Weakening of the Indian Ocean Dipole in the mid-Holocene due to the mean oceanic climatology change

Abstract

The Indian Ocean Dipole (IOD) is one of the leading modes of interannual climate variability in the tropical Indian Ocean (IO). Paleoclimate provides real climate scenarios to examine IOD behaviors and the linkage to basic states. Based on 18 models from the Paleoclimate Modelling Intercomparison Project phase 3 and 4 (PMIP 3/4), the IOD change from the preindustrial period to mid-Holocene is investigated. The multimodel mean reveals that the IOD variability weakens by 14% as measured by the standard deviation of the Dipole Mode Index, which is defined using the zonal sea surface temperature (SST) difference. Such attenuation is dominated by the spatially consistent suppression in the western-pole SST variability, while the eastern pole contributes little due to the opposite-signed changes in its northwestern and southeastern portions. The primary reason for the aforementioned changes comes from the altered climatic background, which displays a positive IOD-like pattern during IOD growing seasons, with intensified westward currents along the equator and northwestward currents in the southeastern equatorial IO. Such changes in the mean-state currents modulate the strength of the IOD-related anomalous advection and subsequently cause alterations in the IOD variability. Further analyses show that the IOD attenuation in the mid-Holocene is likely irrelevant to the concurrently subdued El Niño–Southern Oscillation in the tropical Pacific because of the diminished connections between the two oscillations themselves. The above simulated changes in both the IO mean climatology and IOD variability agree well with the available paleo-records in literature.

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

“…Regarding the IOD, despite uncertainties in projecting changes in amplitudes, it is generally accepted that enhanced thermocline feedbacks due to shoaling thermoclines and weakened atmospheric feedbacks resulting from intensified atmospheric static stability play crucial roles in future IOD changes, while the influence of zonal advection is considered negligible (An et al., 2022; Zheng et al., 2013). However, in the orbitally‐induced warm periods, changes in zonal advection play the dominant role in IOD changes, as clearly shown here and in the literature (Liu et al., 2023). This difference in dominant processes modulating IOD amplitudes suggests that the LIG as a potential analog for future warming does not fully hold to the IOD variability.…”

“…Our study, along with previous research focusing on the mid‐Holocene (Brierley et al., 2023; Liu et al., 2023), highlights common characteristics of IOD changes in orbitally‐induced warm periods: suppressed western warming and westward extended eastern cooling. Partly due to contrasting changes between western and southeastern domains at the early IOD phase during the LIG, the overall IOD amplitude change assessed by the DMI is not more significant than that during the mid‐Holocene.…”

“…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). It was also suggested that there might be no causal links between the weakened IOD and concurrent suppressed ENSO (Brierley et al., 2023; Liu et al., 2023). Apart from the mid‐Holocene, the Last Interglacial (LIG, ∼129–116 ka BP), as another representative interglacial epoch, experiences stronger insolation changes (Berger, 1978) and larger reductions in zonal SST gradients over the IO (Hoffman et al., 2017; Turney 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). 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).…”

“…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). It was also suggested that there might be no causal links between the weakened IOD and concurrent suppressed ENSO (Brierley et al., 2023; Liu et al., 2023).…”