The Indian Ocean Dipole (IOD), is an irregular oscillation of sea surface temperatures in which the western Indian Ocean becomes alternately warmer (positive phase) and then colder (negative phase) than the eastern part of the ocean.
The Indian Ocean Dipole (IOD), is an irregular oscillation of sea surface temperatures in which the western Indian Ocean becomes alternately warmer (positive phase) and then colder (negative phase) than the eastern part of the ocean.
The IOD involves an aperiodic oscillation of sea-surface temperatures (SST), between "positive", "neutral" and "negative" phases. A positive phase sees greater-than-average sea-surface temperatures and greater precipitation in the western Indian Ocean region,[ dubious – discuss ] with a corresponding cooling of waters in the eastern Indian Ocean—which tends to cause droughts in adjacent land areas of Indonesia and Australia. The negative phase of the IOD brings about the opposite conditions, with warmer water and greater precipitation in the eastern Indian Ocean, and cooler and drier conditions in the west.
The IOD also affects the strength of monsoons over the Indian subcontinent. A significant positive IOD occurred in 1997–98, with another in 2006. The IOD is one aspect of the general cycle of global climate, interacting with similar phenomena like the El Niño-Southern Oscillation (ENSO) in the Pacific Ocean.
The IOD phenomenon was first identified by climate researchers in 1999. [1] [2]
An average of four each positive-negative IOD events occur during each 30-year period with each event lasting around six months. However, there were 12 positive IODs between 1980 and 2009, and no negative events between 1980 and 1992. The occurrence of consecutive positive IOD events is extremely rare with only two such events recorded, 1913–1914 and the three consecutive events from 2006 to 2008 which preceded the Black Saturday bushfires. Modelling suggests that consecutive positive events could be expected to occur twice over a 1,000-year period. The positive IOD in 2007 evolved together with La Niña, which is a very rare phenomenon that has happened only once in the available historical records (in 1967). [3] [4] [5] [6] A strong negative IOD developed in October 2010, [7] which, coupled with a strong and concurrent La Niña, caused the 2010–2011 Queensland floods and the 2011 Victorian floods.
In 2008, Nerilie Abram used coral records from the eastern and western Indian Ocean to construct a coral Dipole Mode Index extending back to 1846 AD. [8] This extended perspective on IOD behaviour suggested that positive IOD events increased in strength and frequency during the 20th century. [9]
A positive IOD is associated with droughts in Southeast Asia [10] , [11] and Australia. Extreme positive-IOD events are expected. [12]
A 2009 study by Ummenhofer et al. at the University of New South Wales (UNSW) Climate Change Research Centre has demonstrated a significant correlation between the IOD and drought in the southern half of Australia, in particular the south-east. Every major southern drought since 1889 has coincided with positive-neutral IOD fluctuations including the 1895–1902, 1937–1945 and the 1995–2009 droughts. [13]
The research shows that when the IOD is in its negative phase, with cool western Indian Ocean water and warm water off northwest Australia (Timor Sea), winds are generated that pick up moisture from the ocean and then sweep down towards southern Australia to deliver higher rainfall. In the IOD-positive phase, the pattern of ocean temperatures is reversed, weakening the winds and reducing the amount of moisture picked up and transported across Australia. The consequence is that rainfall in the south-east is well below average during periods of a positive IOD.
The study also shows that the IOD has a much more significant effect on the rainfall patterns in south-east Australia than the El Niño-Southern Oscillation (ENSO) in the Pacific Ocean as already shown in several recent studies. [14] [15] [16]
A positive IOD is linked to higher than average rainfall during the East African Short Rains (EASR) between October and December. [17] Higher rainfall during the EASR are associated with warm sea-surface temperatures (SST) in the western Indian Ocean and low level westerlies across the equatorial region of the ocean which brings moisture over the East Africa region. [17]
The increased rainfall associated with a positive IOD has been found to result in increased flooding over East Africa during the EASR period. During a particularly strong positive IOD at the end of 2019, average rainfall over East Africa was 300% higher than normal. [18] This higher than average rainfall has resulted in a high prevalence of flooding in the countries of Djibouti, Ethiopia, Kenya, Uganda, Tanzania, Somalia and South Sudan. [19] Torrential rainfall and increased risk of landslides over the region during this period often results in widespread destruction and loss of life. [20] [21] [22] [23]
It is expected that the Western Indian ocean will warm at accelerated rates due to climate change [24] [25] leading to an increasing occurrence of positive IODs. [26] This is likely to result in the increasing intensity of rainfall during the short rain period over East Africa. [27]
A 2018 study by Hameed et al. at the University of Aizu simulated the impact of a positive IOD event on Pacific surface wind and SST variations. [28] They show that IOD-induced surface wind anomalies can produce El Nino-like SST anomalies, with the IOD's impact on SST being the strongest in the far-eastern Pacific. They further demonstrated that IOD-ENSO interaction is a key for the generation of Super El Ninos. [29]
A positive IOD cycle is related to multiple cyclones that ravaged East Africa in 2019, killing thousands. The unusually active 2018-2019 South-West Indian Ocean cyclone season was aided by warmer than normal waters offshore (starting with Cyclone Idai and continuing on to the subsequent cyclone season). Additionally, the positive IOD dipole contributed to Australian drought & bushfires (convective IOD cycle brings dry air down on Australia) and the 2020 Jakarta floods (convective IOD cycle prevents moist air from going south, thus concentrating it in the tropics), and more recently the 2019–21 East Africa locust infestation. [30] [31]
El Niño–Southern Oscillation (ENSO) is a global climate phenomenon that emerges from variations in winds and sea surface temperatures over the tropical Pacific Ocean. Those variations have an irregular pattern but do have some semblance of cycles. The occurrence of ENSO is not predictable. It affects the climate of much of the tropics and subtropics, and has links (teleconnections) to higher-latitude regions of the world. The warming phase of the sea surface temperature is known as "El Niño" and the cooling phase as "La Niña". The Southern Oscillation is the accompanying atmospheric oscillation, which is coupled with the sea temperature change.
The Antarctic oscillation, also known as the Southern Annular Mode (SAM), is a low-frequency mode of atmospheric variability of the southern hemisphere that is defined as a belt of strong westerly winds or low pressure surrounding Antarctica which moves north or south as its mode of variability.
The Pacific decadal oscillation (PDO) is a robust, recurring pattern of ocean-atmosphere climate variability centered over the mid-latitude Pacific basin. The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of 20°N. Over the past century, the amplitude of this climate pattern has varied irregularly at interannual-to-interdecadal time scales. There is evidence of reversals in the prevailing polarity of the oscillation occurring around 1925, 1947, and 1977; the last two reversals corresponded with dramatic shifts in salmon production regimes in the North Pacific Ocean. This climate pattern also affects coastal sea and continental surface air temperatures from Alaska to California.
The Western Hemisphere Warm Pool (WHWP) is a region of sea surface temperatures (SST) warmer than 28.5 °C that develops west of Central America in the spring, then expands to the tropical waters to the east.
The Madden–Julian oscillation (MJO) is the largest element of the intraseasonal variability in the tropical atmosphere. It was discovered in 1971 by Roland Madden and Paul Julian of the American National Center for Atmospheric Research (NCAR). It is a large-scale coupling between atmospheric circulation and tropical deep atmospheric convection. Unlike a standing pattern like the El Niño–Southern Oscillation (ENSO), the Madden–Julian oscillation is a traveling pattern that propagates eastward, at approximately 4 to 8 m/s, through the atmosphere above the warm parts of the Indian and Pacific oceans. This overall circulation pattern manifests itself most clearly as anomalous rainfall.
The Atlantic Multidecadal Oscillation (AMO), also known as Atlantic Multidecadal Variability (AMV), is the theorized variability of the sea surface temperature (SST) of the North Atlantic Ocean on the timescale of several decades.
The Monsoon of South Asia is among several geographically distributed global monsoons. It affects the Indian subcontinent, where it is one of the oldest and most anticipated weather phenomena and an economically important pattern every year from June through September, but it is only partly understood and notoriously difficult to predict. Several theories have been proposed to explain the origin, process, strength, variability, distribution, and general vagaries of the monsoon, but understanding and predictability are still evolving.
The Atlantic Equatorial Mode or Atlantic Niño is a quasiperiodic interannual climate pattern of the equatorial Atlantic Ocean. It is the dominant mode of year-to-year variability that results in alternating warming and cooling episodes of sea surface temperatures accompanied by changes in atmospheric circulation. The term Atlantic Niño comes from its close similarity with the El Niño-Southern Oscillation (ENSO) that dominates the tropical Pacific basin. For this reason, the Atlantic Niño is often called the little brother of El Niño. The Atlantic Niño usually appears in northern summer, and is not the same as the Atlantic Meridional (Interhemispheric) Mode that consists of a north-south dipole across the equator and operates more during northern spring. The equatorial warming and cooling events associated with the Atlantic Niño are known to be strongly related to rainfall variability over the surrounding continents, especially in West African countries bordering the Gulf of Guinea. Therefore, understanding of the Atlantic Niño has important implications for climate prediction in those regions. Although the Atlantic Niño is an intrinsic mode to the equatorial Atlantic, there may be a tenuous causal relationship between ENSO and the Atlantic Niño in some circumstances.
The Subtropical Indian Ocean Dipole (SIOD) is featured by the oscillation of sea surface temperatures (SST) in which the southwest Indian Ocean i.e. south of Madagascar is warmer and then colder than the eastern part i.e. off Australia. It was first identified in the studies of the relationship between the SST anomaly and the south-central Africa rainfall anomaly; the existence of such a dipole was identified from both observational studies and model simulations .
The Tropical Atlantic SST Dipole refers to a cross-equatorial sea surface temperature (SST) pattern that appears dominant on decadal timescales. It has a period of about 12 years, with the SST anomalies manifesting their most pronounced features around 10–15 degrees of latitude off of the Equator. It is also referred to as the interhemispheric SST gradient or the Meridional Atlantic mode.
The 1997–1998 El Niño was regarded as one of the most powerful El Niño–Southern Oscillation events in recorded history, resulting in widespread droughts, flooding and other natural disasters across the globe. It caused an estimated 16% of the world's reef systems to die, and temporarily warmed air temperature by 1.5 °C (2.7 °F) compared to the usual increase of 0.25 °C (0.45 °F) associated with El Niño events. The costs of the event were considerable, leading to global economic losses of US$5.7 trillion within five years.
The 2010–2012 La Niña event was one of the strongest on record. It caused Australia to experience its wettest September on record in 2010, and its fourth-wettest year on record in 2010. It also led to an unusual intensification of the Leeuwin Current, the 2010 Pakistan floods, the 2010–2011 Queensland floods, and the 2011 East Africa drought. It also helped keep the average global temperature below recent trends, leading to 2011 tying with 1997 for the 14th-warmest year on record. This La Niña event also led to above-average tropical cyclone activity in the North Atlantic Ocean during the 2010, 2011, and 2012 hurricane seasons, while the Eastern and Western Pacific experienced record low activity in 2010 and below average activity in 2011.
The 2014–2016 El Niño was the strongest El Niño event on record, with unusually warm waters developing between the coast of South America and the International Date Line. These unusually warm waters influenced the world's weather in a number of ways, which in turn significantly affected various parts of the world. These included drought conditions in Venezuela, Australia and a number of Pacific islands while significant flooding was also recorded. During the event, more tropical cyclones than normal occurred within the Pacific Ocean, while fewer than normal occurred in the Atlantic Ocean.
Axel Timmermann is a German climate physicist and oceanographer with an interest in climate dynamics, human migration, dynamical systems' analysis, ice-sheet modeling and sea level. He served a co-author of the IPCC Third Assessment Report and a lead author of IPCC Fifth Assessment Report. His research has been cited over 18,000 times and has an h-index of 70 and i10-index of 161. In 2017, he became a Distinguished Professor at Pusan National University and the founding Director of the Institute for Basic Science Center for Climate Physics. In December 2018, the Center began to utilize a 1.43-petaflop Cray XC50 supercomputer, named Aleph, for climate physics research.
Caroline C. Ummenhofer is a physical oceanographer at the Woods Hole Oceanographic Institution where she studies extreme weather events with a particular focus on the Indian Ocean. Ummenhofer makes an effort to connect her discoveries about predicting extreme weather events and precipitation to helping the nations affected.
Pacific Meridional Mode (PMM) is a climate mode in the North Pacific. In its positive state, it is characterized by the coupling of weaker trade winds in the northeast Pacific Ocean between Hawaii and Baja California with decreased evaporation over the ocean, thus increasing sea surface temperatures (SST); and the reverse during its negative state. This coupling develops during the winter months and spreads southwestward towards the equator and the central and western Pacific during spring, until it reaches the Intertropical Convergence Zone (ITCZ), which tends to shift north in response to a positive PMM.
Ocean dynamical thermostat is a physical mechanism through which changes in the mean radiative forcing influence the gradients of sea surface temperatures in the Pacific Ocean and the strength of the Walker circulation. Increased radiative forcing (warming) is more effective in the western Pacific than in the eastern where the upwelling of cold water masses damps the temperature change. This increases the east-west temperature gradient and strengthens the Walker circulation. Decreased radiative forcing (cooling) has the opposite effect.
Effects of the El Niño–Southern Oscillation in Australia are present across most of Australia, particularly the north and the east, and are one of the main climate drivers of the country. Associated with seasonal abnormality in many areas in the world, Australia is one of the continents most affected and experiences extensive droughts alongside considerable wet periods that cause major floods. There exist three phases — El Niño, La Niña, and Neutral, which help to account for the different states of ENSO. Since 1900, there have been 28 El Niño and 19 La Niña events in Australia including the current 2023 El Niño event, which was declared on 17th of September in 2023. The events usually last for 9 to 12 months, but some can persist for two years, though the ENSO cycle generally operates over a time period from one to eight years.
The recharge oscillator model for El Niño–Southern Oscillation (ENSO) is a theory described for the first time in 1997 by Jin., which explains the periodical variation of the sea surface temperature (SST) and thermocline depth that occurs in the central equatorial Pacific Ocean. The physical mechanisms at the basis of this oscillation are periodical recharges and discharges of the zonal mean equatorial heat content, due to ocean-atmosphere interaction. Other theories have been proposed to model ENSO, such as the delayed oscillator, the western Pacific oscillator and the advective reflective oscillator. A unified and consistent model has been proposed by Wang in 2001, in which the recharge oscillator model is included as a particular case.
The 2020–2023 La Niña event was a rare three-year, triple-dip La Niña. The impact of the event led to numerous natural disasters that were either sparked or fueled by La Niña. La Niña refers to the reduction in the temperature of the ocean surface across the central and eastern equatorial Pacific, accompanied by notable changes in the tropical atmospheric circulation. This includes alterations in wind patterns, pressure, and rainfall. The cold phase of the El Niño Southern Oscillation (ENSO), known as La Niña, typically produces contrasting effects on weather and climate compared to El Niño, which is the warm phase of the same phenomenon.
