Global variability in submesoscale density variability
from historical thermosalinograph data
Kyla Drushka
Applied Physics Laboratory, University of Washington.
Liège Colloquium on Submesoscale Processes – 23 -27 May 2016 – Liège, Belgium
1. Thermosalinograph ( TSG) data from ships
Sources: LEGOS, GOSUD, SAMOS, pangaea.de, IMOS, NOAA
3
6
75 ships, 10 transects, 10 total good datapoints (T and S)
2. Submesoscale density variability
Fig. 3: Binned σρ (computed over 100-km TSG segments)
Fig. 1: Number of 2-km-spacing TSG datapoints per 3°x3° box
σρ, kg/m3
Fig 5
Fig 6
Strongest in regions with:
– strong currents or background gradients
– river outflow, ice melt
Same spatial patterns if computed over segments 10–100 km long
Fig 2
Where does temperature vs. salinity drive density variations?
Define the ”Density variability ratio”,
Fig. 2: TSG data from an example ship transect
Fig. 4: Density variability ratio for regions with strong variability
Temperature
Salinity
AUSTRALIA
TEMPERATURE
DOMINATES
DENSITY
VARIABILITY
Regions with strong
currents and mean
SST gradients
Density
SALINITY
DOMINATES
DENSITY
VARIABILITY
Standard deviation calculated
over each 100 km segment: σρ
(Averaged over 3°x3° boxes to produce Fig 3)
4. Spectral slopes
3. Regional–seasonal patterns
Fig. 7: Slope of surface density wavenumber spectrum
(fit over 20-100 km wavelength)
Fig. 5: South of Greenland: ice melt
Winter
Tropics & highlatitudes (river outflow & ice melt)
Mean sea surface salinity
Summer
–5/3
GREENLAND
slope
SSS, psu
Summer ice-melt
Submesoscale density variability
increases submesoscale
Winter
Summer density variability
Fig. 6: Eastern tropical Atlantic: river outflows from West Africa
Mean sea surface salinity
April-May-June-July
Aug-Sept-Oct-Nov
Expect a seasonal cycle in submesoscale energy
(Callies et al., 2015: k –3 in summer, k –2 in winter )
South of Greenland (i.e., Fig. 5), we
observe a shallower slope in summer
than in winter – due to energetic
small-scale surface variability?
power spectral density
σρ, kg/m3
Expect a slope of –5/3 from surface QG theory (Callies & Ferrari, 2013)
Globally, we observe:
– shallower in the subtropics
– steeper in the tropics
5/3
inverse wavenumber, km-1
WEST AFRICA
SSS, psu
References & Acknowledgements
River outflow increases
Submesoscale density variability
submesoscale density
April-May-June-July
Aug-Sept-Oct-Nov
variability
Callies & Ferrari (2013). Interpreting energy and tracer spectra of upper-ocean turbulence in the submesoscale range (1–200 km). J. Phys. Oceanogr.,
43(11), 2456–2474.
Callies, J., Ferrari, R., Klymak, J. M., & Gula, J. (2015). Seasonality in submesoscale turbulence. Nature communications.
Rudnick and Ferrari (1999). Compensation of horizontal temperature and salinity gradients in the ocean mixed layer. Science, 283(5401), 526–529.
Support from NASA is gratefully acknowledged (NNX14AQ54GW).
σρ, kg/m3
Data sources (thank you!): GOSUD project, http://www.gosud.org/. LEGOS Sea Surface Salinity Observation Service,
http://sss.sedoo.fr/. PANGAEA Data Publisher for Earth and Environmental Science, https://pangaea.de/. SAMOS (Shipboard
Automated Meterological and Oceanographic System), http://samos.coaps.fsu.edu/html/. Sophie Clayton. NOAA Ship of Opportunity Program, http://www.aoml.noaa.gov/phod/tsg/soop/index.php (QC by Clifford Hoang). Australian Integrated Marine
Observing System, https://imos.aodn.org.au/.