Attribution of the summer warming since 1970s in Indian Ocean Basin to the inter‐decadal change in the seasonal timing of El Niño decay phase

Abstract

[1] This paper reports that, on top of the general warming trend in the Indian Ocean Basin (IOB) SST in the past 140 years, there exists an additional multidecadal-scale warming in the period from late 1940s to early 2000s, and this warming in IOB is particularly stronger in summer than in other seasons. Our analysis indicates that, the seasonal timing of the decay phase of El Niño events during this period has been delayed from spring to early summer since 1970s, while the seasonal timing of La Niña decay phase remains in spring season. The direct effect of the later decay of El Niño events is the lengthening of the delayed warming effect of El Niño on IOB SST into summer, attributing to the stronger IOB warming in summer from the late 1940s to the early 2000s. Citation: Li, Q., R.-C.Ren, M. Cai, and G. X. Wu (2012), Attribution of the summer warming since 1970s in Indian Ocean Basin to the inter-decadal change in the seasonal timing of El Niño decay phase, Geophys.

Summer SST anomalies in the Indian Ocean and the seasonal timing of ENSO decay phase

“…To suppress the possible effect of the long-term SST warming trend in the tropical oceans on our results, before the analysis we first remove the long-term linear trend from the original SST records. Considering the time span of the available JRA-55 data, and the time period in which the significant relationship between the timing of the ENSO decay phase and the IOB SST variability was identified by Li et al (2012), we mainly use the SST data records since the 1940 sto demonstrate the ENSO-IOB relationship, and the reanalysis data records since 1958 to analyze the related atmospheric circulation.…”

“…As in Li et al (2012), the ENSO signal is represented by the area-mean SST anomalies in the Niño3 region, and an IOB SST index (denoted as IOBI) is defined as the areamean SST anomalies over the Indian Ocean basin (40°-110°E, 30°S-30°N) following Li et al (2010). All the monthly anomalies are obtained by removing the climatological annual cycle from the total fields.…”

“…The long-term warming over the IOB is commonly attributed to anthropogenic global warming (Barnett et al 2005;Pierce et al 2006). Nevertheless, on top of this long-term warming, the IOB SST also exhibits significant inter-decadal variability, which has been indicated to be closely related to epochal changes of ENSO (Allan et al 1995;Chowdary et al 2012;Cole et al 2000;Li et al 2012;Reason et al 2000;Xie et al 2010). For example, Allan et al (1995) indicated that epochal changes of the mean condition over the SIO are in-phase with ENSO-related correlation patterns.…”

“…In particular, we first examine the decaying evolution of ENSO events since the 1940s on a case-by-case basis to distinguish between normal-decay and later-decay ENSO events, and then diagnose the changes between them of the related air-sea interaction processes over the IOB. We further link the asymmetric interdecadal changes between El Niño and La Niña to the IOB SST warming in recent decades that was identified by Li et al (2012). In addition, while some studies have linked the summer NIO warming in recent decades to longer persistence of El Niño events, less attention has been paid to changes in the delayed effects of La Niña on the IOB SST.…”

“…An and Wang (2000) indicated that since the mid-1970s, the period of ENSO may have become longer than it was before; and Xie et al (2010) showed that the slower decay of El Niño events after the mid-1970s is related to stronger warming anomalies over the NIO in boreal summer. Li et al (2012) further indicated that, on top of the long-term warming of the IOB SST, a significant inter-decadal warming exists over the IOB since the 1970s.They demonstrated that this inter-decadal IOB warming is strongest in the summer months and can be attributed to the inter-decadal delay in the seasonal timing of the decay phase of ENSO. Particularly, the decay phase of El Niño events, but not that of La Niña events, has been significantly delayed since the late 1970s.…”