Equatorial Upwelling & Walker Circulation

Course Context & Lecture Goals

• Closing conceptual gaps from earlier weeks: integrating biology, chemistry, and physical oceanography.
• Focus this week: exceptions to the strict two-layer ocean model (surface vs. deep) and why breaking that stratification matters.
• First and principal exception examined: equatorial upwelling.

Ocean Stratification – Quick Recap

• Surface layer vs. deep layer traditionally viewed as isolated:
  • Surface: warm, well-lit, low in nutrients (due to continual biological uptake).
  • Deep: cold, dark, high in nutrients (remineralization dominates).
• Any process that mixes these layers can drastically change chemistry & biology at the surface.

Upwelling – Definition & Significance

• Upwelling = upward movement of subsurface water to replace diverging surface water.
• Brings:
  • Nutrients (N, P, Fe, Si, etc.)
  • Dissolved $\text{CO}<em>2$ and $\text{O}</em>2$
• Fuels the majority of global marine primary productivity — most oceanic phytoplankton blooms are linked to upwelling events.
• Equatorial upwelling is only one type (others: coastal, divergence at gyre boundaries, etc.) but is among the most persistent and geographically extensive.

Atmospheric Circulation Refresher

• Earth’s atmosphere organized into pressure-driven circulation cells:
  • Hadley (0–30°)
  • Ferrel (30–60°)
  • Polar (60–90°)
• Coriolis deflection turns meridional (N–S) flows into zonal (E–W) winds.
• At the equator Coriolis parameter $f \to 0$ ⇒ no deflection; winds blow straight.

Surface Zonal Winds Over the Pacific

• Persistent easterly (east-to-west) trade winds form a surface zonal wind band along the equator.
• Basin focus: Pacific
  • Eastern boundary ≈ Latin America
  • Western boundary ≈ Southeast Asia/Australia
• Wind stress transfers momentum to the sea surface, physically pushing surface water westward.

Creation of the Pacific Western Warm Pool

• Warm surface water piles up in the western Pacific → Western Warm Pool (WWP).
• Consequences:
  • Thermocline depth increases (isostatic depression) in WWP region.
  • Sea-surface height (SSH) rises by tens of cm above mean sea level in the west.
  • Eastern Pacific experiences the converse: shallow thermocline, cooler SSTs, SSH several cm below mean.

Walker Circulation (Walker Cell)

• Warm, moist air rises over WWP (low density).
• Aloft, flow travels eastward; air cools, condenses, precipitates.
• Cooler, drier, denser air sinks in the east, returns westward at the surface (reinforcing trade winds).
• Net: a longitudinal (zonal) cell superimposed on the global Hadley–Ferrel–Polar system.

Ekman Transport & Surface Divergence at the Equator

• On the equator: Coriolis ≈ 0 → surface flow is due east-to-west wind stress.
• Slightly off the equator: Coriolis deflects moving water:
  • Northern Hemisphere: rightward (northward)
  • Southern Hemisphere: leftward (southward)
• Result: surface water diverges away from the equator on both sides.
• To conserve mass, subsurface water replaces the diverging surface layer → equatorial upwelling.

Mechanics of Equatorial Upwelling

• 3-D conceptual model:
  1. Trade winds push warm water west (surface layer slope forms).
  2. Ekman divergence draws surface water poleward on either side of 0°.
  3. Water is upwelled from depth to fill the equatorial “void.”
• Upwelling strength ∝ wind stress magnitude.
• Upwelling depth = Ekman depth (depth to which wind-driven spiral acts). Consistent over the length of the wind band.
• Interaction with thermocline depth:
  • Western Pacific: Deep thermocline remains above upwelled depth → only nutrient-poor surface water recycled.
  • Eastern/central Pacific: Shallow thermocline intersected by Ekman depth → real deep water (nutrient-rich, high $\text{CO}<em>2$, fresh $\text{O}</em>2$) reaches surface.
• Thus, upwelling in the east/central Pacific is biologically and chemically potent.

Biological & Chemical Consequences

• Rich supply of nutrients → explosive phytoplankton growth ("primary production hotspots").
• Supports higher trophic levels (zooplankton → fish → top predators).
• Drives significant carbon cycling: drawdown of surface $\text{CO}_2$ during blooms; later respiration/export flux returns carbon to depth.

Observational Evidence

• Satellite chlorophyll-a imagery shows a persistent, wide equatorial streak of high chlorophyll across the Pacific.
• Mirrors regions where upwelled deep water meets sunlight.
• Provides real-world confirmation of theoretical physical mechanisms.

Key Terms & Quick Facts

• Upwelling: upward motion of subsurface water replacing diverging surface water.
• Equatorial Divergence: Ekman-driven lateral removal of surface water from 0° latitude.
• Surface Zonal Wind: persistent east-to-west wind along the equator.
• Western Warm Pool: mound of warm water & elevated SSH in W. Pacific.
• Walker Cell: east-west atmospheric circulation cell along equator.
• Ekman Depth (a.k.a. Ekman layer thickness): depth over which wind forcing transmits momentum; determines upwelling source depth.
• Primary Production: synthesis of organic carbon by phytoplankton; majority in the ocean fueled by upwelling.

Practical & Broader Significance

• Fisheries: Many of the world’s richest fishing grounds align with persistent upwelling zones.
• Climate feedbacks: Upwelling modulates ocean-atmosphere $\text{CO}_2$ exchange and thus influences global carbon budgets.
• Variability: Changes in wind patterns (e.g., El Niño/La Niña) can enhance or suppress equatorial upwelling, with cascading socio-economic impacts (weather extremes, fishery collapses/booms).