Global Ocean Circulation (2) Focus on conservation principles: Balancing heat, salt, etc. into and out of boxes Wind-driven gyre-scale circulation of the surface ocean and upper thermocline Global heat and freshwater water transport, conservation properties Water mass properties, polar convection and mixing, tracers, global thermohaline circulation © Donglai Gong

Why should the thermocline slope down toward the west Why should the thermocline slope down toward the west? It does this in the subtropical gyres of all the oceans. Westward propagation of Rossby waves

Sea surface topography variations Phenomenon Typical Surface Expression Period of Variability Comments Western boundary currents (Gulf Stream, Kuroshio) 130 cm/100 km Days to years Variability in position, and 25% variability in transport Large gyres 50 cm/ 3000 km One to many years 25% variability expected Eastern boundary currents 30 cm/100 km 100% variability expected, possible direction reversals Mesoscale eddies 25 cm/100 km 100 days 100% variability Rings 100 cm/100 km Weeks to years 100% variability, growth and decay Equatorial currents 30 cm/5000 km Months to years Tides 100cm/5000 km Hours to years Aliased to low frequency http://jeffreyearly.com/science/qg-eddies-paper/

Box A: Between the Trades and the westerlies the Ekman transport is converging Box B is the same: the reversal in direction of the Ekman transport in the Southern Hemisphere means this is still convergence

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http://ferret. pmel. noaa. gov/nvods/UI http://ferret.pmel.noaa.gov/nvods/UI.vm#panelHeaderHidden=false;differences=false;autoContour=false;globalMin=2.566;globalMax=29.03;xCATID=WOA_05_CAT;xDSID=woa05_monthly;varid=t0112an1;imageSize=auto;over=xy;compute=Nonetoken;tlo=16-Jan-0001;thi=16-Jan-0001;catid=WOA_05_CAT;dsid=woa05_monthly;varid=t0112an1;avarcount=0;ylo=21;yhi=21;xlo=110;xhi=260;ylo=21;yhi=21;zlo=0;zhi=1500;operation_id=Plot_2D_zoom;view=xz

East-west slope of thermocline comes about because of balance between Ekman pumping and westward traveling Rossby waves. With a map of north-south velocity to go with this temperature map, we can calculate total north-south heat transport. Everywhere between the maximum of the westerlies, and the maximum of the Trades, there is convergence of Ekman transport

With a map of north-south velocity to go with this temperature map, we can calculate total north-south heat transport. wind

zone of Ekman convergence max. of westerlies westerlies easterlies zone of Ekman convergence max. easterlies East-west slope of thermocline comes about because of balance between Ekman pumping and westward traveling Rossby waves. With a map of north-south velocity to go with this temperature map, we can calculate total north-south heat transport.

Bowl of subtropical gyre shows westward and eastward flow into and out of the page forming the gyre circulation.

http://ferret.pmel.noaa.gov/nvods Mean SST shows cooler regions in the east, why? Warm Pool. Example slices through World Ocean Atlas Xy: Surface O2 (follows cold water) Salinity at 1000m (Med Sea outflow) Yz: (Atlantic) Salt Silicate Yz: (Pacific) Salt – fresher in far north – tongue of subducting water

With a map of north-south velocity to go with this temperature map, we can calculate total north-south heat transport. wind

Review of air-sea heat exchange processes

Positive implies ocean warming Warming everywhere – of course Shortwave Everywhere warming the ocean (obviously) - Where is greatest shortwave, and why?

Net longwave is less in WPWP than in east yet water is warmer? Positive implies ocean warming Negative implies ocean cooling Net longwave Values are negative => net longwave loses heat from the ocean Sum of downward longwave emitted by clouds and water vapor Why is net upward longwave less in WPWP than East Pacific?

Positive implies ocean cooling Western boundary currents have greatest air-sea temperature difference Sensible Positive implies cooling – the atmosphere cools the ocean on average everywhere Local hot spots in Kuroshio and Gulf Stream have greatest air-sea temperature difference

Positive implies ocean cooling Distribution largely follows SST Latent Positive: ocean is cooling. Distribution largely follows SST Elevated values in WBC southeast Pacific (warm and relatively clear skies – presumably less water vapor) COADS monthly climatology http://ferret.pmel.noaa.gov/NVODS/servlets/dataset?catitem=18098

Shaded is net cooling Shaded is net cooling White is net warming Western boundary currents are all cooling Equator is warming – especially in the east

Western boundary currents are all cooling Shaded is net cooling Western boundary currents are all cooling Equator is warming – especially in the east Shaded is net cooling White is net warming Western boundary currents are all cooling Equator is warming – especially in the east

Shaded is net cooling Shaded is net cooling White is net warming Western boundary currents are all cooling Equator is warming – especially in the east

Shaded is net cooling Shaded is net cooling White is net warming Western boundary currents are all cooling Equator is warming – especially in the east

Oceanic heat transport. Box balance of fluxes integrated across the entire basin from coast to coast. These values agree reasonably with direct estimates from ocean observations. Notice again that the Atlantic is different – the heat transport is northward at al latitudes

RT = Radiation total at the top of the atmosphere from Earth Radiation Budget Experiment OT = ocean transport from net air-sea heat flux AT = atmospheric transport (calculated from RT – OT) Radiation Total (RT) at the top of the atmosphere from the Earth Radiation Budget Experiment (ERBE) satellites. Ocean Transport from air-sea flux divergence. AT from the residual. Similar pattern with atmosphere doing 2 to 3 times as much transport of heat.

East-west slope of thermocline comes about because of balance between Ekman pumping and westward traveling Rossby waves. With a map of north-south velocity to go with this temperature map, we can calculate total north-south heat transport. http://ferret.pmel.noaa.gov/NVODS/servlets/dataset?catitem=5707

Bars show direct estimates from oceanographic cross-sections Oceanic heat transport. Box balance of fluxes integrated across the entire basin from coast to coast. These values agree reasonably with direct estimates from ocean observations. Notice again that the Atlantic is different – the heat transport is northward at al latitudes

Atlantic has northward heat transport everywhere Heat transport from ocean observations constrained by conservation principles Atlantic has northward heat transport everywhere

Ocean heat transport implied from the divergence of the air-sea heat flux. Notice again that the Atlantic is different. These calculations are made assuming conservation of salt.

Atlantic heat transport is so different because there is a vertical over-turning circulation that shows up clearly in patterns of salinity Why the Atlantic is so different Overturning circulation why doesn’t it overturn at the equator?

Where evaporation is strong we tend to have high salinity Low salinity in north but no large negative E-P. What other freshwater sources are there? South North Now consider freshwater balances. Global zonal average – no surprises here. Very low salinity in the northern more latitudes but not especially low E-P what freshwater sources are not included in E-P? - river runoff - floating pack ice being exported from the Arctic

Precipitation minus evaporation (m year-1) Shaded implies net precipitation Earth’s water budget is dominated by P-E over the ocean (80% occurs there) Precipitation minus evaporation (m year-1) Max P-E in East Pacific is not exactly over the equator – why? Earth’s water budget is dominated by E-P over the ocean, about 80% occurs there. White = net evaporation Shaded = net precipitation Max in east pacific is not exactly over the equator, why?

Surface Atlantic is saltier than the Pacific Many large rivers drain into the Atlantic and the Arctic Sea, so why is the Atlantic saltier than the Pacific? Low salinity follows the regions of greatest negative E-P. The mass of salt in the ocean stays constant, with inputs from erosion via rivers balancing deposition to sediments. The atmosphere and ocean can exchange freshwater, but not salt. So conservation of salt principles are coupled to freshwater transport. Atlantic is saltier than the Pacific Surface Atlantic is saltier than the Pacific

Broecker (1997) showed that 0 Broecker (1997) showed that 0.32 Sv of the water evaporated from the Atlantic does not fall as rain on land. It is carried by winds into the Pacific. The quantity is small, equivalent to little more than the flow in the Amazon River. But were this flux not compensated by an exchange of more salty Atlantic waters for less salty Pacific waters, the salinity of the Atlantic would rise about 1 gram per liter per millennium. Many large rivers drain into the Atlantic and the Arctic Sea, so why is the Atlantic saltier than the Pacific? Broecker (1997) showed that 0.32 Sv of the water evaporated from the Atlantic does not fall as rain on land. It is carried by winds into the Pacific. The quantity is small, equivalent to a little more than the flow in the Amazon River. Were this flux not compensated by an exchange of more salty Atlantic waters for less salty Pacific waters, the salinity of the entire Atlantic would rise about 1 gram per liter per millennium.”

© Donglai Gong

(a) (b) Make the plot of the 26N section more obvious with the boundary current Extedn the n-s secion in the quiz document (c) (d)