OS33A-01 INVITED 13:30h
Air-sea interactions and Internal Variability in the Indo-Pacific Basin
The Indo-Pacific basin is the home to ENSO and monsoon phenomena and their interactions. Much of the nonlinearities in these phenomena are often discussed in terms of the internal variability of the atmosphere due to its fast time-scales and its energetics. Here we show that even the ocean has substantial internal variability that contribute significantly to the interannual variability of the SSTs and mixed layer heat budgets. An OGCM forced with climatological winds used to produce oceanic circulation fields of a hundred years. The model is coupled to an advective atmospheric mixed layer so that SSTs are free to evolve without any feedbacks to observations. The analyses shows that the highly nonlinear currents in the Indian Ocean forced by reversing monsoonal circulation produce SST anomalies as large as observed interannual anomalies. Considering the high mean SSTs in the Indian Ocean, these are quite important. The coupled nature of the intraseasonal variability in this region is associated with the oceans ability to generate such internal variability. The tropical Pacific is noted for the mesoscale variability associated with the tropical instability waves which have been known to contribute to the heat budget of the cold-tongue. The hundred year climatologically forced ocean simulation shows that the SST anomalies associated with these waves are not necessarily related to the oceanic heat transport but advection of heat in the atmosphere associated with the gradients of air temperature and humidity. A comprehensive air-sea interaction analyses associated with such variabilities will be presented.
OS33A-02 13:50h
A cautionary note for the interpretation of mechanisms controlling oceanic heat content associated with ENSO and Indian-Ocean diople/zonal mode
El Nino-Southern Oscillation and Indian Ocean dipole/zonal mode are the most prominent climate variability in the Pacific and Indian Oceans on interannual time scales. Many studies have been devoted to the analysis of upper-ocean heat balance associated with these phenomena. Volume integration of local heat budget over a large region is traditionally used to infer the mechanisms controlling the variability of total heat content of the region. In particular, the volume integrations of the directional components of heat advection are often used to describe the relative contribution of horizontal, meridional, and vertical advective processes. In this presentation, it is shown that such volume integrations of directional heat advection could be dominated by internal processes that redistribute heat within the region of integration but do not control the total heat content of the integration domain. A new formulation of heat advection is put forth to to describe external mechanisms that control the heat content of a large region. The differences between the new formulation and the traditional volume integration, as applied to El Nino and Indian Ocean diople/zonal mode events, highlight a need for caution in the interpretation of "mechanisms" controlling upper-ocean heat content associated with these climate events, which is important to the understanding of a complete heat balance involving air-sea heat exchange and oceanic mixing.
OS33A-03 14:05h
On the Dominant Mode of the Tropical Indo-Pacific Climate Variability
Because of the huge warm water pool that straddles across the western Pacific and the eastern Indian Oceans, the climate variations in these two oceans are closely related and need to be studied as a whole. Similar to the tropical Pacific ENSO mode, the tropical Indian region is dominated by a zonal mode of variability on interannual time scales. There is a striking out-of-phase relation between the zonal sea level gradients in these two oceans and, to a lesser extent, between the corresponding SST gradients. This can be explained in simple terms as follows. The Walker circulation ascends above the warm pool, with easterly surface winds on the Pacific side and westerly on the Indian side, which piles waters up in the warm pool and lowers sea level and SST in the eastern Pacific and western Indian Oceans. Thus opposite zonal gradients of sea level and SST are produced in the two oceans. When the Walker circulation weakens or strengthens, these gradients decrease or increase together, and positive feedbacks between the gradients and the Walker circulation take place. This mode of variability, which we refer to as the Tropical Indo-Pacific Mode, is very robust and is the dominant mode of the regional climate change. The present study emphasizes the role of ocean-atmosphere interaction and local ocean dynamics, as well as the importance of the Indo-Pacific connection, thus clarifying some hotly debated issues in the tropical Indo-Pacific climate variability.
OS33A-04 14:20h
Temporal And Spatial Characteristics Of Coastal Waves Around Australia
Temporal and spatial characteristics of coastal waves propagating along Australian coast during 1993 to 1996 are investigated, using observed daily-mean sea level data at 20 tidal stations located at the Australian coast. While the seasonal variation associated with the thermal expansion is significant in the northern part of Australia, short-term sea level variations with a period of less than a month are dominant at the other part of the coast. At the western and southern coasts, the sea level signals associated with the short-term variations propagate anticlockwise to the southeastern corner of Australia with a phase speed of about 10 m/s. There is clear seasonal modification in the amplitude of the short-term variations; the large (small) amplitude appears during the austral winter (summer) season. It turns out that these propagating signals are mainly generated in the western coast by synoptic weather disturbances and, in some cases, are reinforced as they propagate along the southern coast by different synoptic disturbances. The signals along the eastern coast show different characteristics and propagate from south to north with the phase speed of 2.3 m/s in all seasons, as discussed by Hamon (1966). We discuss results from both a simple wave model and a high resolution OGCM run using the Earth Simulator to understand those observed characteristics of the propagating signals.
OS33A-05 14:35h
Barrier layer in the Indian Ocean in a 200-year simulation of the SINTEX-Frontier, CGCM
The CGCM SINTEX-Frontier, originally developed in Europe has been implemented on the Earth Simulator. This model is based on OPA8.2 (ORCA2), ECHAM4.6 (T106L19) and OASIS 2.4.1. We will present results focused on the Barrier Layer in the Indian Ocean, based on 200-year long experiments. We will present the model climatology as a short introduction. Next we will focus on the barrier layer (BL) in the Indian Ocean. In agreement with the observations, a thick BL (more than 20m) is observed (1) in the northern part of the Bay of Bengal from July to march with a maximum extent in January-February, (2) in the south-eastern Arabian Sea in January-February and (3) offshore of Sumatra in December-January. In agreement with our previous studies, off Sumatra, the BL variability is controlled by the Wyrtki jet intraseasonal and interannual variability. The formation mechanisms explored in previous studies are found again in the coupled model. Regarding to the potential impact of the BL on the climate, we found, in agreement with the recent study of Durand et al. [2004], that in the south-eastern Arabian Sea the maximum of BL thickness lead SST maximum by about 1 month suggesting a significant impact of BL though the trapping of heat bellow the mixed layer. From December to February vertical inversion of temperature of more than 2 degree are located in the northern part of the Bay. Regarding the interannual variability, the coupled model shows similar results in comparison with our previous work suggesting the positive impact of equatorial shallow salinity stratification on the IOD amplitude. A set of sensibility experiments has been designed to quantify the impact of the barrier layer and salinity stratification on the Indian Ocean variability. The first results of this work will be presented in the second part of the talk before concluding.
OS33A-06 14:50h
Decadal Indian Ocean Dipole Simulated in an Ocean-Atmosphere Coupled Model
Using an output from 200-year integration of the SINTEX-F1 coupled ocean-atmosphere general circulation model, the decadal climate variability in the tropical Indian Ocean is investigated. A wavelet analysis on the model Dipole Mode Index captures the Indian Ocean Dipole (IOD) events as major variations with a period of about 4 years; the model's capability of resolving IOD (and ENSO events) is already proved. Interestingly, the model also captures lower frequency variations. The first EOF mode of the band-pass (9-35 years) filtered sea surface temperature anomaly represents a basin-wide mode and explains 37 percent of the total variance in the decadal band. Its principal component is correlated well with the decadal Nino3 (90-150W, 5S-5N) SSTA index, suggesting the close connection with the Pacific ENSO-like decadal variability. The second EOF mode, explaining 14 percent of the variance in the decadal band, shows a clear east-west dipole pattern. Although the pattern resembles the interannual IOD, its time scale is much longer. Therefore the latter mode may be named the decadal IOD. We suggest, however, that the decadal air-sea interaction in the tropics is a statistical artifact; the decadal IOD should be interpreted as decadal modulation of interannual IOD events. The modulation includes a) frequency modulation, b) amplitude modulation and, most importantly, c) asymmetric occurrence of positive and negative events.