NADW AABW AAIW Itay Halevy ATLANTIC WATER MASSES NADW AABW AAIW Itay Halevy

OVERVIEW Definition of water masses. Motivation. Classification of water masses. Atlantic water masses and their properties. WHAT ARE WATER MASSES? WHY WE WANT TO TALK ABOUT THEM? THEIR CLASSIFICATION ATLANTIC WATER MASSES AS EXAMPLES

SECTION OF THE ATLANTIC OCEAN. Y COOR. - DEPTH, X COOR. - LATITUDE. EQUATOR IN THE MIDDLE. THIS IS NITRATE. TONGUES/SLABS ARE CHARACTERISTIC OF ATLANTIC SECTIONS - T & S ALSO. STRATIFICATION IMPLIES DENSITY DIFFERENCES

WATER MASSES Water mass mixing is a way of explaining T and S profiles in the ocean. Any T-S profile displays properties that are a result of a mixture of discrete end member waters. Assumption - T and S are conservative properties - i.e. there is no diffusive exchange with the surroundings and change in water properties is through mixing alone. Water masses have a “core depth” where their typical characteristics are located. Vertical mixing creates a range of depths, above and below the core, influenced by the water mass. WATER MASSES ARE A THEORY ON THE OCEAN’S DRIVING MECHANISMS. NAMELY, CHARACHTERISTICS OF WATER (T, S, NO3, O2) ARE PRODUCTS OF MIXING END MEMBERS. ASSUMPTION - DIFFUSION DOEN’T PLAY A MAJOR ROLE. MASS’ CORE IS WHERE TYPICAL PROPERTIES ARE. INFLUENCE OF A MASS IS EXTENDED BY VERTICAL MIXING

WATER MASSES CONT. Low velocities: Replenishment of Typically < 1 cm/sec Up to as much as 15 cm/sec Replenishment of nutrients to surface. SURFACE WATER WELL MIXED - ABUNDANT O2. NUTRIENTS USED BY PHOTOSYNTHETIC ORGANISMS. THEIR DECAY AND RESPIRATION BY HETEROTROPHS RELEASES A NUTRIENT RAIN TO THE DEEPER OCEAN. WHEN DEEP WATER UPWELLS THE SURFACE IS REPLENISHED IN NUTRIENTS.

WATER MASSES CONT. Waters classified by their DEPTH their DENSITY their TEMPERATURE & SALINITY Note: Density has higher dependence on T than on S. CLASSIFIED BY DEPTH. WE KNOW DEPTH DEPENDS ON DENSITY. IN STABLE CONDITIONS WATER WILL BE UNDER ANYTHING LIGHTER THAN IT. DENSITY DEPENDS ON T & S. GREATER DEPENDENCE ON T.

SO WHY DO WE CARE? Differential solar influx  Pole-Equator temperature gradient Differential evaporation and precipitation Requires meridional heat and salinity transport Facilitated by Atmosphere & Ocean Circulation characteristics influence climate EARTH IS SPHERICAL - POLES GET LESS RADIATION BECAUSE OF ANGLE OF INCIDENCE. RESULTANT GRADIENT CAUSES DIFFERENCES IN EVAPORATION AND PRECIP AND THEREFORE SALINITY ANY NATURAL SYSTEM DRIVES ITSELF TO EQUILIBRIUM - TRANSPORT OF T AND S ATMOSPHERIC CIRC PRIMARILY BUT ALSO OCEANIC TRANSPORT. INFLUENCE CLIMATE (T, PRECIP, EVAP)

WHY THE ATLANTIC? Major role in T and S transport: Other oceans form much less deep water. Pacific less saline due to lower SSTs. Pacific also doesn’t reach as far north. Indian too warm. North Atlantic is high-lat and saline enough. WHY ATLANTIC? MAJOR ROLE IN OCEAN CIRCULATION. OTHER OCEANS DON’T FORM DEEP WATER IN SIGNIFICANT AMOUNT.

CLASSIFICATION OF WATER MASSES Divided into SURFACE, INTERMEDIATE, DEEP. Surface water Warm and saline. Lower density - dependence of  on T, S. Nutrient depletion - biotic productivity. Oxygen abundance - mixing. SURFACE WATER SALINE - WHY DOESN’T SINK? T MORE IMPORTANT.

CLASSIFICATION OF WATER MASSES Intermediate water T & S vary greatly. Combination gives relatively high , typically 1027 to 1027.6 kg/m3. DENSITY HIGH ENOUGH TO SINK BELOW SURFACE WATER. EXAMPLE OF AN INTERMEDIATE WATER MASS - AAIW

CLASSIFICATION OF WATER MASSES Antarctic Intermediate Water - AAIW T = 3°C S = 34.25  = 1027.30 kg/m3 Core depth ~ 1000 m Source region: Southern Pacific, Indian and Atlantic Oceans near the Polar Front. Spreads as far as 20°N in all ocean basins. FORMS AT LATS OF ABOUT 50-60 DEG S. MECHANISM IN A FEW SLIDES.

CLASSIFICATION OF WATER MASSES Deep water T = -1 to 3°C S = 34.5 to 35.0  = 1027.7 to 1028.0 kg/m3 Nutrient enrichment. Relative oxygen depletion. SALT WATER FREEZES AT ABOUT -1.5C NOT AS O2 DEPLETED AS SOME SHALLOW-INTERMEDIATE (150-500M) DUE TO RESPIRATION

CLASSIFICATION OF WATER MASSES Antarctic Bottom Water (AABW) T = -1°C S = 34.65  = 1027.87 kg/m3 Core depth ~ 4000 m Source region: Wendell Sea, Ross Sea. Mean production rate: 20 Sv/yr ROSS TO A LESSER EXTENT. WENDELL AND ROSS - SEAS ADJACENT TO ANTARCTIC ICE SHELVES.

UPWELLING BECAUSE OF WESTERLIES (PRESUMABLY NADW). SOME FLOWS NORTH, IS COOLED BY ATMOSPHERE AND FRESHENED BY MELTWATER - AAIW SOME FLOWS SOUTH, IS COOLED BY ATMOSPHERE AND SALINIFIED BY BRINE REJECTED FM FREEZING WATER AABW.

CLASSIFICATION OF WATER MASSES North Atlantic Deep Water (NADW) T = 3°C S = 34.93  = 1027.83 kg/m3 Core depth ~ 2500 m Source region: Imringer Sea, Nordic Sea, Labrador Sea. Mean production rate: 15 Sv/yr AS WARM AS AAIW BUT MUCH SALTIER. COMPRISED OF WARM, EVAPORATED WATER FM GULF STREAM AND MED WATER. COOLED AND SINKS.

SUMMARY Warm saline water cools and sinks in the Northern Atlantic Ocean (NADW). Some of this water upwells near the ACC. Some then flows south is cooled further and sinks to the ocean bottom (AABW). Some flows north and sinks beneath warm, low latitude water (AAIW). ANTARCTIC CIRCUMPOLAR CURRENT

REFERENCES www.ocean.washington.edu/ courses/oc513/ www.iku.sintef.no faculty.washington.edu/pdquay/OC210/ www.ldeo.columbia.edu/dees/ ees/climate/lectures/o_atm.html www.ldeo.columbia.edu/edu/dees/ees/climate/slides www.deas.harvard.edu/climate/eli/Courses/2005fall_a/

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