Subtropical wind systems (trade winds + westerlies) drive a rotating surface-current gyre. Because the Coriolis force deflects moving water, the net wind-driven (Ekman) transport is roughly 90° to the wind rather than along it — and around a closed gyre this transport converges toward the center from every direction, piling up water into a slight dome and pumping it slowly downward (Ekman pumping). Anything floating that can't sink with that downwelling gets left behind, concentrating near the center.
This simulator advects every debris particle through an analytic 2D velocity field — a rotating (circulating) term plus a radial convergence term — by numerically integrating dx/dt = v(x,y) every frame; nothing pulls particles toward the center directly, they simply follow the local current vector wherever they are:
Tangential: v_θ(r) = Ω·r (r ≤ R_core)
v_θ(r) = Ω·R_core²/r (r > R_core)
Radial: v_r(r) = −k·r·e^(−(r/L)²) (inward, peaks near r ≈ L)
Wind drift: v_wind = α·W (constant "windage" push on exposed debris)
Total: dx/dt = v_θ·θ̂ + v_r·r̂ + v_wind
- Ω — Gyre rotation sets how fast the circulation spins (stronger wind stress curl → faster rotation).
- k — Ekman convergence sets how strongly the radial inflow sweeps debris toward the center. Click "Disable convergence" to set k = 0 live and watch particles keep orbiting on closed loops forever with the accumulation index flat — proof that rotation alone never concentrates debris; the inward term is what actually does the trapping.
- α — Windage is the fraction of wind speed directly imparted to debris riding above the water line — real drift-model studies use values of roughly 1–4% for low-profile plastic fragments, higher for objects that catch more wind.
- Accumulation index is the share of particles inside the inner core radius. The live density heatmap under the debris shows the same buildup spatially — cells near the center brighten over simulated time as particles genuinely spend more time passing through and lingering there.
Real-world relevance: this convergence mechanism is why there are five major subtropical accumulation zones — most famously the Great Pacific Garbage Patch in the North Pacific Gyre — and why cleanup and monitoring efforts target the gyre centers rather than the open ocean at large.