Diurnal Variations in Near-Surface Salinity Kyla Drushka 1, Sarah Gille 2, Janet Sprintall 2 1. Applied Physics Lab, Univ. of Washington 2. Scripps Inst. of Oceanography based on Bernie et al Night solar radiation mixed- layer depth depth Night Day hour (local time)

based on Bernie et al 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 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

Hourly data from a TAO mooring in the tropics (buoys at 1-m depth; meteorological sensors) 2 Dec 7 Dec 12 Dec 17 Dec mm/hr 0.4 mm/hr E,2N psu What drives the average diurnal salinity? Salinity (1 m depth) Precipitation Evaporation oCoC Temperature (1 m depth)

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 – :0012:0018:00 psu 156E,2N Nighttime salinity increase (max S at 8am) Daytime freshening (min S at 3pm) 00: ±0.006 psu 1-m salinity

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

Total ∆S: ±0.006 psu hour (local time) mm/hr – – –4 0 – – psu oCoC mm/hr m precipitation –0.004 psu evaporation psu entrainment psu Estimated contributions: 1-m salinity 1-m temperature precipitation evaporation mixed-layer depth deeper 156E,2N

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

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 diurnal rain rate, mm/hr psu/m x N 15S diurnal salinity, psu Observed diurnal salinity

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

Generalized Ocean Turbulence Model (GOTM) (Burchard & Bolding 2001, 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)

Validation: GOTM vs TAO Forcing: Precipitation mm/hr oCoC 30 Model Observation (TAO) Model Observation (TAO) psu 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)

Lens formation under rain with GOTM Idealised rain (Gaussian pulse) + wind (sinusoid) 0 15 mm/hr 0 6 m/s 0 20 z, m 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 Local time

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 z, m :00 06:00 12:00 18:00 m/s Stronger rain (10 mm/hr) 00:00 06:00 12:00 18: psu S' 1m S', psu rain rate wind speed

Varying the strength of wind 0 15 mm/hr z, m :00 06:00 12:00 18:00 m/s 00:00 06:00 12:00 18: psu S' 1m 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

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 Magnitude of salinity anomaly, psu

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

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?

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