Ocean Sciences Meeting, Salt Lake City, Utah, February 20-24, 2012
Ocean striations as a crossroad of multiple physics
Nikolai Maximenko
International Pacific Research Center
School of Ocean and Earth Science Technology
University of Hawaii
Collaborators and contributors: Oleg Melnichenko, Ali Belmadani, Emanuele Di Lorenzo,
and Niklas Schneider
Striations
cm/s
in long-time mean zonal geostrophic velocity at the ocean surface, high-pass
filtered horizontally with two-dimensional 4° filter.
Smearing of eddy signal by time averaging
Schlax and Chelton, (2008) – Figure 2
Maximenko et al. (2005)
Scott et al. (2008)
cm/s
Many striations seem to dynamically correspond to betaplumes, 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.
Azores current
Kida, PhD: Azores current is a beta-plume induced
by overflow of Mediterranean water into
Atlantic
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 and
stationary meanders
cm/s
Mean zonal geostrophic velocity at the ocean surface, high-pass filtered
horizontally with two-dimensional 4° filter.
HLCC: curl-driven zonal jet (slide courtesy of Belmadani)
Ekman flow: w
Chavanne et al. 2002, CJRS
Sverdrup flow: V
v  f
w 1  

z  z
Sverdrup Vorticity Equation
w
 Vorticity produced by vertical stretching +
fluid turbulent stress.
 
f
 Vorticity increased by moving poleward.
Ekman pumping
 Curl drives Ekman pumping / suction.
V
 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).
 

Sverdrup Balance
 Meridional transport driven by curl.
Volume conservation: U
U V

0
x y
Barotropic Continuity Equation
 Nondivergent barotropic flow.
 Zonal transport to the west.
V
dx
xe y
U  
x
Integration from eastern boundary
HLCC: wind-forced β-plume (slide courtesy of Belmadani)
← Rossby waves
Western
boundary
Cyclonic eddies
Curl > 0
V>0
Qiu et al. 1997, JPO
HLC
Island
Anticyclonic eddies
HLCC
NEC
Curl < 0
V<0
 Elongated double-gyre west of Hawaii.
 HLCC: narrow eastward jet embedded in
broad North Equatorial Current (NEC).
 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).
 Other mechanisms: air-sea coupling, islandinduced modified large-scale flow (Qiu Durland 2002,
JPO), mode water intrusions (Sasaki et al. 2012, JO),
etc.
Xie et al. 2001, Science
Spatial-filtered SST & wind
 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.).
Linear β-plume: steady-state barotropic solution (slide courtesy of
Belmadani)
Wind: steady mesoscale anticyclonic vortex
τ
x 
 max e
R

y.e
x2  y2
2 R2
y  
 max e
R

x.e
x2  y2
2R2
, R = 40 km
Meridional transport: Sverdrup balance
V
 

∇ xτ
τ
, ρ = 1025 kg.L-1
β ≈ 1.98 10-11 s-1m-1
Zonal transport: continuity equation
y2
 max e  2 R 2  
V
U  
dx 
y.e
3R 2  y 2

4
xe y
R 
 2
x

xe 2  
x2



  xe 

x



2
2
.erf 
  erf 
  R  x.e 2 R  xe .e 2 R  
 2 R 

 
  2R 

Uana
 Good agreement between analytical and numerical solution.
 1 anticyclonic cell (2 jets) + 2 weak cyclonic cells ⇒ 2+2 x-independent zonal jets.
, xe ≈ 2890 km
ULIN
Nonlinear β-plume: eddies and mean flow (slide courtesy of Belmadani)
38ºN
34ºN
SLNL
30ºN
26ºN
22ºN
38ºN
34ºN
Yr 21
178ºE 178ºW 174ºW 170ºW 166ºW 162ºW 158ºW 154ºW 150ºW
SLNL
30ºN
26ºN
22ºN
Yr 21-30
178ºE 178ºW 174ºW 170ºW 166ºW 162ºW 158ºW 154ºW 150ºW
 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
Effect of background meridional flow : advection
Linear regime
Forcing
Nonlinear regime
Forcing
V0
k=(k,l)
Eddies
induced by
instability of
large-scale flow
Eddies
Induced by
instability of
striations
Induction of
eddies by
forcing
Tilt of striations correlates
with 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
All
eddies
New
eddies
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.