Oceanography PDF

Lecture 10 Covering Material in Webb Chapter 8 -Orographic effect: prevailing wind forces air over a topographic barrier. Windward site gets a lot of rain as air is forced up, cools. Leeward side, dry air sinks, absorbs soil moisture, you get a desert. -Amount of water vapor air can hold depends on...

Lecture 10 Covering Material in Webb Chapter 8 -Orographic effect: prevailing wind forces air over a topographic barrier. Windward site gets a lot of rain as air is forced up, cools. Leeward side, dry air sinks, absorbs soil moisture, you get a desert. -Amount of water vapor air can hold depends on temp. Warm air holds a lot more water than cool air, so as air cools and water reaches 100% humidity you get precipitation (rain). Water vapor is less dense than air so air that is warm AND has a lot of water vapor will readily rise. -Large scale pattern of deserts and rainforests reflects the atmospheric circulation cells. Deserts found between 30°N and S and the ITCZ, underneath the Trade Winds (the lower limbs of the Hadley Cells). Air here is warm and dry and absorbs moisture from the land surface. -Underneath the ITCZ it rains like crazy. Warm moist air rises, cools, and causes precipitation, here you get rainforests. -Atmospheric pressure (or barometric pressure) measures the weight of the column of overlying air. High pressure associated with cooler dense air sinking and generally good weather, low pressure means less dense (warm, moist) air rising, usually means rain or stormy weather. Really low pressure associated with hurricanes. Areas of high and low pressure are marked on weather maps. Lines connecting areas of equal pressure are isobars. -Wind is air moving from high pressure to low pressure. The closer the isobars are spaced on a map, the stronger the wind is moving (towards the low) -Sea breeze: Land heats up during daytime, air rises over land. Wind blows in from ocean, starts a circulation cell. Direction reverses at night as land cools off more than ocean -Monsoon: Same idea on much larger scale. Land warms up in summer more than ocean, wind blows in from ocean. If it rises over a topographic barrier (like Himalayas in India/Tibet), you get lots of rain in the spring/summer. -Hurricanes are fueled by warm ocean water, in excess of 26°C (79°F). The North Atlantic has been anomalously warm for the last two years (and has been in a general warming trend for decades) leading to expectation of stronger hurricanes. -Hurricane frequency peaks in early September when ocean temps in the North Atlantic reach their seasonal maximum. Lecture 11 -Hurricanes start as an area of low pressure off the coast of West Africa, moving west with the trade winds. Warm water will cause air to rise faster, intensification of low. -Hurricanes are a North Atlantic name for Tropical Cyclone. These storms are called Typhoons in the Pacific and “Tropical Cyclones” in the Indian Ocean. These are just regional names for the same weather phenomenon. -Northern hemisphere hurricanes rotate counterclockwise. You get rising air under a strong low pressure system. Wind from all surrounding directions blows in towards this low. Coriolis deflects these winds to the right as they approach the low, causes counterclockwise rotation. Opposite sense of Coriolis deflection causes Southern Hemisphere Tropical Cyclones to rotate clockwise. -Air rising from all surrounding directions when reaching the low forms the eye of the hurricane. Strongest winds right at the eyewall but no wind in the eye itself. Bands of rising air surrounding the eye form the bands of rain associated with hurricanes. -Coriolis causes the track of the hurricane itself to bend to the right, north and away from the equator. Eventually it will get out of the trade wind zone and be carried to the Northeast by the Westerly winds underlying the Ferrel Cells. -There’s a narrow Topical Cyclone free zone on either side of the equator. Hurricane tracks are deflected away from the equator in both hemispheres. Also, no Coriolis effect at all right at the equator so you don’t get rotation (just rising air and rain associated with the ITCZ). -Hazards associated with Hurricanes are high winds, intense rainfall, and storm surge. Flooding related to surge has historically been the most damaging. -Hurricanes start at 74 mph wind (Category 1). Every 20mph or so increase gets into the next Category on the Saffir-Simpson Scale. Above 157mph is Category 5, the max on this scale. (note Hurricane Milton last week reached 180mph, high Category 5, before weakening prior to landfall) -Storm surge caused by both “mounding” of seawater under area of low atmospheric pressure (pressure surge), and by wind pushing water onshore (wind-driven surge). -The “right, front” quadrant of a hurricane is the most dangerous because the rotating wind of the hurricane is added to the forward motion of the storm. This is also where you get the max storm surge. (Note that Milton made landfall south of Tampa Bay, so Tampa was in the “left, front” quadrant at landfall and the rotating winds pushed the water OUT of the bay instead of causing a big storm surge. The big storm surge happened south of the area where the eye made landfall “right, front” quadrant) -Storm surge associated with big Cat 5 storms like Katrina in 2005 can be >7 meters (>24 feet), again in the “right, front” quadrant at landfall, in that case east of New Orleans. Tremendous damage from flooding. -Some cities (like New Bedford, MA) have constructed barriers that can be closed during storm to mitigate the impact of surge. These are expensive but can be effective. Starting Chemical Oceanography (Webb Chapter 5) -Composition of seawater. You should be able to make the two most abundant anions (negatively charged ions (chloride and sulfate) and the two most abundant cations (sodium and magnesium). Chloride and sodium are by far the dominant contributors to salinity. -Seawater salinity is 3.5% on average (3.5g dissolved ions per 100g seawater). This is equivalent to 35 parts per thousand, or 35 PSU (practical salinity units). Range is about 32 to 37 for the open ocean (can be higher or lower in coastal or isolated seas). Lecture 12 -Why is seawater salty? Ions enter the oceans from rivers, sourced by chemical weathering reactions with rocks and minerals on land. Ions leave seawater by precipitation into sediment. Calcium goes into shells of marine plankton and shelly animals. Sodium and chloride are removed from seawater from evaporation, but this is slow and only happens in some environments/geologic periods, so these ions hang around a long time and build up to high concentrations. -Residence time is defined as amount of an ion in the oceans divided by the rate at which the ion is removed (or added). Residence time for the ions most abundant in seawater are in the 10’s of millions of years, much much longer than the mixing time of the oceans (1000 years). These ions are “conservative”, they are well distributed throughout the oceans and always found in constant proportion to one another (sodium, calcium, chloride, magnesium, potassium, sulfate are conservative). -salinity profiles through the water column are measured with a CTD. Conductivity is proportional to salinity. Salinity in the top 500-1000m is usually different (higher or lower) than deeper water underneath. Halocline refers to the part of the water column where the salinity is rapidly changing. Pretty constant with depth below the halocline. -Surface salinity can be measured from satellites. Take a look at the map of surface salinity, get a sense for where it tends to be high and where it tends to be low. -Dissolved oxygen, CO2, nitrate, phosphate are non-conservative, can vary independently of salinity. -Dissolved oxygen is high at the surface because atmospheric O2 gas dissolves into seawater. There is an “oxygen minimum zone” near the boundary between the surface and deep ocean. O2 is consumed by respiration of organic matter sinking out of the surface ocean. Higher at the bottom because deep water doesn’t see as much sinking organic matter, and the high oxygen is inherited from when it was at the surface (think about how deep water is formed). -Dissolved CO2 increases with depth because respiration of sinking organic matter both consumes oxygen and produces CO2 (this is the reverse of photosynthesis). -When CO2 dissolves in water it produces carbonic acid. Higher concentrations of carbonic acid decrease the pH of seawater (ocean acidification). pH of seawater is higher than pure water (7), typically about 8.0 to 8.3. When seawater pH decreases to t the lower end of this range, life gets tough for organisms that make shells from CaCO3 (some plankton, corals, clams, oysters, etc). As atmospheric CO2 gas concentrations have increased from fossil fuel burning, pH of oceans have been decreasing. Ocean acidification is sometimes called “global warming’s evil twin”. -Nitrate and phosphate are “nutrients”, critical to the growth of photosynthetic plankton at the base of the food chain. Concentrations of these nutrients very low at the surface (they are used up), reach at maximum at the same depth as the oxygen minimum zone as they are released back to seawater by respiration. The stuff below is Physical Oceanography, covered in Webb Chapter 6 -Light is absorbed by seawater. In the open ocean most of the light is absorbed by 200m depth. This is the “photic zone”. Photosynthetic plankton can only operate within the photic zone, which is why the nutrients are used up here but present in deeper water. 200m to 1000m is the “twilight zone” where there is just a tiny bit of light. Below that is the “midnight zone”, DARK. Violet and red light absorbed more readily than blue, which is why the oceans look blue (blue is reflected). Lecture 13 -Absorption of light energy in the upper 200m HEATS the surface ocean. Some of this heat can be mixed to depths up to 1000m in some cases. The boundary between the warm surface ocean and cold deep ocean is called the “thermocline”. The depth of the thermocline, and the difference between surface and deep water temperatures can vary both with latitude and with season. -Take a look at the map of sea surface temp to get a sense for where the ocean is warm and where it is cold. Surface temp (like surface salinity) can be measured from space. -Density of seawater is VERY IMPORTANT. Affected by salinity and temperature. Boundary between low density surface ocean and higher density deep ocean is the “pycnocline” -Again, you should take a look at the map of surface ocean salinity. Where do you see high density water and where do you see low density? High density regions are critical for “deep water formation”. We’re on to Webb’s Chapter 9 here -The density stratification of the ocean (low density surface ocean separated by pycnocline from high density deep ocean) means that the two parts of the ocean circulate through distinct mechanisms, and at different speeds (much faster in the surface ocean). -Surface currents are wind driven, deep currents are density driven (thermohaline, temperature and salinity) -Ekman spiral, wind pushes on surface water, which start moving but is deflected right (in northern hemisphere) by Coriolis. Water below the surface is dragged along by surface water slightly slower as energy lost to friction, but it is further deflected right. Result is a spiraling motion with depth. By 1km (roughly thermocline/pycnocline depth) or so there is no effect from wind (surface current drops to zero). -Net effect of Ekman spiral is that surface ocean currents move at 90° to the right (in Northern hemisphere) of prevailing wind direction. This is referred to as “Ekman transport”. -The Gulf Stream is a well-known example of a surface ocean current. Moves around 5-10km/hour. -Eddies form where surface ocean currents take a sharp bend (like Gulf Stream as it comes up East Coast of US). “Meanders” form, can get cut off forming rotating masses of water that move away from the main current. Eddies that break off and move north from the Gulf Stream enclose blobs of warm water, called “warm core eddies”, or “warm core rings”. Bring warm water and sometimes tropical fish to shores of New England. Lecture 14 -“Gyres” are ocean basin scale systems of surface ocean currents. There are 5 (plus the Antarctic Circumpolar Current which is sort of like a gyre). Subtropical gyres form between trade winds and westerlies. Ekman transport produces currents from these winds that set the gyres in motion (clockwise in northern hemisphere, counterclockwise in southern hemisphere). Note that this is opposite the circulation of hurricanes! You can remember which way northern hemisphere gyres rotate if you remember that the Gulf Stream is the western boundary of the North Atlantic Gyre and it brings warm water north along the coast of the US from Florida, then moves east away from the coast. -“Geostrophic flow” contributes to the rotation of gyres. Water flowing “downhill” from the center of the gyre towards the edges is deflected right (in Northern Hemisphere) by Coriolis resulting in clockwise rotation of the gyre. -“Upwelling” is the mechanism that makes deepwater come up to the surface. It’s important because this “overturning” between the deep ocean and surface ocean affects how much heat the ocean can absorb and also how the ocean exchanges CO2 with the atmosphere. -Main spots for upwelling are along coastlines and at the equator. Prevailing winds result in surface ocean currents (affected by Ekman transport) that move away from the coast or away from the equator. Deep water rises as surface water pushed away. -Upwelling brings nutrients to surface (photic zone) resulting in areas of high “productivity” (lots of action at the base of the food web). This can also be measured from space (satellite sensors look for chlorophyll which reflects green light). -“Deep water formation” is the mechanism that makes surface water sink to become deepwater. Need to overcome density stratification of the ocean. Normally surface is less dense but can become more dense by cooling or increasing salinity through salt rejection when sea ice forms. This happens most in North Atlantic and close to Antarctica coast (Southern Ocean). Water will sink until it reaches the bottom or other deep water that is even more dense. Lecture 15 -The ocean basins contain “water masses” of distinct densities that don’t effectively mix. These water masses are produced at different sites of deep water production. These water masses are in constant motion but SLOW (10-20km per YEAR). Motion comes from constant production of deep water pushing existing deep water along the “conveyor belt”. -Be familiar with the major deep water masses. (North Atlantic Deep Water, produced near Greenland, Antarctic Intermediate Water, Mediterranean Intermediate Water, Antarctic Bottom Water, produced near Antarctica, the coldest and most dense) -I showed a slide in class (#6) that had the header “There is deep water formation in the N. Pacific” but that was a typo! There is NO deep water formation in the North Pacific, because the surface water never gets dense enough there. The deep water under the Pacific surface water is produced elsewhere (North Atlantic and around Antarctica mostly) and gets to the Pacific from: -The Great Ocean Conveyor Belt (popularized by Columbia University oceanographer Wally Broecker). Also referred to as Meridional Overturning Circulation (MOC). The global system of lined thermohaline currents that move water slowly around the planet. Now we’re on to Webb Chapter 10 -Waves are moving energy, which travels at the boundary between two fluids with different density. Air-water is the one we’re most familiar with but there are also waves in the atmosphere and internal waves in the ocean. -Most of the waves at the surface of the ocean are wind-driven. Other waves come from splash (landslides, icebergs calving from glaciers), tectonic motions (tsunami), boat wakes (energy from motion of the boat), and tides (which are very long wavelength waves that we will cover in detail) -Components of a wave. Be familiar with this figure. Crest, trough, height, wavelength. Lecture 16 -Wave period (T) is the number of waves passing a given point per unit time. Typical period for ocean waves is ~6 to 16 seconds. Frequency (f) is the inverse of T. -Wave motion involves moving energy, not so much moving water as the wave travels. Parcels of water move in circular orbits as the wave passes. The diameter of the orbit is equal to wave height at the surface and gets smaller with depth. By a depth of 1⁄2 L there is no motion. This is called the “wave base”. -Wave speed (sometimes called celerity) is wavelength (L) times frequency (f, there’s a typo on the slide) or wavelength divided by period. -Wave speed depends only on wavelength for “deep water” waves. Make sure you understand the definition of a deep water wave. -Shallow water wave speed depends only on water depth. Make sure you understand the definition of a shallow water wave and a transitional wave too. -3 factors affecting wave height. windspeed, fetch, and duration. Waves big when all three of these big. -Windspeed influences “sea state”, the appearance of the surface of the ocean in terms of waves, spray, whitecaps. Mariners use the Beaufort Scale to infer windspeed based on sea state. Force 12 is the top, a hurricane. A “fully developed sea” is when sea state meets expectations for current windspeed and fetch (i.e. the duration is long enough for waves to develop to equilibrium size) -Waves coming from different directions or traveling at different speeds can “interfere” with one another. If crests/troughs line up, you have constructive interference and bigger waves that you’d expect. Destructive interference can make waves smaller. Usually waves interfere in complex pattern and you get “mixed interference”, resulting in a distribution of wave sizes, some small, some big with an irregular rhythm. -Given that there is typically a range of wave sizes at any given time and location (due to varying wind speed/direction, wave interference), oceanographers will often report “Significant Wave Height”, defined as the mean height of the largest third of wave height distribution. Many waves will be smaller than SWH. Some will be bigger. -Highest winds and biggest waves are in the Southern Ocean. Fetch is infinite as winds race around Antarctica. Roaring 40’s (south latitude) Furious 50’s, Screaming 60’s. There be dragons. -Maximum size of wind driven waves was once thought to be around 20 meters (65 feet) but observations from the deck of a US ship (Ramapo) in the 1930’s caught in a typhoon accurately determined a height of 34 meters. So waves can get at least that big. -“rogue waves” have been reported by mariners for centuries, but scientists were skeptical. Wave data from an offshore natural gas platform in the North Sea (Draupner) in 1995 during a storm showed significant wave height of 12 meters, and at least one wave of 25.6 meters. This meets thedefinition of a rogue wave, a single wave with a height more than two times SWH. So we know they exist but they are rare. Probably result from rare multiple constructive interference. Lecture 17 -As deep water waves move into shallow water, they slow down (especially at depth due to friction with seafloor), wavelength gets shorter, height increases, so steepness (H/L) increases. -When H/L exceeds 1/7, wave will break. Form of a breaking wave depends on steepness of seafloor towards the coast. Spilling, Plunging, Surging breakers as steepness increases. -Tsunamis are a special kind of wave, triggered by seismic activity or large landslides rather than wind. Energy is propagated as a wave, radially from epicenter of earthquake. -Most deadly natural disaster of the 21st century was the tsunami triggered by the Sumatra earthquake of 2004 -Tsunamis are shallow water waves even though they travel through the open ocean. Wavelength is extremely long, 100’s of km, so depth is much greater than 1⁄2 wavelength. This means wave speed is based on depth, and waves travel at speeds of ~220m/s, 500mph. Wave heights in deep water are no more than a few meters, so you really wouldn’t notice these if they passed under your boat in deep water. -As tsunami waves approach shore, energy concentrated into shallow water, wave height increases as wave slows and wavelength shortens. Orbital motion of water becomes more horizontal, results in huge surge and retreat of large amounts of water onto and off of coastal zone.