Coupled dynamics over the Indian Ocean: spring initiation of the Zonal Mode

Abstract

Atmosphere and ocean model assimilated products, in conjunction with observed precipitation and ocean model estimates of Indonesian Throughflow (ITF) transport and barrier layer thickness, are analyzed to elucidate the role of external (ENSO and ITF) and internal (monsoon) factors in the initiation of the Indian Ocean Zonal Mode (IOZM).The diagnostics show that there exists a natural mode of coupled variability in the eastern equatorial Indian Ocean (EEIO) that is weak on its own but intensifies in boreal spring/early summer, usually when ENSO-like conditions exist in the western Pacific, as implied by the Southern Oscillation Index (SOI). In the EEIO, there exists a 'time window' in the annual cycle-boreal spring-during which the ocean-atmosphere system is particularly sensitive to external forcing. At interannual timescales, spring atmospheric conditions in the EEIO are remotely controlled by SST in the equatorial western-central Pacific. Warm SST anomalies there cause changes in the Pacific Walker circulation and induce subsidence over the EEIO that results in negative precipitation anomalies: (i) Forced by this heat sink, an anticyclone develops in the lower atmosphere over the southeastern Indian Ocean as a Rossby-wave response, and the alongshore upwelling-favorable winds off Java-Sumatra are enhanced. (ii) The reduced surface fresh-water flux and enhanced upwelling reduce the barrier layer in the upper ocean. These processes along-with the reduction of ITF help trigger the IOZM. Once triggered, IOZM grows in summer by the Bjerknes feedback. Its interactions with the monsoon heat source result in enhanced precipitation along the monsoon trough in July-August. This north-south heating gradient favors a local meridional circulation with increased alongshore winds off Sumatra, implying the potential role of the monsoon background cycle.The hypothesis that the equatorial western-central Pacific SST anomalies control the spring precipitation variations in the EEIO/maritime continent is demonstrated by sensitivity experiments with an atmospheric general circulation model. During the spring initiation stages of the IOZM, an analysis of the mixed layer heat budget in an ocean general circulation model indicates that cooling off Java is primarily due to entrainment and also due to latent cooling, both caused by enhanced upwelling-favorable winds. r

A recipe for simulating the interannual variability of the Asian summer monsoon and its relation with ENSO

“…9a). The high correlation, which increases to r SON = 0.62 in boreal fall as noticed by Xie et al (2002), the fact that several IOZM events are identified by significant SSTAs only on one side of the basin (Annamalai et al, 2003), and the absence of appreciable thermocline feedback in WTIO (Feng and Meyers, 2003) are the main ingredients for the interpretation of the IOZM as a forced mode of variability. On the other hand, both models and observations show that subsurface oceanic processes are important in SETIO during strong IOZM episodes, leading to the interpretation of the IOZM as a weak mode of variability, internal to the IO dynamics (Murtugudde et al, 2000;Rao et al, 2002;Gualdi et al, 2003;Saji and Yamagata, 2003;Bracco et al, 2005).…”

“…et al, 1999Webster et al, 1999), a coupled ocean-atmosphere mode of variability considered by many as internal to the Indian Ocean dynamics. The existence of the IOZM, and more so its independency on ENSO has been (and still is) a matter of intense debate (Iizuka et al, 2000;Rao et al, 2000;Allan et al, 2001;Susanto et al, 2001;Baquero-Bernal et al, 2002;Hastenrath, 2002;Saji and Yamagata, 2003;Krishnamurthy and Kirtman, 2003;Annamalai et al, 2003;Gualdi et al, 2003;Yamagata et al, 2003;Bracco et al, 2005, Fisher et al, 2005. In its positive phase, the IOZM is characterized by colder than average SSTs in the south-eastern tropical Indian Ocean, with anomalously strong upwelling off the coast of Java and Sumatra starting in late spring, and warmer anomalies in the central and western Tropical IO during summer and fall.…”

Argo profiles variability of barrier layer in the tropical Indian Ocean and its relationship with the Indian Ocean Dipole

“…[5] Recent model studies [Murtugudde et al, 2000;Annamalai, et al, 2003] find an unusual absence of a BL off Sumatra during September and November (SON) of pIOD years, which favors strong air-sea interactions, suggesting that the evolution of the BL leads the IOD. In agreement, Masson et al [2004] highlight the effect of salinity and a thinner BL on the 1997 pIOD.…”

Contrasting Indian Ocean SST variability with and without ENSO influence: A coupled atmosphere-ocean GCM study

“…On the other hand, there are suggestions that the IOZM is not an independent phenomenon but forced by ENSO through changes in surface heat flux or Indian ocean circulation (e.g., Klein et al, 1999;Chambers et al, 1999;Murtugudde and Busalacchi, 1999;Venzke et al, 2000;Schiller et al, 2000;Huang and Kinter, 2002;Xie et al, 2002). There are also suggestions that the IOZM is a weak natural coupled mode of the Indian Ocean that can be amplified by ENSO during a particular season (e.g., Gualdi et al, 2003;Annamalai et al, 2003). The IOZM is also suggested as a natural part of the Asian summer monsoon and the tropospheric biennial oscillation (TBO) (e.g., Meehl and Arblaster, 2002;Loschnigg et al, 2003).…”