meso to submesoscale!

Transcription

1 meso to submesoscale! horizontal wavenumber spectra! in Drake Passage Cesar B Rocha*! Teresa K Chereskin* Sarah T Gille* Dimitris Menemenlis+ * : SIO, UC San Diego +: JPL, NASA Snapshot of surface relative vorticity from 1/48o MITgcm run thanks to William R. Young visualization credit: Ryan Abernathey

2 hypotheses! what do we expect for the KE spectrum at km? isotropic QG turbulence: interior and surface! inertia-gravity waves! other motions interior QG: k -3 surface QG: k -5/3 real ocean spectrum likely due to combined processes Charney 1970; Blumen 1978; Garrett & Munk 1975; Callies & Ferrari 2013 Isotropic spectrum log10 x log10 space

3 hypotheses! what do we expect for the KE spectrum at km? isotropic QG turbulence: interior and surface! interior QG: k -3 inertia-gravity waves! other motions real ocean spectrum likely due to combined processes inertia-gravity waves surface QG: k -5/3 Charney 1970; Blumen 1978; Garrett & Munk 1975; Callies & Ferrari 2013 Isotropic spectrum log10 x log10 space

4 hypotheses!! what do we expect for the KE spectrum at km? real ocean spectrum likely due to combined processes Challenge: few repeat in-situ observations are available to produce statistically meaningful spectra! In the Gulf Stream, Wang et al. and Callies & Ferrari find interior QG at scales > 20 km. CF also argue for a transition into internal waves at smaller scales Wang et al. 2010; Callies & Ferrari 2013 Transition? Isotropic spectrum Isotropic log10 x log10 spectrum space

5 interpreting 1D spectra! isotropic and 1D spectra follow the same power law k -n Across-track and along-track spectra are related through the scaling exponent Non-divergent flows: Ku = n Kv Assuming horizontal isotropy and homogeneous statistics, we can split the spectra into non-divergent and irrotational components: Bühler s et al. decomposition 1D spectra log10 x log10 space Batchelor 1953; Charney 1970; Rhines 1977; Callies & Ferrari 2013; Bühler et al. 2014

6 interpreting 1D spectra! isotropic and 1D spectra follow the same power law k -n Across-track and along-track spectra are related through the scaling exponent Irrotational flows: Kv = n Ku Assuming horizontal isotropy and homogeneous statistics, we can split the spectra into non-divergent and irrotational components: Bühler s et al. decomposition 1D spectra log10 x log10 space Batchelor 1953; Charney 1970; Rhines 1977; Callies & Ferrari 2013; Bühler et al. 2014

7 velocity measurements US Antarctic Research and Supply Vessel Laurence M Gould! 290 transects ( ) Underway ADCP (150 khz) Upper ocean currents ( m) mean currents and eddy KE Lenn et al. JMR 2007 Lenn & Chereskin JPO 2009

8 ADCP 1D KE spectra Approximately follows a k -3 power law between km k -3 Slightly shallower spectra between km: k -n, 2<n<3 The across-track/along-track KE ratio is <2 between km, and ~1 between km k -2 No dependence on depth in the upper 200 m (not shown)

9 ADCP 1D KE spectra k -3 Bühler s et al. decomposition Non-divergent flows dominate at scales larger than 40 km Irrotational motions account for 45% of the KE at scales between km k -2 Bühler et al. 2014

10 the llc4320 MITgcm simulation! A 1/48 o global simulation spun up from a 1/24 o spun up from a 1/12 o Forced with tides and 6-hourly ECMWF products ~1 km in Drake Passage Internal-tide resolving and submesoscale-admitting ~80 days (Sept.-Nov. 2011) snapshot

11 llc4320 1D KE spectra Model spectra are roughly consistent with ADCP spectra k -3 Approximately follows a k -n 2<n<3 power law between km The across-track/along-track KE ratio is ~1.5 between km, and <1 between 5-20 km k -2 No dependence on depth in the upper 200 m (not shown)

12 llc4320 1D KE spectra Daily-averaging the fields suppresses high wavenumber KE (dashed lines) k -3 KE spectra are steeper: k -3 power law between km The across-track/along-track KE ratio is ~3 between km k -2 No dependence on depth in the upper 200 m (not shown)

13 llc4320 isotropic KE spectra Approximately follow a k -3 power law between km; shallower between km (~ k -2 ) k -3 Surface velocity estimated from SSH assuming geostrophy are consistent with total field between km, but departs dramatically at scales < 40 km k -2

14 llc4320 isotropic KE spectra A Helmholtz decomposition of the 2D velocity field is consistent with Bühler s et al. decomposition of the 1D spectra k -3 Irrotational motions dominate at scales < 20 km k -2 M2 internal tide: peak in irrotational spectrum at ~150 km

15 llc4320 isotropic SSH variance spectra Ageostrophic motions significantly project onto the sea surface Transition from k -5 to k -3 at ~40 km! k -5 Daily-averaging the SSH fields suppresses high wavenumber variance k -3

16 summary k -3 In Drake Passage KE spectra are consistent with isotropic interior QG turbulence masked by ageostrophic flows KE spectra show no signature of isotropic surface QG turbulence KE spectra are typically steeper than k -2 k -2 No depth dependence in the upper 200 m Ageostrophic flows, likely dominated by inertia-gravity waves, account for at least 45% of the surface KE and SSH variance between km! Implication: what will altimeters observe at O(10) km scales? Details and caveats in Rocha et al.: Drake Passage kinetic energy and sea-surface height horizontal wavenumber spectra in the submesoscale range ( km), in prep. for JPO, Please, me for a copy: crocha@ucsd.edu

17 extra slides

18 ADCP 1D KE spectra Approximately follows a k -3 power law between km Slightly shallower spectra between km: k -n, 2<n<3 The across-track/along-track KE ratio is <2 between km, and ~1 between km No dependence on depth in the upper 200 m

19 llc4320 1D KE spectra Approximately follows a k -n 2<n<3 power law between km Slightly shallower spectra between km: k -n, 2<n<3 The across-track/along-track KE ratio is ~1.5 between km, and <1 between 5-20 km No dependence on depth in the upper 200 m

20 llc4320 isotropic KE spectra A Helmholtz decomposition of the 2D velocity field is consistent with Bühler et al. decomposition of the 1D spectra k -3 Daily-averaging the fields suppresses high wavenumber KE k -2 Spectra are consistent with isotropic interior QG turbulence

21 SSH variance spectra from AlTiKa! Spectra are steep: k -4 and k -5 down to about 60 km

22 ADCP 1D KE spectra Spectra based on small subtransects north and south of the 1-std envelope of the Polar Front from Sallée et al Basic characteristics are the same: steep spectra (~k -3 ) and across-track/along-track ratio < 2

23 how isotropic is the flow in Drake Passage? The ratio <u 2 >/<v 2 > is typically close to 1 White pixels denote ratio <u 2 >/<v 2 > between larger than 1.5 The flow is only slightly anisotropic There is no obvious pattern

24 llc4320 1D KE spectra Bühler s et al. decomposition Non-divergent flows dominate at scales larger than 40 km Irrotational motions dominate at scales between 5-20 km

25 llc4320 1D KE spectra Bühler et al. decomposition Non-divergent flows dominate daily-averaged fields at scales larger than 10 km Irrotational motions are efficiently removed by dailyaveraging the fields

26 SQG spectrum depth dependence Spectra following k -5/3 at the surface Significant depth dependence in the upper 200 m Both ADCP and model spectra lack such depth dependence