This lecture will learn the basics of oceanography.
Let’s get started with this question.
how do ocean currents work?
when we learn physics of ocean currents on some text book,
there are many equations and we could not sometimes understand well what they mean.
That’s the same for me.
So, I find an interesting story on ocean currents.
Let me introduce the story.
In 1992, a cargo ship carrying bath toys got caught in a storm.
Shipping containers washed overboard, and the waves swept 28,000 rubber ducks.
So what do you think happened after?
the ducks have since washed up all over the world,
and researchers have used their paths to chart a better understanding of ocean currents.
So let’s think about driving forces of ocean currents.
Ocean currents are driven by many forces.
First, you can come up surface wind stress
Wind drags the ocean surface.
How about tide?
Tide can change a direction of ocean current,
especially for the coastal area.
Of course, we need to think about water density depending on water temperature and salinity.
If we have higher temperature and smaller amount of salt, water density should be lighter and it comes up to the surface.
If we have lower temperature and larger amount of salt, water density should be heavier and it goes down.
The last one is the rotation of the Earth called the Coriolis effect.
In addition, the topography of the ocean floor and the shoreline modifies those motions.
Topography is causing currents to speed up, slow down, or change direction.
Ocean currents fall into two main categories: surface currents and deep ocean currents.
Surface currents control the motion of the top 10 percent of the ocean’s water,
while deep-ocean currents mobilize the other 90 percent.
Though they have different causes, surface and deep ocean currents influence each other.
Near the shore, surface currents are driven by both the wind and tides,
which draw water back and forth as the water level falls and rises.
Meanwhile, in the open ocean, wind is the major force behind surface currents.
As wind blows over the ocean, it drags the top layers of water along with it.
That moving water pulls on the bellow layers.
In fact, water as deep as 400 meters is still affected by the wind at the ocean’s surface.
Here, let’s make an equation for the momentum between the atmosphere and ocean surfaces.
Now, you have the surface wind speed of 10 m/s,
the atmospheric density,
seawater density,
duration, and momentum exchange coefficient that depends on the surface roughness.
The relation of each momentum would be like this,
and you can estimate ocean surface current speed
If you zoom out to look at the patterns of surface currents all over the earth,
you’ll see that they form big loops called gyres,
which travel clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere.
There are five major gyres of the Indian Ocean Gyre,
North Pacific Gyre,
South Pacific Gyre,
North Atlantic Gyre,
and South Atlantic Gyre.
If you zoom in to look at the patterns of surface currents,
you’ll see that there are a lot of eddies.
When we focus on the Indonesian Seas,
one of the most famous eddies is the Halmahera eddy.
Actually, I visited Ternate in the last week.
There is the Sultan Kingdom started from 12th century
and I am very surprised at the fact that they have a historical map drawing the Halmahera eddy in 16th century.
I can see that they had a fairly advanced understanding of oceanography.
By the way, if the earth didn’t rotate,
air and water would simply move back and forth between low pressure at the equator and high pressure at the poles.
But as the earth spins,
air moving from the south Pole to the equator is deflected eastward,
so that the major streams of wind form loop-like patterns around the ocean basins.
This is called the Coriolis Effect.
The Coriolis force is an apparent force that changes the direction of flows and it increases with the higher latitude.
The Coriolis force is represented by these equations.
A zonal component is in proportion to a meridional motion
and a meridional component is in proportion to a zonal motion.
Unlike surface currents,
deep ocean currents are driven primarily by changes in the density of seawater.
As water moves towards the North Pole,
it gets colder.
It also has a higher concentration of salt,
because the ice crystals that form trap water while leaving salt behind.
This cold, salty water is more dense, so it sinks,
and warmer surface water takes its place,
setting up a vertical current called thermohaline circulation.
Thermohaline circulation of deep water and wind-driven surface currents combine
to form a winding loop called the Global Conveyor Belt.
In addition, we have specific surface currents like this.
For example, along the eastern coast of Japan,
we have Kuroshio and Oyashio currents that are the western boundary currents.
Over the equator, there are the equatorial currents
that are predominantly driven by the wind
and they are accompanied by the equatorial counter currents.
All they have a wide range of variations in response to atmospheric variations.
That’s why we need to observe their actual status.
Then, how can we observe the ocean?
we can bring a thermometer by a ship,
and we get a profile if it sink to the bottom.
If you like to get profiles repeatedly,
you can put a wire on the thermometer.
The most famous one is CTD observation to monitor the basic status of seawater.
We did this operation 8 times per day on the R/V Mirai during the YMC campaign.
Another is micro structure profiler to see more precise turbulence of sea water.
If you want to continuously monitor ocean profile for longer period,
you can use a float connected by wire to a sinker.
That is, TRITON buoy observation.
We deployed new one
replacing old one
every a few years at fixed points.
Recently, a persistent mobile data-gathering platform is developed.
That is called wave glider
In addition, if you did radiosonde operation,
you could understand air-sea interaction processes.
R/V Mirai has an automatic launching system for radiosonde like this.
We did 8times per day just before CTD operation.
Now you can get on R/V Mirai by three sixty degree virtual tour.
In this way, we did observational campaigns.
Let me introduce some examples
from the Pre-YMC in 2015
and YMC in 2017.
General features during the two campaigns are these.
In 2015, we did before the MJO passage,
under the situation of the weak easterly wind less than 5 m/s,
and sea surface temperature is more than 30 degree C.
On the contrary in 2017, we did after the MJO passage,
under the situation of the strong westerly wind more than 10 m/s,
and sea surface temperature is less than 29 degree C.
And as for the ocean structure,
we observed haline in 2015,
and we observed eddies in 2017,
I’m very interested in the fact that the eddy has a vertical motion in the mixed layer
and such motion could increase salinity near the surface.
Looking at time-depth sections of the temperature in 2015 and 2017,
they are entirely different.
Look at the mixed layer by the dashed line.
The mixed layer depth in 2015 is very shallow less than 20 m
but that in 2017 is more than 100 m.
Even a very precise ocean model cannot exactly represent this kind of fine surface structure and we need more observations.
Though we have oceanic objective analysis products,
we can improve them after the observations are much more sufficiently assimilated.
Looking at salinity sections,
we have a very strong salinity stratification in 2015
because the small salinity near the skin surface due to the large amount of diurnal rainfall.
On the contrary we have well mixed salinity to 100 m depth in 2017
and drastic increase of salinity from underneath is shown in the latter half.
At the same time, we have a drastic increase of turbulence from underneath in 2017,
in contrast, the turbulence below 50 m depth in 2015 is much smaller that in 2017.
Looking at current direction,
you can see a steady southward flow by warmer colors in 2015
and rotational flow showing stripe cold and warm color pattern in 2017.
That is, there is the alongshore flow under the weak wind situation before the MJO in 2015
but there is the alongshore eddies under the strong wind situation after the MJO in 2017.
Such different features of ocean currents to the west of Sumatra are made from
these different atmospheric features and also reversely feed back to the atmosphere.
That’s why we need your love more and more!
Thank you for coming to Japan and looking forward to seeing you again.
Good bye!
 
