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Home , Deep ocean water

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One of the biggest questions in climate change is the role of the oceans, which presents particular challenges because of the complexities of circulation. Hence, we shall discuss ocean properties and ocean circulation as background to understanding some primary limitations to climate modelling. . Greatest reservoir of on the surface of the Earth . Composition of :

SALINE! (= SALTY) 35 o/oo

Major cation: Na+ sodium

Major anion: Cl- chloride . pH of ocean water: . slightly more than 8 . ocean water slightly alkaline (basic)

COMPOSITION OF SEAWATER Major cations: NaSodium Mg Magnesium Ca Calcium K Potassium Major Anions: Cl Chloride SO4 Sulfate HCO3 Bicarbonate Br Bromide Other components: Sr, SiO2, B, F Ocean Circulation . Surface layer: top 50-300 m stirred by wind well mixed top to bottom relatively depleted in nutrients surface mixed layer directly influenced by atmospheric fluxes. mixing based on long-term combined effects of wind, rotation of the Earth, and gravity—initiate surface currents and rsponsible for major global transport of water. The shallow ocean circulation is relatively difficult to model, particularly because of turbulence. Proper understanding of turbulence in the surface mixed layer is crucial for simulation of air- exchange, sea surface , and sea ice formation.

. Deep ocean colder roughly horizontal layers of increasing density (varied temp., ) rich in nutrients Generally easier to model. . top of deep part of ocean steep decrease temperature Absent in polar regions where dense water forms at surface due to cooling and freezing, and sinks into deep layer Three main forces driving shallow circulation: . effect--deflects motion of water moving north or south from the Equator . Wind (acted on by C. E)-- . Gravity Gyres and boundary currents Zonal winds (e.g., latitudinal wind belts) are the primary factor controlling the surface circulation of the oceans. Other factors: temperature, salinity, topography of sea floor, coriolis effect, size and shape of ocean basins, positions of continents (openings between continents) The combined results of zonal winds and positions of continents are large surface GYRES in each ocean basin = essentially closed loops or rings. Note that the Indian ocean circulation is more complex because of seasonal wind changes due to monsoons Ekman Spirals In N. Hemisphere, velocity of water will be slightly to the right of the wind direction against the water (to left in S. Hemisphere) Top "slab" of water will exert friction on "slab" below it Amount of frictional force decreases with depth Results in an "" Under ideal conditions, the net horizontal motion of the entire wind-driven surface layer is perpendicular to the direction of the wind. An example of effect of Ekman Flow: . Coast of California Why is it too cold to swim along the California Coast??? . prevailing westerlies (wind from west) hit mountains; wind deflected southward . Southward wind causes Ekman flow (to right) offshore . Removal of ocean surface water offshore creates a "hole" such that deep ocean water comes up= Very rich in nutrients (Monterey Bay)

Deep Ocean Circulation . Thermohaline= circulation brought about by differences in temperature and salinity (= density) . Deeper= colder, water more dense . Surface circulation important! . Poles: Surface become dense . Ice forms, salts left behind . Warm water hits cold air =evaporation= more salty Surface water becomes so dense it sinks! In the Northern hem., most of the cold, dense water is trapped in the Arctic basin, except where it escapes over sea rides between Greenland, iceland, and Scotland, into Atlantic. In Southern hemisphere, cold, deep water flows northward more freely. In shallow oceans, water does not often cross the Equator. In deep oceans, density-driven circulation is mostly oriented longitudinally rather than having a strong latitudinal component. Local changes in seafloor topography can greatly alter deep-sea circulation. One would expect that circulation might speed during glacial periods because of greater cooling and density- driven (and vice versa). There may be occasional huge swings in deep ocean circulation; for example, if very hot increase evaporation leading to more saline oceans near the equator. . Other places: . Some slow diffusion of water upward; Upwelling . Models for ocean convection are fairly well developed, and give good results for about the last decade. However, it is 'not certain that current schemes will work equally well in situations that involve substantial changes in the THC' "Global ocean convenyor belt" —Simplified view of ocean circulation and heat exchange Warm surface water is transported from the Pacific and Indian oceans to the North Atlantic where it is cooled and sinks. This cold, saline water travels south at depth to join cold, dense water from the Antarctic and flows eastward. Most flows around Antarctica to the Pacific, slowly rising to the surface and returning to the North Atlantic. Ocean Flows and Residence Times Shallow ocean waters: . Flow few kilometers/hr Deep ocean waters: . Flow few kilometers/month . Mean residence time 200-500 yrs Atlantic 1000-2000 yrs Pacific El Niño Southern Oscillation and La Niña (parts of this are edited from NOAA www site) The ENSO (El Niño/ Southern Oscillation) is a large-scale fluctuation of ocean temperatures, rainfall, and across the tropical Pacific Ocean. These fluctuations are vast in scale. Sea-surface temperatures can span a distance of more than 1/4 the circumference of the Earth; changes in tropical rainfall and winds can span a distance of more than 1/2 the Earth's circumference. El Niño episodes (also called Pacific warm episodes or ENSO) and La Niña episodes (also called Pacific cold episodes) represent opposite extremes of the ENSO cycle. El Niño events exhibit abnormally warm sea surface temperatures across the eastern tropical Pacific. La Niña events exhibit abnormally cold sea-surface temperatures across the eastern tropical Pacific. During normal years, the western Pacific is considerably warmer than the eastern Pacific. However, during a strong El Niño event, ocean temperatures can locally average 2¡C - 5¡C (4¡F - 9¡F) above normal along the west coast of S. America. There are uniform surface temperatures across the equatorial Pacific. During La Niña events, ocean temperatures along the west coast of S. America locally average 1¡C - 4¡C (2¡F - 7¡F). From NOAA www site In normal, non-El Niño conditions (top), the trade winds blow towards the west across the tropical Pacific. These winds pile up warm surface water in the west Pacific, so that the sea surface is ~1/2 m higher at Indonesia than at Ecuador. The SST is ~ 8 oC higher in the west, with cool temperatures off S. America, due to upwelling. This nutrient-rich water supports the major anchovy fishery. The East Pacific is dry, with desert conditions along a good deal of S. America coastline and inland. During El Niño, the trade winds decrease in the central and western Pacific. In the eastern Pacific, the thermocline becomes depressed, and in the western Pacific, the thermocline becomes elevated. Upwelling along the S. American coastline decreases or stops. SST rises, and the lack of nutrients can decimate commercial fisheries (Peruvian Anchovy crisis). Instead of dry conditions along S. American and wet in Indonesia, there can be flooding in S. America and extremely dry conditions (fires!) in Indonesia and even Australia. The eastward displacement of the atmospheric heat source overlaying the warmest water results in large changes in the global , which in turn force changes in weather in regions far removed from the tropical Pacific. How might El Niño be affected by climate change, and vice versa? We do not yet know, primarily because of difficulties in modelling ocean circulation and effects of cloud cover. To date, there is no established single climate model that can accurately predict both El Niño events and changing global climate.