OS14B-01 INVITED
The IOD as an Intrinsic Mode in CGCM Simulations
The Indian Ocean Dipole mode (IOD) is identified as one of the leading modes of climate variabilities in tropics. The IOD is reportedly captured by most of the existing coupled atmosphere-ocean general circulation models (CGCMs). While some CGCM studies reported independent existence of IOD from El Nino/Southern Oscillation (ENSO), a few studies questioned this independent existence. In this study, we examined the intrinsic nature of the IOD using the SINTEX-F1 CGCM. In addition to a control experiment, which realistically simulates the IOD and ENSO variabilities, a decoupled experiment is carried out to verify the observational inferences derived from statistical analyses. In this decoupled experiment, named as the non-ENSO experiment, the tropical Pacific Ocean is decoupled from the atmosphere.In the non-ENSO experiment, the ocean-atmosphere conditions including the characteristic east-west dipole in sea surface temperature (SST) anomalies related to the IOD are realistically simulated. As in the observation and control experiment, the IOD peaks in boreal fall. This confirms that the dipole mode in the Indian Ocean is mainly determined by intrinsic processes within the basin. Furthermore, EOF analysis of SST anomalies reveals that the IOD is a dominant mode of variability instead of the basin-wide monopole mode in the non-ENSO experiment. It is also found that the IOD variability becomes dominantly biennial in absence of ENSO. In the years of IOD and ENSO co- occurrences the ENSO is found to affect the periodicity, strength and formation processes of the IOD. The amplitudes of SST anomalies in the western pole of concurrent IODs are aided by dynamical and thermodynamical alterations related to the ENSO-induced wind variability. Anomalous latent heat flux and vertical heat convergence associated with the modified Walker circulation contribute to the modification of western anomalies. It is found that 42% of IOD events in control experiment are affected by changes in the Walker circulation related to the tropical Pacific variabilities including ENSO. The IOD formation is delayed until boreal summer for those IODs, which otherwise form in boreal spring as in the non-ENSO experiment. In the rest 58% of cases, positive IOD events evolve from favorable equatorial wind anomalies internal to the Indian Ocean. Additionally, the western anomalies substantially affect intra-seasonal disturbances aiding the IOD formation further. Based on these findings it can be deduced that the IOD predictability is higher in years when IOD evolves independently as compared to those years when IOD evolves through ENSO interactions.
OS14B-02
Atmosphere/ocean Variations Over Indian Ocean Associated With Indian Ocean Dipole and ENSO
Based on the data analysis of the 1000 hPa wind, SST and SSH anomalies, it is revealed that the atmospheric variations associated with Indian Ocean Dipole (IOD, or referred as Indian Ocean Zonal Dipole Mode, IOZDM) consist of a pair of anticyclones closely north and south of the equator with accompanying intense equatorial easterly anomalies, while the atmospheric variations related to ENSO include a strong anticyclone in the southeastern tropical Indian Ocean (TIO) at higher latitudes with strong along-shore wind anomalies near Java- Sumatra coast. The different atmospheric forcing patterns lead to the fact that oceanic thermocline variations associated with IOD/IOZDM are more closely confined to the region north of 10S, while ENSO-induced thermocline variations are dominant south of 10S.
OS14B-03
Identifying ENSO Impact on Indian Ocean Interannual Variations From Numerical Experiments
Since ENSO and Indian Ocean Dipole (IOD) are not orthogonal to each other, their footprints on the Indian Ocean interannual variations are entangled, which limits our thorough understanding of the dynamics of IOD and its relation with ENSO. The problem is tackled with the help of specially designed numerical experiments, which permits the regional (Indian Ocean and Pacific Ocean respectively) decoupling between ocean and atmosphere. The atmospheric/oceanic signals associated with individual ENSO and IOD are identified and further compared with previous understanding based on the partial correlation examination of the historical data sets.
OS14B-04 INVITED
Seasonal climate predictability in a coupled OAGCM
Predictabilities of the tropical climate variations are investigatedusing a relatively high-resolution SINTEX-F coupled GCM. Nine ensemble forecast members are generated by perturbing the model coupling physics which accounts for the uncertainties of both initial conditions and model physics. Because of the model good performance in simulating the climatology and ENSO in the tropical Pacific, a simple coupled SST-nudging scheme generates realistic thermocline and surface wind variations in the equatorial Pacific. Several westerly and easterly wind bursts in the western Pacific are also captured. Hindcast results for the period 1982-2004 show a high predictability of ENSO. All past El Nino and La Nina events, including the strongest 1997/98 warm episode, are predicted successfully with anomaly skill scores above 0.7 at 12-month lead time. The model forecasts also show a ``spring prediction barrier'' similar to the observations. Spatial SST anomalies, teleconnections, and global drought/flood during three different phases of ENSO are successfully predicted at 9-12 months lead. The Indian Ocean Dipole has also profound socio-economic impacts on not only the countries surrounding the Indian Ocean but also various parts of the world. The model predicted the extreme positive IOD event in 1994 at 2-3 seasons lead. Retrospective ensemble forecasts of IOD index for the past two decades showed skillful scores up to 3-4 months lead and a winter prediction barrier associated with its intrinsic strong seasonal phase-locking. Prediction skills of SST anomalies in both the eastern and western Indian Ocean are higher than those of the IOD index; this is due to influences of the highly predictable ENSO. In addition, extended ENSO predictions up to two years lead and real time forecasts with more ensembles for 2005 and 2006 will also be briefly introduced.
OS14B-05
Intraseasonal variability in the meridional current in the equatorial Indian Ocean simulated in a high-resolution OGCM
Spatial distribution and characteristics of intraseasonal variability in the meridional current field within the equatorial Indian Ocean is investigated by use of a high-resolution Ocean General Circulation Model, called as OFES, which is driven by daily-mean QuikScat wind stresses. Spectral analysis for the simulated meridional currents indicates three significant intraseasonal variability; ~15 days, 25-30 days, and 30-70 days period. The composite analysis for the 15-day variation demonstrates that the horizontal velocity structure of the signal is consistent with that for the mixed Rossby-gravity wave, with the westward phase speed of about 4 to 5 m/s and the typical wavelength of 3000 to 4000km. This variability shows strong association with the meridional component of the wind stresses. In addition, the energy of the waves propagate downward to the bottom of the ocean along the ray-path for the mixed Rossby-gravity wave. Longer time-scale variability of 25- to 70-day period, on the other hand, appears in the deeper layer blow the strong pycnocline, located at the depth of about 120m, in the central and eastern Indian Ocean. The distinct 25 to 30 days variability is associated with the mixed Rossby gravity waves generated by the internal instability of the western boundary current system near the African coast. The energy of the wave again propagates downward along the ray-path, although it is strongly affected by the Maldive Island chain. Another signal at the 30 to 70 days period is related to the southward propagation of eddy-like features originated from the region southeast of Sri Lanka. Comparison with the OFES results forced by climatological wind stress demonstrates that these variability in the deeper layer is nothing to do with the direct wind forcing of the intraseasonal time-scale.
OS14B-06
Impact of atmospheric submonthly oscillations on sea surface temperature of the tropical Indian Ocean
Impacts of atmospheric intraseasonal oscillations (ISOs) at submonthly periods (10-30 days) on Indian Ocean sea surface temperature (SST) are studied using satellite observed outgoing long wave radiation, QuikSCAT winds, SST and an ocean general circulation model for the period of 1999-2004. The results suggest that submonthly ISOs can cause significant 10-30 day SST changes throughout the equatorial basin and northern Bay of Bengal, with an amplitude of as large as 0.5C and standard deviation of exceeding 0.2C for a 4-year record. Impact of the submonthly ISO associated with the Indian summer monsoon is separately examined. It is associated with basin-scale SST evolution with distinct spatial structures. The SST variation results mainly from submonthly wind forcing, which causes changes in oceanic processes and surface turbulent heat fluxes. Radiative fluxes can also have large influences in some regions for some ISO events.
OS14B-07
Intraseasonal Variations of the Ocean Mixed Layer Depth Derived From Argo Data
Stimulated by the advances in the satellite microwave SST observation, much progress on the understanding of the oceanic variations with relevance to the intra-seasonal oscillation (ISO), especially in tropical Indian Ocean (TIO), has been seen recently. However, spare observations over open ocean, except for TMI and AMSR-E SST, limit our ability to thoroughly explore the oceanic variations below surface associated with ISO. Increase of Argo floats in TIO provides us the first chance of obtaining the quasi-simultaneous 3-D oceanic observation at basin scale and at intra-seasonal time resolution. Here we present the preliminary results on the oceanic mixed layer depth variations with relevance to ISO. This analysis is based on the five-day mixed layer depth products derived from TIO Argo profiles. Together with OLR, QSCAT wind and TMI/AMSR-E SST data, we illustrate how the ocean mixed layer response to ISO and also discuss the potential feedbacks to the atmosphere, which helps to understand the role of ocean for the development of ISO.