Physical Oceanography: Thermohaline Circulation PDF

Physical Oceanography: Thermohaline Circulation Prof. Ogbaekwe Office Hours: 10-12 on Mondays Thermohalin e circulation Thermohaline circulation = currents driven by density named as such because density is determined by temperature (“thermo”) and salinity (“haline”) The Great Ocean C...

Physical Oceanography: Thermohaline Circulation Prof. Ogbaekwe Office Hours: 10-12 on Mondays Thermohalin e circulation Thermohaline circulation = currents driven by density named as such because density is determined by temperature (“thermo”) and salinity (“haline”) The Great Ocean Conveyor Belt  masses Being able to interpret this plot is super important! T-S diagrams distinguish water masses T-S diagrams distinguish water masses What is the main factor involved in changing density?  T-S diagrams distinguish water masses Intermediate water from the Indian Ocean in the Red Sea T-S diagrams distinguish water masses Why this works: - Once seawater sinks below the surface, its temperature and salinity essentially do not change - Thus, temperature and salinity are considered to be “conservative” - Temperature and salinity mix linearly on a T-S diagram T-S diagrams distinguish water masses and c e n urfa s ee betw AIW ixing A M T-S diagrams distinguish water masses Mixin g AAI W to AABW T-S diagrams distinguish water masses IW AA he s no re wa re the IF Water- column stratification One Argo float near Hawaii Water- column stratification Surface mixed layer Thermocline (a rapid change in temperature Within the water column) One Argo float near Haw Water- column stratification Water- column stratification One Argo float near Haw  Density can only increase with depth (stratification) Any instance of higher density above lower density will quickly be alleviated by buoyancy One Argo float near Haw  Density can only increase with depth Pycnocline = layer in (stratification) which density changes rapidly Pycnocline - “thermocline” and “halocline” are similar, but for temp. and salinity ne Argo float near Hawaii  Density can only increase with depth In situ vs potential density (stratification) One Argo float near Haw T-S diagrams distinguish water masses What is the main factor involved in changing density? T-S diagrams show mixing All of the HOT data Deep-water formation sites  Deep-water formation sites Deep water can only form in the absence of density stratification  Deep-water formation sites Deep water can only form in the absence of density stratification  Deep-water formation sites Deep water can only form in the absence of density stratification  Deep-water formation sites Sea surface salinity Deep water can only form in the absence of density stratification How???  Deep-water formation sites Deep water can only form in the absence of density stratification How???  Deep-water formation sites Deep water can only form in the absence of density stratification. How does this occur? At the surface: low temperature + high salinity = high density = less stratification Deep-water formation sites Deep-water formation sites Deep-water formation sites Deep-water formation sites How do we know this pathway?  How do we know this pathway? Thermohaline circulation is slow! - ~10 km per year (= 0.03 cm/s) - Measured in some instances with deep-sea drogues - But mainly by chemical tracers!  Implications for climate The site of deep-water formation in the North Atlantic change? is one of the few places where surface temperatures are cooling! Why? Rate of change in sea surface temperature (SST) From 1870 to 2016  Implications for climate The site of deep-water formation change? in the North Atlantic is one of the few places where surface temperatures are cooling! Why? Melting ice from Greenland is freshening this region, which increases stratification Rate of change in sea surface temperature (SST) From 1870 to 2016  Implications for climate AMOC  Atlantic Meridional overturning circulation change? Includes both deep and surface currents The melting of the glacial ice in Greenland has weakened the AMOC. The layer of lower density water prohibits the formation of deep water. What about the end of the pathway?  What about the end of the pathway? What goes down, must come up…. There must be an equal amount of water rising elsewhere to balance the water sinking at the deep-water formation sites These vertical currents are far too slow to measure - Occurs through upwelling - along coastlines - along the equator - Occurs through slow mixing into the surface layer T-S diagrams distinguish water masses AA IW nd r face a en su t we ng be Mixi Mixing AAIW to A ABW If no mixing occured If everything was completely mixed Reality: slow, gradual mixing The temperature of the entire deep ocean depends on the temperature where deep water forms warm deep water, analogous to past warm climates Sofianos and Johns (2015) warm deep water, analogous to past warm climates Sofianos and Johns (2015) warm deep water, analogous to past warm climates Sofianos and Johns (2015) warm deep water, analogous to past warm climates Sofianos and Johns (2015)