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 in long-time mean zonal geostrophic velocity at the ocean surface, high-pass filtered horizontally with two-dimensional 4° filter. cm/s

Smearing of eddy signal by time averaging Schlax and Chelton, (2008) – Figure 2 Maximenko et al. (2005) Scott et al. (2008)

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. Azores current cm/s 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. Jets off California coast cm/s Centurioni et al, 2008: beta-plume induced by interaction between mean Ekman flow and stationary meanders

Mean zonal geostrophic velocity at the ocean surface, high-pass filtered horizontally with two-dimensional 4° filter. cm/s

Sverdrup Vorticity Equation HLCC: curl-driven zonal jet (slide courtesy of Belmadani) Ekman flow: w Sverdrup flow: V Chavanne et al. 2002, CJRS Sverdrup Vorticity Equation Vorticity produced by vertical stretching + fluid turbulent stress. Vorticity increased by moving poleward. Ekman pumping Curl drives Ekman pumping / suction. Thermocline is lifted / depressed. Cyclonic / anticyclonic eddies (e.g., Calil et al. 2008, DSR). Sverdrup Balance Meridional transport driven by curl. Rossby waves propagate anomalies (e.g., Sasaki et al. 2010, OD). Volume conservation: U Nondivergent barotropic flow. Barotropic Continuity Equation Zonal transport to the west. Integration from eastern boundary

Spatial-filtered SST & wind HLCC: wind-forced β-plume (slide courtesy of Belmadani) HLCC NEC HLC Qiu et al. 1997, JPO Curl > 0 V > 0 ← Rossby waves Cyclonic eddies Western boundary Island Anticyclonic eddies 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). Xie et al. 2001, Science Other mechanisms: air-sea coupling, island-induced modified large-scale flow (Qiu Durland 2002, JPO), mode water intrusions (Sasaki et al. 2012, JO), etc. 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) τ ∇xτ Wind: steady mesoscale anticyclonic vortex , R = 40 km Meridional transport: Sverdrup balance , ρ = 1025 kg.L-1 β ≈ 1.98 10-11 s-1m-1 Zonal transport: continuity equation , xe ≈ 2890 km Uana ULIN Good agreement between analytical and numerical solution. 1 anticyclonic cell (2 jets) + 2 weak cyclonic cells ⇒ 2+2 x-independent zonal jets.

Nonlinear β-plume: eddies and mean flow (slide courtesy of Belmadani) SLNL 178ºE 178ºW 174ºW 170ºW 166ºW 162ºW 158ºW 154ºW 150ºW 38ºN 22ºN 30ºN 34ºN 26ºN Yr 21 Yr 21-30 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 Nonlinear regime Forcing Forcing k=(k,l) V0 Induction of eddies by forcing Eddies induced by instability of large-scale flow Eddies Induced by instability of striations

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.