1 SOEE3410: Lecture 13 Ocean Mixed Layer

2 What is the mixed layer? Why is it important? How do the atmosphere and ocean-surface interact? Example of atmosphere – ocean-surface coupling Evidence of climate change? SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

3 Idealised vertical density profiles Mixed Layer SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics Pycnocline Halocline Thermoocline TemperatureSalinity Density Depth (km)

4 Cool skin Warm layer residual mixed layer Thermocline Surface temperature Solar radiation Evaporation ~1-10 cm ~ metres < 1 mm ocean ~ ~ When wind-driven mixing is very low, solar radiation warms a thin layer of water near the surface. This increases local stability and further suppresses mixing. Evaporation of water at the surface causes evaporative cooling, producing a cooler ‘skin’ layer at the top of the warm layer. This is convectively unstable, promoting mixing within the warm layer, and limiting the extent of the cooling in the skin layer.

5 Mixed layer depth: January-March Mixed layer depth: June-August SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics LEVITUS94: World Ocean Atlas 1994, an atlas of objectively analyzed fields of major ocean parameters at the annual, seasonal, and monthly time scales. S94/ Mixed Layer Depths

6 Isotherms slope equatorward in eastern in subtropics Isotherms slope poleward in eastern subpolar basins Confined high salinity regions in subtropics. S Atl > S Pac High salinity ‘escapes’ northward in N Atlantic Levitus et al. World Ocean Atlas 2005 Observed surface temperature and salinity

7 Ocean Mixed Layer What is the mixed layer? Why is it important? How do the atmosphere and ocean-surface interact? Example of atmosphere – ocean-surface coupling Evidence of climate change? SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

8 Heat in the oceans Increase in the heat content of the oceans is an order of magnitude larger than the increase in the atmospheric and cryospheric heat content (Levitus et al., Science, 2001) Global climate change in response to long-term natural and/or anthropogenic forcing depends on the effectiveness of the ocean as a heat reservoir… But, the effectiveness of the ocean as a reservoir is curtailed by increasing thermal stratification which limits the extent to which surface signals can be transmitted to depth via mixing SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

9 Incoming Short wave radiation Q sw Net Long-wave radiation Q lw Net latent heat Q lat Net sensible heat Q sen Q net = Q sw +Q lw +Q lat +Q sen +Q ad Heat fluxes into a region of ocean: Q ad Advection Ocean Atmosphere Air-sea interface +ve -ve Heat budget equation SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics Turbulent Fluxes

10 Climatic importance of the mixed layer Ocean - Atmosphere Interface: ~70 % of Earth’s surface The largest source of water vapour to the atmosphere Huge heat capacity of the oceans slows down climate change (thermal inertia) Deep ocean's slow response  temperature may continue to rise for centuries after stabilization of greenhouse gas levels SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

11 Ocean Mixed Layer What is the mixed layer? Why is it important? How do the atmosphere and ocean-surface interact? Example of atmosphere – ocean-surface coupling Evidence of climate change? SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

12 Ocean–Atmosphere interactions: Oceans and atmosphere are interacting continuously Transferring: momentum, heat, fresh water SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics Wind stress Heat Q net Evaporation Precipitation & run-off

13 Momentum transfer: Wind Stress:  w = C D  a u 2 where:  w = Wind stress (Nm -2 )  a = Density of air ~ 1 (kgm -3 ) C D = Drag coefficient ~ (Dimensionless) u = Wind speed (ms -1 ) Air-sea Boundary => Wind and Wave Driven Mixing SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics ~100m Wind Current velocity ~2% wind speed

14 Heat transfer Solar heating: Short wave radiation penetrates into the water column to heat the water Latent, sensible, longwave radiation (tend to cool the ocean at the surface): Latent heat flux: associated with evaporation of water at the surface Sensible heat flux: associated with temperature difference between the atmosphere and the ocean and the turbulent transfer of heat between the two fluids (can heat or cool surface) Long wave radiation: thermal (infra red) radiation associated with the ocean’s surface temperature Heat transfer within water column results from turbulent heat fluxes (i.e. dependent on wind forced mixing or convection) In addition, heat exchanges between the mixed layer & deeper waters Mostly restricted to specific geographical regions: formation of cold, saline deep waters in polar regions; wind-driven upwelling of cold waters along coastal regions. SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

15 Ocean mixed layer: polar regions Summer: shallow warm surface mixed layers isolate the newly formed deep water from the atmosphere; mean currents and mesoscale eddies steadily transfer the newly formed deep water into the abyssal ocean Winter: deep convection affects local SST directly, and larger scales indirectly through its effect on water mass properties and circulation The maximum depth of convection occurs at the end of the cooling season (Think: plot of heat loss in Central Labrador Sea, THC lecture) Deep convection depends on the balance between cumulative air-sea fluxes i.e. ice-melt, precipitation, and the oceanic advection of buoyancy SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

16 Ocean Mixed Layer What is the mixed layer? Why is it important? How do the atmosphere and ocean-surface interact? Example of atmosphere – ocean-surface coupling Evidence of climate change? SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

17 What is the El Nino-Southern Oscillation? oscillation in ocean-atmosphere system that occurs in tropical Pacific; occurs every 2-7 years; associated with unusually warm waters in eastern Pacific; impacts on weather around the globe – e.g. increased rainfall in southern US, bush-fires in Australia  important to understand the driving mechanisms and to be able to predict the onset. El Nino – Southern Oscillation (ENSO) Source: NOAA website

18 Normal conditions: strong easterly trade winds across Pacific – from South Pacific High to Indonesian Low warm waters pile up in West Pacific El Nino conditions: Indonesian Low and S Pacific High both weaken trade winds relax sea-surface “collapses” Understanding ENSO Source: NOAA website thermocline sea-surface 0.5m E WE W sea-surface thermocline

19 Monitoring ENSO Source: TOA website Tropical Atmosphere-Ocean Project 1982/83 el Nino event was particularly strong & not expected since 1984, an array of moorings has developed now 70 moorings returning real-time oceanographic / meteorological data to shore via satellite Lots of information / data at:

20 Ocean Mixed Layer What is the mixed layer? Why is it important? How do the atmosphere and ocean-surface interact? Example of atmosphere – ocean-surface coupling Evidence of climate change? SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

21 Climate implications: atmosphere-ocean boundary Both coupled climate models and observations suggest: the heat content of the ocean is increasing Rising SST and atmospheric water vapour is increasing : potentially enhancing tropical convection, including thunderstorms, and the development of tropical storms SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

22 Barnett et al., SCIENCE, VOL 309, 8 JULY, 2005 Ocean Warming (Internal/Solar/Volcanic Variability) SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

23 Barnett et al., SCIENCE, VOL 309, 8 JULY, 2005 Ocean Warming (Anthropogenic Forced) SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

24 (Webster et al., 2005, Science, Vol 309) SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics Ocean feedback on atmosphere: hurricane frequency & intensity I

25 (Webster et al., 2005, Science, Vol 309, ) SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics Ocean feedback on atmosphere: hurricane frequency & intensity II

26 SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics Ocean feedback on atmosphere: hurricane frequency & intensity III

27 Ocean mixed layer is the connection between ocean and atmosphere Climatically, the ocean acts as a heat reservoir Air-sea fluxes transfer momentum, heat, freshwater => affect properties of mixed layer El Nino – example of large-scale effects of air-sea interaction Evidence of climate impact on ocean, and ocean on atmosphere Summary – ocean mixed layer