SIO 210 Typical distributions (2 lectures) Fall 2014 Reading: DPO Chapter 4 Chapter S7 (or 7) sections 7.4.1, 7.8.5, 7.10.1, 7.10.2 First lecture 1. Definitions - structures 2. Concepts 3. Water masses 4. 4-layer structure Second lecture 1. Upper layer 2. Intermediate layer 3. Deep and bottom layers 4. Time scales Talley SIO210 (2014)

The approximately layered structure of the top-to-bottom ocean: Now in more detail Upper ocean (down through the permanent pycnocline) a. Surface mixed layer b. Pycnocline/thermocline c. Pycnostad/thermostad embedded in pycnocline (“mode water”) Intermediate layer Deep layer Bottom layer Talley SIO210 (2014)

Mixed layers Surface layer of the ocean is almost always vertically mixed to some degree In summer, calm, warm conditions, the mixed layer might be very thin (several meters) At the end of winter, after the full season of cooling and storms, mixed layers reach their maximum thickness Mixed layers are created by Wind stirring (max. depth of such a mixed layer is around 100 m) Cooling and evaporation (increasing the density of the surface water), which creates vertical convection. Max. depth of these mixed layers can range up to about 1000 m, but is mainly 200-300 m. Talley SIO210 (2014)

Maximum mixed layer depth (mainly late winter in each location) Typically 20 to 200 m Thicker (> 500) in some special locations, notably in (1) band in the Southern Ocean and (2) northern North Atlantic Repeated from last lecture Using delta T = 0.2°C DPO Fig. 4.4c from Holte et al Talley SIO210 (2014)

Mixed layer development Winter development of mixed layer: Wind stirring and cooling erode stratification, gradually deepening the mixed layer to maximum depth at the end of winter (Feb. to April depending on location) Summer restratification: Warming at the top adds stratified layer at surface, usually leaves remnant of winter mixed layer below. DPO Figure 4.8 Large, McWilliams and Doney (Rev. Geophys 1994) Talley SIO210 (2014)

Mixed layer development Winter development of mixed layer: Wind stirring and cooling erode stratification, gradually deepening the mixed layer to maximum depth at the end of winter (Feb. to April depending on location) Summer restratification: Warming at the top adds stratified layer at surface, usually leaves remnant of winter mixed layer below. DPO Figure 7.3 Talley SIO210 (2014)

The approximately layered structure of the top-to-bottom ocean Upper ocean (down through the permanent pycnocline) a. Surface mixed layer b. Pycnocline/thermocline c. Pycnostad/thermostad embedded in pycnocline (“mode water”) Intermediate layer Deep layer Bottom layer Talley SIO210 (2014)

Thermocline (pycnocline) (1) DPO Fig. 4.5 Two separate physical processes: Vertical balance: mixing between warm, light surface waters and cold, dense deep waters, plus upwelling (diffusive process) Circulation of denser surface waters down into interior and thus beneath the lower density surface layers (subduction) (advective process) Talley SIO210 (2014)

Thermocline (pycnocline) (2) DPO Fig. 7.15 Two separate physical processes: Vertical balance: mixing between warm, light surface waters and cold, dense deep waters, plus upwelling (diffusive process) Circulation of denser surface waters down into interior and thus beneath the lower density surface layers (subduction) (advective process) Talley SIO210 (2014)

Creation of the thermocline through subduction Iselin (1939): equivalence of surface properties on transect through N. Atlantic with properties on a vertical profile in the subtropical gyre --> hypothesized that properties are advected into the interior from the sea surface Circles: section 1 Squares: section 2 Continuous plots: vertical profiles x x Talley SIO210 (2014)

The approximately layered structure of the top-to-bottom ocean Upper ocean (down through the permanent pycnocline) a. Surface mixed layer b. Pycnocline/thermocline c. Pycnostad/thermostad embedded in pycnocline (“mode water”) Intermediate layer Deep layer Bottom layer Talley SIO210 (2014)

Thermostad development: Subtropical Mode Water (Eighteen Degree Water) WHP Atlas Atlantic Pot. Temp. q Section across Gulf Stream Thickening of isotherms/isopycnals is the thermostad/pycnostad Forms at surface as a thick mixed layer near Gulf Stream in late winter. Circulates into the interior south of the Gulf Stream along isopycnals Neutral density Talley SIO210 (2014)

Mode water: definition, location and development Pycnostads/thermostads embedded in the pycnocline occur in identifiable regions They usually occur on the warm (low density) side of strong currents Example (previous slide): Gulf Stream has a pycnostad/thermostad at about 18°C on its south (warm) side. Because a pycnostad has a large volume of water in a given temperature-salinity interval, these waters were termed “Mode Waters”, to indicate that the the mode of the distribution of volume in T/S space occurs in these particular T/S ranges. Talley SIO210 (2014)

Mode Waters Hanawa and Talley (2001); DPO 14.12 Location of especially strong, permanent thermostads/pycnostads - derived from thick winter mixed layers that then spread into the interior along isopycnals (subduct) Gulf Stream’s Eighteen Degree Water (Subtropical Mode Water of the North Atlantic) from previous slide Hanawa and Talley (2001); DPO 14.12 Talley SIO210 (2014)

Importance of mode waters for dissolved gas inventories Chlorofluorocarbon (CFC) water column inventory (conservative anthropogenic tracer) Willey et al. (GRL 2004) Talley SIO210 (2014)

Importance of mode waters for dissolved gas inventories Anthropogenic CO2 Khatiwala et al. (Biogeosciences 2013) Talley SIO210 (2014)

The approximately layered structure of the top-to-bottom ocean We are using four layers to describe the world’s oceans. Upper ocean (down through the permanent pycnocline) Intermediate layer Deep layer Bottom layer Talley SIO210 (2014)

Pacific intermediate waters Intermediate depth (500-2000 m), vertical salinity minima Antarctic Intermediate Water (AAIW) North Pacific Intermediate Water (NPIW) DPO Fig. 4.12 Talley SIO210 (2014)

Intermediate water masses Pacific intermediate waters from previous slide Intermediate water production sites DPO Fig. 14.13 Labrador Sea Water: salinity minimum, deep convection in Labrador Sea Mediterranean Overflow Water: salinity maximum, evaporation and cooling in Mediterranean Sea, overflow Antarctic Intermediate Water: salinity minimum, medium convection in Drake Passage region Red Sea Overflow Water: salinity maximum, evaporation in Red Sea, overflow North Pacific Intermediate Water (Okhotsk Sea): salinity minimum, brine rejection in the Okhotsk Sea Talley SIO210 (2014)

Atlantic intermediate waters Intermediate depth (500-2000 m), vertical salinity minima AND maximum Mediterranean Water (MW) Antarctic Intermediate Water (AAIW) Labrador Sea Water (LSW) DPO Fig. 4.11 Talley SIO210 (2014)

Intermediate water masses Atlantic intermediate waters from previous slide Intermediate water production sites DPO Fig. 14.13 Labrador Sea Water: salinity minimum, deep convection in Labrador Sea Mediterranean Overflow Water: salinity maximum, evaporation and cooling in Mediterranean Sea, overflow Antarctic Intermediate Water: salinity minimum, medium convection in Drake Passage region Red Sea Overflow Water: salinity maximum, evaporation in Red Sea, overflow North Pacific Intermediate Water (Okhotsk Sea): salinity minimum, brine rejection in the Okhotsk Sea Talley SIO210 (2014)

Atlantic intermediate waters viewed in Potential temperature-salinity Mediterranean Overflow Water Labrador Sea Water North Atlantic Deep Water Antarctic Intermediate Water Antarctic Bottom Water Blue: N. Atlantic > 15°N Red: 15°S-15°N Green: S. Atlantic < 15°S Talley SIO210 (2014)

The approximately layered structure of the top-to-bottom ocean Four layers to describe the world’s oceans. Upper ocean (down through the permanent pycnocline) Intermediate layer Deep layer Bottom layer Talley SIO210 (2014)

Bottom properties Potential temperature: high in N. Atlantic and eastern S. Atlantic (other highs are due to shallower bottom) Salinity: high in N. Atlantic and Indian DPO 14.14b,c (Mantyla and Reid, 1983) Talley SIO210 (2014)

Deep and bottom water DPO Fig. 14.14a Deep and bottom water production sites sites DPO Fig. 14.14a North Atlantic Deep Water: high salinity, high oxygen; mixture of NSOW, LSW and MOW; formed at sea surface through deep convection Antarctic Bottom Water: very cold, high oxygen; formed near sea surface along coast of Antarctica through sea ice formation-brine rejection Indian and Pacific Deep Waters: low oxygen, high nutrients; slow upwelling and slow deep mixing of inflowing NADW and AABW Talley SIO210 (2014)

Atlantic deep and bottom waters Antarctic Bottom Water (AABW) Cold bottom waters from Antarctic region Talley SIO210 (2014)

Atlantic deep and bottom waters North Atlantic Deep Water (NADW) (high salinity in tropics and S. Atlantic) Antarctic Bottom Water DPO Fig. 4.11 Talley SIO210 (2014)

Atlantic deep and bottom waters (This very low O2 is due to intense biological activity and not age) Labrador Sea Water North Atlantic Deep Water (NADW) (high oxygen) Antarctic Bottom Water DPO Fig. 4.11 Talley SIO210 (2014)

NADW and AABW in the abyssal ocean NADW and AABW both occupy the deep and bottom layers, although AABW clearly dominates at the bottom. Maps of the fraction of water at mid-depth and at the bottom that are NADW or AABW. (Only two water masses were included in the analysis: these are the surface source waters.) (Johnson et al., 2008) DPO 14.15 Talley SIO210 (2014)

Atlantic deep/bottom waters viewed in Potential temperature-salinity Mediterranean Overflow Water Labrador Sea Water North Atlantic Deep Water Antarctic Intermediate Water Antarctic Bottom Water Blue: N. Atlantic > 15°N Red: 15°S-15°N Green: S. Atlantic < 15°S Talley SIO210 (2014)

Pacific deep and bottom waters Remnant NADW (high salinity) Antarctic Bottom Water (lower salinity) DPO Fig. 4.12 Talley SIO210 (2014)

Pacific deep and bottom waters Pacific Deep Water (low oxygen, old water) Antarctic Bottom Water (high oxygen) DPO Fig. 4.12 Talley SIO210 (2014)

Pacific deep and bottom waters Pacific Deep Water (extreme carbon-14, and no CFCs – very old water) DPO Fig. 4.24 Talley SIO210 (2014)

Ventilation age (years) based on radiocarbon (Broecker et al., 2004) Based on difference in radiocarbon age between surface and deep water. (Taking into account anthropogenic (bomb) radiocarbon in the surface waters, the actual deep Pacific age should be more like 1250 years.) (Broecker et al., 2004) Broecker et al. (2004) Talley SIO210 (2014)

Global deep water potential temperature-salinity Worthington, 1982 4°C 0°C Pacific Deep Water (or Common Water) North Atlantic Deep Water Indian Deep Water Antarctic Bottom Water DPO 4.17b Talley SIO210 (2014)

Deep and bottom water DPO Fig. 14.14a Deep and bottom water production sites DPO Fig. 14.14a Nordic Seas Overflow Water (contributor to North Atlantic Deep Water): high oxygen; deep convection in the Greenland Sea, overflow North Atlantic Deep Water: high salinity, high oxygen; mixture of NSOW, LSW and MOW Antarctic Bottom Water: very cold, high oxygen; brine rejection along coast of Antarctica Indian and Pacific Deep Waters: low oxygen, high nutrients; slow upwelling and slow deep mixing of inflowing NADW and AABW Talley SIO210 (2014)