Indian Ocean variability changes in the Paleoclimate Modelling Intercomparison Project

Abstract

Abstract. The Indian Ocean exhibits multiple modes of interannual climate variability, whose future behaviour is uncertain. Recent analysis of glacial climates has uncovered an additional El Niño-like equatorial mode in the Indian Ocean, which could also emerge in future warm states. Here we explore changes in the tropical Indian Ocean simulated by the Paleoclimate Model Intercomparison Project (PMIP4). These simulations are performed by an ensemble of models contributing to the Coupled Model Intercomparison Project 6 and over four coordinated experiments: three past periods – the mid-Holocene (6000 years ago), the Last Glacial Maximum (21 000 years ago), the last interglacial (127 000 years ago) – and an idealized forcing scenario to examine the impact of greenhouse forcing. The two interglacial experiments are used to characterize the role of orbital variations in the seasonal cycle, whilst the other pair focus on responses to large changes in global temperature. The Indian Ocean Basin Mode (IOBM) is damped in both the mid-Holocene and last interglacial, with the amount related to the damping of the El Niño–Southern Oscillation in the Pacific. No coherent changes in the strength of the IOBM are seen with global temperature changes; neither are changes in the Indian Ocean Dipole (IOD) nor the Niño-like mode. Under orbital forcing, the IOD robustly weakens during the mid-Holocene experiment, with only minor reductions in amplitude during the last interglacial. Orbital changes do impact the SST pattern of the Indian Ocean Dipole, with the cold pole reaching up to the Equator and extending along it. Induced changes in the regional seasonality are hypothesized to be an important control on changes in the Indian Ocean variability.

Indian Ocean Dipole Changes During the Last Interglacial Modulated by the Mean Oceanic Climatology

“…It is noteworthy that despite the minor amplitude change assessed by the DMI, structural changes are still expected to reshape climatic responses to the IOD, inducing westward expanded drying over the east and wetter responses over the Indian peninsular (Brierley et al, 2023). Furthermore, by examining various scenarios including the LIG and mid-Holocene, Brierley et al (2023) demonstrated that IOD amplitudes are linearly correlated to the seasonal cycle of IO SST zonal gradients but less influenced by the ENSO strength. Their results further support the dominant influence of regional ocean-atmosphere coupling on IOD changes in orbital interglacial periods, consistent with our study.…”

“…Previous proxies have indicated co‐occurrence of enhanced IOD activities and positive IOD‐like mean‐state changes during the seventeenth century and a close coupling between IOD and ENSO amplitudes during the last millennium (Abram, Wright, et al., 2020). However, these relationships might not fully hold at orbital time scales (Brierley et al., 2023). For instance, the modeling studies have reported that the IOD weakens during the mid‐Holocene notwithstanding positive IOD‐like responses in the SST (Brierley et al., 2023), because enhanced westward currents lead to reduced variability in the WEIO (Liu et al., 2023).…”

“…However, these relationships might not fully hold at orbital time scales (Brierley et al., 2023). For instance, the modeling studies have reported that the IOD weakens during the mid‐Holocene notwithstanding positive IOD‐like responses in the SST (Brierley et al., 2023), because enhanced westward currents lead to reduced variability in the WEIO (Liu et al., 2023). The peak cooling over the SEIO during the mid‐Holocene was suggested to remain unchanged by both simulations (Liu et al., 2023) and proxies (Abram et al., 2007).…”

Atmospheric Methane: Comparison Between Methane's Record in 2006–2022 and During Glacial Terminations

“…Although there are many possible explanations of this sequence of events, the pattern is consistent with a wider scenario beginning with initial modest Hadley cell expansion during interdecadal variation, bringing about more methane emission from wetter outer tropical swamps ($${\delta }^{13}{\mathrm{C}}_{\mathrm{C}\mathrm{H}4}$$ perhaps very roughly −60‰ in cool conditions compared to today). Then in response, changes in AMOC, the Walker circulation and the Indian Ocean Dipole (IOD) (Abram et al., 2020; Brierley et al., 2023), may have brought about intensification of equatorial rainfall and consequent rapid growth in equatorial wetland emissions, initiating methane's abrupt rise.…”

Long-range, time-varying statistical prediction of annual precipitation in a Mediterranean remote site

“…For this purpose, we adopted an index that represents key climate variability in the Mediterranean: the DMI. Orbital changes affect the Indian Ocean dipole's sea surface temperature pattern, with induced changes in regional seasonality (Brierley et al 2023). In particular, a dominant mode of the interannual variability of Indian summer monsoon rainfall shows a west-east dipole pattern, which is linked to the active Azores High (Yadav 2021).…”