The Little Ice Age: How Climate Made History 1300-1850 by Brian Fagan Read Free Book Online

index
    The "NAO index" expresses the constant shifts in the oscillation from
year to year and decade to decade. A high NAO index signals low pressure
around Iceland and high pressure off Portugal and the Azores, a
condition that gives rise to persistent westerly winds. These westerlies bring
heat from the Atlantic's surface to the heart of Europe, together with powerful storms. The same winds keep winter temperatures mild, which makes
northern European farmers happy and produces dry conditions in southern
Europe. A low NAO index, in contrast, brings shallower pressure gradients,
weaker westerlies, and much colder temperatures over Europe. Cold air
from the north and east flows from the North Pole and Siberia, snow blankets Europe, and Alpine skiers have a wonderful time. NAO's winter oscillations account for about half the variability in winter temperatures in northern Europe and also exercise an important effect on summer rainfall.
A high NAO index brings more rain in the summer, as it did after 1314.

    Low North Atlantic Oscillation index
    The seesaw swings unceasingly, in cycles that can last seven years or
more, even decades, or sometimes much less. The swings are unpredictable and sudden. An extreme low NAO index causes a reversal of
winter temperatures between Europe and Greenland. When pressure in
Greenland is higher than in Europe-the "Greenland above" (GA) effect-persistent blocking high pressure systems form between Greenland
and Scandinavia. Temperatures are above average in western Greenland
and lower than usual in northwestern Europe (and also in eastern North
America). Winter in western Europe is bitterly cold. When pressure over
Greenland is lower than that in Europe-"Greenland below" (GB)then temperatures are reversed and the European winter is milder than
normal.
    The extreme swings of the NAO are part of the complex atmospheric/ocean dynamics of the North Atlantic, which include sea-surface temperature anomalies, the strength of the Gulf Stream, atmospheric
wave structure, and the distribution of sea ice and icebergs. These interactions are poorly understood, but there seems little doubt that many of the
swings in the NAO result from changes in sea surface temperatures in the
North Atlantic. One day, computer simulations of dynamic ocean temperatures and atmospheric relationships may allow meteorologists to predict winter rainfall in Europe several years in advance, a scientific breakthrough of momentous importance.

    Greenland below effect
    The chaotic atmosphere over the North Atlantic plays a major role in
the NAO's unpredictable behavior. So do the mild waters of the Gulf
Stream, which flow northeast off North America and then become the
North Atlantic current, bringing warm water to the British Isles and adjacent coasts as far north as Iceland and Norway. The warmer surface waters
of the north flush regularly, sinking toward the bottom, carrying atmospheric gases and excess salt. Two major downwelling sites are known, one
just north of Iceland, the other in the Labrador Sea southwest of Greenland. At both these locations, vast quantities of heavier, salt-laden water
sink far below the surface. Deep subsurface currents then carry the salt southward. So much salt sinks in the northern seas that a vast heat pump
forms, caused by the constant influx of warmer water, which heats the
ocean as much as 30 percent beyond the warmth provided by direct sunlight in the north. What happens if the flushing fails? The pump slows
down, the warm North Atlantic current weakens, and temperatures fall
rapidly in northwestern Europe. When downwelling resumes, the current
accelerates and temperatures climb again. The effect is like a switch, triggered by the interaction of atmosphere and ocean. For example, in the
early 1990s, the Labrador Sea experienced vigorous downwelling, which
gave Europe mild winters. In 1995/96, the NAO index changed abruptly
from high to low, bringing a cold winter