixing schemes of Earth system models and general ocean circulation models. The data sets describe upper-ocean mixing during near-inertial wave events in the eastern tropical North Atlantic, seasonal-average mixing in the equatorial Atlantic and mixing during the presence of Tropical Instability Waves in the equatorial Atlantic.



In the eastern tropical North Atlantic, surface-intensified near-inertial waves (NIW) are more widespread than previously thought, particularly between 8°N and 20°N. In the near-surface velocity data from 22 about one-month-long research cruises to this region, 23 NIW events were detected. The near-surface velocity associated with NIWs varied between 0.1 and 0.5ms−1. Their average frequency was 1.1f, where f is the Coriolis frequency. While NIWs are present in the mixed layer, turbulent mixing is enhanced by about an order of magnitude in the upper 30 m of the stratified water column below the mixed layer and average mixed layer heat loss is 120W m−2. Considering the probability to encounter NIWs in this region, NIWs account for a mixed layer heat loss of about 35W m−2.They therefore contribute significantly to the cooling of the SST. The data set is disseminated at doi: 10.1594/PANGAEA.964317.

The vertical distribution of seasonal-average upper-ocean equatorial mixing was determined from 151 microstructure profiles collected during 6 research cruises within 1.5° of the equator at 23°W. The cruises were completed in different years during different months encompassing the seasonal cycle. The average dissipation rate of turbulent kinetic energy at 23°W is enhanced between a factor of 5  and 50 in the upper 60m of the stratified water column below the mixed layer. Deep cycle turbulence is pronounced in the upper 40m of the water column and night time dissipation rates exceed day time dissipation rates by about a factor of 3. It is found that average upper-ocean dissipation rates are best parameterized by the product of wind stress magnitude and vertically averaged shear between the mixed layer and the core of the Equatorial Undercurrent. The data set is disseminated at doi: 10.1594/PANGAEA.964376.

Velocity, stratification and turbulence times series from a mooring at the equator, 23°W (2014-2018) were used to determine a composite Tropical Instability Wave (TIW) from 12 elevated individual events. On average, the dissipation rate of turbulent kinetic energy is enhanced within TIWs. 5 of the 12 events exhibit pronounced dissipation rate maxima during the southward flow phase that are due to reduced upper-ocean stratification and enhanced vertical shear of zonal current. The changes in stratification and shear are associated with the arrival of the TIW tail-edge front at the equator, i.e. the transition of cold water masses to warm water masses that are advected by the TIW. During the presence of TIWs, modulation of air-sea interaction is dominated by variability of the latent heat flux, which in turn is due to variations of relative humidity. The data set is dissimated at doi: 10.1594/PANGAEA.964316.