The ENSO is the surface oscillation of the equatorial Pacific Ocean. It causes large-scale changes in sea pressures, sea surface temperatures, precipitation and winds, not only in the tropics but also in many other parts of the globe. It describes the natural interannual variations of the ocean and atmosphere in the tropical Pacific. Sea surface temperatures (SSTs) in the central and eastern equatorial Pacific also vary.

ENSO results in states of average, below or above average SSTs being established. Thus, when SSTs in the central and eastern equatorial Pacific are significantly higher than usual an El Niño situation occurs. When the waters of the central and eastern equatorial Pacific are significantly colder than usual, a La Niña condition occurs. A neutral situation occurs when the temperatures are equivalent to the established average.

For ease of observation, the tropical Pacific is divided into several boxes from which SST indices are calculated. Usually, the anomalies are calculated with respect to a 30-year base period.  The Niño 3.4 index and the Ocean Niño Index (ONI) are the most commonly used indices to define El Niño and La Niña events.

Nino zone observation of ocean surface temperatures in the equatorial Pacific

The Southern Oscillation Index (SOI) is defined as the normalized pressure difference between Tahiti and Darwin. There are several slight variations in the SOI values calculated in different centers.  It is associated with the El Niño and La Niña phases of ENSO.

https://psl.noaa.gov/gcos_wgsp/Timeseries/Data/soi.long.data

https://psl.noaa.gov/gcos_wgsp/Timeseries/SOI/

Tropical Atlantic Variability (TAV) is the fluctuation of the ensemble of variations in sea surface temperature (SST), sea level pressure (SLP), Hadley circulation, inter-tropical convergence zone (ITCZ), and changes in wind direction. Its clearest manifestation usually occurs between March and May (MAM). This is dominated by interannual and decadal changes.

The VAT is often called “Atlantic El Niño” and is associated with Trade Winds as El Niño in the Pacific. Depending on the strength of the Trade Winds, Southeast, it warms the ocean water alternately south of the equator, then north, then south of the equator again.

NB : The significant rainfall anomalies in the Sahel (Lamb, 1978) and many other regions, are related to the variation of the tropical Atlantic.

The oceanographic indices of the Tropical Atlantic are :

The Atlantic Multidecadal Oscillation (AMO) is a large-scale cyclic variation in the atmospheric and oceanic current in the North Atlantic Ocean that combines to alternately increase and decrease the surface temperature of the Atlantic Ocean (SST).

This multi-decadal oscillation varies on a 50-70 year scale with positive anomalies for about 40 years, followed by negative anomalies for about 20 years in the North Atlantic with an approximate difference of 0.6°C between the extremes.

The causes of the AMO are currently not well known because this oscillation is long.

The index is calculated by averaging the SST from the North Atlantic Ocean to the Atlantic Ocean equator, between 0°N and 60°N, then 75°W and 7.5°W. These low frequencies are associated with precipitation changes on the Sahel and on the North-East of the Brésil, the frequency and intensity of hurricanes in the North Atlantic and the summer climate of Europe and North America.

The difference in atmospheric pressure between the north and the south at the surface of the Atlantic Ocean leads to an exceptionally strong wind in South-East Africa. This wind anomaly weakens the Northeast Trade Winds. When the trade winds are weak, the surface of the ocean cools less and less to make room for warming. As the temperature increases, the pressure difference between the North and South Atlantic increases, causing a stronger wind anomaly. The opposite phenomena occur if the trade winds are strong.
The presence of warming in the North Atlantic Ocean, leads to the intensification of winds near the equator. As a result, the ITCZ (the intertropical convergence zone where significant precipitation occurs) moves northward.

The AMO climatic phenomena influence the rainfall of West Africa. Indeed, droughts in the Sahel occurred mainly when the AMO index was negative, i.e. when the temperature anomaly in the North Atlantic was negative and vice versa in the South Atlantic.

Access link to AMO data: http://www.cdc.noaa.gov/data/correlation/amon.us.long.data

When AMO is positive, positive air-sea feedback develops..

The variation of surface temperatures between the East and West of the Indian Ocean creates a dipole called Indian Ocean Dipole (IOD). The variation of the IOD leads to an interaction phenomenon between the ocean and the atmosphere in the Indian Ocean. This has significant impacts on the climate of India and the surrounding countries according to the anomalies of the surface temperature of the Indian Ocean.
The Dipole Mode Index (DMI) is an indicator of the East-West temperature gradient in the tropical Indian Ocean, related to the dipole or zonal mode of the Indian Ocean. It is calculated as the difference between the WTIO (Western Tropical Indian Ocean) and SETIO (South-eastern Tropical Indian Ocean) indices. The intensity of the Indian Ocean Dipole is represented by its calculated index. The September-October-November rainfall extremes over tropical East Africa have been associated with periods of persistent high DMI.

Link to the calculated indices of the DMI HadISST1.1 :

https://psl.noaa.gov/gcos_wgsp/Timeseries/Data/dmi.had.long.data

As previously stated, the SPI was designed to quantify precipitation deficit at multiple time scales, i.e., averaging over a sliding window. These time scales reflect the impacts of drought on different resource types and meet the needs of different decision makers. Weather and soil moisture (agriculture) respond relatively quickly to precipitation anomalies, at time scales of, for example, 1 to 6 months, while groundwater, streamflow, and reservoir storage volumes are sensitive to longer-term precipitation anomalies, i.e., at time scales of more than 6 months. The time period of the SPI will therefore vary according to the type of drought under analysis and application: for example, the SPI will be taken over 1 to 2 months for a meteorological drought, over 1 to 6 months for an agricultural drought and over 6 to 24 months, or even longer, for a hydrological drought. Read more

Other indices