A climate index is here defined as a calculated value that can be used to describe the state and the changes in the climate system. The climate at a defined place is the average state of the atmosphere over a longer period of, for example, months or years. Changes on climate are much slower than on the weather, that can change strongly day by day.
The first classical climate indices of the atmosphere have been defined already about approximately a hundred years ago, for example the North Atlantic Oscillation (NAO), the first found teleconnection pattern. The NAO index and the NAO pattern can be determined from monitoring station data or identified by means of EOF analysis.
Climate indices allow a statistical study of variations of the dependent climatological aspects, such as analysis and comparison of time series, means, extremes and trends.
Climate elements and climate indices
Each climate index is based on certain parameters and describes only certain aspects of the climate, so there are a variety of climate indices that have been defined and examined in numerous publications. For each climate index there is a defining equation that uses the so-called climate elements. These are measurable parameters that influence the properties of the climate system, primarily, for example, atmospheric parameters such as air pressure, air temperature, precipitation and solar radiation, but also non-atmospheric parameters such as sea surface temperature or ice cover.
Means and extreme values, linear trends and standard deviations of longtime time series can be calculated for each of these climate parameters. These results are a simple form of climate indexes, as they already describe changes in climate. In addition, special climate indices exist for many parameters from which are some described below.
Long time series of station data are required to calculate the climate indices for air temperature and precipitation described below. These results correspond to the chosen station, but in many cases are transferable to the environment, dependent on the environments homogeneity. Some results, such as the minimum temperature or rain events, are already in a small space highly variable and can not simply be transferred to larger areas.
Typical climate indices that use air pressure are of a different type. Here matters not the absolute value of a station but the pressure gradient between locations. This means that at least two stations are needed, but in most cases field data are used to calculate atmospheric patterns and time series of climate indices. The same applies to the sea surface temperature.
Climate base period
The so called climate base period is used for calculation of some climate indexes. It is a reference period, which should include 30 years of data on the recommendation of the WMO (World Meteorological Organization). The currently most used time period is from 1961 to 1990. The mean values of different parameters for this period are used for further calculations. For comparisons between results it is important that all refer to the same base period.
Climate indices for air temperature
The air temperature is an atmospheric parameter that is recorded since the start of weather recording. The air temperature is measured daily in 2 m height above ground and long time series exist for many stations. Additional to the current temperature the daily minimum and maximum values are recorded that are used for index calculations as well.
- Number of frost and ice days
- Number of summer days and tropical nights
- Extreme values during a specific period
- Exceeding specific limits (Percentile)
- Daily temperature range
- Heating degree day
- Warm and cold spell duration
- Growing season length
Climate indices of precipitation
Similar to air temperature, the recording of precipitation amounts started a begin of weather recording. After some technical developments the duration of a precipitation event was recorded, too. Additionally information about the type of precipitation (rain, snow, hail) was collected by the observer and can nowadays be detected automatically. The following climate indexes are for the precipitation amount:
- Maximum 1- and 5-day precipitation per Year
- Simple precipitation intensity index
- Annual count of days when the precipitation is greater than a defined limit
- Maximum length of dry spell
- Maximum length of wet spell
- Annual total precipitation when rain rate is above a defined limit (Perzentil)
- Annual total precipitation in wet days
The variation from the mean value of the precipitation sum at a certain location during a specific time period has a strong influence, because low rainfall means drought, while too much rain may be associated with floods. The Standard Precipitation Index, SPI, is well suited to study these variations in rainfall:
Climate indices for air pressure
The climate indices for the air pressure compiled here are calculated from large-scale air pressure fields or the geopotential. They are also known as the teleconnection pattern of air pressure. The term "teleconnection" was introduced because air pressure anomalies at different locations within the considered area can be related to one another using this method. The exact nature of these relationships is examined using the indices. The most important ones are:
- North Atlantic oscillation (NAO)
- Southern Annular Mode (SAM)
- Arctic oscillation (AO)
- Antarctic oscillation (AAO)
- Pacific-North American pattern (PNA)
- Southern Oscillation (SO)
The ILD Index, that describes the east-west-shift of the NAO is a CEN product.
The "blocking" is also dependent on air pressure and has a strong influence on the weather that can last for several weeks. The calculation of a blocking index is based on the pressure gradient between mid- and high lattitudes.
Climate indices for sea surface temperature
- Pazifische Dekaden Oszillation (Pacific Decadal Oscillation, PDO)
- El Niño / La Niña - Southern Oscillation (ENSO)
- Indian Ocean Dipole (IOD)
- Atlantic Multidecadal Oscillation (AMO)
- Trends of the SST