Here are the electronic lecture notes for lectures 9-12. -Matt ********************************* AOS/IES 171 - Prof. Hitchman - Spring 2002 Notes for "Lectures 9-12"
**Water, Heat, Salt; The Oceans, ENSO**
1. When water evaporates from the subtropical oceans, it leaves behind saltier water. When it recondenses at higher latitudes the water vapor adds heat to the atmosphere (the latent heat of vaporization is recovered), while the associated rain will freshen the higher latitude ocean surface. Greater salinity can also result from brine rejection from freezing sea ice. Similarly, melting sea ice will freshen the upper layer of the ocean. It is interesting that water vapor and sea salt each make up from 0 - 40 parts per thousand in the atmosphere and ocean, respectively, but they each are crucial in determining where the fluid rises or sinks. In the atmosphere over Madison, the partial pressure of water vapor varies from about 3 mb in winter to about 18 mb in summer. The summer desert air contains much more water vapor than does air in winter! This is another manifestation of the exponential dependence on temperature for the capacity of air to hold water vapor.
If 500 kg of water evaporated from 1 m^3 and there was 40 kg of salt to begin with, the salinity would increase from 40 to 80 parts per thousand. Sea water density increases when the temperature cools off and if it is more saline. Thus, the formation of North Atlantic bottom water depends on evaporation in the subtropical Atlantic, followed by poleward-moving water in the Gulf Stream, cooling and brine rejection in the high latitude winter. This water spreads throughout the bottom of the world's oceans, the beginning of the thermohaline circulation. 1 t of salt can fit into 1/2 c of water.
2. Movie of first ocean general circulation model. This historical movie was made by Drs. Chervin and Semtner at the National Center for Atmospheric Research (NCAR). It shows the global distribution of temperature and salinity at different depths for 10 years with an annual cycle driven by realistic atmospheric winds and heat fluxes through the ocean surface. One feature of interest is the cold tongue of water extending from South America along the equator into the Pacific. Note also that the Atlantic is very salty, a result of considerable evaporation in the subtropical ocean and influx from the Mediterranean Sea. There is as incredible variety of ``ocean weather", or wavy variations, with some eddies lasting several years.
3. Building blocks of the El Nino Southern Oscillation (ENSO) phenomenon. Air over warm water holds considerably more water vapor than air over cold water. This is why the coasts near the poleward moving warm currents tend to be rainier (east coasts of continents), while the west coasts, near equatorward moving cold water, tend to be much drier. In the tropics Indonesia acts like a "maritime continent", while the eastern tropical Pacific is relatively cooler. Clouds and rain are usually more common over Indonesia. The circulation along the equatorial Pacific acts like a monsoon circulation, with air moving westward at the surface toward low pressure, rising over Indonesia, moving eastward in the upper troposphere, and sinking over high pressure off the coast of Peru. This is called the Walker circulation. This circulation strengthens and weakens on time scales of 3-7 years, giving a "Southern Oscillation".
When the high pressure system off the coast of Peru weakens, the wind-driven offshore transport of ocean water weakens, so upwelling of cold nutrient-rich water from the deep ocean stops, and the waters get warmer (Fig. 9.1). The phytoplankton are starved for nutrients and don't grow, so the zooplankton that eat them don't thrive, and the fish go away. Clouds and rain are also more common. This situation of warm water off the coast of Peru is often noticed around Christmas time and is named after the Christ Child, or El Nino. The opposite phase occurs when the Walker Circulation is stronger than normal, with a stronger high pressure system off of Peru, stronger upwelling and colder water. This phase is called La Nina. The coupled atmosphere/ocean variation is called the El Nino Southern Oscillation (ENSO) phenomenon.
4. Most of the ocean is within 5 C of zero, but within the top few hundred meters the sunlight can warm the ocean to over 30 C. The region of rapid upward increase in temperature is called the thermocline. In the tropical Pacific warm water is piled up at the western edge against Indonesia by the wind, giving a deep thermocline. At the eastern edge near Peru, the thermocline is shallow and the waters are colder. So normally the thermocline slopes upward to the east. During El Nino the thermocline gets deeper to the east as the surface waters warm and upwelling ceases. In concert with this, rainclouds are seen much more often in the eastern tropical Pacific. Thunderstorms are very energetic and can affect global weather patterns. Thus, this ocean/atmosphere climate phenomenon known as ENSO can affect the climate world-wide. By predicting ENSO, countries may plan for global redistribution of food. We can also better prepare ourselves for epidemics of diseases which are more common with higher sea surface temperatures (cholera) and warmer, wetter weather (yellow fever).
5. GSFC ENSO movie. The movie of ENSO by the Goddard Space Flight Center illustrates the power of modern satellite and drift-buoy observations, capturing the 1997-1998 El Nino - La Nina sequence in incredible detail. Quantities shown in 3D include sea surface temperature, ocean color (an indicator of phytoplankton), ocean surface altitude, thermocline depth, and smoke. Fires were a bad problem in Indonesia due to the dryness in the region during El Nino. The strong changes in the Indian Ocean and along the west coasts of the Americas associated with ENSO are also apparent.
**Climate Change**
1. Concepts from Gore, Chapter 3. Climate change has guided the evolution of species and and our civilization. Many examples of temporary climate change caused by volcanic eruptions are given. This deterministic theory regards such a perturbation as an identifiable cause, with clear effects: cooling of the planet, changes in weather patterns, often with crop failures. He notes the likely synergy between climate change, food supply, and outbreaks of disease. He cautions against narrowing the genetic diversity of crops, that it is foolhardy in the face of inevitable climate change. Since natural climate change has had such a large effect on us in the past, we should expect that anthropogenic climate change could also affect us adversely.
2. The Paleoclimate Record Major climate events (Figs. 10.1-10.3; see McK2 pp. 276, 280). On time scales of millions to billions of years ago, factors such as the solar output, carbon dioxide loading, rate of crustal recycling, positions of the continents, and possible clustering of asteroid impacts are believed to exert primary controls over our climate. The Cretaceous period of about 100 million years ago was about 10 K warmer, sea level was about 300 m higher, and it is likely that there was a much higher atmospheric carbon dioxide concentration.
Major glacial/interglacial events of the last 100,000 years or so include the Emian interglacial of 130,000 ypb, the Wisconsin ice age peaking at 20,000 ypb, and the Holocene warm period of 8,000-4,000 ypb. Current theory holds that these are related to changes in Milankovich orbital parameters, amplified by feedbacks due to greenhouse gases, ice, and plants. The climatic record suggests that pronounced natural global temperature oscillations occurred on time scales of a hundred years or so commonly in the past, and could well do so again. An example is the Younger Dryas event of about 11,000 ybp. This event may have been due to the interplay between Laurentide ice sheet volume, flow of fresh water out the St. Lawrence River, turning on and off the formation of North Atlantic bottom water, and the strength of the Gulf stream (the thermohaline circulation oscillator theory).
There was a "Little Ice Age" between 1400 and 1800 A.D. Volcanic eruptions seem to have been more common during the earlier part of this period, while the Maunder minimum in sunspots was observed during 1645 to 1715.
Since 1880 the global average surface temperature has increased by more than 0.8 K. Scientists think that anthropogenic trace gas increases may be related to the temperature rise over the past 100 years.
2. Younger Dryas Event Figure 10.4 illustrates the rapid climate change that took place in the Younger Dryas event of about 11,000 years ago (11 kybp). Such rapid temperature oscillations have occurred naturally in the past. One possible explanation is the "North Atlantic Oscillator" theory, which includes feedbacks among the cryosphere, ocean and atmosphere (Fig. 10.5).
3. Climate theory.
Ice core and deep sea sediment core records suggest that during most of the past 800,000 years our planet has been colder than present, with the range in globally averaged temperature being about 6 K. There is the further suggestion of multiple equilibria (a glacial state and an interglacial state) between which the climate oscillates. An equilibrium is found when the net forcing of climate is zero. The stability is neutral if there is no clear dominance by positive or negative feedbacks, unstable if positive feedbacks dominate, and stable if negative feedbacks dominate. Perturbations to the climate system include orbital parameter variations, volcanic eruptions, changes in solar output, asteroids, and human activities. A positive feedback amplifies an initial perturbation, fostering instability. A negative feedback opposes the initial perturbation, fostering stability. Examples of negative feedback include: the curvature of the earth will help make it so that continental glaciers expanding equatorward will experience strong sunlight and tend to melt. Another is the tendency for continental glaciers to make cold, high pressure regions, which do not favor further snowfall. Examples of positive feedbacks which can help explain how we go from a glacial to an interglacial state include greenhouse gas feedbacks, the ice-albedo feedback, and the boreal forest feedback. One concern is that our activities are changing the nature of climate feedbacks, reducing the complexity of the "earth organism", reducing its resiliency, making it more likely for our climate to reach a state that hasn't occurred in millions of years.
4. Chaos Theory versus Determinism
Natural variability is the tendency to wander from one climate state to another without any obvious "external" perturbations causing the changes. Chaos theory, pioneered in 1961 by meteorologist Ed Lorenz, holds that much of observed variability is simply due to interactions between different parts of the climate system, rather than being clearly linked to specific perturbing influences (determinism). The example of a water wheel with leaky buckets illustrates the essential unpredictability of even simple systems (Fig. 10.6). A key notion in chaos theory is that a small uncertainty in the initial state of a system will amplify, so that the detailed evolution of the system rapidly becomes unpredictable. This is called the butterfly effect. Whether it flaps its wings or not will eventually affect the timing of a cold frontal passage weeks later in some other part of the world! Deterministic components within the system will lead to preferred general states (attractors), such as glacial and interglacial states, but the timing of transitions from one to the other, and the details of global climate within each state are unpredictable, according to chaos theory.
5. Milankovich Theory
An example of deterministic climate theory is Milutin Milankovich's 1930 theory of climate change. He showed that the distribution of sunlight on the earth depends on three aspects of how the earth goes around the sun: axial tilt, eccentricity of the orbit, and precession of the axis (Fig. 10.7). The axial tilt ranges from 22 to 24.5 degrees, with a period of 41,000 years. Ice ages are favored by a modest tilt, since in a mild winter more snow will fall on northern hemisphere land masses (cool air holds more water vapor to make snow than colder air), and mild summers imply less melting. The eccentricity of the orbit varies from 3-9% with a 100,000 year period, implying a variation in solar intensity around the orbit of 6% at small eccentricity and 20% at high eccentricity. High eccentricity coupled with a low axial tilt can enhance the tendency to build up continental glaciers. Our axis precesses (wobbles) every 23,000 years, which controls where the northern hemisphere winter occurs in its orbit. If northern winter occurs when the earth is farthest from the sun in its orbit, this will enhance glaciation too.
It is important to note that changes in orbital parameters do not affect the total radiation received by the earth each year. The northern hemisphere land masses control when glacial ages occur. Positive feedbacks are required to change the climate based on orbital perturbations alone. It takes many thousands of years to build up ice sheets, so there is an expected time lag from the most favorable orbital configuration to when it is actually coldest. Based on Milankovich theory, we expect a small axial tilt about 8,000 years from now, hence an ice age perhaps 10,000 years from now (Fig. 10.8).
6. Proxies of paleoclimate
We didn't have thermometers and rain gauges way back then to measure climate. Instead, we depend on piecing together a variety of proxy evidence for past climate.
a) Fauna and flora. Dendrochronology (tree ring comparisons) enables dating back to around 40,000 years before present (ybp), where thicker rings are found during wet warm years. Lichenometry allows dating back to around 10,000 ypb, where the presence of a large, old lichen proves that no glacier scraped the rock clean while it was living. Plant pollen types found in lake varves show when the climate favored warm or cold-adapted species (e.g., the Dryas flower). The presence of fossil corals, which live at temperatures between 24-29 C, show when the water was warm. Other organisms living in the upper ocean which died and settled to the bottom are sensitive in body type to water temperature, such as foraminifera and radiolarians.
b) Chemical methods. For the last 50,000 years or so it is possible to date the age of material that was once alive by examining the amount of C-14 isotope in the sample. The much more common C-12 has 6 protons and 6 neutrons in the nucleus. C-14 (6 protons and 8 neutrons) is produced in the atmosphere by cosmic ray bombardment of N-14, converting one proton into a neutron. It is then incorporated into live organisms via metabolic processes. Upon death the organism can be subsequently buried. C-14 decays radioactively to C-12 with a half-life of 5730 years. Measuring the ratio of C-14 to C-12 gives a good estimate of when the organism died, enabling dating of climatic events. Other radioactive elements may be used to date events from longer ago.
A handy temperature proxy is derived from relative amounts of two isotopes of oxygen, which aren't radioactive. It is harder for the heavier isotope of oxygen, O-18 (8 protons and 10 neutrons) to evaporate as part of a water vapor molecule H2O-18, than it is for the lighter isotope O-16 to evaporate as H2O-16. When it is colder this ``isotope fractionation" is even more pronounced. During cold times more O-18 is left behind and is enriched in ocean sediment cores, while the ligher isotope evaporates and falls as snow on glaciers, enriching the O-16 found in ice cores. During interglacial times, more O-18 is found in glaciers and more O-16 is found in deep sea cores. The anticorrelation of oxygen isotope records in deep sea cores and ice cores throughout the past is confirmation of the global nature of ice ages and interglacials.
c) Rocks and dirt. Aolean dust is blown from continental interiors during cold, dry times and deposited into the oceans and thereby into bottom sediments. Rafted rock debris at the bottom of the ocean indicates a colder climate. Terminal moraines and erratics (boulders moved far from their parent rock) with lichens on them indicate when the glacier retreated. Ancient sea and lake levels are recorded as shelves above or below the present shoreline. Care must be taken in measuring sea level rise. In places where there were once thick glaciers, the plastic asthenosphere underlying the continents upon which the heavy ice rested causes an "isostatic rebound". Central Canada (including the Great Lakes region) and northern Scandinavia are two places still rebounding upward after the last glaciation.