Indian Ocean Variability in the CMIP5 Multimodel Ensemble: The Basin Mode

Abstract

This study evaluates the simulation of the Indian Ocean Basin (IOB) mode and relevant physical processes in models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Historical runs from 20 CMIP5 models are available for the analysis. They reproduce the IOB mode and its close relationship to El Niño–Southern Oscillation (ENSO). Half of the models capture key IOB processes: a downwelling oceanic Rossby wave in the southern tropical Indian Ocean (TIO) precedes the IOB development in boreal fall and triggers an antisymmetric wind anomaly pattern across the equator in the following spring. The anomalous wind pattern induces a second warming in the north Indian Ocean (NIO) through summer and sustains anticyclonic wind anomalies in the northwest Pacific by radiating a warm tropospheric Kelvin wave. The second warming in the NIO is indicative of ocean–atmosphere interaction in the interior TIO. More than half of the models display a double peak in NIO warming, as observed following El Niño, while the rest show only one winter peak. The intermodel diversity in the characteristics of the IOB mode seems related to the thermocline adjustment in the south TIO to ENSO-induced wind variations. Almost all the models show multidecadal variations in IOB variance, possibly modulated by ENSO.

Inhomogeneous warming of the Tropical Indian Ocean in the CMIP5 model simulations during 1900–2005 and associated mechanisms

“…10 The linear trend of a wind effect on latent heat flux from atmospheric forcing part (Q E W , colors, Wm −2 /10 yr) and surface wind (vector, ms −1 /10 yr), c SST (colors, K/10 yr) and surface wind (vector, ms −1 /10 yr), e sea surface height (SSH, colors, cm/10 yr) and surface wind (vector, ms −1 /10 yr), and g precipitation (colors, mmd −1 /10 yr) and surface wind (vector, ms −1 /10 yr) from ensemble means of the all-forcing runs from 16 CMIP5 models during the northern TIO and the northwesterly anomalies reduce the southeasterly trades over the southern TIO, which are favorable for the non-uniform IOBM-like warming pattern. As stated by Du et al (2013), the anomalous SST warming over the south TIO induced by the down-welling ocean Rossby wave and enhanced atmospheric advection with positive precipitation anomalies (Fig. 10h), reflecting the slowdown of the Walker circulation under global warming Vecchi et al 2006), trigger the anti-symmetric atmospheric circulation.…”

Cold Tongue and Warm Pool ENSO Events in CMIP5: Mean State and Future Projections

“…As a consequence, the dissipation of the WP events seems to occur via ocean heat discharge, in a similar way to CT events, rather than via advection of mean SST gradients by anomalous zonal currents as seen in observations. Recent studies have also attributed biases in ENSO simulations to remote influences, particularly climate over the Indian Ocean basin (Du et al 2013;Santoso et al 2012, and references therein). Okumura and Deser (2010) suggest that remote forcing from the Indian Ocean can influence the asymmetry in the duration of El Niño and La Niña.…”

Tropical Indian Ocean response to the decay phase of El Niño in a coupled model and associated changes in south and east-Asian summer monsoon circulation and rainfall

“…For example, Du et al (2013) suggested that half of CMIP5 models capture the key processes responsible for TIO basin-wide warming associated with El Niño. Saji et al (2006) found that the association between El Niño and TIO basin-wide warming is well resolved in many of the CMIP3 models.…”