Sustained changes in the difference between sea surface temperatures of the tropical western and eastern Indian Ocean are known as the Indian Ocean Dipole or IOD. The IOD is one of the key drivers of Australia’s climate and can have a significant impact on agriculture. This is because events generally coincide with the winter crop growing season. The IOD has three phases: neutral, positive and negative. Events usually start around May or June, peak between August and October and then rapidly decay when the monsoon arrives in the southern hemisphere around the end of spring.
Positive IOD phase
Westerly winds weaken along the equator allowing warm water to shift towards Africa. Changes in the winds also allow cool water to rise up from the deep ocean in the east. This sets up a temperature difference across the tropical Indian Ocean with cooler than normal water in the east and warmer than normal water in the west.
Generally this means there is less moisture than normal in the atmosphere to the northwest of Australia. This changes the path of weather systems coming from Australia’s west, often resulting in less rainfall and higher than normal temperatures over parts of Australia during winter and spring.
Negative IOD phase
Westerly winds intensify along the equator, allowing warmer waters to concentrate near Australia. This sets up a temperature difference across the tropical Indian Ocean, with warmer than normal water in the east and cooler than normal water in the west. A negative IOD typically results in above-average winter–spring rainfall over parts of southern Australia as the warmer waters off northwest Australia provide more available moisture to weather systems crossing the country.
Interplay between IOD and El Nino on Indian Monsoon
The Indian summer monsoon rainfall is influenced by a system of oscillating sea surface temperatures known as the Indian Ocean Dipole (IOD) in which the western Indian Ocean becomes alternately warmer and then colder than the eastern part of the ocean. While the existence of three types of IOD is well known, a recent study published in the journal Natural Hazards attempts to determine the effects on monsoon rainfall of each of the three types. A positive IOD occurs when the sea surface temperatures are greater than normal in the Arabian Sea and less than normal in the tropical eastern Indian Ocean. When the reverse is the case, a negative IOD is said to have developed.
A positive IOD leads to greater monsoon rainfall and more active (above normal rainfall) monsoon days while negative IOD leads to less rainfall and more monsoon break days (no rainfall).
The co-occurrence of the different types of El Nino events over the tropical Pacific (weak to very strong) and the result of their interaction with the three types of IOD on monsoon rainfall were studied. It was found that during weak El Nino years (1994 and 2006) there were no break events in the peak monsoon months of July and August (early IOD), whereas in the case of moderate (1987), strong (1972) and very strong El Nino years (1982, 1997) more break events were observed during the mid-monsoon months.
The Walker circulation is an ocean-based system of air circulation that influences weather on the Earth. The Walker circulation is the result of a difference in surface pressure and temperature over the western and eastern tropical Pacific Ocean.
Normally, the tropical western Pacific is warm and wet with a low pressure system, and the cool and dry eastern Pacific lie under a high pressure system. This creates a pressure gradient from east to west and causes surface air to move east to west, from high pressure in the eastern Pacific to low pressure in the western Pacific. Higher up in the atmosphere, west-to-east winds complete the circulation. The warm waters of the western Pacific Ocean in East Asia heat the air above it and supply it with moisture. On average, the air rises, forms clouds, and then flows to the east across the Pacific, losing moisture to rainfall. The air then sinks off the west coast of South America and returns to the west along the surface of the ocean, back to the western Pacific Ocean.
The Walker circulation contributes to normal weather conditions in the tropical Pacific Ocean: warm, wet weather in the western Pacific and cool, dry weather in the eastern Pacific.
The Walker circulation usually brings areas of low pressure to the western IndianOcean but, in years when El Niño occurs, this pattern can get shifted eastward, bringing high pressure over India and suppressing the monsoon, especially in spring when the monsoon begins to develop.
Madden-Julian Oscillation (MJO)
The Madden-Julian Oscillation (MJO) is the major fluctuation in tropical weather on weekly to monthly timescales. The MJO can be characterised as an eastward moving ‘pulse’ of cloud and rainfall near the equator that typically recurs every 30 to 60 days.
A disturbance of clouds, wind and pressure, moving eastward at a speed of 4-8 metres per second, MJO goes around the globe in 30-60 days on average. Sometimes, it can take 90 days. As it moves, strong MJO activity often splits the planet in to two — one in which the MJO is in active phase and brings rainfall, and the other in which it suppresses rainfall. In the active phase, MJO results in more than average rainfall for that time of the year, while in the suppressed phase, the area receives less than average rainfall.
An active phase is generally followed by a weak or suppressed phase, in which there is little MJO activity. Three active MJO periods are witnessed every year on average.
Pacific Decadal Oscillation
The Pacific Decadal Oscillation (PDO) is a pattern of Pacific climate variability similar to ENSO in character, but which varies over a much longer time scale. The PDO can remain in the same phase for 20 to 30 years, while ENSO cycles typically only last 6 to 18 months.
The PDO, like ENSO, consists of a warm and cool phase which alters upper level atmospheric winds. Shifts in the PDO phase can have significant implications for global climate, affecting Pacific and Atlantic hurricane activity, droughts and flooding around the Pacific basin, the productivity of marine ecosystems, and global land temperature patterns. Experts also believe the PDO can intensify or diminish the impacts of ENSO according to its phase.
If both ENSO and the PDO are in the same phase, it is believed that El Niño/La Nina impacts may be magnified. Conversely, if ENSO and the PDO are out of phase, it has been proposed that they may offset one another, preventing “true” ENSO impacts from occurring.
The PDO spatial pattern and impacts are similar to those associated with ENSO events. During the positive phase the wintertime Aleutian low is deepened and shifted southward, warm/humid air is advected along the North American west coast and temperatures are higher than usual from the Pacific Northwest to Alaska but below normal in Mexico and the Southeastern United States.
Winter precipitation is higher than usual in the Alaska Coast Range, Mexico and the Southwestern United States but reduced over Canada, Eastern Siberia and Australia. The PDO along with the AMO strongly influence multidecadal droughts pattern in the United States, drought frequency is enhanced over much of the Northern United States during the positive PDO phase and over the Southwest United States during the negative PDO phase in both cases if the PDO is associated with a positive AMO.
The Asian Monsoon is also affected, increased rainfall and decreased summer temperature is observed over the Indian subcontinent during the negative phase