CIRENE

Air-Sea interactions at several timescales in the Indian Ocean

by J. Vialard *(a) and the CIRENE team  

^* Corresponding author : Jérome Vialard

^(a) : LOCEAN UMR 7159, Paris France

The Indian Ocean was once thought as a relatively passive ocean from a climate perspective, in comparison to the neighbouring Pacific Ocean that hosts the powerful El Niño phenomenon (McPhaden et al. 2006). But recent studies deeply changed this view. First, an intrinsic mode of interannual climate variability called Indian Ocean Dipole (IOD) has been discovered (e.g. Saji et al., 1999). The IOD strongly influences climate variability around the Indian Ocean (e.g. Yamagata et al., 2004) and may also affect the evolution of El Niño (Izumo et al., 2010). Second, the Madden-Julian Oscillation (MJO), i.e. the main mode of atmospheric variability at intraseasonal timescale (e.g. Zhang 2005), initially develops in the tropical Indian Ocean, before propagating eastward and affecting tropical rainfall along its track. 

For those reasons, the Indian Ocean has come back at the forefront of the climate science scenes in recent years. One region that has received prime attention is the TRIO (Thermocline Ridge of the Indian Ocean) region, around [55°E-80°E, 10°S-3°S]. In this region, the mean structure of the wind promotes an open ocean upwelling and shallow thermocline. In austral summer, there are at the same time high Sea Surface Temperatures (SST), that allow deep atmospheric convection. This promotes a strong air-sea coupling in this region. In consequence, a strong variability at several timescales clearly appears (Vialard et al., 2009) : it is a cyclogenesis region, a region where the MJO is associated with SST variations of several degrees noted over 10-30 days, and a region that is strongly affected by the IOD, with clear climate teleconnections. However, the processes of this SST variability and its impact on the atmosphere are hardly known mainly because of the lack of in situ measurements, while such data are needed to confirm scenarios of mechanisms built on simulations by numerical modelling. 

Several international field experiments supported by the Climate Variability (CLIVAR) World Meteorological Organization (WMO) program have hence been organized in the Indian Ocean after 2000. Amongst them, the CIRENE program involved several  oceanographic cruises in the TRIO region, with the main cruise in January-February 2007, in order to collect oceanic, atmospheric and fluxes measurements. The main objectives of the CIRENE experiment was to document the processes of SST variations and the ocean-atmosphere coupling at several timescales in the TRIO region.  

The CIRENE cruise was the oceanographic component of the VASCO-CIRENE experiment. CIRENE involved 6 French laboratories or institutions ( LOCEAN, LMD, CETP, CNRM, US025 from IRD and DT INSU) and 4 USA laboratories (PMEL, WHOI, RSMAS, and ODU). The CIRENE campaign was supported by the CLIVAR Indian Ocean Panel, and has been funded by GMMC in 2007. 

Scientific Interest

The western Pacific and Indian oceans are home to the largest pool of warm water (in excess of 28°C) and the largest region of atmopsheric deep convection of the world ocean. Over the Indian Ocean, this deep convection is most active during the boreal winter, with a maximum amplitude between the equator and 15°S. This intraseasonal variability (ISV) of the deep convection is one of the most organized and reproducible large-scale perturbations in the Tropics : the perturbation, generally referenced as the Madden-Julian oscillation (MJO, see Madden and Julian 1994 for a review), propagates eastward from the West Indian Ocean to the Central Pacific.

But the mechanisms for the generation and the evolution of the ISV of the deep convection over the Indo-Pacific region are not perfectly understood, even if 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).  Some 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).

A recent study of Wentz et al., 2000 based on Tropical Rainfall Measuring Mission’s (TRMM) Microwave Imager (TMI) data shows that tropical intraseasonal perturbations of the deep convection may be associated to Sea Surface Temperature (SST) variations of several degrees, especially south of the equator in the western Indian Ocean during boreal winter. Other studies based on satellite measurement of the SST confirmed large SST perturbations in the Indo-Pacific region (Harrison and Vecchi 2001; Duvel et al 2004; Duvel and Vialard 2007), particularly south of the equator in the Indian Ocean during northern hemisphere winter, in association with relatively thin mixed layer (fig.1).

Figure 1 : Amplitude of the SST response to the Madden-Julian Oscillation (from Vialard et al, 2013)

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 had to be established. Since there are very few in situ observations in this region, an experimental campaign was needed to confirm the hypotheses that can be built using numerical modeling. This scientific context determined the choice of the region and the season for the VASCO-CIRENE experiment, whose aim is to trace the physical source of the intraseasonal perturbation of the SST in the Western Indian Ocean south of the equator during the January- March season. If some hypothesis are made on the probably major role of heat fluxes to drive the SST response to the MJO, questions remain on the role of vertical mixing and Ekman pumping and an experimental campaign was needed to help describing the vertical scale of these temperature anomalies. This experiment was also an opportunity to sample the other strong SST signals in this region at other timescales (due to the IOD or to cyclones).

Methodology

The CIRENE experiment (PI / J. Vialard) was an oceanographic campaign with the Ifremer ship « Le Suroît », starting from the Seychelles (55°E) to the Chagos (70°E). It involved two legs, both starting from Seychelles, from the 5th of January to the 21st of February 2007. This scientific cruise, within the TRIO region, has been synchronised with the Vasco experimental program (PI: J-P Duvel, Aeroclippers and pressurised balloons launched from the Seychelles).

XBTs provided by the Coriolis program were deployed every 30 miles along both legs. A north-south section across the thermocline ridge was performed at 67°E. PROVOR profilers provided by the Coriolis program were deployed at 3°S (3 profilers), 5°S (3 profilers), 7°S (3 profilers) and 9°S (1 profiler). The groups of 3 profilers were deployed with standard Argo profiling characteristics (e.g. one profile every 10 days, 1000m parking depth, 0-2000m profiles), but were programmed to profile every third alternate-day, hence providing an increased temporal resolution during the first few months after the cruise. A mooring was deployed at 8°S, 67°E as part of the RAMA program (Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction ; Mc Phaden et al., 2009). Most of the cruise consisted of a long station at 8°S, 67°30’E, with intense air-sea fluxes, and high-frequency ocean profiles sampling (see Vialard et al. 2009 for a more detailed description of the cruise).

In addition, the diurnal cycle has been measured thanks to ASIP instruments (Air-Sea Interaction Profiler, B. Ward's), and biogeochemical measurements (nutrients, chlorophyll) have been collected because of the useful information they can give on the physical processes at work. 

Preliminary Results 

The CIRENE cruise science objectives were to monitor cyclones, the MJO and the oceanic signature of the IOD. We were lucky with two of those three objectives during the cruise itself.

1)- A strong IOD event occurred in late 2006, and generated strong thermocline depth anomalies in the eastern half of the Indian Ocean, between 5°S and 12°S. By the time of the cruise, these anomalies had propagated westward as planetary waves and we monitored its vertical structure in the region of largest sea level interannual anomalies, as well as anomalously fresh water in the surface layer and 0.2 m.s^-1 eastward current anomalies down to 600m (Vialard et al. 2009). 

2)- the DORA cyclone captured during the CIRENE cruise :

On January 27 2011, a tropical disturbance formed almost exactly over the mooring and Suroît location. The system was upgraded to a moderate tropical storm named Dora on January 29. Dora meandered southward (see Figure 2 for the Dora track) over the next two days while strengthening, and Météo-France upgraded it to a tropical cyclone early on February 1. Dora reached its strongest intensity (930 hPa and 190 km/h winds) on February 3, at about 15°S and remained intense until the 9th. The Vasco-Cirene program provided unique observations of both the oceanic response to the storm (Cuypers et al. 2013) in a region of strong air-sea interactions, and of surface winds in the storm eye wall from Aeroclippers further south (Duvel et al. 2009).

Nevertheless, the whole duration of the CIRENE cruise was associated with inactive MJO, but the instruments deployed during the cruise (RAMA mooring, Argo profilers) allowed to monitor and analyse the processes of a very strong MJO event in late 2007. The RAMA mooring especially allowed us to analyse mechanisms of the SST signature of the MJO, and to conclude that air-sea heat fluxes were the dominating process (Vialard et al., 2008).

Figure 2 : (Left) GLORYS2 SST analysis on the 10th February 2007. (Right) TMI-AMSR-E microwave SST estimates on the same date. The full track of the Dora cyclone is indicated by the black line. The recorded positions of the cyclones on that day are indicated by black disk (the average cyclone maximum winds on that day was 25 ms^-1).

Related links

VASCO-CIRENE webpage : http://www.lmd.ens.fr/vascocirene/

VASCO webpage : http://www.lmd.ens.fr/vasco/

CLIVAR Indian Ocean Pannel : http://www.clivar.org/organization/indian/indian.php

MISMO campaign : http://www.jamstec.go.jp/iorgc/mismo/index-e.html

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