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Predictability of intraseasonal convective perturbations in the Indo-Pacific tropical area
Role of air-sea coupling in this intraseasonal variability
Physical origin of the intraseasonal SST variability in the western Indian Ocean

Summary

Recent studies based on Tropical Rainfall Measuring Mission’s (TRMM) Microwave Imager (TMI) data shows that tropical intraseasonal perturbations of the deep convection (such as the Madden-Julian oscillation) may be associated to Sea Surface Temperature (SST) variations of several degrees, especially south of the equator in the western Indian Ocean (55°E-80°E, Eq-15°S) during boreal winter. While this variability is partly reproduced by forced or coupled Ocean models, the relative role of different physical processes (warm layer formation, Ekman pumping, sub-surface cooling due to vertical mixing, surface fluxes) in these intraseasonal SST perturbations still has to be established.

Since there are very few in situ observations in this region, an experimental campaign is needed to confirm the hypotheses that can be built using numerical modeling. The objective of the VASCO-CIRENE campaign is to measure the impact of the different physical processes listed above on SST perturbations from diurnal (warm layer) to intraseasonal time-scales. This aims to better explain (i) the mechanisms of the intraseasonal variability of the SST and (ii) the feedback of these SST variations on the atmosphere.

The Scientific context

The intraseasonal variability (ISV) of the deep convection has maximum amplitude over the Indo-Pacific region and is one of the most organized and reproducible large-scale perturbations in the Tropics. The ISV of the convection may have a strong impact on the seasonal predictability in the tropics. During winter, the maximum amplitude of the convective perturbation is located between the equator and 15°S (Fig.1a). This perturbation propagates eastward from the West Indian Ocean to the Central Pacific. This winter variability is generally referenced as the Madden-Julian oscillation (MJO, see Madden and Julian 1994 for a review). These perturbations are associated with westerly wind bursts generating important surface flux perturbations (e.g. Weller and Anderson 1996; Duvel et al. 2004) and that may play a role in the onset of El Niño events (e.g. McPhaden 1999; Lengaigne et al. 2002).

The mechanisms for the generation and the evolution of the intraseasonal variability of the deep convection over the Indo-Pacific region are not perfectly understood. However, recent modelling studies suggest that air-sea interactions could play an important role both during summer and winter (e.g. Waliser et al. 1999; Inness and Slingo 2003; Maloney and Sobel 2004). Observations also have revealed SST perturbations up to 3K in relation with the ISV of the convection in the China Sea (Kawamura 1988), in the Bay of Bengal (Sengupta and Ravichandran 2001) and in the western Pacific (e.g. Anderson et al, 1996).

Recent satellite measurement of the SST by the Tropical Rainfall Measuring Mission’s (TRMM) Microwave Imager (TMI) (Wentz et al 2000) also revealed large SST perturbations in the Indo-Pacific region (Harrison and Vecchi 2001; Duvel et al 2004; Duvel and Vialard 2006). These SST perturbations are particularly strong south of the equator in the Indian Ocean during NH winter (Fig.1c) and is associated to a relatively thin mixed layer (Fig.1d). This determined the choice of the region and the season for the VASCO-CIRENE experiment. These large SST variations, identified with the TMI satellite dataset, were confirmed by in situ data and show a potentially important role of warm layers in the intraseasonal amplitude of the SST (Fig.2).

Figure 2 from Duvel et al (2004): (left) Comparison between the SST measurement of drifting buoys and the deduced TMI SST along the path of the buoy. TMI SST at the location of the buoy is obtained by horizontal bi-linear interpolation and linear temporal interpolation in daily TMI fields. The inlay shows the trajectory of the WMO 14549 between February 25 and March 30 1999. (right) Amplitude of the TMI SST perturbation for a strong intraseasonal event in March 1999. The path of the buoy is indicated in blue.

The physical origin of these large SST perturbations over the south equatorial Indian Ocean may be due (i) to vertical and horizontal heat transport in the ocean mixed layer, (ii) to the surface fluxes and, (iii) to the formation of warm layers prior to the cooling event that might contribute to enhance the SST perturbation at intraseasonal time-scales. The shallow thermocline between 5°S and 10°S, due to average Ekman pumping during NH winter, certainly is a fundamental feature to explain the large observed SST perturbations. This shallow thermocline makes cold water readily available to cool the surface by vertical mixing or local upwelling and it also limits strongly the depth of the mixed layer, making it more responsive to surface forcing.

This surface forcing perturbation itself is due to various physical processes that may have different phasing relative to the maximum convective activity. For the surface heat fluxes, the western Indian Ocean during winter is more affected by the surface wind (Fig.1b) associated to convection further East than to the local convection (the opposite is observed over the Eastern Indian Ocean). Duvel et al (2004) showed that the latitudinal position of maximum SST variability was the result of a consensus between the position of the region of maximum flux perturbation (spanning the equator) and the region where the thermocline is shallow (between 5°S and 12°S). Results of the forced OGCM also showed that the salinity perturbation induced by the strong rain under convection and the intraseasonal Ekman pumping perturbation could play some role in the large SST response by limiting the mixed layer deepening induced by the wind perturbation.

Both the scientific objectives and the geographical location of this campaign fit into CLIVAR IOP objectives and the CIRENE region is now recognized as a key region for the development of intraseasonal events.