Ocean Circulation II

I. Thermohaline Circulation

• need to understand temperature-salinity relationships in order to understand how they can influence current patterns
• at the surface, temperature of coldest waters is controlled by temperature of initial freezing of seawater, while warmest water temperatures are controlled by evaporation of water
• salinity at the surface also shows a relatively limited range, between 33 and 37
• below surface, temperature and salinity vary together in characteristic ways for the region in which the waters formed
• the curve of temperatures and salinities are called a T-S curve and can be used to identify any given water mass and to determine how water masses mix

• thermohaline circulation is composed of currents below pycnocline
• these currents are driven by density differences (cause water masses to sink or rise to their appropriate density level)
• density differences are caused by both temperature and salinity, which can counteract each other (e.g., salinity has a minimal effect in equatorial regions because surface temperatures are high enough to maintain a low water density at the surface and to prevent water from sinking)
• ocean waters are arranged in a series of horizontal layers of increasing density--from surface waters to the ocean floor
• 3 principal depth zones:
1. surface zone (mixed layer): approximately 100-200 m thick; generally uniform or nearly uniform in density
2. pycnocline zone: where water density changes markedly with depth; top of zone corresponds approximately to 10°C contour while its bottom corresponds to 4°C contour; is a stable zone because the vertical movements of water in the surface zone and seasonal changes in temperature or salinity do not penetrate the pycnocline
• zone in pycnocline where density is controlled primarily by changes in temperature is the thermocline (this occurs in open ocean water because salinity changes little there)
• zone in pycnocline where density is controlled primarily by changes in salinity is the halocline (this occurs in coastal ocean areas where salinity changes dominate due to runoff and temperature changes are less important)
• in high latitudes (other than the North Pacific and Arctic Ocean), there is no pycnocline because heat is lost from the oceans exceeds heat gained from solar radiation; thus surface waters are cooled and increase in density -- this forms the deep ocean water masses
3. deep zone: contains about 80% of ocean's volume; except at high latitudes, the deep zone is separated from atmosphere -- this isolation prevents interactions with the atmosphere and warming of deep ocean water by solar heating; thus deep zone retains its low temperature (3.5°C)
• colder, higher density water sinks to the depth at which the water below has an even higher density and the water above has a lower density -- it then spreads laterally to form a thin layer extending out from the source and pretty much stays as a separate water mass (called salt lenses)
• the lateral spread can be extensive because density differences inhibit vertical movement and mixing; less energy is needed for this flow than is needed for mixing or vertical movement
• densest water masses are created by cooling or freezing of surface waters in only a few locations at high latitudes; formed by ice exclusion:
• sea ice can only incorporate about 15% of seawater's salt -- salt remaining in unfrozen water beneath ice forms brine

• occurs in the Weddell Sea (bay on Antarctica opposite south end of the Atlantic Ocean); forms the cold, high salinity water mass (-0.5°C; 34.65‰) called Antarctic Bottom Water which sinks to the deep ocean floor and is transported eastward around Antarctica; as it sinks it moves northward into each of the three major ocean basins up to 45°N in N. Atlantic and 50°N in Pacific
• also occurs in the Arctic Ocean, but topography of that basin prevents most of it from escaping -- can't flow into Pacific because of a shallow sill between the Aleutian Islands and the shallow, narrow Bering Strait (which connects the Bering Sea and the Arctic Ocean) -- can't flow into Atlantic Ocean either because of shallow ridges between Scotland and Greenland
• however, get cold water masses in the Norwegian Sea and Greenland Sea which forms the North Atlantic Deep Water (NADW) that sinks and flows south in vast quantities --is one of the most voluminous masses
• NADW has water of higher than average salinity because large amounts of high salinity water are introduced from the Mediterranean Sea (where more water is evaporated and less is added via precipitation) and are carried into high latitudes by the Gulf Stream; this makes NADW distinguishable from Antarctic Bottom Water, and, because it is less dense, it flows over Antarctic Bottom Water

• ocean thermohaline circulation has been portrayed as a global conveyor where ~1000 years is needed to make a complete circuit -- amount of water moved is ~20 times the combined flow of all the world's rivers
• major feature of this model is that deep circulation is driven by the formation of highly saline deep waters in the N. Atlantic
• conveyor begins in the North Atlantic near Iceland and Greenland where surface waters are chilled by the dry winds of the Canadian arctic -- evaporation occurs here due to the dryness, and leaves salt behind; this causes the waters to become more dense, sink, and form the NADW which flows southward
• once NADW reaches Antarctica, it flows eastward around the continent and gets mixed with Antarctic Bottom Water (1 part NADW and 2 parts Antarctic water; the mixing is called caballing) -- it loses its identity about halfway around the continent
• from there deep waters flow northward into the Indian and Pacific Oceans -- here they gradually mix with other warmer waters
• some return to the surface via upwelling around Antarctica
• these upwelled surface waters return to the Atlantic in known and unknown ways, but they mix with the surface waters to form the mixed layer current system
• some waters return by flowing through the islands of Indonesia and then enter the Agulhas Current to return to the Atlantic via the Cape of Good Hope (this transfers heat from the Indian and Pacific Oceans to the Atlantic Ocean, which is why northern Europe is warmer than it should be for its latitude location)
• eventually the waters return to Greenland, where they are cooled and begin the cycle again

• ocean sediment records show that the NADW conveyor belt has been turned off and on during the past tens of thousands of years and that these switches have coincided with abrupt changes in climate and extinction of marine forms
• the hypothesis for this is related to postglacial periods where glaciers melted and their freshwater runoff emptied into the ocean -- the cold melted "fresh" water formed a stable, low density surface layer over a large area of the North Atlantic Ocean; this formed a cap, severely restricting formation of the NADW current and slowing the conveyor belt; last ice age ended 13,500 years ago, but 11,000 years ago there was a short cold period where the northwestern part of Europe's climate became colder within a matter of decades and then rewarmed to its former condition
• once the glaciers finished melting, runoff was reduced, low density surface water eventually became mixed, and the NADW conveyor was able to start up again
• the turn off of the conveyor belt causes a new ice age as northern climates cool without the input of warmer Indian and Pacific Ocean water
• it is unknown if changes in the conveyor belt cause climate changes or if climate changes cause conveyor belt changes; BUT in 1980's, the annual rate of formation of the Greenland Sea Deep Water (one source of NADW) decreased by about 90 percent (which suggests that climate change may drive conveyor belt changes since greenhouse gases increased dramatically during the same time)

II. Ancient Current Patterns

• can reconstruct surface currents because the prevailing winds are primarily a result of the heating of Earth's surface in low latitudes and the cooling of the surface near the pole
• ancient surface currents would thus reconstruct as follows:
• in narrow basins, winds and wind-driven currents would parallel the sides of the basins; thus in the newly formed Atlantic, winds were restricted to a north-south movement and so were currents

• as the basin widened, winds blowing along the north-south axis would weaken to form gyres via the Ekman spiral

• after the Americas separated from Eurasia and Africa, the connection between N. and S. America was still submerged -- once the Tethys Sea opened between Africa and Eurasia (~100 mya), there was a globe-circulating equatorial current system
• this equatorial current system continued until the ends of the Mediterranean Sea closed when Africa and Eurasia came together about 30 mya
• then the land connecting N. and S. America emerged and further disrupted the equatorial current system
• 40 mya Antarctica separated from Australia and Australia moved northward and helped to demark the Indian Ocean
• 30 mya the Drake Passage between S. America and Antarctica deepened permitting the Circum-Antarctic Current to flow around Antarctica, which isolated Antarctica initiating the formation of present ice cap and causing the present glacial climate

III. Ancient Subsurface Currents

• have greater difficulty reconstructing subsurface and bottom water formation sites because these are controlled by details of the basin shape and by climate
• when present cycle of mid-ocean ridge spreading began (200 mya), earth had a warmer climate than present (poles were much warmer and no sea ice formed); thus conditions did not favor formation of cold, dense water masses because there was no chilling of surface waters and no increases in salinity resulting from salts being excluded from newly formed sea ice
• dense seawater probably formed from evaporation in basins at arid mid-latitudes that resulted in warm, but highly saline waters (like the present Mediterranean and Red Seas)
• during the past 50 mya earth's climate cooled; at the beginning of this period, deep waters were probably 13°C but have since cooled to ~0°C
• once the Circum-Antarctic Current isolated the shallow seas around Antarctica the climate cooled enough to permit formation of sea ice (with ice exclusion of salts); Antarctic Deep Water formed and moved northward into the Atlantic, Pacific, and Indian oceans
• 40 mya the Iceland-Faeroe Ridge (between Greenland and Scotland) submerged which then allowed Arctic waters to flow into the North Atlantic; this allowed the formation of the NADW and marked the development of the 3 layered ocean