Increased frequency of extreme Indian Ocean Dipole events due to greenhouse warming

Abstract

The Indian Ocean dipole is a prominent mode of coupled ocean-atmosphere variability, affecting the lives of millions of people in Indian Ocean rim countries. In its positive phase, sea surface temperatures are lower than normal off the Sumatra-Java coast, but higher in the western tropical Indian Ocean. During the extreme positive-IOD (pIOD) events of 1961, 1994 and 1997, the eastern cooling strengthened and extended westward along the equatorial Indian Ocean through strong reversal of both the mean westerly winds and the associated eastward-flowing upper ocean currents. This created anomalously dry conditions from the eastern to the central Indian Ocean along the Equator and atmospheric convergence farther west, leading to catastrophic floods in eastern tropical African countries but devastating droughts in eastern Indian Ocean rim countries. Despite these serious consequences, the response of pIOD events to greenhouse warming is unknown. Here, using an ensemble of climate models forced by a scenario of high greenhouse gas emissions (Representative Concentration Pathway 8.5), we project that the frequency of extreme pIOD 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. We find that a mean state change--with weakening of both equatorial westerly winds and eastward oceanic currents in association with a faster warming in the western than the eastern equatorial Indian Ocean--facilitates more frequent occurrences of wind and oceanic current reversal. This leads to more frequent extreme pIOD events, suggesting an increasing frequency of extreme climate and weather events in regions affected by the pIOD.

Monsoon-Induced Biases of Climate Models over the Tropical Indian Ocean

“…The tropical IO is unique in its strong interaction with the great monsoon of Such an IOD-like future change in the mean state is climatically important, which would result in more frequent occurrences of extreme positive IOD events in the future (Cai et al 2014), with a potentially increasing risk of extreme climate and weather events in the regions affected by the IOD such as Indonesia and East Africa (Saji et al 1999;Hashizume et al 2009;Cai 2013a, 2013b). …”

“…This climate change pattern has important consequences to the IO and its surrounding countries (Saji et al 1999;Hashizume et al 2009;Cai 2013a, 2013b;Cai et al 2014). For example, attributed to such an IOD-like future change for the mean state in the global warming scenario, the CMIP5 ensemble of CGCMs forced by a scenario of high greenhouse gas emissions (RCP 8.5 runs) commonly projects an increased frequency of extreme positive IOD events by almost three times from about one event every 17 years during the twentieth century to about one event every 6 years during the twenty-first century (Cai et al 2014), with an increasing risk of extreme climate and weather events over Indonesia and East Africa (Saji et al 1999;Hashizume et al 2009;Cai 2013a, 2013b).…”

Deconstructing Demand: The Anthropogenic and Climatic Drivers of Urban Water Consumption

“…Indeed, because the mismatch between modeled and observed per-capita consumption was highest at 5−11 year cycles (e.g., between 0.09 and 0.2 cpy, Figure S6), the following climate modes warrant further evaluation: the El Nino Southern Oscillation (ENSO; 3−7 year return period) and the Indian Ocean Dipole (IOD; 6−17 year return period). 77 Interactions among climate modes (e.g., ENSO, IOD, the Interdecadal Pacific Oscillation, and the Southern Annular Mode) should also be evaluated, as the relationship among modes shifted mid1970s, changing drought dynamics 78 (and potentially, urban water consumption) in the Southern Hemisphere.…”

The positive Indian Ocean Dipole–like response in the tropical Indian Ocean to global warming

“…The future pIOD event will develop and terminate earlier than the canonical pIOD due to an earlier onset of the Asian summer monsoon associated with the weakened Indian Ocean Walker circulation (Cai et al, 2013). The frequency of extreme pIOD events will also increase significantly due to climatologically stronger westminus-east SST gradients and easterly winds along the equatorial Indian Ocean (Cai et al, 2014).…”