(AMO)Atlantic Multidecadal Oscillation

1st PART







The AMO (the Atlantic Multidecadal Oscillation) is a cyclic variation of the Oceanic and atmospheric current in large time scale in the North Atlantic Ocean, that combines to alternately to increase and drop the temperature of the sea surface Atlantic (SST). This oscillation that is multidecadal varies on a time scale from 50 to 70 years with positive anomalies for approximately 40 years followed by negative anomalies of SST for approximately 20 years in the North Atlantic with a difference of approximately 33,08°F (0,6°C) between the extremes. The AMO is a quasi-periodic cycle, related to the variability of the ocean thermohaline circulation. Its index shows that from 1856 to today that there was a warm persistent from 1856 to 1900, from 1930 to 1960 and from 1995 to day and then a cold persistence from 1900 to 1920 and from 1960 to 1995. The causes of the AMO are currently not well known because this oscillation is long.

The AMO index is calculated by averaging the SST between the North of Atlantic Ocean and the equator of the Atlantic Ocean, between 0°N and 60°N, 75°W and 7,5°W.

The annual index of AMO from 1856 to now. The red parts of the graph show that
the temperature of the sea surface (SST) of North Atlantic is warmer than average
and vice versa for the South Atlantic SST and blue parts show instead that the
North Atlantic SST was below normal while the South Atlantic SST was above
normal. The graphic was created and is set to days of the NOAA data.

The evolution of the AMO on a moving average of 10 years reconstructed from tree rings
in North America of Eastern Europe and Western Scandinavia from 1572 to 1980 according
to the NOAA data and which is updated each year according to other above NOAA data.


    • The AMO has many effects on the climate of the Northern Hemisphere. It has been shown that it is responsible for approximately 15% of the warming of 33,8°F (1°C) on the France during these last 15-20 years ! Which is due to the exchange of energy between the ocean and the atmosphere.


      This climatic phenomenon also affects the number and see the power of tropical cyclones in the North Atlantic on a long-term time scales while the QBO, ENSO... have effects on an annual basis.

      During the positive phase of the AMO as from 1995 to present, there are more hurricanes than during the negative phase as from 1960 to 1995. Because when AMO is in positive phase then the ocean circulation is faster, the westerlies (the west wind in the mid-latitude) are shifted to the north and the trade winds more warm up more the ocean which is favorable for the formation of hurricanes. While during the negative phase the thermohaline circulation is lower, the westerly winds are curved farther to the south. This causes a vertical wind shear which reduces the number of tropical hurricanes in the North Atlantic.

Comparison of the evolution of the AMO index (June to October) and the
evolution of the number of tropical storms and hurricanes in the North Atlantic.
The graph was created and it is updated according to the data of the AMO
NOAA and the number of hurricanes from UNISYS WEATHER


      The AMO is responsible for regional atmospheric circulation changes and therefore associated with the precipitations and temperatures anomalies in the United States, South of the Mexico and probably in Western Europe.

      During the positive phase of the AMO most of the United States has less rainfall and more droughts in the central west. But to the south-western of the United States during the warm phase precipitations are greater. And when the negative phase of the AMO it is quite the opposite.

      Between the warm and cold phase of the AMO, the flow of the Mississippi varies from 10%, while the inflow to Lake Okeechobee, Florida varies from 50-40%. During cold phases (AMO-) the average of the influx of the lake is approximately 2 feet while during the warm phases (AMO+) influx is about 4 feet. The geographical pattern of variability is influenced by changes in rainfall mainly in the summer.

      On the right you have a comparison between of the AMO index (top right) and the percentage of rainfall in Florida (the shaded area at the bottom right) and the percentage of the water level in Lake Okeechobee Florida (the blue curve below right)..

      By studying the relationship between the data records of the cut of the tree and the variation of the climate of the past hundred years in North America and in Europe, there are links with Atlantic Multidecadal Oscillation.



      The climate of North America is influenced by long-term variability of the AMO as seen above but also by the climate phenomenon of the Pacific, the PDO. And so these two long periods oscillations may offset or the reverse and thus amplify the anomalies of rainfall, temperature and atmospheric pressure.

      On the right there is a comparison of the indices of the two climate phenomena : "A" the PDO and "B" the AMO.


      Sectors smaller frequencies (blue <25%) and higher (red> 25%) of droughts are associated
      with climate phenomena of the PDO and AMO. Note the greater magnitude of drought
      in the United States linked to the warming of SST in North Atlantic Ocean.

      Two of the most severe droughts of the 20th century occurred during the positive AMO between 1925 and 1965 : The "Dust Bowl", the name given to a series of dust storms in the 1930s and the last major drought in the Midwest, in the mid 1950s.


      The evolution of the rainfall anomaly in the Sahel from 1900 to now
      compared to the average from 1950 to 1979 and the evolution of the
      AMO. The graph was created and is updated with NOAA and JJASO data.

      When the AMO is positive, a positive feedback of air-sea develops.

      Atmospheric pressure difference on the surface of the sea between North and South lead an exceptionally strong wind by the southeast of Africa. This wind anomaly weakens the Northeast tradewinds. When the trade winds "tradewinds" are weakened they less cool water in the North, and the water grows warmer. The hotter temperature increases the difference in the pressure between the North and South Atlantic, and causes a stronger wind anomaly. The feedback continues.

      Due to the hot water in the North Atlantic, near the Equator wind intensifies, the ITCZ (Intertropical Convergence Zone) where significant rainfall occurs moves northward.

      The climate in the Northeast Brazil becomes drier than normal. During a period of drought, many Brazilians emigrate to more humid areas. Meanwhile, the Sahel region of Africa gets more rain than in the years 80s. Each decade, these wet and dry climates move of places because the sea surface temperature of the North and South Atlantic ocean reverses. And therefore their effect on the climate reverses.

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