EAS 447 Tropical Meteorology

Ocean circulation and dynamics, Monsoon details

Ocean density

Ocean layering • Layered according to density • Three-layered structure• I. Surface mixed zone

• Sun-warmed zone • Zone of mixing • Shallow (300 meters)

Near Surface Ocean Energy in Surface Mixed Zone

Oceanic Heat Content (OHC)• Basically, integral of water temperature in excess

of 26 °C from surface downwards: OHC = ρ * cp* §Z26

0(surface) (T-26) dz• Large values (especially those greater than 100

kJ/cm2) generally indicate a deep layer of warm water, which would be less prone to upwelling of very cold water from large waves

• Maintenance of warm water at the surface despite large waves would minimize the loss of any static instability and loss of evaporation

Ocean density

Three-layered structure (continued)• II. Transition zone

• Between surface layer and deep zone • Thermocline and pycnocline

• III. Deep zone • Sunlight never reaches this zone except parts of

it at high latitudes• Temperatures are just a few degrees above

freezing • Constant high-density water

Variations in ocean water temperature with depth

Variations in ocean water density with depth

Stratified; stable (little vertical motion, with light water above dense)

Neutral, with vertical mixing possible

Layering in the ocean

Ocean Balances• U2/L - fV = - (1/ρ) (δP/L) • Remember gradient balance.• Near shore, ocean flows are dominated by smaller

scale flows, and small eddies may become important. Nevertheless, a large, continuous river plume will tend bend to the right along a coastline in the Northern Hemisphere.

• Flows are much, much slower (100-1000 times) than in the atmosphere, so U/foL < 1 at much smaller length scales.

Ocean Balances

• U2/L - fV = - (1/ρ) (δP/L) • Further offshore from the coast, wind

primarily drives surface water motions.• In the open ocean, gyre flows prevail, which

are geostrophic.

Ocean water movements

Surface circulation of gyres• Ocean currents are masses of water that flow from

one place to another • Surface currents develop from friction between

the ocean and the wind that blows across the surface, also conservation of potential vorticity due to differential heating and rotation of earth

• Huge and slowly moving

The oceans of Earth

Ocean water movements Surface circulation

• Five main gyres • North Pacific Gyre • South Pacific Gyre • North Atlantic Gyre • South Atlantic Gyre • (South) Indian Ocean Gyre

• Related to atmospheric circulation • It takes 6 years for an object to circle in the

mixed layer the Pacific ocean and 3 years to do the same in the Atlantic

Average ocean surface currents in February-March

Ocean Circulation

Ocean water movements Surface circulation

• Importance of surface currents • Climate

• Influence of cold currents is most pronounced in the tropics or during the summer months in the middle latitudes; weaker than warm currents

• Upwelling • The rising of cold water from deeper layers • Most characteristic along west coasts of continents,

as water is effectively pulled off coast there• Brings greater concentrations of dissolved nutrients to

the ocean surface

Ocean water movements Deep-ocean circulation

• A response to density differences• Factors creating a dense mass of water

• Temperature – cold water is dense• Salinity – density increases with increasing salinity, more

important to generate sinking water for deep oceans• Called thermohaline circulation • Most water involved in deep-ocean currents begins in

high latitudes at the surface • A simplified model of ocean circulation is similar to a

conveyor belt that travels from the Atlantic Ocean, through the Indian and Pacific Oceans and back again

Idealized “conveyor belt”model of ocean circulation

Ocean currents– Role of thermohaline circulation: transport heat from

equator to poles– Deep-sea currents – move both vertically and

horizontally, though horizontal in most regions• Thermohaline circulation – “conveyor belt” circulation

caused by density differentials– Temperature –> cold water is more dense than warm– Salinity –> more salt = denser water

• Surface currents change in both temperature and salinity as they move• Water takes several hundred to several thousand years to

make a vertical loop

Ocean Circulation

Fig. 16.12, p.406

Land-Sea Breeze

• Land and sea breezes are strong in tropics due to:

-strong solar radiation -weak Coriolis -generally weak winds

Orographic Winds

• Upslope during day and downslope at night– Coriolis acceleration generally not important– Wind field important– Static stability important– Terrain characteristics important

Monsoons• At least 120° shift in wind direction between

“summer” and “winter” along with significant amount of persistence in a single direction for both seasons

• Utilize sea and land breeze winds often coupled with orographic winds to enhance intensity

• Coriolis IS important, as are interhemispheric airmass differences often, too

• Release of latent heat in summer hemisphere deepens monsoon circulation

Features of Indian Monsoon

• SE trades SH cross equator and turn SW (blow from cool to warm to hot); southerly LLJ in Eastern Africa

• Subsidence and midlevel warming due to dust absorption over Arabia prevents NW progress of monsoon, channels it over India; connected with subsidence from neighboring convection?

• LH release at mid and upper levels over Tibetan Plateau, Tropical Easterly jet aloft

Importance of Moist Convection• Moist convection results in latent heat release

which warms the mid and upper troposphere.• This causes upper level high pressure and

enhanced outflow which, in turn, strengthens the surface monsoon flow.

• Moist convection will eventually stabilize lapse rates in a given area, so propagation of convection and consistency of conducive large scale flow is generally needed to maintain monsoonal deep convection.

West African Monsoon

• ITCZ shifts considerably between seasons over Africa due to intense heating and cooling of land surface

• In boreal summer, northernmost position of ITCZ bounded by Sahara desert and cT air to the north; very hot and dry

• mT air to south of ITCZ; warm and moist

West African Monsoon• Hotter air to north: thermal wind is to the east• Midlevel African Easterly Jet (AEJ), along and just south of

ITCZ position, which marks cT/mT boundary and typically bends equatorward with height

• Convection along and south of African Easterly Jet but north of the equator, in area with relatively high moisture at low levels

• African Easterly waves may develop in this frontogenetic environment (horizontal temp grad) and cyclonic vorticity due to shear on south side of jet and may also extend partially north of the AEJ due to barotropic instability at times

West African Monsoon• African Easterly Jet can create energy for waves via cyclonic

vorticity on the south side; barotropic instability possible on north side; temperature gradient provides for energy in general

• African easterly jet can also increase vertical wind shear to tropical waves, which reduces large-scale organization and may also advect the dry, hot, dusty Saharan Air Layer (SAL), which can also weaken the waves

• SAL occurs at midlevels: it is hot, which decreases low-level lapse rates and dry, which increases potential for cool downdrafts and boundary layer stabilization in the presence of moist convection

• SAL also exhibits anticyclonic vorticity, typically

Southwest North American Monsoon

• Best developed in NW and W Mexico; also extends into Southwest US and Intermountain West

• Thermal low at low levels due to heating of high terrain

• Instability from strong solar heating, advection of Pacific moisture at low levels, and upslope flow helps create to rainfall in the region during the summer and early fall and relatively high pressure at upper levels due to moist convection

Southwest North American Monsoon

• Land cools off considerably in winter • Due to stability and reduced moisture (also,

reduced SST), rainfall is suppressed, especially in W Mexico, where extratropical cyclones generally track to the north

End of Section

• Continue with hurricane climatology and conditions for and disturbances involved with tropical cyclogenesis