What are ocean currents?
Movement of water in the main water bodies of the earth.
- affected by cold and warm sources
- Modified by Coriolis Force
- Play a key role in warm water transport poleward, cold water transport equatorward
- conform to continental boundaries
Wind stress
- Winds and wind stress push water initially in the direction of the wind
- coriolis force deflects water to the right at the surface
- net current is about 45 degrees to the right of the wind
Balanced water flow associated with the Atlantic Gyre
-Balance between water pressure and Coriolis force
aligns the current closer to the wind direction.
c = -(g/f) (dh/ds)
h=height of ocean sfc
Sub-Surface Currents
Driven by:
- Continental boundary effects - large scale upwelling, downwelling
- Thermohaline effects - sinking of cold and salty water, rising of fresh and warm water
Effects of Coastal Winds
-upwelling & downwelling
Tides
- Lunar and Solar effects
- Ebb and flood
Impacts of currents
- heat transport and enhancement of ocean storms
- enhanced ocean waves when winds are opposed
- coastal water cooling/warming - low clouds, fog; convection
- ship routing decisions
Three Factors in Wave Formation
- Wind strength
- Fetch
- Duration
Ocean Wave Formation - wind
- Microscopic capillary waves
- Slight ripples are pushed upward by wind
- Wave height increases based on speed and duration
Energy Transfer
- Kinetic energy of air creates potential energy in the water
- ½ mV^2 -> mg H
- Thus, there is a proportionality between wave height and velocity squared
- H (ft) = 0.02 V^2 (H is significant wave ht; V kts)
- Other factors enter in such as wave shape, steepness, and wind drag or shadowing
The Fetch
- Fetch to attain a fully developed sea (FDS):
Fetch (nm) = 3.17 x Wind^(4/3) kts…..for 30kts, 295nm - Fetch for a Significant Wave H (ft):
Fetch for H (nm) = 43 x H^(2/3)….for 18ft wave, 295nm - Equations are theoretical and assume a totally unperturbed sea
- Reality is that shorelines have far more incipient wave activity
- Result is that waves grow faster, require less distance to grow.
- These equations are not useful for lakes or bays.
Duration
-Wind over a particular patch of water must blow for an
extended period to generate largest waves
- Biggest waves form in slow moving storms
- Fast moving or brief windstorms (squalls) don’t
linger long enough to generate significant waves
Wind duration for a fully developed sea
Time (hrs) = 6.4 x Wind^(1/3) (kts)…..30kts, 20hrs
= 12.3 x H^(1/6) (ft)
Significant Wave Height
- Most often-used wave forecasting parameter
- Useful for mariners because if favors the larger waves that might be encountered allowing for better preparation, roughly the top 6th of waves
- Average wave height is 62% of significant wave height
- There is a 1% chance of getting a wave twice as high as the significant wave height
Wave Composition
- A typical ocean wave is made up of a series of individual wave lengths
- Once formed the wave starts to disperse with the longer wave lengths traveling faster
- This is why distant storms will be evidenced by long period swells
Superposition of Swells
- Real ocean waves come from superimposed swells.
- They are rarely regular and orderly
- They are more like oscillating hillocks
- The wave that “bites you” usually arises seemingly out of nothing
Limits on wave height
-Let’s say wind is 100kts
-Significant wave ht is 200ft…why don’t we see waves this high on the open ocean?
-Because they break…a wave will break if its height is appx 10% of its length
-Wave length L = 0.15 x Wind^(2) (kts)
-For 100kts L = 1500 ft,
waves break at 150 ft
-Near shore waves break if H > 80% depth (h)
Dangerous Wave Situations
- Strong slow-moving lows
- Lows moving at certain speeds
- Currents with wave motion opposed
- Waves near land - breaking
