Ocean and Atmosphere Dynamics

where is the warmest ocean water located at

along the equator in the western ocean basins

where is the coldest water located at

near the poles

temperature of the atlantic ocean

• warm thin surface layer

• cold thick deep layer

• thermocline separates the warm and cold layers

where is the saltiest water

in the subtropics

which ocean is the saltiest

atlantic

which ocean is the freshest

north pacific

deep ocean salinity at higher latitudes

more uniform and generally matches the surface salinity at higher latitudes, specifically northern high latitudes

how do large subtropical gyres rotate

• rotate clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere

how do smaller subpolar gyres rotate

counterclockwise in the northern hemisphere and absent in the southern hemisphere

deep ocean circulation

cold surface water sinks at high latitudes and moves to lower latitudes at depth 

deep-water returned to the surface layer through broad diffusive upward vertical mixing

how to determine the direction of coriolis force

• point your nose in the direction the ocean current (or wind) is moving

• stick your hand directly out from your side to get the direction for the coriolis force 

• right hand for northern hemisphere and left hand for southern hemisphere

where is coriolis force acting on in the northern hemisphere

right of the water parcel motion

where is coriolis force acting on in the southern hemisphere

left of the water parcel

how is the atmosphere heated

• the visible radiant energy from the sun (short wavelength radiation) passes through the atmosphere without being absorbed → earth’s surface absorbs a lot of the short wavelength sunlight energy so it becomes warm

• warm earth surface radiates infrared radiation (long wavelength radiation) from its surface and is absorbed by the atmosphere. atmosphere is heated

how does atmospheric convection occur

• warm ocean surface heats the atmosphere from below

• warm surface air is able to hold a lot of water vapor so it becomes moist → it has low density so it rises and moves higher. it then expands and cools

• cooling of air aloft causes water vapor to condense and precipitate 

• condensation releases latent heat that warms the surrounding air and drives the upward convection higher

where is the most intense solar heating

at the equator where solar radiation is the most direct

where is atmospheric convection and precipitation the strongest

along the equator

why is there a thick band of clouds along the equator

strong surface heating by the sun along the equator drives upward atmospheric convection, condensation and precipitation

is a column of high amounts of water vapor heavier or dry air?

dry air since N2 has a heavier molecular weight than H2O

what is the idealized hadley circulation

• surface air at the equator warms and rises aloft. this air spreads north/south and becomes more dense as it cools and dries due to precipitation. it sinks and warms at about 30 degree latitude

• warm and dry surface air at 30 degrees spreads out north and south, picks up moisture, by 60 degree latitude, it has moistened to the point where it rises, cools precipitates and spreads out aloft north and south

• near the poles, the air aloft becomes very cold and dense so it sinks over the poles and spreads out towards the equator

which latitudes have high sea level pressure

north pole and 30 degree latitude

which latitudes have low sea level pressure

equator at 60 degrees latitude

why should we worry about the vertical distribution of seawater density in the ocean

• the vertical distribution of seawater density directly influences the vertical movement of water in the ocean

• salt concentration and water temperature jointly determine seawater density so the concepts of ocean salinity and ocean heat content are presented in some detail

how does precipitation affect salt concetration

rain dilutes it

how does evaporation affect salt concentration

it concentrates salt concentration

what is surface ocean temperature is determined by

exchange of heat across the air-sea boundary

what is the most dominant method for the sea to lose heat

evaporative cooling

what is ocean temperature a measure of

heat energy contained in the ocean

surface ocean temperature in spring/summer for temperate ocean regions

more heat enters the ocean due to the sunlight than leaves so the surface ocean warms

surface ocean temperature in winter for temperate ocean regions

more heat leaves than comes in from sunlight so surface ocean cools

light penetration depth in the clearest open-ocean waters

up to 150-200 meters

light penetration in turbid coastal waters

light can only penetrate up to 50m

at which depth does most of the overall heating occur in

0-30 meters

how far does vertical mixing of near-surface water reach

mixing only reaches to at most ~500 meters

what is the temperature of seawater below 500 meters

uniformly cold

what is the permanent thermocline

broad region centered at around 500 meters where seawater temperature changes from warm to uniformly cold

seasonal thermocline

a much shallower thermocline that forms only in the summer and is erased in the winter

what is the typical vertical density profile

less dense water floats above more dense water

what is seasonal pycnocline

• it is a region of strong change in density with depth

• it comes and goes with net heat gained in spring/summer and net heat losses in fall/winter

what is a permanent pycnocline

• remains in place (at 500 meters) and is the result of long-term balance between the downward heating/mixing at the surface and upward mixing of the cold water below

ekman spiral

• spiraling of thin ocean currents within the ekman layer

ekman transport

it is the rate of total water transported in the ekman layer (which is 50-100 meters thick)

• derived by summing all the individual thin current sheets over the entire ekman layer 

what is the angle of ekman transport to the wind direction

90 degrees to the right (northern hemisphere) or left (southern hemisphere) of the wind direction

response of fluid in the ocean due to the pressure gradient force and coriolis force

• fluid first response to a pressure gradient force and moves down the pressure gradient to lower pressure

• coriolis force pushes it to the right of its direction of travel

• fluid continues to be pushed by coriolis until the coriolis force is equal and opposite of the pressure gradient force. the two forces cancel each other out so there is no acceleration of the fluid but there is also no friction to bring this steady motion to a halt

what is geostrophic balance

the point at which currents move with a steady speed, with coriolis and pressure gradient forces in perfect opposition

what is geostrophic current

resulting current from geostrophic balance

formation of subtropical gyres

if a mound of water builds up at the surface of the ocean, a central high pressure develops under the mound and circular geostrophic currents develop

what do the dark lines represent

contours of constant pressure that decrease away from the center of the mound region

what do the yellow arrows represent

direction of fluid motion for a northern hemisphere mound

what do the light blue arrows represent

they point out the directions of the pressure gradient force, toward lower pressure

what do the red arrows represent

point out the coriolis force and are directed to the right of the fluid motion, and this case to the center of the higher pressure

combining ekman transport and geostrophic currents

• subtropical gyres develop in response to the westerly and trade wind belts 

• westerly winds and trade winds drive the ekman layer to the center of the gyre to create a mound of surface water

• center of high pressure develops under the mound of surface water

• fluid initially moves away from the center of high pressure

• the push of coriolis turns the fluid until it reaches geostrophic balance

how can you find the center of the subtropical gyre

if you put your nose in the direction of the trade winds and stick your right hand out, you will point to the center of the subtropical gyre

western boundary currents

• currents on the western side of all subtropical gyres

• very swift and narrow jets that bring warm water from the tropics to high latitudes

eastern boundary currents

• currents on the eastern side of all subtropical gyres

• broad and slow and bring cold water from the high latitudes toward the tropics

where is the center of the gyre

shifted slightly towards the west

which part of the subtropical gyre does the coriolis force point towards

• center of high pressure 

• in the northern hemisphere, right hand rules apply so it requires a concentric clockwise motion to achieve a center directed coriolis force

• a counterclockwise motion is needed in the southern hemisphere with its left hand coriolis rule

what is the seasonal variation of heating and cooling of the surface ocean at mid-latitude regions

• march to aug: gaining heat 

• september - february: losing heat

what are the 2 foundational concepts of a subtropical gyre

1. ekman transport

2. geostrophic currents

direction travelled by ekman spiral

• the top-most current moves at 45 degrees to the wind forcing because of wind, coriolis force and friction

• bottom-most current moves directly opposite of the wind direction

equatorial upwelling

• north east trade winds drive ekman transport to the north

• south east trade winds drive ekman transport to the south

• the “parting” of the surface water to the north and south means that deep water rises upward from the base of the ekman layer to the surface along the equator

• surface water also piles up on the western side of the ocean, forcing a downward tilt to the thermocline

• water is drawn up from the base of a relatively shallow ekman depth

what type of water is drawn up to the surface if thermocline is close to the ekman depth

cold

what type of water is drawn up when the thermocline is deep/further away from the ekman depth

warm

how did the eastern equatorial pacific cold tongue form

due to equatorial upwelling AND the close proximity of the thermocline to the ekman depth

how did the western equatorial pacific warm pool form

upwelling still occurs but thermocline is much deeper than the ekman depth so upwelling draws more warm water to the surface

coastal upwelling along the washington oregon coast

• wind blowing out of the north drives ekman layer to the right (northern hemisphere) which is offshore

• offshore transport of the ekman surface layer is replaced by upwelling of deeper cold water allong the coast

downwelling

• wind blowing out of the south drives the ekman layer again to the right (because northern hemisphere) which is onshore

• onshore transport of ekman surface layer is driven downward

atlantic ocean temperature at depth (meridional section from iceland to antarctica)

1. warm thin surface layer

2. cold thick deep layer

3. thermocline separates the warm and cold layers

atlantic ocean salinity at depth

1. high salinity in surface layer in subtropics

2. deep-ocean salinity more uniform and generally matches the surface salinity at higher latitude - especially northern high latitudes

temperature salinity of north atlantic deep water

close to 35 degrees celsius
and 1027.75% salinity

temperature salinity of antarctic bottom water

roughly 34.85 degrees celsius
and 1028.0% salinity

temperature salinity of antarctic intermediate water

roughly 34 degrees celsius and 1027.25% salinity

speed of circulation of deep water in the global ocean

• cold dense water sinks in the north atlantic and around antarctica to form deep abyssal water

• north pacific salinity is too low to sink (though temperatures are just as cold as north atlantic) 

• deep water in north atlantic moves south and merges with antarctic water in the southern ocean as they both swing around antarctica and up into indian and pacific basin

how old are the deepest waters and where are they found

2000 years old and in deep pacific

schematic of heat transport by the global conveyor belt circulation

1. heat is gained by the surface ocean at low latitudes and transported via the wind-driven surface circulation (western boundary currents) to higher latitudes 

2. heat is given up by the ocean to the atmosphere at higher latitudes to warm these regions

3. surface water sinks and enters the thermohaline deep circulation and is returned to the surface to pick up more heat and the cycle repeats

what currents aid in most heat transport to high latitudes

strong western boundary currents

what is the result of winters due to global conveyor belt circulation

• western europe experiences mild winters, relative to northern canada

• westerly winds then carry warm coastal air over land

what is the level of AMOC circulation

weakest in more than 1000 years

AMOC collapse

• recent analysis suggests that critical thershold is closer than the 2019 IPPC special report indicated

• the bistable nature of the phenomenon means that it will find a new equilibrium in its off state and turning it back on would require a shift in the climate far greater than the changes that triggered the shutdown

what is el nino

the roughly periodic (3-7 years) occurrence of prolonged (ca. 8 months) warming of coastal waters off of peru and ecuador

idealized hadley circulation

• winds at ocean surface

  • rush toward the equator

  • coriolis force turns the winds to cause them to blow from east to west (classic trade winds) 

• winds aloft

  • rush away from the equator

  • coriolis force turns the winds to cause them to blow from west to east (opposite of trade winds)

why does the east-west pattern of hadley circulation occur

• differences due to unequal heating of land and ocean

• land heats more strongly than adjacent ocean in summer and cools more than adjacent ocean in winter - due to differences in specific heat of rock and seawater

walker-circulation cell (east-west atmospheric circulation cell)

• winds at ocean surface move toward the equator but are deflected by coriolis toward the west to form surface trade winds

• winds aloft move away from the equator but are deflected by coriolis toward the east to complete the walker-circulation cell

• discovered by gilbert walker and that it periodically reversed direction

what is the southern oscillation

the coherent change (periodic reversal - every 3 to 7 years) in the east-west circulation cell over the pacific (walker cell) resulting in coherent changes in atmospheric pressure patterns, precipitation patterns and wind direction

• high / low SLP (atmosphere pressure at sea level) patterns also change

sea surface temperature (la nina)

• exceptionally strong trade winds

• equatorial upwelling and cooling in the eastern pacific

sea surface temperature under normal conditions

• normal trade winds

• upwelling and cooling in the eastern pacific

sea surface temperature (el nino)

• exceptionally weak or reversed trade winds

• little or no equatorial upwelling

• strong warming in eastern pacific

warming during el nino

• warms surface water off peru

• also has a greater warming event, covering large parts of eastern tropical pacific and reaching down to the thermocline

what happens to the global average temperature when the thick warm pool of water in western pacific spreads out over a large area of the equatorial pacific during an el nino period?

it goes up as the area of warm water in contact with the atmosphere increases and the ocean gives up some of its stored heat to the atmosphere

direct impacts of el nino’s conditions on pacific precipitation and storm patterns

• relative cooling in the western tropical pacific creates drought conditions in australia and indonesia during an el nino

• relative warming in central and eastern tropical pacific heat energy for formation of intense precipitation and storms

  • seen in flooding of california/western US

el nino’s direct effect on biological productivity off of peru and ecuador

less biologically productive

what type of trend is human-caused global warming

• a multi-decadal trend

• other natural climate variations that can temporarily warm and cool the earth are superimposed on this multi-decadal trend

what is the current status of the el nino/lanina conditions

• tropical pacific is now enso-neutral after a few months of la nina conditions

• la nina conditions will emerge in the next couple of months and continue through the northern hemisphere winter

how has el nino affected coral reefs

• bleaching of coral reefs

• 2015-2016 and 2023-2024 el nino events were disastrous for coral reefs

jet stream’s north/south position relative to el nino conditions

• jet streams have wave-like properties 

• the regional disturbance in the jet stream position propagate around the globe in a wave-like fashion

• enso produces such a disturbance in the jet stream that can propagate around the world