EF

oceanlecture10upperocean2025

Upper Ocean Circulation

Overview

  • Focus on understanding the physical dynamics of upper ocean circulation, primarily driven by wind.

Global Wind and Ocean Dynamics

Global Mean Wind Speed

  • Analysis of wind work from 1991-2011 showing total kinetic energy in upper ocean acceleration.

  • Reference: Hu et al. (2020) Science Advances.

Measuring Ocean Currents

I. Current Measurement

  • Currents can be measured in terms of either Velocity (m/s) or Flux (Sv = 10^6 m³/s).

A. Mechanical Current Meters

  • Fixed instruments moored in place.

  • Utilizes an impeller and directional vane to measure current direction and speed.

B. Doppler Acoustic Current Meters

  • Uses sound waves reflected from suspended particles to determine depth.

  • Doppler shift analysis provides speed of particles; four beam comparison indicates flow direction.

C. Floats

  • Allow to drift naturally with ocean currents.

  • Types:

    • Buoys: Trackable via radio or satellite.

    • Neutrally buoyant floats: Transmit data when at the surface.

    • Drift cards: Information collected when found by individuals.

Example Study: Bottle Release

  • Hundreds of bottles released at sea to study ocean currents.

  • Each bottle carries a request for finder information to aid study by the Woods Hole Oceanographic Institution.

Accidental Floats

  • Significant events involving accidental floats:

    • 1990: 21 containers of Nike shoes (40,000 pairs).

    • 1992: 29,000 bath toys.

    • 2011: 1.5 million tons of debris from the Honshu tsunami.

Wind-driven Gyre Circulation

II. Characteristics of Wind-driven Gyres

  • Surface currents are primarily within the upper 400 m and account for ~10% of ocean volume.

  • Dominated by subtropical gyres with dynamics influenced by three main factors:

    • Ekman transport

    • Geostrophic balance

    • Westward intensification

A. Ekman Transport

  • Result of the balance between wind stress and the Coriolis force.

  • Wind stress acts in direction of wind, and Coriolis increases with water velocity and latitude.

  • Flow adjusts until Coriolis balances wind stress; in the Northern Hemisphere, net flow is 90° to the right of the wind direction.

Ekman Spiral

  • Describes the distribution of flow direction with depth, demonstrating reduced velocity with increasing depth.

  • At surface, water moves ~45° to the right of wind direction; depth effects cause additional rightward movement.

Clicker Question on Ekman Transport

  • Asks to characterize net water movement between easterly trade winds and westerlies; possible answers include divergent, convergent, or transform.

Ekman Convergence

  • Convergence creates a 'hill' of water leading to high sea surface pressure (e.g., observed in North Atlantic Subtropical Gyre).

B. Geostrophic Balance

  • Balancing act between pressure gradients and Coriolis effect

  • Flow moves from high to low pressure; Coriolis alters flow courses.

  • Subtropical gyres characterized as 'anti-cyclonic', employing a circulation timescale of just a few years.

C. Westward Intensification

  • Gyre highs pile against western boundaries, mainly due to increasing Coriolis force with latitude, leading to stronger pressure gradients and faster flows in the west.

Ocean Currents Characteristics

Fast and Warm Currents

  • Fast, deep, narrow currents (e.g., California Current).

    • Average speed: ~1-2 m/s.

    • Depth: ~1000 m, width: ~70 km.

Cold and Slow Currents

  • Slower, shallow, wider currents (e.g., Canary Current).

    • Average speed: ~0.1-0.2 m/s.

    • Depth: ~100 m, width can reach ~1000 km.

Clicker Question on Subpolar Gyre Flow

  • Evaluates knowledge of the flow direction of subpolar gyre resulting from westerlies and polar easterlies in the high-latitude North Atlantic regions.

D. Subpolar Gyres

  • Located in North Atlantic and North Pacific regions.

  • Ekman divergence creates a sea level low with a cyclonic counterclockwise flow, resulting in productive regions due to upwelling.

Antarctic Circumpolar Current (ACC)

III. Overview and Dynamics

  • Unique current flows around the globe south of ~40°S; lacks land boundaries.

  • Flow is not the fastest (~1 m/s), but has the largest volume flux (~130 Sv).

A. Dynamics of ACC

  • Depth of ~2-4 km; breadth of up to 2000 km.

  • Experiences strong Ekman divergence influenced by westerlies and polar easterlies, leading to a band of low sea surface pressure and resultant clockwise geostrophic flow around Antarctica.

B. Consequences of ACC

  • Strong divergence induces upwelling, markedly enhancing marine productivity- leading to a significant thermal barrier around Antarctica, impacting chemical concentrations (e.g., phosphate).

Ocean Dynamics Variability

IV. Eddies in Ocean Flow

  • Ocean flow is heterogeneous, characterized by meanders and eddies formed from cut-off meanders spinning geostrophically.

  • Eddies range from tens to hundreds of kilometers and can persist for days to months, playing crucial roles in ocean mixing.