1. Diurnal Variations in Near-Surface Salinity Kyla Drushka 1, Sarah Gille 2, Janet Sprintall 2 kdrushka@apl.uw.edu 1. Applied Physics Lab, Univ. of Washington 2. Scripps Inst. of Oceanography based on Bernie et al. 2007 Night solar radiation mixed- layer depth 0 4 8 12 16 20 24 depth Night Day hour (local time)

2. based on Bernie et al. 2007 Mixed layer shoals rapidly at dawn Surface cooling; mixed layer deepens slowly Thin "diurnal warm layer" traps surface fluxes A B C Night solar radiation mixed- layer depth 0 4 8 12 16 20 24 depth Night Day A B C hour (local time) cool warm cool Motivation: The diurnal cycle in solar radiation modulates air-sea fluxes Motivation 2: Sorting out the day/night bias in satellite salinity

3. Hourly data from a TAO mooring in the tropics (buoys at 1-m depth; meteorological sensors) 2 Dec 7 Dec 12 Dec 17 Dec2010 33.5 34 0 20 10 mm/hr 0.4 mm/hr 0 34.5 156E,2N psu What drives the average diurnal salinity? Salinity (1 m depth) Precipitation Evaporation 28 30 oCoC Temperature (1 m depth)

4. Diurnal cycles from hourly TAO mooring data (high-pass filter, bin by hour of day; after Cronin & McPhaden 1999) hour (local time) *showing two cycles 6:00 12:00 18:00 00:00 –0.005 0 6:0012:0018:00 psu 156E,2N Nighttime salinity increase (max S at 8am) Daytime freshening (min S at 3pm) 00:00 +0.005 ±0.006 psu 1-m salinity

5. precipitation evaporation entrainment Salinity at 1-m (buoy) depth ≈ advection assume h p ~3m assume h E ~1m h = mixed- layer depth negligible

6. Total ∆S: ±0.006 psu hour (local time) 0 6 12 18 24 mm/hr –0.005 0.005 0 –0.1 0.1 0 4 –4 0 –0.2 0.2 0 –0.01 0.01 0 6 12 18 psu oCoC mm/hr m precipitation –0.004 psu evaporation +0.001 psu entrainment +0.003 psu Estimated contributions: 1-m salinity 1-m temperature precipitation evaporation mixed-layer depth deeper 156E,2N

7. –0.2 0.2 0 Diurnal salinity amplitude (psu) 0 0.01 60E 120E 180E 120W 60W0 60E 0 15N 15S 0 0.01 0.02 0 0.01 0.02 R = 0.89 slope = 0.6 ± 0.2 R = 0.73 slope = 0.06 ± 0.03 R = 0.91 slope = 0.3 ± 0.1 0 0.01 0.02 0 0.01 0.005 Observed ∆S, psu Precipitation contribution, psu Evaporation contribution, psu Entrainment contribution, psu Precipitation & entrainment consistently dominate diurnal salinity

8. Where diurnal salinity is expected to be strong: 60E 120E 180E 120W 60W 060E 0 30N 30S 0 30N 30S Diurnal rain rate amplitude (TRMM 3-hrly data) Climatological "stratification strength" ( ) from the MIMOC product, an Argo-based climatology 0 0.1 diurnal rain rate, mm/hr psu/m x 10 -3 0 2 0 15N 15S diurnal salinity, psu 0 0.01 Observed diurnal salinity

9. Next step: better estimate of global diurnal salinity First, need to understand: – how important are wind variations? – does the phasing of the forcing matter? … sensitivity tests with a 1-dimensional model

10. Generalized Ocean Turbulence Model (GOTM) (Burchard & Bolding 2001, www.gotm.net) 1-d model: – 2-parameter k-ε turbulence closure scheme – forced with hourly TAO observations (shortwave flux, wind, rain) – T and S profiles initialized once per day (at sunrise) – COARE bulk formula – surface wave-breaking (Burchard, 2001) and internal wave parameterizations (Large et al., 1994) – <5cm resolution within the top 5m (<30cm within the mixed layer) – 1-min time step – has been used for diurnal/surface layer studies (e.g., Jeffery et al., 2008; Pimental et al., 2008)

11. Validation: GOTM vs TAO Forcing: Precipitation 0 10 20 mm/hr 28 32 oCoC 30 Model Observation (TAO) Model Observation (TAO) 32 34 psu 33 14 Sept 21 Sept 1-m temperature 1-m salinity Fresh events captured (R 2 =0.77 over 8-month run) Diurnal warming well reproduced (R 2 =0.89 over 8-month run)

12. Lens formation under rain with GOTM Idealised rain (Gaussian pulse) + wind (sinusoid) 0 15 mm/hr 0 6 m/s 0 20 z, m -0.05 0.05 psu wind speed (4±1 m/s) 10 rain rate (peak 5mm/hr) 5 salinity anomaly relative to no-rain case 00:00 06:00 12:00 18:00 psu salinity anomaly at z=1 m 0 -0.1 -0.2 Local time

13. Varying the strength of rain Rain rate affects: – Strength of salinity anomaly – Lens thickness (h p ) Weaker rain (2 mm/hr) 0 15 mm/hr 0 10 0 20 z, m 0 15 0 10 0 00:00 06:00 12:00 18:00 m/s Stronger rain (10 mm/hr) 00:00 06:00 12:00 18:00 0 -0.2 psu S' 1m 0 -0.2 -0.05 0.05 S', psu rain rate wind speed

14. Varying the strength of wind 0 15 mm/hr 0 10 0 20 z, m 0 15 0 10 0 00:00 06:00 12:00 18:00 m/s 00:00 06:00 12:00 18:00 0 -0.2 psu S' 1m 0 -0.2 -0.05 0.05 S', psu rain rate wind speed Weaker wind (5 m/s) Stronger wind (10 m/s) Wind speed controls: – Strength of salinity anomaly – Duration of salinity anomaly

15. Strongest salinity anomalies when: – rain coincides with weak winds – surface mixed layer is thin (mid-day) 00:0012:0006:00 18:00 00:00 12:00 06:00 18:00 Hour of peak rain Hour of peak wind 0.05 0.10 0.15 Magnitude of salinity anomaly, psu

16. Summary TAO mooring data show a significant, weak diurnal salinity cycle driven by rain + entrainment (Drushka et al., JGR, 2014) A 1-d turbulence model shows that wind & rain strength significantly affects lens formation & salinity anomaly Observed diurnal ∆S Precipitation contribution Evaporation contribution Entrainment contribution wind weak rain strong weak strong

17. Open question: can cold rain cause instability? (And do models get this right?) – Rain is cold + fresh: competing buoyancy effects. – If temperature affect > salinity effect, instability. – Asher et al. (2014) measurements show this occurs at rain rates < 6 mm/hr. – But GOTM never shows instability from cold rain(COARE flux formula: rain has wet-bulb temperature). … is rain actually colder than COARE predicts?

18. References Asher, William E., Andrew T. Jessup, Ruth Branch, and Dan Clark. 2014. “Observations of Rain-Induced near-Surface Salinity Anomalies.” J. Geophys. Res.119 (8): 5483–5500. doi:10.1002/2014JC009954. Bernie, D., E. Guilyardi, G. Madec, J. Slingo, and S. Woolnough. 2007. "Impact of resolving the diurnal cycle in an ocean-atmosphere GCM. Part 1: A diurnally forced OGCM." Clim. Dyn., 29(6), 575–590, doi:10.1007/s00382-007- 0249-6. Burchard, Hans, and Karsten Bolding. 2001. “Comparative Analysis of Four Second-Moment Turbulence Closure Models for the Oceanic Mixed Layer.” Journal of Physical Oceanography 31 (8): 1943–68. doi:10.1175/1520- 0485(2001)031 2.0.CO;2. Cronin, M. F., and M. J. McPhaden.1999. "Diurnal cycle of rainfall and surface salinity in the western Pacific warm pool." Geophys. Res. Lett., 26(23), 3465–3468. Fairall, C. W., E. F. Bradley, D. P. Rogers, J. B. Edson, and G. S. Young. 1996. “Bulk Parameterization of Air-Sea Fluxes for Tropical Ocean-Global Atmosphere Coupled-Ocean Atmosphere Response Experiment.” Journal of Geophysical Research: Oceans 101 (C2): 3747–64. doi:10.1029/95JC03205. Jeffery, C. D., I. S. Robinson, D. K. Woolf, and C. J. Donlon. 2008. “The Response to Phase-Dependent Wind Stress and Cloud Fraction of the Diurnal Cycle of SST and Air–sea CO2 Exchange.” Ocean Modelling 23 (1–2): 33–48. doi:10.1016/j.ocemod.2008.03.003. Pimentel, S., K. Haines, and N. K. Nichols. 2008. “Modeling the Diurnal Variability of Sea Surface Temperatures.” Journal of Geophysical Research: Oceans 113 (C11): C11004. doi:10.1029/2007JC004607.