Clima e oceanos

Rui Rosa

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Clima e oceanos

Rui Rosa, Ph.D.

Centro de Oceanografia,

Laboratório Marítimo da Guia,

Edição: Faculdade de Ciências da Universidade de Lisboa

ISBN: 978-989-98296-2-6

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MAJOR FEATURES OF PRESENT CLIMATE SYSTEM

Energy from the sun drives circulation patterns in both the oceans and the atmosphere. Atmospheric circulation is driven by the principle that warm air is less dense than cool air and therefore rises. Ocean circulation is driven by both temperature and salinity.

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Warm water rises, cool water sinks, and salty water is more dense than freshwater, leading salty water to sink and less salty water to rise.

The sun’ s heat received at the equator causes the Earth’ s atmosphere to mix — warm hot air rises and builds up in the tropics, pushing toward the cooler poles.

As air masses move from the tropics toward the poles, they cool, descend, and eventually return to the tropics in a giant loop. This movement of heat, known as heat transport, creates large, systematic patterns of circulation in the atmosphere.

In the atmosphere, these circulation patterns are known as Hadley cells.

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There are two Hadley cells between the equator and each pole.

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Hadley cells have both vertical and horizontal structures. Viewed in cross section, air masses in a Hadley cell rise at the equator, move toward the pole, and then descend.

From above, the circulation is clockwise, as moving air is deflected by the Coriolis effect imparted by the Earth’ s rotation.

Coriolis Effect If the Earth did not rotate and remained stationary, the atmosphere would circulate between the poles (high pressure areas) and the equator (a low pressure area) in a simple back-and-forth pattern. But because the Earth rotates, circulating air is deflected. Instead of circulating in a straight pattern, the air deflects toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere, resulting in curved paths. This deflection is called the Coriolis effect.

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In complement to the Hadley cells, in the tropics there are East – West-oriented circulation cells. These circulation patterns arise when pressure differences across ocean basins drive surface winds in one direction, balanced by transfers aloft in the opposite direction. Over the Pacific Ocean, the circulation is known as Walker cell circulation or the “Southern Oscillation.” It drives easterly surface winds across the Pacific. Breakdown in Walker cell circulation in the tropical Pacific results in an El Niño event.

Trade winds (“ventos alísios”) In the Northern Hemisphere, warm air around the equator rises and flows north toward the pole. As the air moves away from the equator, the Coriolis effect deflects it toward the right. It cools and descends near 30 degrees North latitude. The descending air blows from the northeast to the southwest, back toward the equator. A similar wind pattern occurs in the Southern Hemisphere.

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Thus, they move westward along the equator in both the Northern and the Southern Hemisphere.

Where the trade winds (“ventos alísios) converge along the equator, a zone of uplift and cloud formation results, which is known as the Intertropical Convergence Zone.

The trade winds are balanced by return flows in the mid-latitudes by west-to-east blowing winds known as the westerlies.

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Follow the wind patterns, forming large gyres with east-to-west flow along the equator and west-to-east flow at the mid-latitudes.

There are five major ocean-wide gyres—the North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Ocean gyres. Each is flanked by a strong and narrow “western boundary current,” and a weak and broad “eastern boundary current”

When surface ocean currents strike continents, they deflect and follow the shoreline, forming boundary currents.

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OCEAN CIRCULATION PATTERNS

However, ocean current direction varies from wind direction by 15 – 45° progressively with depth, an effect known as the Ekman spiral.

The Ekman spiral occurs as a consequence of the Coriolis effect. When surface water molecules are moved by the wind, they drag deeper layers of water molecules below them. Like surface water, the deeper water is deflected by the Coriolis. As a result, each successively deeper layer of water moves more slowly to the right or left, creating a spiral effect. Because the deeper layers of water move more slowly than the shallower layers, they tend to “twist around” and flow opposite to the surface current.

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Upwelling

Upwelling results when along-shore winds move ocean water. The wind-driven surface movement is deflected by the Ekman spiral, resulting in transport of water away from the coast.

This moving water has to be replaced, so water from depth is drawn to the surface. The movement of this cold, nutrient-rich water from depth to the surface is referred to as upwelling.

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Winds drive ocean currents in the upper 100 meters of the ocean’s surface. However, ocean currents also flow thousands of meters below the surface.

These deep-ocean currents are driven by differences in the water’s density, which is controlled by temperature (thermo) and salinity (haline). This process is known as thermohaline circulation.

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THERMOHALINE CIRCULATION

Thermohaline circulation drives a global-scale system of currents called the “global conveyor belt.”

The conveyor belt begins on the surface of the ocean near the pole in the North Atlantic. Here, the water is chilled by arctic temperatures. It also gets saltier because when sea ice forms, the salt does not freeze and is left behind in the surrounding water

The cold water is now more dense, due to the added salts, and sinks toward the ocean bottom. Surface water moves in to replace the sinking water, thus creating a current.

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THERMOHALINE CIRCULATION

This deep water moves south, between the continents, past the equator, and down to the ends of Africa and South America. The current travels around the edge of Antarctica, where the water cools and sinks again, as it does in the North Atlantic. Thus, the conveyor belt gets "recharged."

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THERMOHALINE CIRCULATION

As it moves around Antarctica, two sections split off the conveyor and turn northward. One section moves into the Indian Ocean, the other into the Pacific Ocean.

These two sections that split off warm up and become less dense as they travel northward toward the equator, so that they rise to the surface (upwelling).

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THERMOHALINE CIRCULATION

They then loop back southward and westward to the South Atlantic, eventually returning to the North Atlantic, where the cycle begins again.

The conveyor belt moves at much slower speeds (a few centimeters per second) than wind-driven or tidal currents (tens to hundreds of centimeters per second).

It is estimated that any given cubic meter of water takes about 1,000 years to complete the journey along the global conveyor belt.

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THERMOHALINE CIRCULATION

In addition, the conveyor moves an immense volume of water—more than 100 times the flow of the Amazon River. The conveyor belt is also a vital component of the global ocean nutrient and carbon dioxide cycles. Warm surface waters are depleted of nutrients and carbon dioxide, but they are enriched again as they travel through the conveyor belt as deep or bottom layers. The base of the world’s food chain depends on the cool, nutrient-rich waters that support the growth of algae and seaweed.

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THERMOHALINE CIRCULATION

The influence of the thermohaline circulation is especially strong in the North Atlantic, bringing in massive quantities of heat from the equator. This portion of the thermohaline circulation is known as the Gulf Stream. When the Gulf Stream shuts off, it robs heat from two major landmasses near the poles, greatly accelerating ice buildup.

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THERMOHALINE CIRCULATION

Glacial periods appear to end when the Gulf Stream strengthens, pumping energy northward to melt the ice sheets. Whereas the onset of glacial periods appears to be more gradual, the end of the glacial periods can be dramatically rapid.

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THERMOHALINE CIRCULATION and CLIMATE CHANGE

Shutdown of the thermohaline circulation is one factor that clearly drives rapid climate change.

Meltwater from land ice in Greenland and North America enters the North Atlantic during warming, causing the waters of the Gulf Stream to become less salty.

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THERMOHALINE CIRCULATION and CLIMATE CHANGE

This less saline water is less dense and thus cannot sink and complete the return trip to the equator.

The thermohaline circulation shuts down, stopping transport of heat from the equator. The net result of the shutdown is colder conditions throughout the North Atlantic, especially in Europe.

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An example of thermohaline shutdown took place during the transition out of the last ice age. As conditions warmed, continental ice melted and meltwater entered the North Atlantic.

The thermohaline circulation shut down, plunging Europe into a sudden cold snap lasting approximately 1000 years

The closing decades of the twentieth century and the early years of the present century were unusually warm.

Globally speaking, the last 30 years have been the warmest since accurate records began somewhat over 100 years ago.

Twelve of the 13 years 1995 to 2007 rank among the 13 warmest in the instrumental record of global surface air temperature that began around 1850, the years 1998 and 2005 being the warmest.

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IS THE CLIMATE CHANGING?

Warming of the climate system is unequivocal, as is now evident from observations of:

- increases in global average air and ocean temperatures,

-widespread melting of snow and ice, and

- rising global average sea level .

The Intergovernmental Panel on Climate Change in its 2007 Assessment states:

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The period has also been remarkable for the frequency and intensity of extremes of weather and climate.

Few examples: E.g.1. An extremely unusual heatwave in central Europe occurred in the summer of 2003 and led to the premature deaths of over 20 000 People. E.g2. Periods of unusually strong winds have been experienced in western Europe. During the early hours of the morning of 16 October 1987, over 15 million trees were blown down in southeast England and the London area. The storm also hit northern France, Belgium and the Netherlands with ferocious intensity; it turned out to be the worst storm experienced in the area since 1703. Storm-force winds of similar or even greater intensity but covering a greater area of western Europe have struck since – on 4 occasions in 1990 and 3 occasions in December 1999 .

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But those storms in Europe were mild by comparison with the much more intense and damaging storms other parts of the world have experienced during these years. About 80 hurricanes and typhoons – other names for tropical cyclones – occur around the tropical oceans each year, familiar enough to be given names:

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- Hurricane Gilbert caused devastation on the island of Jamaica and the coast of Mexico in 1988, - Typhoon Mireille hit Japan in 1991, -Hurricane Andrew caused a great deal of damage in Florida and other regions of the southern United States in 1992, - Hurricane Mitch caused great devastation in Honduras and other countries of central America in 1998 and - Hurricane Katrina caused record damages as it hit the Gulf Coast of the United States in 2005

All are notable recent examples .

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-Low-lying areas, such as Bangladesh, are particularly vulnerable to the storm surges associated with tropical cyclones The combined effect of intensely low atmospheric pressure, extremely strong winds and high tides causes a surge of water which can reach far inland. In one of the worst such disasters in the twentieth century over 250 000 people were drowned in Bangladesh in 1970.

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The increase in storm intensity during recent years has been tracked by the insurance industry, which has been hit hard by recent disasters. Until the mid 1980s, it was widely thought that windstorms (or hurricanes) with insured losses exceeding $US1 billion (thousand million) were only possible, if at all, in the United States.

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But the gales that hit western Europe in October 1987 heralded a series of windstorm disasters that make losses of $US10 billion seem commonplace. Hurricane Andrew, for instance, left in its wake insured losses estimated at nearly $US21 billion (1999 prices) with estimated total economic losses of nearly $US37 billion.

Figure shows the costs of weather-related disasters over the past 50 years as calculated by the insurance industry. It shows an increase in economic losses in such events by a factor of over 10 in real terms between the 1950s and the present day .

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Rainfall patterns, which lead to floods and droughts especially in tropical and semi-tropical areas, are strongly influenced by the surface temperature of the oceans around the world, particularly the pattern of ocean surface temperature in the Pacific off the coast of South America.

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EL NIÑO EVENTS

About every three to five years a large area of warmer water appears and persists for a year or more. Because they usually occur around Christmas these are known as El Niño (‘the boy child’) events.

They have been well known for centuries to the countries along the coast of South America because of their devastating effect on the fishing industry; the warm top waters of the ocean prevent the nutrients from lower, colder levels required by the fish from reaching the surface.

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A particularly intense El Niño, the second most intense in the twentieth century, occurred in 1982–3; the anomalous highs in ocean surface temperature compared to the average reached 7 °C. Droughts and floods somewhere in almost all the continents were associated with that El Niño.

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Like many events associated with weather and climate, El Niños often differ very much in their detailed character; that has been particularly the case with the El Niño events of the 1990s.

For instance, the El Niño event that began in 1990 and reached maturity early in 1992, apart from some weakening in mid 1992, continued to be dominated by the warm phase until 1995. The exceptional floods in the central United States and in the Andes and droughts in Australia and Africa are probably linked with this unusually protracted El Niño.

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This, the longest El Niño of the twentieth century, was followed in 1997–8 by the century’s most intense El Niño which brought exceptional floods to China and to the Indian sub-continent and drought to Indonesia – that in turn brought extensive forest fires creating an exceptional blanket of thick smog which was experienced over 1000 miles away! A scientific question that is being urgently addressed is: - the possible link between the character and intensity of El Niño events and global warming due to human-induced climate change.

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The effect of volcanic eruptions on temperature extremes

Natural events such as volcanoes can also affect the climate. Volcanoes inject enormous quantities of dust and gases into the upper atmosphere. Large amounts of sulphur dioxide are included, which through photochemical reactions using the Sun’s energy are transformed to sulphuric acid and sulphate particles. Typically these particles remain in the stratosphere (the region of atmosphere above about 10 km in altitude) for several years before they fall into the lower atmosphere and are quickly washed out by rainfall.

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OTHER NATURAL EVENTS

During this period they disperse around the whole globe and cut out some of the radiation from the Sun, thus tending to cool the lower atmosphere. One of the largest volcanic eruptions in the twentieth century was that from Mount Pinatubo in the Philippines on 12 June 1991 It injected about 20 million tonnes of sulphur dioxide into the stratosphere together with enormous amounts of dust. This stratospheric dust caused spectacular sunsets around the world for many months following the eruption. The amount of radiation from the Sun reaching the lower atmosphere fell by about 2%. Global average temperatures lower by about a quarter of a degree Celsius were experienced for the following two years.

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We know for sure that because of human activities, especially the burning of fossil fuels, coal, oil and gas, together with widespread deforestation, the gas carbon dioxide has been emitted into the atmosphere in increasing amounts over the past 200 years and more substantially over the past 50 years. Every year these emissions currently add to the carbon already present in the atmosphere a further 8000 million tonnes, much of which is likely to remain there for a period of 100 years or more.

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. Because carbon dioxide is a good absorber of heat radiation coming from the Earth’s surface, increased carbon dioxide acts like a blanket over the surface, keeping it warmer than it would otherwise be. . With the increased temperature the amount of water vapour in the atmosphere also increases, providing more blanketing and causing it to be even warmer. The gas methane is also increasing because of different human activities, for instance mining and agriculture, and adding to the problem.

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Being kept warmer may sound appealing to those of us who live in cool climates. However, an increase in global temperature will lead to global climate change. If the change were small and occurred slowly enough we would almost certainly be able to adapt to it. However, with rapid expansion taking place in the world’s industry the change is unlikely to be either small or slow. The estimate is that, in the absence of efforts to curb the rise in the emissions of carbon dioxide, the global average temperature will rise by about a third of a degree Celsius or more every ten years – or three or more degrees in a century.

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This may not sound very much, especially when it is compared with normal temperature variations from day to night or between one day and the next. But note: it is not the temperature at one place but the temperature averaged over the whole globe. The predicted rate of change of 3 °C a century is probably faster than the global average temperature has changed at any time over the past 10 000 years. And as there is a difference in global average temperature of only about five or six degrees between the coldest part of an ice age and the warm periods in between ice ages, a few degrees in this global average can represent a big change in climate. Many ecosystems and human communities will find difficult to adapt to this very rapid rate of change (especially those in developing countries)

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Not all the climate changes will in the end be adverse! 1) While some parts of the world experience more

frequent or more severe droughts, floods or significant sea level rise, in other places crop yields may increase due to the fertilising effect of carbon dioxide.

2) Other places, perhaps for instance in the sub-arctic, may become more habitable. Even there, though, the likely rate of change will cause problems: large damage to buildings will occur in regions of melting permafrost, and trees in sub-arctic forests like trees elsewhere will not have time to adapt to new climatic regimes.

. Scientists are confident about the fact of global warming and climate change due to human activities. . However, uncertainty remains about just how large the warming will be.

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. What will be the patterns of change in different parts of the world? . Although useful indications can be given, scientists cannot yet say in precise detail which regions will be most affected. Intensive research is needed to improve the confidence in scientific predictions .

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