Unraveling Causes for the Changing Behavior of the Tropical Indian Ocean in the Past Few Decades

Abstract

Observations show that decadal (10–20 yr) to interdecadal (>20 yr) variability of the tropical Indian Ocean (TIO) sea surface temperature (SST) closely follows that of the Pacific until the 1960s. Since then, the TIO SST exhibits a persistent warming trend, whereas the Pacific SST shows large-amplitude fluctuations associated with the interdecadal Pacific oscillation (IPO), and the decadal variability of the TIO SST is out of phase with that of the Pacific after around 1980. Here causes for the changing behavior of the TIO SST are explored, by analyzing multiple observational datasets and the recently available large-ensemble simulations from two climate models. It is found that on interdecadal time scales, the persistent TIO warming trend is caused by emergence of anthropogenic warming overcoming internal variability, while the time of emergence occurs much later in the Pacific. On decadal time scales, two major tropical volcanic eruptions occurred in the 1980s and 1990s causing decadal SST cooling over the TIO during which the IPO was in warm phase, yielding the out-of-phase relation. The more evident fingerprints of external forcing in the TIO compared to the Pacific result from the much weaker TIO internal decadal–interdecadal variability, making the TIO prone to the external forcing. These results imply that the ongoing warming and natural external forcing may make the Indian Ocean more active, playing an increasingly important role in affecting regional and global climate.

Causal Links on Interannual Timescale Between ENSO and the IOD in CMIP5 Future Simulations

“…Significant IOD influence on ENSO found in several models shows an agreement with past studies (e.g., Cai et al, 2011;Jourdain et al, 2016;Luo et al, 2010;Santoso et al, 2012). The results described here show a support for the suggestion in previous studies (e.g., Zhang, Han, & Sienzn, 2018) that the IOD is more active and is having greater influence on global climate. While natural external forcing (i.e., volcanic forcing) and global warming make the IO more active, the warming trend over the IO is the main contributor and mostly in phase with the trend of the global mean SST (Roxy et al, 2014).…”

“…In addition, IOD events might be induced by the low-frequency variability of thermocline (Zhang, Du, et al, 2018). Furthermore, future IOD changes might be influenced by anthropogenic forcing and volcanic forcing (Zhang, Han, et al, 2018). Figure 1).…”

Indian Ocean Warming Trend Reduces Pacific Warming Response to Anthropogenic Greenhouse Gases: An Interbasin Thermostat Mechanism

“…The different interbasin warming contrast across different ensemble members of CESM‐LE is mainly due to variations in the simulated Pacific Ocean warming trend, which is due to the influence of the Interdecadal Pacific Oscillation (IPO). The spread of the Indian Ocean warming in CESM‐LE is much smaller, indicating that the anthropogenic warming trend dominates its relatively weak decadal variability (Zhang et al, ). By contrast, in the CMIP5 models, magnitudes of the warming trends in the two ocean basins are tightly linked, with a large (small) Indian Ocean warming corresponding to a large (small) Pacific Ocean warming, which may be related to different model sensitivities to the anthropogenic GHG forcing.…”

“…The majority of climate models that participated in the Coupled Model Intercomparison Project phase 5 (CMIP5) simulated an El Niño-like sea surface temperature (SST) warming pattern in response to GHG forcing, with a larger warming appearing in the eastern tropical Pacific relative to the western tropical Pacific (Figure 1d; Vecchi & Soden, 2007b;Xie et al, 2010;Zhang & Li, 2014). The long-term SST trends in observational data sets, however, exhibit different patterns from those in the CMIP5 models (Coats & Karnauskas, 2017;Zhang, 2016;Zhang et al, 2018). While some observational data sets show a weak La Niña-like SST trend pattern with a small negative SST trend in the eastern tropical Pacific (Figures 1a and 1b), others show negligible changes in the zonal SST gradient (Figures 1c and S1; Solomon & Newman, 2012).…”

The global warming hiatus has faded away: An analysis of 2014–2016 global surface air temperatures

“…The hiatus has attracted great attention because it contrasts with the steady increasing trend of greenhouse gas emissions, and various mechanisms have been proposed to explain it. Numerous studies have suggested that the hiatus is a decadal climate fluctuation or variation induced by radiative forcing (Lean and Rind, ), upper or deep ocean heat uptake (Chen and Tung, ; Nieves et al , ), or internal variability modes of the climate system including the El Niño‐Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), the Pacific Decadal Oscillation (PDO)/Interdecadal Pacific Oscillation (IPO), and the Atlantic Multidecadal Oscillation (AMO) (Kosaka and Xie, , ; England et al , ; Dai et al , ; Steinman et al , ; Zhang, ; Yao et al , ; Zhang et al , ). Mechanistically, a negative phase of PDO/IPO leads to an unprecedented acceleration of the Walker circulation via strengthening of the trade wind in the central‐eastern tropical Pacific, resulting in a La Niña‐like cooling which contributes to the hiatus (Kosaka and Xie, ; McGregor et al , ).…”