CONTINENTAL divergence Ocean- Continent Convergence Ocean Divergence Ocean-Ocean Convergence.
Ocean striations as a crossroad of multiple physics Nikolai Maximenko International Pacific Research...
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Transcript of Ocean striations as a crossroad of multiple physics Nikolai Maximenko International Pacific Research...
Ocean striations as a crossroad of multiple physics
Nikolai Maximenko
International Pacific Research CenterSchool of Ocean and Earth Science Technology
University of Hawaii
Ocean Sciences Meeting, Salt Lake City, Utah, February 20-24, 2012
Collaborators and contributors: Oleg Melnichenko, Ali Belmadani, Emanuele Di Lorenzo, and Niklas Schneider
Striations in long-time mean zonal geostrophic velocity at the ocean surface, high-pass filtered horizontally with two-dimensional 4° filter.
cm/s
Schlax and Chelton, (2008) – Figure 2
Scott et al. (2008)
Smearing of eddy signal by time averaging
Maximenko et al. (2005)
Many striations seem to dynamically correspond to beta-plumes, induced by local vorticity forcing at their eastern tips
cm/s
Mean zonal geostrophic velocity at the ocean surface, high-pass filtered horizontally with two-dimensional 4° filter.
cm/s
Azores current
Kida, PhD: Azores current is a beta-plume inducedby overflow of Mediterranean water intoAtlantic
Mean zonal geostrophic velocity at the ocean surface, high-pass filtered horizontally with two-dimensional 4° filter.
cm/s
Jets off California coast
Centurioni et al, 2008: beta-plume induced by interaction between mean Ekman flow andstationary meanders
Mean zonal geostrophic velocity at the ocean surface, high-pass filtered horizontally with two-dimensional 4° filter.
cm/s
HLCC: curl-driven zonal jet (slide courtesy of Belmadani)
Chavanne et al. 2002, CJRS
Curl drives Ekman pumping / suction.
Thermocline is lifted / depressed.
Cyclonic / anticyclonic eddies (e.g., Calil et al. 2008, DSR).
Rossby waves propagate anomalies (e.g., Sasaki et al. 2010, OD).
Ekman flow: w Sverdrup flow: V
zz
wfv
1
Sverdrup Vorticity Equation
V
Sverdrup Balance
Vorticity produced by vertical stretching + fluid turbulent stress.
Vorticity increased by moving poleward.
Meridional transport driven by curl.
Volume conservation: U
dxy
VU
x
xe
0
y
V
x
U
fw
Ekman pumping
Barotropic Continuity Equation
Nondivergent barotropic flow.
Zonal transport to the west. Integration from eastern boundary
HLCC: wind-forced β-plume (slide courtesy of Belmadani)
Xie et al. 2001, Science
Island
Curl < 0 V < 0
Curl > 0 V > 0
Cyclonic eddies
Anticyclonic eddies
Western boundary
HLCC
NEC
HLCQiu et al. 1997, JPO
β-plume (Rhines 1994, Chaos) = Sverdrup gyre driven by compact vorticity source (momentum, heat, mass).
HLCC = wind-forced β-plume (Jia et al. 2011, JGR).
HLCC: narrow eastward jet embedded in broad North Equatorial Current (NEC).
Elongated double-gyre west of Hawaii.
← Rossby waves
Other mechanisms: air-sea coupling, island-induced modified large-scale flow (Qiu Durland 2002, JPO), mode water intrusions (Sasaki et al. 2012, JO), etc.
HLCC advects warm SST → far-field curl dipole (Xie et al. 2001, Science; Hafner Xie 2003, JAS; Sakamoto et al. 2004, GRL; Sasaki Nonaka 2006, GRL, etc.).
Spatial-filtered SST & wind
Linear β-plume: steady-state barotropic solution (slide courtesy of Belmadani)
Wind: steady mesoscale anticyclonic vortex
2
22
2max . R
yx
x eyR
e
2
22
2max . R
yx
y exR
e
, R = 40 km
Meridional transport: Sverdrup balance
V
Zonal transport: continuity equation
2
2
2
2
2
2
222224
max ..22
.32
. R
x
eR
xeR
yx
x
e
e
exexRR
xerf
R
xerfyRey
R
edxy
VU
, ρ = 1025 kg.L-1
β ≈ 1.98 10-11 s-1m-1
, xe ≈ 2890 km
Good agreement between analytical and numerical solution.
1 anticyclonic cell (2 jets) + 2 weak cyclonic cells ⇒ 2+2 x-independent zonal jets.
τ
∇xττ
Uana ULIN
SLNL
SLNL
178ºE 178ºW 174ºW 170ºW 166ºW 162ºW 158ºW 154ºW 150ºW
178ºE 178ºW 174ºW 170ºW 166ºW 162ºW 158ºW 154ºW 150ºW
38ºN
22ºN
30ºN
34ºN
26ºN
38ºN
22ºN
30ºN
34ºN
26ºN
Yr 21
Yr 21-30
Nonlinear β-plume: eddies and mean flow (slide courtesy of Belmadani)
With stronger forcing, nonlinearities and instabilities grow, mesoscale eddies are shed from the forcing region.
Mean circulation is modified and becomes a dipole with 3 jets and a broader y-scale.
Heterogeneity of eddy trajectories
Schlax and Chelton (2008) – Figure 1
Heterogeneity of eddy trajectories
Schlax and Chelton (2008) – Figure 1
Space correlation functions of U’ and ζ’ reveal long zonal correlations and indicate that eddies may be exaggerated and striations may be suppressed during mapping
From AVISO data From AVISO mapping function
k=(k,l)
V0
Linear regimeForcing
ForcingNonlinear regime
Induction ofeddies by forcingEddies
Induced by instability ofstriations
Eddiesinduced by instability oflarge-scale flow
Effect of background meridional flow : advection
Tilt of striations correlateswith the direction of background glow
Effect of background meridional flow : instability
Spall (2000): Generation of strong mesoscale eddies by weak ocean gyres
Stage 1: Instability of meridional flow produces strong zonal jets
Stage 2: Instability of zonal jets produces heterogeneous, isotropic eddies.
Formation of new eddies is not completely random
Alleddies
Neweddies
Histogram of anticyclones relative to crests in striations
Histogram of cyclones relative to troughs in striations
Concluding remarks:
1. Eddies are important (and most energetic) players in visualizing striated patterns.
2. Striations reflect higher organization of eddies (preferred paths, eddy trains, etc.).
3. Understanding striations may be more feasible through dynamical rather than kinematic study.
4. “Anchoring” processes that still need to be understood:- anchoring of source regions to topographic features;- anchoring of new eddy formation to striations;- possible fixation of western tips of striations;- air-sea interaction.