Thanks to global warming, we can expect more frequent torrential rains and floods in east coast of Africa and cold dry conditions and drought in Indonesia. This revelation comes from a study lead authored by Dr. Wenju Cai, in today’s issue of Nature. Dr. Cai is a climate scientist with the Commonwealth Scientific Industrial Research Organization (CSIRO).
These extreme climatic events in the Indian Ocean region are caused by increased Sea surface temperatures (SSTs) in the western Indian Ocean, off the East coast of Africa and lowered SSTs in the eastern Indian Ocean off the Sumatra Java coast, Indonesia. Such scenarios witnessed in the past — 1961, 94 and 97 — have had disastrous consequences for both the east and west regions.
Such extreme events occur when the difference between the western and eastern temperature anomalies is large. However, if the sea surface temperature anomalies are reversed — decreased SSTs in the western Indian Ocean and increased SSTs in the in the eastern Indian Ocean, the climatic events too get reversed; more frequent torrential rains and floods would be seen in the eastern Indian Ocean and cold dry conditions and drought in the western Indian Ocean.
The study used climate models forced by a scenario of high greenhouse gas emissions and projected that the frequency of such extreme events will increase by almost a factor of three, from one event every 17.3 years over the twentieth century to one event every 6.3 years over the twenty-first century.
The role of winds and ocean current reversal in causing these extreme events was examined.
The winds and ocean currents usually flow from the west to the east.
When these winds and oceanic currents weakened due to faster warming in the western equatorial Indian Ocean compared to the slow warming in the eastern equatorial Indian Ocean there are more frequent occurrences of wind and oceanic current reversal.
The significance of reversal is that an extreme event results from westward extension of cold and low rainfall anomalies from the east.
“Under global warming the East Indian Ocean is warming slower than the Western Indian Ocean. The weakening makes it easier to reverse, and is therefore conducive for extreme positive event,” notes Dr. Cai in an email.
When the wind and current direction is reversed and the flow is from east to west, heat is transferred horizontally (advection) to the westward flowing wind. This flow carries warm water and convection in western longitudes of the Indian Ocean region, further west, resulting in extreme rainfall in East African countries.
Rainfall anomalies are used in a statistical analysis using a statistical tool called EOF (empirical orthogonal function) to study the cold, dry east pole conditions which characterise an extreme event. Two EOFs; EOF1 and EOF2 are used.
“EOF1 and EOF2 are two anomaly patterns: EOF1 features cold and dry anomalies in the eastern Indian Ocean, and EOF2 features cold and dry anomalies along the equator. An extreme event is indicated by cold anomalies not only in the East Indian Ocean but also along the equator, so the superimposition of the two EOFs is necessary,” Dr Cao clarifies in an email to this correspondent.