Geography discussion 2

Based on our learning thus far, when you see the Earth’s image from the space, you would notice that a line of convective clouds mark the ITCZ north of the equator, with clear areas to the north and south of the ITCZ.

Also, as we all know, the ocean, which covers over 70 % of the Earth surface, contributes to the Earth’s climate.

Global Winds

Westerlies

Westerlies

90°N (North Pole)

90°S (South Pole)

60°N

30°N

0° (Equator)

30°S

60°S

Polar Easterlies

Polar Easterlies Polar Front

Polar Front

Trade Winds NE Trade Winds

Trade Winds SE Trade Winds

subtropical high "horse latitudes"

subtropical high "horse latitudes"

L

L

rising air masses

rising air masses

L

H

sinking air masses

sinking air masses

H

H

H

Prevailing winds create a drag (wind stress) on the ocean surface, and some of this momentum is transferred to the water, causing it to move. As the surface water moves, friction among the water molecules causes the momentum to be transferred deeper into the water column, but energy is lost with increasing depth. As a result, the velocity of the current at the surface decreases with greater depth. Interestingly, not only the velocity, but the direction of the current changes with depth as well.

Ekman Transport Prevailing wind

Net transport of surface water All vectors (magnitude

and direction) of the Ekman spiral yield a net current direction that is ~90 degree to the prevailing wind. This composite current is the Ekman transport and it controls the motion of the surface ocean.

Such change in the direction is the result of the Coriolis Effect, which will affect moving water in the same way that it does the winds.

The Coriolis Effect causes the moving water to be deflected away from its direction of travel (to the right in the Northern Hemisphere). The surface current is deflected ~45 degree from the direction of the prevailing wind. A decrease in current speed, coupled with continuous Coriolis deflection with increasing depth cause an apparent spiral of moving water called the Ekman spiral. All vectors (magnitude and direction) of the Ekman spiral yield a net current direction that is ~90 degree to the prevailing wind. This composite current is the Ekman transport and it controls the motion of the surface ocean.

Trade Winds NE Trade Winds

Westerlies

Polar Easterlies

Trade Winds SE Trade Winds

Westerlies

Polar Easterlies

90°N (North Pole)

90°S (South Pole)

60°N

30°N

0° (Equator)

30°S

60°S

L

H

L

H

L

ITCZ

H

H

Ekman Transport ~90o to the

prevailing winds

to the right of the prevailing winds in the N. Hemisphere, to the left of the prevailing winds in the S. Hemisphere

The energy derived from the prevailing winds set the uppermost water column in mo7on. This movement of the upper water masses is the wind-driven circula.on, and the mo7on is in a direc7on to the right of the prevailing winds in the Northern Hemisphere and to the le< of the prevailing winds in the Southern Hemisphere.

Ekman transport causes near-surface waters to converge (pile-up) in subtropical regions thereby crea7ng subtle ”hills” on the ocean surface, which causes water to diverge (move apart) in subpolar regions and along the equator, crea7ng “depression”. These subtle highs and lows on the ocean surface are not visible because the relief is less than 2 meters (<6.6 <.) higher or lower than the average level of the sea over broad areas of the ocean (see more detail in next slide).

http://www.seos-project.eu/modules/oceancurrents/oceancurrents-c06-s02-p01.html

The prevailing winds provide the energy to drive the surface currents of the world ocean. Ekman transport and the Coriolis effect cause surface water to converge (“pile-up”) in the subtropics and diverge (move apart) at the equator and in subpolar waters. This creates subtle “hills” and “valleys” on the ocean surface of <2m (<6.6ft.). Gravity acts on the water to pull it back from these hills or into these valleys. This continuous tug-of-war between opposing forces results in a partial balance or equilibrium that keeps water moving around these subtle domes and valleys (= geostrophic flow/current).

Geostrophic currents flow around subtle �hills� and �valleys� on the ocean surface

Caused by prevailing winds and Colioris effect, resulting in Ekman transport on surface ocean, gyres (white solid arrows circling the ocean) are the large horizontal wind-driven current systems that circulate around the subtle domes and depressions on the ocean surface.

Ocean Circula,on

North Equatorial Current

This figure shows the surface water circulation pattern of the Earth. Let’s take a look of the Atlantic Ocean. The subtropical gyres, for instance, represent large circulation cells around the hills created by convergence in the subtropics. This subtropical gyre in the North Atlantic starts when the Trade Winds blow out of the northeast towards the Equator and initiate the westward-flowing North Equatorial Current. When this current encounters the Caribbean Islands and North America, the Coriolis effect deflects the current to the right (north) as the Gulf Stream.

As we learned in previous lecture, this surface ocean current is a wind driven surface current.

h"ps://s-media-cache-ak0.pinimg.com/736x/00/88/d9/0088d94516b288d4bf347cebee62257d.jpg

Gulfstream

The Gulfstream is one of the strongest warm current in the world.

Please watch this suggested video: http://media.pearsoncmg.com/bc/bc_0media_geo/geo_animations/gulf-stream- meanders/meanders.html

http://theresilientearth.com/?q=content/conveyor-belt-model-broken

We learned about how the surface ocean moves, driven largely by the energy of the prevailing winds. There is a circulation of intermediate and deep (or bottom) water as well.

Suggested video; http://media.pearsoncmg.com/bc/bc_0media_geo/geo_animations/deep- water-circulation/deep-water-circulation.html

Because ocean circulation depends on temperature (density) and salinity, it is referred to as thermohaline circulation (thermo is Greek for “heat”, and haline comes from the Greek hals, for “salt”).

And shutting down or slowing down of this thermohaline circulation is speculated to have a significant impact on abrupt climate change…. (see next slide)

If you have watched “The Day A3er Tomorrow”, which is a Hollywood movie,

you may now no=ce that it uses this scien=fic reasoning of thermohaline

circula=on!

Check its official trailer “hDp://www.youtube.com/watch?v=MFLncfCvPeY”

J

Although the thermohaline circula=on hypothesis is accurate, unfortunately,

this movie is less realis=c.

Why?

As you know by now, the mel=ng of con=nental ice is a slow-responding

system, and it does not occur in the range of “hours”…!

Global map showing where 2017 heat content in the top 700 meters (2,300 feet) of the ocean was higher (orange) or lower (blue) than the 1993–2017 average. NOAA Climate.gov map, adapted from State of the Climate in 2017.

Modeled increase in the heat content of the upper 700 meters (2,300 feet) of the oceans, based on observations from 1993 to 2017 average. Oceans store much of the excess solar energy delivered to earth and so, they are effectively buffering us.

The connection to hurricanes is obvious. Warmer water means greater potential for evaporation, which means more fuel for hurricanes. It is often postulated that global warming will lead to more INTENSE, not frequent, hurricanes.

Please visit this site for further reading: https://www.climate.gov/news-features/understanding-climate/climate-change- ocean-heat-content

The figure shows differences from the long- term average global ocean heat content (1955- 2006) in the top 700 meters of the ocean.

Major surface currents are set in motion by ________.

A) the wakes of ships B) salinity differences C) winds D) density differences E) shapes of coastlines

The answer is C.

Ocean gyres rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

A) True B) False

The answer is A.