Ocean mixing and stratification

Why is vertical ocean mixing so much slower than horizontal mixing?

DENSITY: much larger changes in density with depth than horizontal Stratified waters mix vertically much more slowly than unstratified waters --- stratification inhibits turbulent mixing ----*** tropics are highly stratified Over most of the globe, seawater density increases or becomes constant with water depth creating a stable water column * image shows tropics = highly stratified * high latitude area = straight down graph, less stratified

What drives vertical changes in seawater density?

Global surface sea temperatures - sea surface temperature: ------ can be measured by satellite ------- reflect balance of heat gain (solar) and loss (albedo, conduction, evaporation) ------- net heat gain below ~ 30 degrees latitude ------- net heat loss > 30 degrees latitude Global sea surface salinity - Seawater salinity variations driven by: ------- the balance of freshwater gain (precipitation, runoff, ice melt) and loss (evaporation, ice freezing). ------- PSU (practical salinity units) - dimensionless measure of salinity ------- in sub-tropics evaporation > precipitation (means HIGH salinity) *** North Atlantic is saltier than North Pacific -> this is partly why the North Pacific doesn't form deep water ----- dry trade winds evaporate freshwater from atlantic making it salty *away from sea surface, T and S are more constant

Ocean mixing reflects (diffusion types)

Molecular diffusion - a cm scale process. Turbulence diffusion (chaotic flow with irregular fluctuations in speed and direction) - a mm to 100s km process. •wind driven waves •current shear (interfaces between different water bodies flowing in different directions or at different speeds) •movement over an irregular seabed •movement along an irregular sea coast tidalcurrents etc * horizontal mixing is 10^5 to 10^8 faster than vertical mixing

molecular diffusion

Molecular diffusion rates are temperature dependent and are ~10-5 cm2 s-1 in seawater To mix salinity, salt ions have to diffuse amongst the water molecules. (areas with high salinity go to less salinity) To mix heat, high energy ions/molecules (from hotter water) can also diffuse amongst low energy ions/molecules (from cooler water) but heat can also be transferred by high energy ions/molecules interacting with low energy ions/molecules and transferring their energy. Diffusion of heat occurs as high energy molecules (warm water) transit energy to low energy molecules (cooler water). Diffusion of heat is ~ x100 faster than diffusion of salt.

Ocean mixing and stratification Summary

Ocean temperature and salinity are most variable in the upper ocean ( top 500-1000m). The mixed surface layer is up to 200-300 m deep. Below this lies the permanent thermocline (if present). Temperature and salinity control seawater density. Ocean mixing occurs by molecular diffusion and by turbulence. Horizontal mixing is much more rapid than vertical as vertical density changes inhibit mixing.

Salt fingering

Sinking cells of salty water alternate with rising cells of fresher water happens during double diffusion The sinking salty cells have lost their heat and now sink while the rising fresh cells has gained heat so rise. Fingers are 1-5 cm wide and 10s cm long. This is oceanic overturning on a cm scale. Salt fingering is very common in the temperate latitudes as surface waters are SALINE and WARM (increased evaporation) and overlie COLDER FRESHER waters. At 2400 m depth, there is salt fingering in the Atlantic, driven by the saline outflow from the Mediterranean (Mediterranean Intermediate Water). Salt fingering contributes about 20% or more of the new (not recycled) nitrogen supply in some parts of the Atlantic and Indian Oceans (Fernadez-Castro et al Nature Communications 2015)

What is impact of climate change on ocean stratification and primary production?

Surface warming: → increased water stratification → decreased vertical mixing/overturning of surface waters → decreased primary production In the winter the temperature change between the surface and 200 m deep is small (~3 degrees) and there are more phytoplankton (chlorophyll). The temperature change increases to ~8 degrees in the summer and there are less nutrients (there are less phytoplankton. Good correlation between stratification and chlorophyll on a seasonal basis No significant correlation between stratification and primary production on a decadal scale (Lozier et al, 2011)

double diffusion

Temperature will diffuse faster than salt so temperature will become constant throughout the water column. At some point temperature will be constant but salinity will be lower at the surface and higher at depth. The column will become more stable. impact of double diffusion: So at some point temperature will be constant throughout the water column but salinity will not be constant - salinity will be higher at the water top of the column. So the water column will become unstable and will overturn and mix. The temperature gradient will diffuse away in about 1% of the time it takes the salt gradient to diffuse away. The remaining salinity gradient will be unstable and overturning will occur. ****This is a cm scale process

Pacific vs Atlantic (T and Salinity)

The far North Pacific is both colder and fresher than the North Atlantic Paradoxically the cooler temperatures in the North Pacific reduce evaporation so that waters do not become dense enough to form deep waters Both the north and south Atlantics are far saltier than their Pacific counterparts. Dry trade winds (moving east to west along the equator) evaporate freshwater from the Atlantic making it salty. In the North hemisphere this evaporated freshwater crosses Central America and is delivered to the Pacific Ocean making it fresher. This isn't the whole story - water vapour removed from the South Atlantic subtropics moves south (due to a sw directed wind) and rains into the Antarctic Circumpolar Current. The South Atlantic gets saltier and the excess freshwater is carried into the Indian and ultimately to the Pacific Ocean by the Antarctic Circumpolar Current.

Temperature depth profiles

Thermocline = The section of the profile where temperature changes very rapidly Mixed Surface layer = The surface section above this, where temperature shows little variation with depth (typically 10s - ~300m deep) ----- most solar energy is absorbed within a few metres of the ocean surface ***** Heat can be transmitted deeper in the water column by: turbulent mixing (winds, waves), conduction (usually a very small component), downwelling (occasionally) Most of the oceans have a permanent thermocline Seasonal thermoclines: may also form particularly in mid latitudes ----- At mid latitudes, the surface is usually cold and wind speeds are higher in the WINTER and it mixes down to the permanent thermocline ------In SUMMER, temperatures are warmer and conditions become less rough, So there is less efficient mixing of surface warming with the waters at depth ***** The water becomes increasingly stratified (layered) and a seasonal thermocline develops. There is no permanent thermocline at high latitudes, although seasonal thermocline may develop.

Stratification and primary production:

phytoplankton bloom in the North Atlantic over the summer. Note the tropical Atlantic does not show the boom - this area has a permanent thermocline so nutrients at depth are not recycled into the surface layer. The northern area has a seasonal thermocline which breaks down over the winter allowing nutrients in deeper waters to be recycled to the surface. Strong signal from N Sea throughout year is likely a false positive (sediment etc) (sea Lecture 3, slide 33)

Seawater density profiles

sea water density can be calculated using Salinity and Temp ----- Lines joining points of equal density are called isopycnals pynocline = The section of the profile where density changes very rapidly The equator profile has a very low density at the surface (warm surface waters tend to have low density). The tropics profile is similar but less pronounced. The high latitude profile shows little density change with depth - this is an area of deep water formation so water is moving from the ocean surface to the ocean floor with little change in density.

Salinity depth profiles

typically decreases with depth (in the top ~1000m) at low and mid latitudes. (Salinity can also increase with depth) Halocline = The section of the profile where salinity changes very rapidly The mixed surface salinity layer is usuallysimilar in depth to the mixed surface temperature layer. **Salinity depth profiles are more variable