Phys 2300 Finals Review PDF

Intro to Physical Oceanography Midterm 1 Review Slides PHYS/OCSC 2300 Fall 2024 Dr. Joe Fitzgerald  Earth’s Oceans Atlantic Ocean Half the size of the Pacific Ocean Shallower than the Pacific Ocean Named for Atlas Indian Ocean Smaller than the A...

Intro to Physical Oceanography Midterm 1 Review Slides PHYS/OCSC 2300 Fall 2024 Dr. Joe Fitzgerald  Earth’s Oceans Atlantic Ocean Half the size of the Pacific Ocean Shallower than the Pacific Ocean Named for Atlas Indian Ocean Smaller than the Atlantic Ocean Similar depth as the Atlantic Ocean Primarily in the Southern Hemisphere Named for proximity to India  Earth’s Oceans Arctic Ocean Seven percent the size of the Pacific Ocean Shallowest world ocean Permanent layer of sea ice a few meters thick Named for the constellation Ursa Major (arktos=bear) Question: What time of year would you expect sea ice to reach it’s annual minimum? Why?  Nebular Hypothesis Gravity concentrates material at center of cloud (Sun). Nebula evolves into a rotating disk Pizza dough analogy Earth’s equatorial bulge (40 km) Protoplanets form from smaller concentrations of matter (aided by eddies). Protoearth Protoearth becomes hot: Radioactive heat Spontaneous disintegration of atoms Fusion reactions Heat from contraction (protoplanet shrinks due to gravity) Heat from collisions Protoearth partially melts Melting enables density stratification (layered Earth) Density Stratification High density = heavy for its size (mass per unit volume) Early Earth experienced gravitational separation: High-density materials (iron and nickel) settled in core. Less dense materials formed concentric spheres around core.  Earth’s Internal Structure Crust Low-density, mainly silicate minerals 3-km thick: skin of an apple Oceanic vs continental crust Mantle Mainly iron (Fe) and magnesium (Mg) silicate minerals Extends to ~2885km depth Core High-density, mainly iron (Fe) and nickel (Ni) -metals From ~2885km – 6371km (Earth’s radius) Development of Earth’s Oceans  Evidence for Continental Drift Wegener proposed Pangaea—one large continent existed 200 million years ago (pan = all, gaea = Earth) Panthalassa—one large ocean, including the Tethys Sea Noted puzzle-like fit of modern continents Evidence for Continental Drift Glacial Evidence Evidence of glaciation in now tropical regions Alternative explanation? Global climate change (snowball earth) Direction of glacial flow and rock scouring  Snowball Earth During periods of extreme cold in Earth’s climate system, the surface of Earth may have been entirely frozen Thought to have occurred ~650 MYA (million years ago) Observational evidence? Artist’s rendition from Wikipedia Dropstones are one example Mechanism is the Ice-Albedo Feedback More ice -> more reflected radiation -> cooler earth -> More ice -> … How to escape from this state? Mudball (dust on ice surface reduces albedo/increases absorption) High CO2 in atmosphere Dropstone: A rock of a different type than its surrounding rocks that was dropped into a region from ice Plate Continental crust is pushed away from mid-ocean ridge Tectonic Processes Mid-ocean ridge— spreading center * young c ks Subduction zones—oceanic trench site of crust destruction Subduction can generate deep ocean trenches. Convecting mantle rocks are hot but not molten! * old rocks Age of Ocean Floor Youngest oceanic rocks at mid-ocean ridges Oldest rocks near subduction zones Hydrogen Bonding Polarity means small negative charge at O end Small positive charge at H end Attraction between positive and negative ends of water molecules to each other or other ions Hydrogen Bonds in Three States of Water Water’s Heat Capacity Heat Capacity—amount of heat required to raise the temperature of 1 gram of any substance by 1°Celsius Water has a high heat capacity—can take in or lose a relatively large amount of heat without changing temperature – hydrogen bonding Specific Heat—heat capacity per unit mass 1 calorie = 4.184 J (Note we use kilocalories for nutrition but still call them calories for historical reasons) Latent Heat: Heat absorbed or released during phase change The Real Version: Evaporation-Precipitation Eq: E-P0 Evaporation latitudes Midlatitudes /Polar: E-P S = (1/.55)*C ~ 1.818*C (Difference from 1.80655 is an empirical correction due to inexactness of constant proportion rule) Determining Salinity Salinometer Measures water’s electrical conductivity More dissolved substances increase conductivity  Salinity Variations Open-ocean salinity is ~33-38 ppt In coastal areas, salinity varies more widely. Brackish waters (~10 ppt) Influx of fresh water from rivers or rain lowers salinity. Occurs near coastlines Hypersaline What is the effect of having your legs out of the water while floating in the Dead Sea? High evaporation conditions Easier/harder to stay afloat? Great Salt Lake salinity = 280 ppt Dead Sea salinity = 330 ppt Archimedes’ Principle Archimedes of Syracuse (287-212 BC) Greatest mathematician of ancient history Quantification of the Force of Buoyancy Archimedes Principle Force of Buoyancy = Weight of the Medium (e.g., water) Displaced F = (Density of displaced seawater)*(Volume of displaced seawater)*g g = 9.8 m/s^2 Processes Affecting Salinity SmartTable 5.3 Processes that affect seawater salinity Salinity Effect on salt Effect on increase How Adds or in H2O in or Process accomplished removes seawater seawater decrease? Precipitation Rain, sleet, hail, Adds None More H2O Decrease or snow falls very directly on the fresh ocean water Runoff Streams carry Adds Negligible More H2O Decrease water to the mostly addition of ocean fresh salt water Icebergs Glacial ice Adds None More H2O Decrease melting calves very into the ocean fresh and melts water Are icebergs fresh or salty? Why? Fresh – icebergs calve off glaciers formed by snowfall which is freshwater Processes Affecting Salinity (3 of 3) SmartTable 5.3 [Continued] Effect on Effect on Salinity How Adds or salt in H2O in increase or Process accomplished removes seawater seawater decrease? Sea ice Sea ice melts in Adds Adds a More H2O Decrease melting the ocean mostly small fresh amount of water salt and some salt Sea ice Seawater Removes 30% of salts Less H2O Increase forming freezes in mostly in seawater cold ocean freshwat are areas er retained in ice Evaporation Seawater Removes None Less H2O Increase evaporates very pure (essentially in hot climates water all salts are left behind)  Earth’s Hydrologic Cycle Processes that affect seawater salinity Recycles water among ocean, atmosphere, and continents Water in continual motion between water reservoirs Huge volumes of water (hundreds of thousands of cubic kilometers) move through these pathways each year Cycling of Dissolved Seawater Components Volcanic activity Adsorption onto surfaces Biology Precipitation (in the sense of a chemical precipitate) River discharge contains ions Dust deposition (Sahara dust storms) Hydrothermal activity Sea spray Chemical analysis: The entire ocean volume is recycled in mid-ocean ridges every 3 million years!  Real Ocean Equation of State Seawater EOS is simple conceptually: ”Official” Ocean EOS: TEOS-10 (Thermodynamic equation of seawater) This manual is Some of the >200 pages! computer codes involved  Approximate ocean EOS: The ‘linearized equation of state’ Example: Use the linearized EOS to estimate the density of seawater with temperature T=21 C and salinity S = 36 ppt kg/m^3 Quite accurate! Compare to ‘exact’ result kg/m^3 from TEOS-10 Surface Salinity Variation High latitudes Low salinity Sea ice melting, precipitation, and runoff Mid latitudes/sub-tropics High salinity Warm, dry, descending air increases evaporation (Descending branch of the Hadley Cell (cover later)) Low latitudes near equator Local salinity minimum High precipitation and runoff Note that salinity dips due to precip but is still relatively high  Salinity Variation with Depth Mixed layer at surface Winds & surface cooling drive convection/turbulence that mixes the near-surface waters Halocline: region of abrupt change in salinity with depth Low latitudes: S decreases with depth. High at surface (high evaporation), decreases below. low lat ↓ as X - = High latitudes: S increases with depth. Low at surface (runoff/precip), increases below. muh lat ↑as] = Deep ocean: Salinity fairly consistent globally Temperature and Density Variations with Depth Mixed layer (0-100m): Homogeneous density and temperature due to mixing driven by turbulence (convection/waves/wind) Thermocline/Pycnocline (100-1000m): Sharp decrease in temperature/increase in density. Low latitudes: Density mostly controlled by temperature (mirror images in top plots) High latitudes: Low temperatures, high densities, little structure in the vertical. Terminology: Pycnocline—abrupt change of density with depth Thermocline—abrupt change of temperature with depth  Earth’s Seasons Earth’s rotation axis is tilted 23.5 degrees with respect to plane of the ecliptic (plane of orbit around Sun) Key times of year: Winter solstice (Dec. 22) Vernal equinox (Mar. 21) Summer solstice (Jun. 21) Autumnal equinox (Sep. 23) Equinoxes: Sun directly overhead at equator Solstices: Sun directly overhead at tropic of cancer/Capricorn Arctic/Antarctic Circles No sunlight during winter  Distribution of Solar Energy Due to all these factors, intensity of radiation (number of photons absorbed per unit area) is greatly reduced in high latitudes compared to equatorial region Concentration of photons (area effect) Thickness of atmosphere vs latitude Albedo including angle- dependence Net surface solar radiation = incoming - reflected  Heat Gained and Lost by Oceans High latitudes—more heat lost than gained (to atmosphere and space) Ice has high albedo Low solar ray incidence Low latitudes—more heat gained than lost Equator-to-pole temperature gradient Circulation of ocean and atmosphere transfer heat to maintain equilibrium Physical Properties of the Atmosphere Composition Mostly nitrogen (N2) and oxygen (O2) Other gases significant for heat- trapping properties (greenhouse effect) Methane Water vapor Nitrous Oxide (N2O)  Temperature Variation in the Atmosphere Troposphere—lowest layer of atmosphere Atmosphere mostly transparent to solar radiation which is instead absorbed at surface: atmosphere is heated from below! Where all weather occurs Temperature decreases with altitude Extends from surface to about 12km up Stratosphere – next layer of atmosphere Temperature increases with height Ozone heating (absorption of solar radiation) Upper atmosphere Very thin (low density) Ideal gas law breaks down  Density Variations in the Atmosphere Convection cell—rising and sinking air Warm air rises Less dense Cool air sinks More dense At what rate? Free-body diagram Moist air: More or less dense than dry air? H2O has less mass than N2 and O2 Ideal gas law PV=nRT for 1 kg of air Dry air = higher molar mass = smaller n = smaller V = higher density Moist air = lower molar mass = larger n = larger V = lower density  Forces on Air/Water Parcels: Pressure Force If pressure is higher on one side of a Pressure is a “field”, meaning water parcel than the other, it will feel the pressure of the seawater a “push” toward lower pressure and depends on the location and away from higher pressure the time we measure it: The Coriolis Effect: The Formula Why does the Coriolis force depend on: Force in East-West Direction: 1) Mass (Flow in North-South Direction) 2) Velocity? Force in North-South Direction: (Flow in East-West Direction) U = East-West velocity component V = North-South velocity component Theta = latitude  Vector Form of Coriolis Force (15 m/s directly north) Mathematical representation of a Force in East-West Direction: velocity vector: (Flow in North-South Direction) Force in North-South Direction: (Flow in East-West Direction) Mathematical representation of the Coriolis force vector: Combining & Calculating Pressure and Coriolis Forces Forces acting on this slab of air? 10 m/s 13 m/s 15 m/s 18 m/s Combining & Calculating Pressure and Coriolis Forces Pressure & Coriolis are ~equal and opposite. They ”balance one another” Pressure force is slightly larger at the surface so flow moves toward lower pressure, deflected strongly to the right by Coriolis  Three-Cell Model of Atmospheric Circulation Circulation cells—one of each in Northern and Southern Hemisphere Hadley Cell: 0–30 degrees latitude Ferrel Cell: 30–60 degrees latitude Polar Cell: 60–90 degrees latitude Rising and descending air from cells generate high and low pressure zones at the surface. Need to know: Cells, wind bands, pressure zones Three-Cell Model of Atmospheric Circulation Circulation cells—one of each in Northern and Southern Hemisphere Hadley Cell: 0–30 degrees latitude Ferrel Cell: 30–60 degrees latitude Polar Cell: 60–90 degrees latitude Rising and descending air from cells generate high and low pressure zones at the surface. Real picture: Hadley cell stronger than Ferrel and Polar cells Seasonal variations are strong!! Cancellations occur when we average DJF and JJA. From Peixoto and Oort (or Held’s GFD Notes) 1 Sv = 10^6 m^3/s Zonal mean zonal surface winds: Positive = Westerly, Negative = Easterly Prevailing westerlies Horse latitudes Polar front Doldrums Prevailing (weak) (weak) (weak) westerlies Zero Win d Horse line latitudes (weak) Polar Polar Polar front easterlies easterlies (weak) Trades (easterly)  Why is weather hard to predict? The dynamics of the atmosphere is chaotic The existence of this possibility (called sensitive dependence on initial conditions) was first noticed by Ed Lorenz (MIT) in the 1960s Cited about 30 000 times Ed Lorenz Sensitive Dependence on Initial Conditions Let’s look at two simulations whose initial conditions differ by 0.1%  Sensitive Dependence on Initial Conditions (SDIC) Implications for weather prediction: Step one: Observe atmospheric state (weather balloons). This will have slight errors due to sparseness and instrumental accuracy limits Step two: Run a weather computer model forward (like the Lorenz model but more complex) to predict future state of atmosphere Due to our tiny errors we cannot distinguish nearby initial states These turn into large differences over time due to chaos/SDIC Fundamental predictability limit  The ‘Climate’ of the Lorenz Model On the other hand, climate can possibly be predicted and understood Represent the state of convection (X,Y,Z) as a point/trajectory in 3D space (’phase space’) Graph this trajectory The state of the system generally lies near a ‘surface’ called the Lorenz attractor/Lorenz butterfly Statistical description of the system is possible, detailed future predictions are not possible for times far into the future “Lorenz butterfly”/Strange Attractor  Weather Systems Cyclonic flow low Counterclockwise around a low in Northern Hemisphere Clockwise around a low in Southern Hemisphere Anticyclonic flow High Clockwise around a high in Northern Hemisphere Counterclockwise around a high in Southern Hemisphere In NH, flow keeps the LOW on the LEFT (and consequently the high on the right) Geostrophic balance again! Circulation partially FILLS UP the low (ascent) and is REPELLED OUT of the Coriolis force and pressure high (descent). forces balance one another  Sea and Land Breezes Land and sea warm differently under equal insolation due to different heat capacities of land and water delta T = Q/C_V Sea breeze From ocean to land Land warms à Rises Filled in by cool breeze from the sea Land breeze From land to ocean Land cools - > Sinks Blows out to sea where pressure is lower (warm air) Quidi vidi wharf is a great place to experience this.  Jet Stream Jet Stream— narrow, fast- moving, eastward flow PressureNL At middle latitudes just Force below top of Low pressure side troposphere Geostrophic Flow NL balance applies Direction High pressure side Chart shows mid- tropospheric pressure Coriolis NL map: Jet stream is where the pressure lines Force are close together circumnavigating the pole (note the tiny flags showing wind direction)  Tropical Cyclones (Hurricanes) Large rotating masses of low pressure that form in the tropics and then propagate into the extratropics Not to be confused with normal weather systems, which are sometimes called extratropical cyclones Strong winds, torrential rain L Classified by maximum sustained wind speed Low pressure NL Typhoons—alternate name in North Pacific CCW Rotation Geostrophic Cyclones—name in Indian Low on left Ocean  Hurricane Origins Low pressure cell Winds feed water vapor Latent heat Air rises, low pressure deepens Release of latent heat accelerates ascent, warming, and the deepening of the low Winds circle the low and flow into it, capturing more water vapor from the ocean surface Cycle repeats => Storm develops Latent heat heat of vaporization stores energy in water vapor that is released within the hurricane circulation  Hurricane Facts want don't want About 85 tropical storms form worldwide each year Require: Ocean water warmer than 25°C Warm, moist air The Coriolis effect Initial low-level disturbance Hurricane season is June 1– November 30 Warm surface waters Low wind shear Wind shear = change of wind strength with height wind bad strong = Presence of initial disturbances Historical Storm Tracks Hurricane Movement: Effect of the Bermuda High Dr. Gus Alaka (NOAA) Hurricane Anatomy Diameter typically less than 200 k m ilo eters Larger hurricanes can be 800 k m ilo eters Eye of the hurricane Low pressure center Spiral rain bands with intense rainfall and thunderstorms Outflow at the top merges with upper- level flow Other Factors Impacting Hurricane Formation Warmer waters favor hurricane development Global warming may impact El Niño: (more to come later) Warming of equatorial Pacific surface waters -> more hurricanes in the Pacific Enhanced wind shear in equatorial Atlantic -> fewer hurricanes in the Atlantic La Niña: Enhanced wind shear in Pacific -> fewer Pacific hurricanes Weaker wind shear in Atlantic -> more Atlantic hurricanes Hurricane Destruction High winds Intense rainfall Storm surge—increase in shoreline sea level Storm surge is almost entirely driven by winds: the low pressure itself has a minimal effect Measuring Surface Currents Direct methods Floating device tracked through time Called ‘Lagrangian’ Measure speed using distance/time Fixed current meter Called ‘Eulerian’ Measure speed e.g. using a propellor  Measuring Surface Currents Direct methods Drifting objects Sneakers fall off cargo ship Washed ashore: Alaska BC Washington Oregon California Hawaii Measuring Surface Currents Indirect methods Pressure gradients Measured using pressure sensors Flow inferred as in meteorological chart (low on left, etc) Doppler flow meter Doppler shift of sound waves off moving particles gives estimate of velocity ’Acoustic Doppler Current Profiler’ (ADCP)  Measuring Surface Currents Indirect methods Radar altimetry Ocean dynamic topography From satellites High and low topographies act like high and low pressures in the atmosphere Measuring Deep Currents Argo Global array of free-drifting profiling floats measuring temperature T, salinity S, depth D/pressure P, other characteristics Free floating submersible device tracked through time Drift at a programmed depth, rise to surface every ten days to transmit data Measuring Deep Currents Tracer methods Tracer: particles or dissolved substance that moves with the currents Tritium Hydrogen isotope From nuclear tests in the 1960s Chlorofluorocarbons (CFCs) Characteristic temperature and salinity of the water From Toggweiler (The Ocean’s Overturning Circulation)  Wind Belts and Surface Current Movement Gyres—Large, circular loops of moving water Subtropical gyres centered around 30 degrees latitude That’s why they are called subtropical gyres (+/- 30 is the subtropics) Bounded by Equatorial current Western Boundary currents Northern or Southern Boundary currents Eastern Boundary currents Wind Belts and Surface Current Movement Physics of gyres: Distribution of continents Influences flow in each ocean basin Other current influences Gravity/Pressure differences Friction/wind Coriolis effect Wind Belts and Surface Current Movement Rotation: Clockwise in NH Counterclockwise in SH Mechanism Trade winds push water west near equator Coriolis force deflects water North in NH and South in SH Prevailing westerlies bring water back across the basin Coriolis force deflects water South (NH) and North (SH)  Subtropical Gyres & D LHS RIS = = Warm cold North Atlantic, South Atlantic, North Pacific, South Pacific, Indian Ocean PHYS/OCSC 2300 Fall 2024 Midterm 2 Review Ekman Spiral and Ekman Transport Observation that Arctic Ocean ice moved at a 20- to 40-degree angle to the right of the wind Vagn Walfrid Southern Hemisphere movement to the left of the wind Ekman Fridtjof Nansen  Ekman Spiral Results from a balance between friction and Coriolis effect Describes direction and flow of surface waters at different depths Conceptual model: Momentum injected at the surface by winds Diffuses downward Rotates due to Coriolis Self-cancellation at depth  Ekman Transport Ekman transport Average movement of surface waters 90 degrees to right in Northern Hemisphere 90 degrees to left in Southern Hemisphere Geostrophic Hill of water in centre of subtropical gyre is created by Ekman transport Currents Surface water flows downhill and is deflected. Right in Northern Hemisphere Geostrophic current Left in Southern Hemisphere Balance of Coriolis effect and gravitational forces Moves in circular path downhill Ideal geostrophic flow: Circular path around high or low SSH anomaly SSH = sea surface height Actual geostrophic flow with friction: Spiraling out of high or into low SSH anomaly  Geostrophic Currents Hill of water resides closer to western boundary than eastern boundary Broad, slow, cool return flow toward the equator on eastern side Narrow, rapid, deep warm current on western side Western intensification Diverging Surface Water Surface waters move away from area. Equatorial upwelling (Remember Ekman!) Divergence of currents at equator generates upwelling and high productivity.  Coastal Upwelling and Downwelling Coastal Upwelling Ekman transport moves surface seawater away from shore. (Remember: Ekman is to the right of the wind!) Cool, nutrient-rich deep water comes up to replace displaced surface waters. Western United States (California) is a good example to think of  Coastal Upwelling and Downwelling Coastal Downwelling Ekman transport moves surface seawater toward shore. Water piles up and moves downward in water column. Lack of marine life  Antarctic Circulation/Southern Ocean Antarctic Circumpolar Current (ACC) Also called West Wind Drift Only current to completely encircle Earth Moves more water than any other current East Wind Drift Polar Easterlies Creates surface divergence with opposite flowing Antarctic Circumpolar Current  Meanders or loops may cause loss of water volume and generate: Gulf Stream Warm-core rings—warmer Sargasso Sea water trapped in loop surrounded by cool water Cold-core rings—cold water trapped in loop surrounded by warmer water  Loop Current Warm ocean surface current in Gulf of Mexico Generates warm loop current eddies Hurricanes intensify when passing over warm cores. Cyclonic (CCW) Low SSH Cold core Anticyclonic (CW) High SSH Warm Core In-class example: Geostrophic balance  Monsoons— Indian Ocean Circulation: the Monsoon seasonal reversal of winds over northern Indian Ocean Heat Capacity Differential Northeast monsoon: Winter Southwest monsoon Summer Only place on Earth that reversing seasonal winds cause currents to reverse direction (Somali current & SW Monsoon) Indian Ocean Circulation: Monsoon is like a giant sea breeze  Indian Ocean Monsoon Affects seasonal land weather Affects seasonal Indian Ocean current circulation Affects phytoplankton productivity Remember Ekman transport is to the right of the wind Offshore Ekman transport drives coastal upwelling  Normal El Nino Conditions Condition Walker cell circulation disrupted High pressure in eastern Pacific weakens. Weaker/reversed trade winds and Walker cell Warm pool migrates eastward. Thermocline deeper in El Nino eastern Pacific Conditions Downwelling/lack of upwelling Lower biological productivity Peruvian fishing suffers.  El Nino Temperature Anomalies in the Pacific Temperature anomalies during strong 1998 El Nino Cooling in western Pacific (warm water flows away toward the east) Strong warming in eastern Pacific (depression of thermocline, warm waters from the west flow east) Warming as much as 4 degrees Celsius (huge) La Niña Conditions Normal Condition Walker cell circulation strengthened High pressure in eastern Pacific strengthens Stronger trade winds Cold tongue expands into western Pacific Thermocline extremely shallow in eastern La Nina Pacific Conditions Strong upwelling off Peru High biological productivity ”Opposite of El Nino” Strengthened version of normal conditions. La Niña Conditions: Temperature Anomalies in the Pacific Temperature anomalies during strong 2000 La Nina Warming in western Pacific Strong cooling in eastern Pacific as cold tongue spreads across the basin due to enhanced Walker circulation Eastern Pacific cools several degrees (major cooling)  ENSO (El Nino-Southern Oscillation) Index El Niño warm phase about Strong El Arrows: Major every 2–10 years Nino El Ninos years Highly irregular Phases usually last 12–18 months ENSO may be connected to other forms of climate variability  Predicting El Niño Events TropicalOcean−Global Atmosphere (TOGA) program Motivated by damage + loss of life in 1982 El Nino Started in 1985 Monitors equatorial South Pacific System of buoys TropicalAtmosphere and Ocean (TAO) project Continues monitoring, instrumentation completed in 1994 70 moorings in tropical Pacific ENSO still not fully understood Possible mechanisms: Large-scale equatorial waves influenced by rotation of the earth propagate between eastern and western Pacific  Thermohaline Circulation Originates in high latitude surface ocean Cooled, now dense surface water sinks and changes little. Recall high latitudes do not have thermocline/halocline, permitting easier downwelling Deep-water masses identified on temperature– salinity (T–S) diagram. Identifies deep water masses based on temperature, salinity, and resulting density Sources of Deep Water Cold water forms in both polar regions (NH and SH). AABW is the densest (cold and salty) and slowly spreads throughout the low levels of the basin NADW flows south in ocean interior and upwells in Southern Ocean Thermocline/Pycnocline exist away from the poles It may take 1000 years for AABW to return to the surface! Sources of North Atlantic Deep Water Labrador Sea Irminger Sea Mediterranean Sea Conservation of Volume For every parcel of water that sinks to depth, an equal volume must rise to the surface somewhere Sinking of dense water occurs in localized regions like the Labrador Sea and near Antarctica Upwelling is thought to occur more diffusely throughout basin interior Turbulent mixing and breaking internal waves near mid-ocean ridges is thought to be important ‘Pumping’ of NADW to the surface by winds/Ekman in the ACC is also an important mechanism  Conveyor Belt Circulation Deep water formation in localized polar regions Flow of deep water throughout basins Broad upwelling of water back to the surface Warm/light surface currents supply water to areas of deep water formation Atlantic Branch of Conveyor: AMOC (Atlantic Meridional Overturning Circulation) Upper branch (red) involves NADW formation From Nikurashin and Lower branch (blue) involves AABW formation Vallis 2012 Atlantic Branch of Conveyor: AMOC (Atlantic Meridional Overturning Circulation) Gradual upwelling associated with mixing at mid-ocean ridges Upper branch (red) involves NADW formation From Nikurashin and Lower branch (blue) involves AABW formation Vallis 2012 Atlantic Branch of Conveyor: AMOC (Atlantic Meridional Overturning Circulation) Abrupt upwelling occurs in the ACC driven by winds: this is the ‘pumping’ effect Upper branch (red) involves NADW formation From Nikurashin and Lower branch (blue) involves AABW formation Vallis 2012  Observing the AMOC with OSNAP Moorings Gliders Floats Current meters measure velocities directly Geostrophic currents inferred from temperature and salinity measurements from gliders (density) Overturning in the Subpolar North Atlantic Program  Observing the AMOC with OSNAP Moorings Gliders Floats Current meters measure velocities directly Geostrophic Year-to-year currents inferred changes of more from temperature than 100%! and salinity measurements from gliders (density) 1 Sv = 10^6 m^3/s  Modelling the AMOC Shutdown: “Hosing Experiments” Freshwater OGCM: Ocean General added Circulation Model here Simulates global ocean flow using physics equations 1995 version has coarse continents shown, modern models much better ”Hosing experiment”: Artificially add freshwater to the surface ocean Reduces salinity and makes water less dense at surface Inhibits NADW formation Rahmstorf, Nature 1995  Hysteresis/Threshold Behaviour: “Tipping Points” Bifurcation Start Small changes in forcing can here lead to significant changes in system state because the Gradual decline system may switch from one equilibrium configuration to another abruptly Abrupt collapse Hysteresis Abrupt Once a threshold has been restart past, reversing the change in forcing may not restore the Full system back to its original state. LARGE changes may collapse Irreversibility to 0 be required to restore the system. AMOC may be sensitive to additions of freshwater (hydrological cycle, Greenland melt) Response of the model AMOC to Freshwater forcing in Labrador Sea Sea Ice Formation Seaice forms directly from seawater. Needle-like crystals become slush. Slush becomes disk- shaped pancake ice in calm waters. Pancake ice freezes together to form ice floes Floe= pack of floating ice more than 20m across  Pressure Ridges Large, heavy sheets/floes of ice collide with one another driven by winds and ocean currents. Ridges of ice form at collision sites. Iceberg Formation Icebergs break off of glaciers. Floating bodies of ice Different from sea ice Greenland Ice Sheet: Marine Terminating Glacier Grounded ice (on land) Crevasses/large cracks and trenches in the ice Meltwater from summer heating drains from surface down to base ”Basal hydrology system” of channels ‘Melange’ of ice buttresses the glacier From Todd et al, 2019 Shelf Ice Antarctica—glaciers cover continent Edges break off Plate-like icebergs called shelf ice  Spatial Wave Characteristics Crest Wave height (H) Base of waves even from storms is only Trough Wavelength (L) the top 150m. Submarines below this Wave base depth no longer feel the surface waves. Still water level  Temporal Wave Characteristics Wave Period T (units of s) Wave frequency f = 1/T (units of per second)  Wave Speed (Phase Speed) Wave speed = L/T = wavelength / period Circular Orbital Motion Wave particles move in a circle. Waveform travels forward. Wave energy advances.  Breaking Waves Wave steepness = H/L ’Stable waves’: H/L < 1/7 à H < (1/7)*L (Can’t be too tall relative to their length) If wave steepness > 1/7 (H too large or L too small), waves break  Deep Water Waves All wind-generated waves in open ocean are Deep Water Waves Occur when water depth is greater than L/2 (half wavelength) L/2 is the approximate wave base of surface waves Very weak flow at depth, motion concentrated near surface  Water depth (d) is less than (1/20)L Shallow-Water Waves d < L/20 Wave ‘feels’ sea floor strongly Parcel orbits distorted near bottom: back and forth motion Wave speed: c = sqrt(gd) g = 9.8 m/s^2 d = water depth Speed doesn’t depend on Example: For a wave with L = 200 m (wavelength) wavelength L/20 = 10 (nondispersive) So a 200 m wave feels the sea floor strongly when the water is 10 m deep. Concept of the Dispersion Relation k = 2*pi/L Omega = 2*pi/T For surface gravity waves, there is a specific relationship between frequency and wavelength that is captured in an equation called the Dispersion Relation. Approximations to the Dispersion Relation: Deep and Shallow Water Limits Deep water case: Because depth is much larger than wavelength, kd>>1 So tanh(kd) ~1 For deep water waves, the wave speed increases as the square root of the wavelength Approximations to the Dispersion Relation: Deep and Shallow Water Limits Shallow water case: Because depth is smaller than wavelength, kd.01.1 1 10 50 concentration is low in some areas is because of low micro-nutrients, Remote sensing map of chlorophyll, with high especially limited for iron (other concentrations in redocean and low nutrients like N and P may be concentrations in blue (mg Chl/m^3) plentiful)  Experiments in the Real Ocean Between 1993-2005, 12 large- scale ocean iron enrichment experiments were carried out in HNLC regions Blooms of phototrophic microorganisms were observed So is it a good idea to use iron fertilization to sequester carbon in the deep ocean? Remote sensing map of ocean chlorophyll in North Pacific HNLC region during an iron fertilization experiment. Components of a climate model GCM: Global Climate Model General Circulation Model Sub-models: Atmosphere Ocean Cryosphere (if Earth System mode, ESM) Land use/vegetation (affects albedo) Radiation code (insolation, greenhouse effect)  How do these sub-models work? Atmosphere component Concept of discretization Break up continuous reality into a large number of discrete ‘grid boxes’ Each climate variable (e.g., wind velocity, temperature, pressure, humidity) has a single value in each grid box Concept of “state vector” (List of ALL values) Coupling between sub-models Atmosphere-ocean interaction: Surface winds => surface wind stress (N/m^2) Affects the ”Change” term for the surface gridboxes of the ocean and atmosphere Drives the surface layer of ocean flow Drag on the lowest layer of the atmosphere Formula: tau = (constant)*u_(10 m) (10 meter altitude atmospheric winds) Affects the momentum part of the system Atmosphere/ocean temperature difference => surface heat flux (W/m^2) Cold atmosphere (winter) cools surface ocean layer Warm atmosphere (summer) warms the surface ocean layer Formula: Q = (constant)*(T_atm – T_ocean) Affects the thermodynamic part of the system How many grid points do we need? Kolmogorov scale, L_k Size of the smallest eddies in the atmosphere and ocean For the ocean, roughly one millimeter!! To grid the whole Earth at this fine level of detail we would need approximately (H/L_k)*(2*pi*R/L_k)*(2*pi*R/L_k) = 10^27 total grid boxes How much computer memory is this? Each variable (temp, salinity, pressure, velocity) have a value at each grid box 10^28 total “floating point” numbers roughly 4 bytes to a float Total memory: 10^16 Terabytes!!! Not possible to simulate climate directly due to memory and processor limitations.  Parameterizations Because we cannot simulate the entire climate directly, to do climate prediction we need to make simplified, non-exact models of many climate processes Examples: Small-scale turbulence (called subgrid-scale, or SGS, parameterization) Turbulence is modelled as enhanced diffusion Diffusion coefficient can depend on the state vector, especially the kinetic energy of the flow (Smagorinsky closure) Clouds Cloud microphysics Raindrop formation Droplet mergers Different kinds of precipitation (rain, snow, hail) Uncertainties in climate modelling Two main kinds of uncertainties Model uncertainty Inexact parameterizations, coarse grid, different models do things different ways Coupled Model Intercomparison Project (CMIP) compares models to try and assess this uncertainty The uncertainty of chaos Slightly different initial conditions can lead to very different climate predictions This type of error would occur even with a hypothetical ‘perfect’ model The big question: All climate predictions are inexact and subject to error and uncertainty What is reliable and what is not reliable? The Butterfly Effect in Climate Trends NCAR Large Ensemble: Take the same climate model and run it many times with tiny tiny perturbations in the initial conditions (machine precision changes to temperature) 30-year future temperature trends predictions are significantly affected by tiny Dr. Clara Deser noise in the initial conditions (NCAR) This is the butterfly effect at the scale of thirty year regional climate change!  Physics of Sound Propagation Sound is a Pressure wave that pushing its neighboring molecules. Vibration compression (high pressure) and expansion (low pressure) Sound travels much faster in water than in air. Propagate in a wave-like motion 33 Spreading of energy in space is an Important process that weakens sound intensity 34 Natural Sources of Underwater Sound Wind noise Rain Manmade sources of Underwater Sound Earthquake Sonar ping Ice cracking Small Ship Rainfall Large Ship Dolphin Air gun  Fundamentals of Ocean Acoustics: Underwater Sound Propagation (Refraction/bending of sound waves) At some locations you may hear a source that you cannot hear somewhere else! Direction of sound arrival doesn’t mean that is where the sound came from! “Sound is lazy”: Sound tends to bend toward where the sound speed is lower (same as surface waves bending toward shallows (lower surface wave speed)) 36  Essential Ocean Acoustic Instruments Active Sonar: Precise mapping, Ranging, etc. It risks detection in military scenarios. Passive Sonar: Stealth operations, detecting natural sound sources. Used in environmental studies and surveillance. 37