Natural Climate Variability
By natural variability, we mean variations that occur without the interactions of humans and their activities.
Natural variability.
Diurnal. Temperatures change dramatically between day and night due to the influence of solar radiation.
Seasonal. Temperature and precipitation changes occur as Earth revolves around the sun.
Stratospheric wind oscillation. The stratospheric winds in the middle latitudes change direction twice every year. In northern hemisphere (NH) winter, they are from the west (eastward, or westerly).
Quasi-Biennial Oscillation. About every two years, the stratospheric winds from 18 – 50 kilometers change direction. They shift first at higher altitudes and the shift progresses downward over the course of about two years.
El Nino Southern Oscillation (ENSO). ENSO is a Pacific Ocean phenomenon that has global effects. It occurs about every 2 to 7 years and lasts about 12-18 months. El Nino: waters warm off the coast of Peru, cool in the western and central Pacific. El Nino indicates what’s happening to the ocean. Southern Oscillation: The Sea Level Pressure (SLP) is lower at Tahiti than at Darwin during El Nino. This results from convection that occurs because of the warmer water.
La Nina refers to the cooling of waters off the Peruvian coast and may extend into the central Pacific. A La Nina will usually occur after a large El Nino because the heat in the Pacific Ocean reservoir is reduced by the El Nino.
North Atlantic Oscillation (NAO). This oscillation occurs on the decadal time scale. It is determined as the difference in the sea level pressure between the Azores and Iceland. A positive index means lower SLP near Iceland. A negative index means higher SLP near Iceland.
During the positive index, which has persisted for about the last 20 years, the following effects have occurred:
Orbital parameters. As Earth’s orbit changes, the amount of sunlight that different parts of the Earth receive changes. These changes in the distribution of sunlight alter climate. This theory was first proposed by Milutin Milonkovich, a Serbian mathematician, in 1920.
Eccentricity. The orbit around the sun is an ellipse. As a result, Earth is closer to the sun at some times than at others. About every 100,000 years, the eccentricity (eccentricity = distance from center to focus/distance from center to curve on long axis). This number varies from 0 to about 0.08.
Obliquity. The angle between the axis of Earth’s rotation and the normal to the plane of Earth’s orbit about the sun changes over a period of about 41,000 years. The obliquity varies from 22-24.5o.
Precession. Earth’s axis rotates about its tilt. This is caused by the pull of the sun and planets and has a period of roughly 19,000-24,000 years.
Anthropogenic climate change.
We have put considerable amounts of greenhouse gases into the atmosphere, so that their concentrations now higher than they have been in a thousand years. These greenhouse gases include: