Essentials of Oceanography, Thurman and Trujillo

Chapter VI: Air-Sea Interaction

Earth’s seasons

Earth’s axis is tilted 23½º from vertical (tropic of Capricorn/Cancer)

Northern and Southern Hemispheres are alternately tilted toward and away from the Sun (6 months apart)

Earth is tilted the same direction during entire sun orbit (precession is the slow turning of the direction)

Causes longer days and more intense solar radiation during summer

Figure 6-1Earth Sun Animation

Uneven solar heating on EarthSolar energy in high latitudes:

Has a larger “footprint”Is reflected to a greater extent (albedo)Passes through more atmosphereIs less than that received in low latitudes

Temperature is constant over longer periods of time. Heat must be transferred from low latitudes to high.All weather is the result of this transfer of heat

Figure 6-1

Oceanic and Atmospheric heat flow

A net heat gain is experienced in low latitudes A net heat loss is experienced in high latitudesHeat gain and loss are balanced by oceanic and atmospheric circulation Net heat is the difference between incoming shortwave radiation (sun) and outgoing longwave (black body) radiation.

Figure 6-3

Table 6.2

Physical properties of the atmosphere: Temperature

Troposphere is:Lowermost part of the atmosphereWhere most weather occursContains all earth’s surface

Temperature of troposphere cools with increasing altitudeTroposphere is ripe for convection!Stratosphere is:

contains ozone layerTemperature of stratosphere warms with increasing altitude

Tropopause is the boundary between the two

Figure 6-4

Physical properties of the atmosphere: Density

Warm, low density air rises (why heaters are near the floor)

Warm air holds moisture, as it rises it cools, can’t hold the same moisture, rains

Cool, high density air sinks (why air conditioner outlets should be near the ceiling)

Cold air can’t hold the moisture. Descending air warms, can hold more moisture, doesn’t rain.

Creates circular- moving loop of air (convection cell)

Figure 6-5

Physical properties of the atmosphere: Pressure

A column of warm, less dense air causes low pressure at the surface, which will lead to rising air (High pressure above)As air rises, air is replaced with air along the earths surfaceA column of cool, dense air causes high pressure at the surface, which will lead to sinking air (low pressure above)Air moves horizontally from H go L pressure Figure 6-6

Physical properties of the atmosphere: Water vapor

Cool air cannot hold much water vapor, so is typically dry (Cool air is a HIGH pressure)

Descending air is cool (does not hold vapor)

Warm air can hold more water vapor, so is typically moist (Warmer air is a LOW pressure)

Ascending air is warm (does hold vapor)

Water vapor decreases the density of air (this is sort of strange, but water vapor is light! H2O vs N2 vs O2)

So even if same temperature, wet air will rise

Physical properties of the atmosphere: Movement

Air INITIALLY flows horizontally from high-pressure regions toward low-pressure regions

Moving air is called wind

Sea Breeze in San Diego

San Diego’s air conditioner

Sea Breeze is quickly reduced as you move inland (and with it the cooler temps)

So where is the High Pressure ?

Figure 6.13

Summer/day

Winter/night

A hypothetical non-spinning earth

Figure 6.7

The Coriolis effect (force)

Newton’s second law: a body in motion will continue in motion (unchanged) unless acted upon

liquid (water) and gas (air) is not attached to the ground so obeys this law.

We are ‘attached’ to the ground thus rotate away from bodies that are unattached.

Causes moving objects to APPEAR to follow curved paths:

In Northern Hemisphere, curvature is to right

In Southern Hemisphere, curvature is to left

A merry-go-round as an example of the Coriolis effect

A merry-go-round is rotating at angular velocity Omega (like Earth)

Throw a ball (it follows Newton’s law and does not change direction or speed once it leaves your hand)

To an observer above the merry-go-round (not on earth), objects travel straight (Path B or C)

To an observer on the merry-go-round, objects follow curved paths. You rotate away, but your sight is still into the center of the merry go round)

The ball does not curve…you curve!

Internet video of balls being rolled across a moving merry-go-round

If you have a some vorticity (spin) in the direction of , coriolis will occur.

Figure 6-8

Planetary Vorticity (Component in the z (along axis) direction)

=90o

=0o =30o

Standing at the pole, the earth spins you around like an ice skater. You have a large vorticity given to you by the earth [f= f0• sin(90)=f0]

Step off the pole, you have just reduced the component of spin PARALLEL to the earth’s axis…but not by much.

By 30o latitude you have reduced your vorticity by 1/2. [f =f0 • sin(30)=f0/2]

By the equator you have no vorticity given to you by the earth [f0•sin(0)=0]

The Coriolis effect

Changes with latitude:

No Coriolis effect at Equator (Recall Vorticity is ZERO)

Maximum Coriolis effect at poles (Recall Vorticity is MAXIMUM)

f is positive (curvature to right) in the Northern Hemisphere (Recall Vorticity is positive)

f is negative (curvature to left) in the Southern Hemisphere (Recall Vorticity is negative)

Missile paths demonstrate the Coriolis effect

The missiles do not change path, you rotate (over your left shoulder in the Northern Hemisphere)

This occurs in all compass directions (east and west as well)

As long as your vorticity (spin) has a component along the earth’s axis, coriolis will occur.

Would you see this if the earth was a cylinder?

Figure 6-9b

Animation

An ADDITIVE effect complementary to Coriolis

ONLY in N-S directionAs Earth rotates, different latitudes travel at different speeds

[Speed is v = • r where v: speed in the east direction, r: distance from the earth’s axis]

Missile will keep eastward speed from the latitude it was launched from as it moved to other latitudes

What about a cylinder? A cone?

Figure 6-9a

Table 6.3

….. coriolis when rolling a ball across a table?

The time is not enough for the rotation of the earth to be important. Friction, inertia is more important over short times.

Coriolis is a ‘weak’ force, it only becomes apparent over a large amount of TIME!!!! A super-fast missile will not see coriolis.

...coriolis in a toilet? No. It is too small and the time of the flush is too fast.

How much time is necessary? 1/2 day to be apparent

Why don’t we see…….

A hypothetical non-spinning earth

Figure 6.7

Wind belts of the world

Figure 6-10

Coriolis turns winds to the right

Weird/Silly Historical Legacy: In describing winds the direction is where the winds are coming FROM (atmospheric convention)

In describing ocean currents the direction is where the currents are going TO (oceanographic convention)

Global Wind Animation

Table 6.4: Characteristics of the Wind Belts and Boundaries

Evaporation/Precipitation Latitudes: Comparison with winds

Figure 5.8Figure 6.10

Surface Salinity

Figure 5.20

Surface salinity variation

Pattern of surface salinity:

Lowest in high latitudesHighest in the tropicsDips at the Equator

Surface processes help explain pattern

Figure 5-19

Coriolis effect influences air movementright hand rule (northern hemisphere)What about southern? Southern Hemisphere questions are always good questions on exams, if you understand the process involved you should be able to keep the hemispheres straight.

Figure 6-12Cyclonic Animation

Weather Maps

Pressure Map

Wind Map

Weather Map

Pressure Map

H: Sinking air

L: Rising air

Precipitation Map

Air masses that affect U.S. weather

Figure 6-14

Sea-Level Mean Atmospheric Pressures in January

Figure 6.11 High pressure over continents/oceans in winter/summer hemisphere; Low pressure over continents/oceans in summer/winter hemisphere. WHY?

Weather Pattern Animation

Fronts: Contact between cold and warm air masses

Regardless if the cold front is moving into warm air or if warm air is moving towards cold; the warm air rises.

Cold air moving: produces a sharp front and REAL thunderstorms

L L

Fonts animation

Weather maps

The cold front is the dividing line between warmer and colder air masses.

In this case we have cold air moving into warm air. The warm air is due to the high pressure winds coming from the SW bringing warm temps.

A very weak front.

Jet Stream

Area of fastest winds which travel around the north (and south) poles.

The Jet Stream is high in the atmosphere

It ‘steers’ air masses about.

If you know where the Jet Stream is you can basically tell what our weather will be.

http://squall.sfsu.edu/crws/jetstream.html

Origin and paths of tropical cyclones

Tropical cyclones are intense low pressure storms created by:

Warm waterMoist airCoriolis effect

Includes:Hurricanes (western hemisphere)Cyclones (indian)Typhoons (eastern pacific) Figure 6-16

Hurricane occurrence

Hurricanes have wind speeds of at least 120 kilometers (74 miles) per hour (tropical storms below)Worldwide, about 100 storms grow to hurricane status each yearIn the Northern Hemisphere, hurricane season is generally between June 1 and November 30Current state of the tropical oceansIs there a strong chance of SD experiencing a hurricane?

Hurricane structure

Hurricanes have:Circular cloud bands that produce torrential rain

The ability to move into the mid-latitudes

A central eye

Low pressureFigure 6-17

Figure 6-19a

Hurricane Animation

Hurricanes produce storm surge

Storm surge:Is a rise in sea level coming ashoreCan be up to 12 meters (40 feet) highCauses most destruction and fatalities associated with hurricanesDoes it have to be a hurricane?

Figure 6-18

Storm Surge

Other Storms can push water toward the downwind side of lakes/bays causing flooding. Bays that are wide at the entrance and narrow in the direction of the wind are especially vulnerable. animation

Climate regions of the ocean

Figure 6-20

Why do temperate regions appear to extend further north (Northern hemisphere) on the east side of the ocean? The Atlantic is the best example.

Short wave vs Longwave Radiation

Visible light is shortwave, Ultraviolet radiation is even shorter.

Infrared is longwave, and this is where the earth loses heat (night vision goggles) Black Body radiation

How a greenhouse works (the greenhouse effect)

Sunlight passes through the clear covering of a greenhouseIt converts to longer wavelength heat energy (black body radiation)Heat cannot pass through the covering and is trapped inside

Figure 6-23

Figure 6.24 The Earth’s Heat Budget

Anthropogenic gases that contribute to the greenhouse effect

Greenhouse Gas Relative contribution

Carbon dioxide (CO2) 60%

Methane (CH4) 15%

Nitrous oxide (N2O) 5%

Tropospheric ozone (O3) 8%

CFC-11 4%

CFC-12 8%

What was the percentage of CO2 in todays atmosphere?

Anthropogenic greenhouse gases

What was the percentage of CO2 in today’s atmosphere? Oxygen = ~21% etc.

Carbon dioxide is increasing in the atmosphere

As a result of human activities, carbon dioxide in the atmosphere has increased by 30% since 200 years ago

Wait a minute…why do we think it is due to humans?

Figure 6-26

Longer term CO2 history

Recent measurements are around 360ppmv. Do we now have higher concentrations or are the methods used to get the below type of time series flawed?

Earth’s average temperature is rising

Earth’s average surface temperature has risen at least 0.6°C (1.1°F) in the last 130 yearsEarth’s average ocean temperature has increased in the last 30 years… especially in higher latitudes.May be related to increase in atmospheric carbon dioxide Lots of wiggles! Are we sure!

LIKE Figure 6-27

Feedback: A secondary change to a primary change.

Negative- tends to counter the primary cause

Volcanos erupt emitting ash/soot into upper atmosphere, blocks incoming sunlight

Ash emitted by coal burning (etc) does similar

Increased CO2, makes plants grow, removes CO2. Ash also creates particles for water to condense around (clouds)

Positive- tends to amplify the primary cause

Ocean warms releasing additional CO2 to atmosphere Tundra melts releasing additional CO2 to atmosphere.

Glaciers (Antarctica, Arctic?) melt releasing fresh water (light) shutting off deep water formation which would have sequestered CO2

Medieval warm period and Little Ice Age in England

Variability of 1oC (but over ~300 years)

Predicted changes with increased greenhouse warming

Higher than normal sea surface AND deep ocean temperatures that could affect world climate

As oceans warm it releases more CO2 (assign)More severe droughts or increased precipitationWater contamination and outbreaks of water-borne diseasesLonger and more intense heat wavesShifts in the distribution of plants and animalsPotential melting or enlargement of polar ice capsIs science sure of these?

Climate Change wrap-up

What we know for sure!

The process (science) of global warming is real. It has happened throughout time and continues to this day.

We have pumped a great deal of Carbon Dioxide into the atmosphere since the beginning of the industrial revolution.

The earth (ocean and atmosphere) has warmed since the industrial revolution.

What we don’t know for sure!

Is the increase of temperatures due to us (we think so, or it is a mighty big coincidence) or is this just natural variability.

What are the feedbacks (positive and negative) that will occur as the earth warms

End of Chapter VI

Essentials of Oceanography 8th Edition

More Realistic Climatological (average) Winds