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.