by Luisa Cristini, PhD, University of Hawaii at Manoa
[Note from the editor: This is the eleventh in a series of blog entries that will focus on introductory topics in climate dynamics and modeling, and will be a great insight into the current understanding of the science.]
Since the beginning of Earth’s history, climate has changed on all timescales. Over millions of years, it has swung between very warm conditions, with annual mean temperatures above 10°C in Polar Regions, and glacial climates, in which the ice sheets covered the majority of the mid-latitude continents. Fluctuations have been observed on scales shorter than a year (interannual) and of tens, no year being exactly the same as a previous one.
The timescale of all these variations is partly set up by the forcing, as discussed earlier (see Part 9: What drives climate changes?). For example, because of its own stellar evolution, the radiation emitted by the Sun has increased by roughly 30% over the 4.5 billion years of the Earth’s history. Variations in the energy emitted by the Sun (the total solar irradiance) on shorter timescales have smaller amplitude. The changes in the Earth’s orbit characteristics have modified the amount of solar energy received in a particular season on every point on the Earth’s surface. Individual volcanic eruptions have produced a general cooling during the years following the eruption. Increased volcanic activity related to plate tectonics has lead to strong forcing lasting thousands to millions of years.
However, mechanisms internal to the climate system can also play a very important role in determining the variability of the Earth’s climate. Natural climate variability is produced through interactions of the atmosphere with the land and ocean surfaces on seasonal and longer timescales; predominantly occurring with preferred geographical patterns and timescales. Such patterns are often called regimes, modes or teleconnections (i.e., patterns with the same or opposite characteristics in different regions of the planet). Important examples are the El Niño-Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO) and the Southern Annular Mode (SAM).
ENSO is a basin-wide warming of the tropical Pacific Ocean associated with a fluctuation of a global-scale tropical and subtropical surface atmospheric pressure pattern. It is a coupled atmosphere-ocean phenomenon, with preferred timescales of 2 to 7 years. It is often measured by the surface pressure anomaly difference between Darwin, Australia, and Tahiti and the sea surface temperature in the central and eastern Equatorial Pacific. This event affects the wind, sea surface temperature and precipitation patterns in the tropical Pacific. It has climatic effects throughout the Pacific region and in many other parts of the world, through global teleconnections. During an ENSO event, the prevailing trade winds weaken, reducing upwelling and altering ocean currents such that the sea surface temperatures warm, further weakening the trade winds. The cold phase of ENSO is called La Niña. ENSO events are major source of interannual variability in atmospheric CO2 growth rate due to their effect on fluxes through land and on surface ocean temperature and precipitation.
NAO consists of opposing variations of surface atmospheric pressure near Iceland and near the Azores. It corresponds to fluctuations in the strength of the main westerly winds across the Atlantic into Europe, and thus to fluctuations in the associated cyclones with their frontal systems.
SAM is an interannual to centennial climatic variability that influences zonal (i.e., along a latitude circle, in the west-east direction) winds, sea ice formation and ocean circulation. During the initial (positive) phase, SAM is associated with enhanced westerly winds over the Antarctic Circumpolar Current, which flows around Antarctica. This favors the strengthening of the current and the northward expansion of sea ice. To compensate there is enhanced pole-ward transport and rising of deep waters at Antarctic continental margin. During the opposite (negative) phase a weakening of both zonal and meridional (i.e., along a meridian, in the north-south direction) ocean transport is observed.
Goosse H., P.Y. Barriat, W. Lefebvre, M.F. Loutre and V. Zunz, (2012). Introduction to climate dynamics and climate modeling. Online