trio science plan v4 - upmcjv/trio/trio_science_plan_v4.pdf · trio science plan september 2008...

TRIO Science plan September 2008

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TRIOTRIO Thermocline Ridge of the Indian Thermocline Ridge of the Indian

OceanOcean

Role of the Indian Ocean Thermocline Ridge in the

Ocean-Atmosphere Variability at Interannual, Intraseasonal and Synoptic scales

September 2008

PIs: J. Vialard (LOCEAN) and J-P. Duvel (LMD)

Science plan for a proposal submitted to LEFE/IDAO (Institut National des Sciences de l’Univers)


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Abstract The TRIO (“Thermocline Ridge of the Indian Ocean”) project and cruise is in continuity of the

Vasco-Cirene project. It will explore air-sea interactions at synoptic (cyclones and tropical storms), intraseasonal (Madden-Julian Oscillation) and interannual timescales in the 5°S-15°S band of the Indian Ocean. In 2009-2010, TRIO will focus on modelling studies and analysis of existing data. The TRIO cruise, in early 2011, will contribute to the development of the RAMA array (Indian Ocean counterpart of TAO and PIRATA) and interact with several satellite programs (Megha-Tropiques, SMOS, Altika). TRIO will also have strong interactions with the synchronous SWICE (South West Indian Ocean Cyclone Experiment) project.

The 5°S-10°S band in the Indian Ocean is a region where several phenomena of significant climatic influence build up. It is a cyclogenesis region for tropical cyclones striking inhabited islands of the Indian Ocean and the African coast. It was recently shown that it is one of the regions of the globe where atmospheric intraseasonal variability (e.g. Madden Julian Oscillation, MJO) is associated with the strongest oceanic response. Finally, there is an important interannual variability over this region (e.g. Indian Ocean Dipole, IOD), which has significant implications on the rainfall over India during the following monsoon. There are interactions between these phenomena at different time scales. For example, the IOD modulates the heat content in the southwestern tropical Indian Ocean and influences the cyclone distribution near La Réunion and Madagascar. Phenomena developing over this region have some impacts over remote regions, in particular the Pacific Ocean. The strong prevalence of ocean atmosphere interactions at a variety of timescales in this region is due to average wind structure that lifts the thermocline. The elevated thermocline leads to different oceanic processes increasing the SST response to atmospheric perturbations. In addition, while upwellings are generally associated with lower sea surface temperature, the ocean surface remains warm here, enabling the development of deep atmospheric convection. This gives strong air-sea interaction associated with the convective perturbations at different time scales.

The TRIO (Thermocline ridge of the Indian Ocean) project aims at analysing the coupled processes associated with these phenomena (i.e. cyclones, MJO, IOD), their scale interactions and their predictability. TRIO is an integrated project that continues and expands the Vasco-Cirene programme. TRIO will combine modelling, analysis of past observations and a new field experiment. The field experiment is mostly based on a cruise in the 5°S-10°S band and will be coordinated with SWICE (South West Indian Ocean cyclone experiment), with three satellite programs (Altika, SMOS and Megha-tropiques) and with the development of a mooring Array in the Indian Ocean (the RAMA array). The TRIO cruise and SWICE are scheduled for late 2010 / early 2011. This takes opportunity of the Atalante presence in the western Pacific in late 2010 and of two other cruises planned in the Indonesian region in late 2010. The TRIO cruise will cover the 5°S-10°S band in the Indian Ocean and the northwestern Australian basin. These two regions have recently been identified as the two regions with the strongest surface temperature signals associated with the MJO.


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1. Thethermoclineridge .................................................................................................................................................42. Questionsrelatedtothethermoclineridge .........................................................................................................52.1. Meanstateandseasonalcycle..........................................................................................................................52.2. Cyclones ...................................................................................................................................................................62.3. MaddenJulianOscillation .................................................................................................................................72.4. Interannualvariability(ENSOandIOD) .......................................................................................................82.5. OceanicintraseasonalvariabilityandRossbywaves...............................................................................92.6. Scaleinteractions .............................................................................................................................................. 102.6.1. Thediurnalcycleandtheintraseasonalvariability ........................................................................................ 102.6.2. Mixingbyinternalgravitywaves ............................................................................................................................ 11

2.7. Atmosphericdrylayers ................................................................................................................................... 122.8. Biogeochemicalprocessesandhalieutics ................................................................................................. 12

3. Nationalandinternationalcontext ...................................................................................................................... 123.1. Observationalnetworks:IndooSandRAMA ............................................................................................ 123.2. Satelliteprograms ............................................................................................................................................. 133.2.1. Megha‐tropiques............................................................................................................................................................. 133.2.2. Altika.................................................................................................................................................................................... 133.2.3. SMOS .................................................................................................................................................................................... 13

3.3. Linkwithotherfieldexperiments ............................................................................................................... 143.3.1. SWICE .................................................................................................................................................................................. 143.3.2. Japanesecruisesinlate2010.................................................................................................................................... 143.3.3. CoordinatedplanningwithtwootherFrenchcruises ................................................................................... 14

4. TRIOImplementationplan ..................................................................................................................................... 144.1. AnalysesandpreparationoftheTRIOcruise .......................................................................................... 144.1.1. Vasco‐Cirenedataanalysis......................................................................................................................................... 144.1.2. Otherexistingobservationsandmodelling ........................................................................................................ 15

4.2. Designoftheoceanographiccruise............................................................................................................. 155. References .................................................................................................................................................................... 17


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1. The thermocline ridge This section presents the phenomenology and the scientific questions investigated in TRIO. In some part, we have

used excerpts from the Vialard et al (2008a) paper that already summarized the scientific objectives and the results of the Cirene cruise.

While easterly trade winds blow year-round over the southern Indian Ocean, surface winds experience a striking reversal north of 10°S. During boreal summer, the low-level easterly flow penetrates northward, is deflected when crossing the equator, and forms the strong Indian monsoon or Findlater jet. During boreal winter, northeasterly winds bend northwesterly while crossing the equator southward, and form a weak low-level westerly jet between the equator and 10°S (Fig. 1a). The cyclonic circulation at the meeting point of these two wind regimes gives an Ekman pumping that is responsible for the formation of the “Seychelles Chagos Thermocline ridge” (hereafter SCTR, Hermes and Reason 2008).

Figure 1. a) Average JFM wind vectors and synchronous 0-300m average temperature; b) Meridional section of JFM temperature at 67°E; c) SST and d) mixed layer depth for the JFM season.

The Seychelles-Chagos thermocline ridge (SCTR) is located between 5°S and 10°S and east of 50°E in the Indian Ocean (Fig. 1a). It corresponds to a region where the thermocline rises close to the surface (Fig. 1b). This year-round feature is more pronounced in boreal winter, and can be explained by the surface wind pattern (e.g McCreary et al. 1993). South of the SCTR, easterly winds drive a southward Ekman transport and north of the SCTR, westerly winds are associated with a northward Ekman transport (Fig. 1a). The resulting Ekman pumping lifts the thermocline and results in this ridge, extending roughly along the northern edge of the Easterlies. A more detailed explanation of the formation of this ridge and its annual cycle can be found in (Hermes and Reason 2008; Yokoi et al 2008).

In most upwelling regions (e.g. the eastern equatorial Pacific or Atlantic), the SST is below 25°C, because of the input of cold thermocline water. The SCTR is quite unique in that the average surface temperature is above 27°C for almost all seasons and above 28.5°C during the Austral summer. In the Tropics such a high surface a temperature favors the atmospheric convective instability and the development of deep convective systems organized at different scales. Because of the shallow thermocline, the mixed layer is shallow and very responsive to atmospheric heat fluxes perturbation related to organized deep convection (e.g. Duvel et al. 2004). The cold thermocline water can also easily be brought to the surface by mixing in certain conditions. It is the combined presence of this shallow thermocline (that promotes SST changes) and of the high SST that makes the SCTR a region favorable to strong air-sea interactions (Xie et al. 2002), in particular at the MJO time scale (Duvel et al. 2004, Duvel and Vialard 2007).

The SCTR also marks the limit between two current systems. Along the SCTR axis, the average water column temperature is colder than at other latitudes, and the sea surface is thus depressed (colder water having a smaller specific volume). There is a northward near surface pressure gradient to the south of the SCTR, which, combined with the effect of the Coriolis force, results in a westward current named “South Equatorial Current” (Fig. 1a, Schott and McCreary, 2001). For similar reason, there is an eastward current, most marked in boreal winter, named “South Equatorial Counter-Current” to the north of the SCTR.


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This region has attracted attention since it is home to distinct oceanic and atmospheric variability at multiple timescales, each time with significant climatic consequences. Anomalously warm sea surface temperature (SST) in the SCTR region is associated with increased cyclonic activity near Madagascar and La Réunion (Jury et al. 1999; Xie et al. 2002). It also induces above-average rainfall along the Western Ghats of India during the following monsoon (Vecchi and Harrison 2004; Izumo et al. 2008). Atmospheric model experiments suggest that these SST anomalies force a substantial fraction of interannual precipitation anomalies over the west Pacific and maritime continent (Annamalai et al. 2005) and influence the northern hemisphere extratropical circulation during boreal winter (Annamalai et al. 2007). These numerous climatic consequences are an incentive to better understand the various climate phenomena that affect SST in this region.

Figure 2. Average 0-30m salinity from Argo profilers along 8°S. The seasonal modulation of the freshwater front is clearly seen. Data from the ATLAS mooring at 8°S, 67°E also clearly show the front. Its seasonal cycle, interannual variability and driving processes remain to be explored. The salinity front oscillates between 60°E and 95°E, and should be found between 65°E and 80°E in Jan-Feb (planned period for TRIO).

2. Questions related to the thermocline ridge

2.1. Mean state and seasonal cycle Although there have been recent studies investigating the semi-annual cycle of the subsurface temperature

variability in the SCTR (Hermes and Reason 2008; Yokoi et al 2008), the processes controlling the mean state and annual cycle of the surface temperature has hardly be investigated. A challenging question related to the SCTR (and other similar regions like the Guinea and Angola dome in the Atlantic and Costa Rica and Mindanao dome in the Pacific oceans) is that high SST may persist during long period despite the occurrence of upwelling. A recent budget estimates suggests indeed that a cooling through the exchanges with the subsurface comparable to that of the central Pacific (Vialard et al., 2008b). The heat budget of this region needs to be examined through significantly longer periods, both in observations and coupled models, in order to understand the coupled feedbacks that maintain such a high SST despite the occurrence of upwelling.

The SCTR (5°-12°S) is located in the region of influence of water masses coming from the Pacific in the Indonesian throughflow. Recent papers have suggested that the input of throughflow water varies seasonally and has an impact on, e.g., the stratification and the generation of instabilities in the SCTR (Zhou et al., 2008). The data from the Cirene cruise furthermore suggests that there is a clear salinity front in the SCTR between fresh water (coming from the throughflow and the ITCZ precipitations) in the east and saltier water in the west, as was detected at the edge of the western Pacific warm pool (e.g. Vialard and Delecluse, 1998).


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→→ Question A: What are the specific processes enabling a shallow thermocline and a high SST in the SCTR region and other similar “domes”? How air-sea interactions feedbacks and other processes are involved in maintaining this peculiar state?

→→ Question B: How does the throughflow or other features modulates the longitudinal position of the fresh-water front in the SCTR. What is the role of the throughflow in the SCTR mean state?

2.2. Cyclones The cyclone season in the south-western Indian ocean (west of 90°E) extends from November through April. There

is an average of 10 tropical storms and 5 named cyclones every year that are a major threat to many places, such as Maritius, La Réunion, Madagascar or Mozambique. The cyclonic activity in this region is as intense as in the north Atlantic, but with much less attention has been paid to this region. Most of the tropical storms that later become cyclones are formed over the 5-10°S band. In addition to being the SCTR, this region is also the mean position of the ITCZ over the Indian Ocean in austral summer (see figure 3). Indian cyclone genesis and intensification has been suggested to be modulated by large-scale oceanic and atmospheric features. Recent studies have suggested that changes in the heat content in the SCTR region can influence the number of cyclone days in the southwestern Indian ocean (Xie et al. 2002; Chowdary et al., 2008). The are also indications that more cyclone genesis occurs during the convectively instable phase of the MJO. In both cases, however, the influence has been detected from the statistical analysis of data, and no detailed suggestion of the processes explaining these potential links between climate variability and cyclones have been proposed. These questions will be investigated in coordination with the SWICE project (South-West Indian Ocean Cyclone Experiment) that especially aims at understanding the relationship between the tropical cyclones and their atmospheric and oceanic environment in the south-west Indian ocean.

Figure 3. Cyclone tracks for the Indian Ocean (1985-2005) and mean OLR during DJFM. The 5°S-15°S band has been highlighted. The TRIO cruise will follow the ITCZ that is also the generation region of most tropical storms and depressions that later transform into cyclones.

Tropical storms and cyclones are associated with large heat uptake from the ocean and instigate isolated blast of vigorous mixing in the upper ocean (both local and non-local through generation of internal gravity waves). Calculations using the observed record of tropical cyclones indicate that the amount of net column heating required to restore the cold surface wakes left by these storms is on the order of the oceans’ poleward heat transport (Emanuel 2001), rendering this a potentially important interaction. But the uncertainties are large, making conclusive evidence for a dominant role elusive. In addition, as the MJO has shown to modulate synoptic variability (including cyclones and tropical storms), there is an intriguing possibility that the changes associated with tropical storms and cyclones account for a large fraction of the total heat uptake during an active phase of the MJO. The integrated effect of storms and cyclones on the seasonal cycle of the SCTR and in phenomena like the MJO should be explored.

→→ Question C: How do intraseasonal (MJO) and interannual (heat content and SST anomalies in the SCTR region) climate anomalies influence cyclones in the southern Indian Ocean, and by which mechanisms?

→→ Question D: How do tropical storms and cyclones influence the long-term climate variability in the SCTR region?


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2.3. Madden-Julian Oscillation

Figure 4. SST, OLR and Sea Level anomalies in the SCTR region (60°E-80°E, 5°S-10°S). OLR is shown only for DJFM. The dashed lines indicate the MJO events with the largest SST impacts. The grey bar indicates the period of the Cirene cruise. No strong MJO was observed during Vasco-Cirene (Vialard et al., 2008a), but a large interannual anomaly could be monitored by the ATLAS mooring in late 2007 and early 2008 (Vialard et al., 2008b).

The most striking SST variability in the SCTR is at intraseasonal timescales (20 to 90 days). Surface cooling events of 1 to 1.5°C may last for more than a month, as during the austral summers of 1999 to 2002, and in 2008. Such events follow by a short lag a sharp increase in atmospheric convective activity (Fig. 4b). These strong cooling events where hardly noticed by satellite SST measurements in the infrared window, for which the screening effect of clouds prevents an accurate estimate of ocean cooling below convective systems (Duvel and Vialard, 2007). The advent of microwave instruments, like the Tropical Rainfall Measuring Mission Microwave Instrument (TMI, Wentz et al. 2000), clearly showed links between these SST variations in the SCTR region and large-scale convective perturbations propagating eastward south of the equator (Harrison and Vecchi, 2001; Duvel et al. 2004). These intraseasonal perturbations, with time scales between 20 and 90 days, are known as Madden-Julian Oscillations (MJO) (see e.g. Madden and Julian 1994; Zhang 2005). The MJO explains a large fraction of the variance of tropical convection during the austral summer, and is associated to modulations of the cyclonic activity, increased convective instability is given phase of the MJO being associated with more cyclones (Liebmann et al. 1994 ; Bessafi and Wheeler 2006). The large-scale convective perturbation (the MJO convective phase) is associated with westerly wind bursts and significant surface flux perturbations west of the convective event (e.g., Duvel et al. 2004); it develops over the Indian Ocean and then propagates into the western Pacific, where wind bursts can play a major role in the onset of El Niño events (e.g., McPhaden 1999).

The SCTR is a region of strong SST intraseasonal variance (Fig. 5) (Duvel and Vialard, 2007). The shallow thermocline in this region favors the strong SST response because a) colder water is more readily brought to the surface by upwelling or mixing, and b) the mixed layer is constrained to be shallow because of the underlying thermocline and is thus more responsive to atmospheric fluxes (Harrison and Vecchi, 2001; Saji et al. 2006; Duvel and Vialard 2007; Vialard et al., 2008a). Modeling studies suggest that considering the MJO surface temperature signature can improve simulations (e.g. Waliser et al. 1999; Maloney and Sobel, 2004) and forecasts (Woolnough et al., 2007) of the MJO.

The SCTR, with its high surface temperature and strong cooling events, may thus be a key region for understanding the processes responsible for the genesis of intraseasonal or MJO events. A recent study (Xavier et al 2008), based on DEMETER coupled model hindcasts, shows that the Coupled GCMs tend to underestimate the spatial organization of the convection at the intraseasonal time scale. This is associated with a poor reproducibility of the intraseasonal perturbation pattern compared to observation. A key question to understand and control the representation of the intraseasonal variability in GCMs is thus certainly to understand the processes leading to a large-scale perturbation of the convection and the associated dynamical response. It is especially interesting to study the role of air-sea interactions in this large-scale organization (see also the section on the oceanic diurnal warm layers).


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Figure 5. Standard deviation of DJFM 10-90 day filtered SST. In some regions, like at the equator between 40°E and 50°E, the SST intraseasonal variability is linked to oceanic internal instabilities. But in others, like the SCTR or the NWAB (North West Australian Basin), it is linked to the Madden-Julian Oscillation.

The latest heat budget analysis from Cirene observations (Vialard et al., 2008b) suggest that the MJO SST response is largely driven by heat fluxes, but that there might also be a significant contribution from subsurface cooling, that remains to be more precisely quantified. The influence of the SST signature on the MJO itself and the interaction between the MJO and interannual anomalies in the region remain to be better quantified. There is a second region of strong SST response to the MJO between Australia and Indonesia (Fig. 5) (Duvel and Vialard 2007; Vialard et al., 2008a), which needs to be more thoroughly investigated.

→→ Question E: What are the physical processes of the SST perturbation at intraseasonal time scales in the SCTR and eastern tropical Indian Ocean and how do they vary regionally?

→→ Question F: How SST perturbations may organize large-scale convective perturbations and trigger MJO events?

→→ Question G: Do interannual variability of the subsurface SCTR influence the amplitude of the SST perturbation and the triggering/evolution of the MJO?

2.4. Interannual variability (ENSO and IOD) By displacing the western Pacific warm pool and the associated atmospheric deep convection over distances of

thousands of kilometers, El Niño disrupts weather patterns at the global scale and can induce SST changes in remote regions via the so-called “atmospheric bridge”. Most regions of the Indian Ocean tend to warm in response to the surface heat flux perturbations induced by El Niño. However, the observed SST anomalies due to El Niño is more variable in the SCTR region, suggesting that internal ocean dynamics or other large-scale phenomena may have a larger influence here (Klein et al. 1999; Lau and Nath 2000).

The “Indian Ocean dipole” (IOD; e.g. Saji et al. 1999; Webster et al. 1999; Murtugudde et al. 2000) is also believed to grow as a result of coupled ocean-atmosphere instability. When the eastern Indian Ocean cools (so-called positive IOD), there is a reduced convection to the east and an easterly wind anomaly in the central basin. This wind anomaly drives westward equatorial currents that lift the thermocline and maintain a cool surface in the east. In 2006, this lead to a significant cold anomaly in the east and anomalous eastward winds in the central basin (Fig 6a). The largest IOD anomalies occur during September-November. While such IOD events occurred during an El Niño event, there is no systematic relation.


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Figure 6. Panel a) and b) show the wind (vectors) and SST (panel a) and SLA anomaly (panel b) in November 2006 at the peak of the 2006 dipole. Panel c and d show the SST and Sea Level interannual anomalies during the Cirene cruise (Jan-Feb 2007). Panel e shows the Temperature anomaly from the section at 67°E obtained from Cirene XBTs. Cirene allowed to observe large anomalies in the SCTR after the 2006 dipole (warmer and fresher upper ocean, anomalous currents down to 800m) that remain to be explained in details (Vialard et al., 2008a).

The IOD has a strong signature in the SCTR region (Vinayachandran et al. 2002), while El Niño induced variability is strongest south of 10°S (Yu et al. 2005). The eastward wind anomaly associated with the IOD drives southward currents, which accumulate mass and deepen the thermocline to the south, resulting in a strong positive sea level anomaly (Fig. 6b). Under the effects of earth rotation, this anomaly propagates westward as oceanic Rossby waves (Masumoto and Meyers 1998). This leads to a deeper thermocline in the SCTR region until April-May the following year (e.g. Fig. 6d). The deeper thermocline results in a diminished connection between subsurface cold waters and the surface, and causes warm SST anomalies (e.g. Fig. 6c). These SST anomalies may feedback to the wind (Xie et al. 2002), potentially making the SCTR the place of a full-fledged air-sea interaction process. (Izumo et al., 2008) suggested that the air sea interaction initiated in the SCTR could have impacts on the Somalia upwelling during the next monsoon onset and then on rainfall over the western Ghats of India, which is what seems to have happened during the unusual 2007 Indian monsoon. The IOD/ENSO generation of a Rossby wave in the SCTR could thus provide, similarly to ENSO, a mechanism by which predictability of cyclones in the South Western Indian Ocean and rainfall over India could be achieved months in advance thanks to the slow deterministic oceanic Rossby wave propagation.

→→ Question H: What are the respective impacts of ENSO and the IOD on the generation of an Interannual Rossby wave in the SCTR region? What are the processes of the dynamical and thermodynamical response to ENSO/IOD in the SCTR?

→→ Question I: Is there any predictability of the usual signatures of this Rossby wave (e.g. cyclones in SWIO, monsoon rainfall over western Ghats of India, Tuna catches, …) several months in advance (e.g., from November onward: the peak month of the IOD)?

2.5. Oceanic intraseasonal variability and Rossby waves Clear Rossby waves intraseasonal signals in the South-eastern tropical Indian Ocean have long been observed and

discussed (e.g. Figure 7, Morrow and Birol, 1998; Feng and Wijfels, 2002). Both barotropic and baroclinic instabilities seem to contribute to the generation of these Rossby Waves (e.g. Yu and Potemra, 2006). Their impact on SST (e.g. Figure 7, Morrow and Birol, 1998) and ocean biogeochemistry (Kawamiya and Oschlies, 2001) has been studied. The signature of this variability is clearly visible on the records of the ATLAS mooring deployed during the Cirene cruise


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(Vialard et al., 2008a). While internal instabilities are large contributors to these Rossby Wave, there is also a clear modulation of the sea level associated with the MJO (i.e. with the intraseasonal variability of Ekman pumping). A more detailed study of the case investigated in (Vialard et al., 2008b) even suggests that the thermocline modulation linked to these internal instabilities / forced response might control some of the mixed layer depth variations and hence influence the SST.

Figure 7. Intraseaonal SST (left) and sea level (right) along 8°S from January 2007 to April 2008. The two types of intraseasonal SST signals in the SCTR are clearly visible. The dominant signal is at large scale (dashed ellipse) and is linked with the MJO. But there is also ~600 km zonal scale intraseasonal signals with clear westward propagation in both SST and SLA. Those Rossby waves are largely forced by internal instabilities, but the wind curl also contributes.

→→ Question J: What are the respective parts of forcing and instabilities in intraseasonal variations of sea level and SST in the SCTR? Does the subsurface intraseasonal variability play any role in controlling the SST response?

2.6. Scale interactions

2.6.1. The diurnal cycle and the intraseasonal variability During convectively suppressed periods, low surface wind lead to reduced vertical mixing. Incident solar flux

generates stable stratification at the surface that results in a strong diurnal variation of the SST (Ward 2006; Stramma et al 1986 and Fig. 8d). Because of the asymmetry of the mixing processes (heating concentrated in the first meter during the day, and cooling spread over the thickness of the mixed layer during the night), diurnal warm layers induce a higher SST than the one expected from daily average air-sea heat fluxes. During the convectively active phase of the MJO, there are strong surface winds and no warm layer can form. As a result, diurnal warm layers tend to increase the amplitude of intraseasonal perturbations of the SST (e.g. Shinoda and Hendon 1998; Bernie et al 2005). These diurnal warm layers influence the atmospheric variability from diurnal to intraseasonal time scales and improve the MJO predictability in a GCM (Woolnough et al 2007). During Cirene, a similar mechanism was observed but in relation to cyclone development (Fig 8, Vialard et al., 2008a), which raises the question of the role of these warm layers in the cyclogenesis. The example of the diurnal cycle illustrates how important fine scale mechanisms could potentially be to understand variability at the scale of the MJO or IOD.

→→ Question K: How much does the diurnal cycle contribute to intraseasonal SST variability in the SCTR? Is the diurnal cycle important in accounting for the main observed properties of the MJO? Does the diurnal cycle interact with the cyclogenesis?


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Figure 8. The left column shows Cirene data before, during and after the passage of Dora (before it became a named tropical cyclone). The right column shows a sample run from a high resolution coupled model. Shortwave flux, rain, wind and SST are shown. Both illustrate the modulation of the diurnal cycle by synoptic features or, similarly, by the MJO. The coupled model can be used as a tool to study the scale interaction between the diurnal cycle and, e.g., the MJO.

2.6.2. Mixing by internal gravity waves

Figure 9. Conversion of barotropic M2 (12 hour) tide to baroclinic waves. The SCTR region is one of the regions where interaction with the topography generates a lot of internal tides. The resulting internal gravity waves travel around and generate mixing in the thermocline when they break.

In the ocean, small-scale processes such as turbulence induced by internal gravity wavebreaking may be important to understand the upper ocean heat budget at longer timescales. The internal wave field in the upper ocean as inferred from time series from the Cirene experiment revealed that both atmospheric forcing and barotropic tides are efficient to generate these waves. In particular the strong variability at semi-diurnal frequency (M2) in the thermocline is the signature of internal tides. These internal tides are generated by the interaction of barotropic tides with bottom topography in regions of rough topography and travel upward toward the thermocline. Both regions of interest of TRIO (the western SCTR and the NWAB) are regions of strong generation of baroclinic tides (Figure 9). The measurements performed along the 8S parallel will aim to provide estimates of the amount of energy that is dissipated locally in the


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generation regions and of that which is radiated away and dissipated in the far field. Internal gravity waves can also be generated by abrupt wind changes like those associated with cyclones or wind bursts. The current shear they generate there results in enhanced turbulence and can affect the rate of exchange between mixed layer and thermocline water, thus impacting the upper ocean heat balance. Preliminary computations suggest that these internal waves induce a twofold increase of mixing in the thermocline that probably needs to be considered in order to close the upper ocean heat budget over long timescales.

→→ Question L: Can we better estimate mixing in the Indian Ocean thermocline by taking into account the non-local effects associated with internal gravity waves?

2.7. Atmospheric dry layers During the Cirene cruise, atmospheric profiles showed dry layers around 700 hPa (Vialard et al., 2008a). These were

probably related to dry-air intrusion produced by subsiding upper-level extratropical air. These dry intrusions were previously observed in the western Pacific (Mapes and Zuidema 1996), in the Indian Ocean (Zachariasse et al 2001) and over West Africa (Roca et al 2005). A dry layer in the lower troposphere can inhibit deep convection (Mapes and Zuidema 1996) and the ability of tropical cyclones to strengthen (Dunion and Velden 2004). How these dry intrusions interact with cyclogenesis or the MJO in the Indian Ocean has to be explored.

→→ Question M: What is the impact of dry layers on cyclogenesis and the MJO in the SCTR region?

2.8. Biogeochemical processes and halieutics There were interesting results on this topic obtained thanks to the Cirene measurements, in particular on the

influence of the IOD and of the MJO. This part will be further developed following discussions with the US participants.

3. National and international context

3.1. Observational networks: IndooS and RAMA

Figure 10. Schematic of RAMA. Solid symbols indicate those sites occupied so far. Color indicates national support, with dates of first involvement for contributing countries or bilateral partnerships shown in the upper right box. Open symbols indicate sites that are yet to be instrumented. Sites at 8°S and 12°S, 55°E will be occupied in November 2008. The red circle shows the mooring deployed during Cirene. The black box shows the RAMA mooring that can be serviced or deployed during TRIO. Note that there is no planned RAMA mooring for the region of strong SST intraseasonal variability northwest of Australia.

The International GOOS program and the Climate Variability and Predictability (CLIVAR) component of the World Climate Research Program (WRCP) established an Indian Ocean Panel (IOP) in 2004 to design, and guide the


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implementation of, a basin-scale, integrated Indian Ocean observing system for climate research and forecasting. The IOP focused on developing a strategy for in situ measurements to complement existing and planned satellite missions for surface winds, sea level, SST, rainfall, salinity, and ocean colour. The resulting system, referred to as IndOOS, is based on proven technologies, including moorings, Argo floats, ship-of-opportunity measurements, surface drifters, and tide gauge stations (CLIVAR-GOOS Indian Ocean Panel, 2006; Meyers and Boscolo, 2006). In addition, IndOOS provides a long-term, broad scale spatial and temporal context for short duration, geographically focused process studies, such as the Mirai Indian Ocean cruise for the Study of the MJO convection Onset (MISMO; Yoneyama et al, 2008), the Validation of the Aeroclipper System under Convective Occurrences (VASCO)-Cirene program (Duvel et al, 2008; Vialard et al, 2008a). The outline of the TRIO project was presented at the 2008 Indian Ocean Panel meeting in Bali, Indonesia. The present document has been submitted to the IOP in order to obtain an endorsement.

A key element of IndOOS is the RAMA (“Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction”) basin-scale moored buoy array (Fig. 10). There had been no plan until now for a coordinated, multi-national, basin-scale sustained mooring array like TAO/TRITON in the Pacific and PIRATA in the Atlantic. RAMA addresses the need for such an array. It is designed specifically for studying large-scale ocean-atmosphere interactions, mixed-layer dynamics, and ocean circulation related to the monsoons on intraseasonal to interannual and longer time scales. The planned array consists mainly of 38 surface moorings eight subsurface moorings (see box). The mooring at 8°S, 67°E was deployed during the course of the Cirene cruise in January 2007, and has been serviced in August 2008 by the Marion Dufresne, in the framework of the Vasco-Cirene project. The TRIO cruise, in early 2011, will also contribute to the RAMA array by deploying/servicing moorings in the SCTR region (most notably along 8°S). There is presently no mooring planned in the region of strong intraseasonal SST variability between Australia and Java. We plan to deploy a mooring there during TRIO (in collaboration with PMEL) for a process study of one or two years. This mooring may later be included in the RAMA design, after evaluation by the CLIVAR IOP.

3.2. Satellite programs Some satellite products will give useful information to complement the TRIO measurement campaign. Also, TRIO

can provide useful measurements to evaluate satellite products.

3.2.1. Megha-tropiques Megha-Tropiques (MT) is an Indo-French satellite project that aims at improving the knowledge of the water cycle

in the tropics. MT is schedule for launch in 2009. It will provide a high temporal resolution of the tropical band (more than 3.5 visibilities per day of each point of the zone situated between 22°S and 22°N). MT associates three radiometric instruments measuring simultaneously water vapour and condensed water (clouds and precipitations) profiles, and radiative fluxes. Estimates of surface winds and rain over the ocean will be also deduced from MT measurements.

The TRIO cruise will follow the Inter Tropical Convergence Zone (Fig. 3) and will sample the highly spatially variable structures of atmospheric convection, surface winds and rainfall. TRIO can provide radiosondes during MT overpasses in order to do some specific inversion algorithm verifications, for example in presence of dry layers. On the other hand, MT data will be very helpful to study the perturbation of the moisture and cloud profiles associated with different phase of the intraseasonal MJO events, especially for the dry phase preceding the triggering of large-scale organized convection. The high temporal repetition of MT measurements will give also invaluable information to better understand the evolution of deep convective cloud systems into tropical storms or cyclones.

3.2.2. Altika AltiKa is also an Indo-French satellite project. AltiKa is an altimeter with improved horizontal resolution and

accuracy. TRIO can contribute to evaluate the AltiKa products by providing CTDs or XCTDs during overpasses of the satellite, or by providing sections through mesoscale features like internal gravity waves or Rossby waves (cf section 2.5). TRIO will use AltiKa measurements in many ways. It will help to localise propagating Rossby Waves, to be studied during the cruise, but will also provide a large scale picture of SLA anomalies, in particular in response to MJO events.

3.2.3. SMOS ESA's Soil Moisture and Ocean Salinity (SMOS) mission has been designed to observe soil moisture over the

Earth's landmasses and salinity over the oceans.

TRIO can contribute to evaluate the salinity estimated by the ESA's Soil Moisture and Ocean Salinity (SMOS) mission. TRIO will indeed provide in-situ data in a highly perturbed tropical environment under the ITCZ. The ITCZ is a region where the fine spatial scales of the convection and rainfall, and periods of low winds, can lead to very fine salinity structures, both in the horizonal and vertical. Such a situation is a stringent test for the retrieval capacity of SMOS. At the same time, the strong salinity front suggested by the Cirene data is also a good test – along with the front of the western Pacific warm pool – of the SMOS capacities. On the other hand, the SMOS data will be precious to study


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the longitudinal position of this salinity front, the surface salinity signature of the MJO and the interannual variability. There is thus also a large potential benefit for TRIO from using SMOS data.

3.3. Link with other field experiments

3.3.1. SWICE The goal of the SWICE (South-West Indian Ocean Cyclone Experiment) project is to estimate the value of new data

from surface stations and spaceborne instruments to understand the relations between the tropical cyclones and their atmospheric and oceanic environment and to test the capacities of numerical models to represent the structure and evolution of wind and precipitation. In January and February 2011, the coordination of new satellite observations, dedicated airborne missions, enhanced radiosounding network, Aeroclipper and oceanic measurements will provide an unprecedented data set in this basin. While SWICE will focus mainly on atmospheric observations and modelling, TRIO will provide an opportunity to complete this by a more thorough examination of the Ocean. It is known, for example, that the heat content along the SCTR can influence the cyclone statistics in the SWIO (Xie et al., 2002). The RAMA array will provide a monitoring of these heat content analyses. The TRIO cruise will also provide a platform for supplementary observations in the SCTR region (a cyclogenesis region). Lagrangian platforms deployed from the ship (gliders, Argo profilers and possibly additional Aeroclippers: Duvel et al., 2008) will also provide an opportunity for direct observations along the cyclone track. We anticipate strong interactions between the TRIO and SWICE projects, both during the observation period and the analysis of the data.

3.3.2. Japanese cruises in late 2010 MISMO (P.I. K. Yoneyama) was a Japanese cruise dedicated to the study of the tropical convection and its

intraseasonal variability (Ocober-November 2006). The follow-up cruise to MISMO will be named CINDY (2 months in late 2011). Shortly after CINDY, another cruise (P.I. A. Murata) along 8°S with the same ship (2 months in late 2011 or early 2012) will focus on CO2 measuremens (CLIVAR/CO2 plan, WOCE I2 and I10 re-visit plan). Discussions are underway to promote interactions between the TRIO cruise and the two Japanese cruises (exchange of data, exchange of personnel to have some instruments present on both cruises roughly one year apart, etc…).

3.3.3. Coordinated planning with two other French cruises The R/V Atalante will be in the western Pacific in late 2010. Building on this opportunity, three geographically

coordinated cruises in late 2010 / early 2011 were proposed. The NiuGuini-Papua-Exp (PIs M-H. Radenac and C. Menkes) aims at understanding the pathways and water masses characteristics (including trace metals) feeding the EUC in the far western Pacific with emphasis on the coastal current systems off New Guinea, New Britain and New Ireland. INDOMIX (PI G. Madec) proposes to travel from Halmhera to Bali and to sample vertical mixing associated with tides in several basins in the Indonesian throughflow. The TRIO cruise would then start from Bali. In addition to the potential advantages in terms of planning of the schedule of Atalante, the coordination of the three cruises will diminish equipment transport costs for instruments used by several cruises (INSU/Meteo France Flux Mast, VMP5500 microstructure profiler, CTD, etc…). In addition, INDOMIX and TRIO have a common interest for mixing by tides.

4. TRIO Implementation plan The year 2009-2010 will be dedicated to the continuation of the Vasco-Cirene data analysis, and to the preparation

of the scientific exploitation of the TRIO measurements using both existing observations and modelling. The TRIO cruise will start in early 2011.

4.1. Analyses and preparation of the TRIO cruise

4.1.1. Vasco-Cirene data analysis The first set of publications using data from the Vasco-Cirene cruise (Vialard et al. 2008ab, Duvel et al. 2008,

McPhaden et al. 2008) will be complemented soon by publications on ongoing studies dealing with more in-depth scientific investigations. This effort will be pursued in 2009-2010 and will help to refine the TRIO scientific questions and cruise plans. The main ongoing research using Vasco-Cirene data are summarized below. These studies use Cirene data complemented by other data sources (e.g. satellite data, Argo data, etc…) and modelling approaches.

1. Processes of the upper ocean perturbation associated with intraseasonal MJO events (to follow up Vialard et al., 2008): important questions remain to be solved about the importance of non-local processes and the modulation of the upper ocean response by interannual anomalies of the thermocline depth.


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2. Seasonal heat budget in the SCTR using the 8°S, 67°E data to further analyse the intensity of the upwelling and how high surface SSTs are maintained in the SCTR.

3. Origin of the longitunal seasonal migration of the salinity front at 8°S (modelling actually suggests that zonal advection is largely the cause).

4. Processes explaining the dynamical and thermodynamical ocean anomalies observed during the 2006 dipole: what are the exact causes of the upper ocean temperature and salinity anomalies observed during Cirene? Can the vertical structure of the current anomalies be explained by the linear theory?

5. Ocean response to the passage of Dora, including the processes of SST changes, the cancellation of diurnal cycle, and the generation of inertial waves.

6. Impact of tidal mixing by breaking of internal gravity waves and of the extra mixing caused by salt fingering is underway.

7. Impact of radiometric against bulk SST in computing air-sea fluxes and impact of high frequency variations (inclucing the diurnal cycle) on these fluxes. Diurnal cycle of the upper ocean and its impact on the boundary layer.

8. Process of development and inhibition of the deep convection, role of dry layers.

4.1.2. Other existing observations and modelling The different questions above will be addressed using Cirene data and with other observational sources (satellite

data, Argo, XBT lines, balloons, ocean and atmosphere re-analyses…). Those themes will be pursued in 2009-2010 and will help to focus TRIO science questions and to improve the planning of the TRIO cruise.

In order to better understand the role of the spatial organisation of the convection on the intraseasonal variability, some tests will be done using different configurations (change of sensitive parameters in the physical parameterization) of an atmospheric GCM in coupled and forced simulations. This will help to define important processes deserving further careful measurements during TRIO. The different intraseasonal phases, and in particular the dry phase preceding the large-scale convective perturbation, will be studied in detail based on events detected using a Multivariate Local Mode Analysis of satellite data (OLR, precipitation, SST, SLA, DWL amplitudes) and interpreted based on meteorological re-analyses and in-situ data (including Cirene data).

Coupled experiments with the SINTEX-F model and satellite / re-analysis observation analysis are underway to study the effect of the interannual variability in the SCTR on the MJO. It seems that two categories of MJO (fast equatorial mode and slower mode at 10°S) exist and that their respective amplitude depends on interannual variations in the large-scale ocean-atmosphere state. The mechanisms for this selection are under study, but the diurnal cycle seems to play a strong role in favouring the slower variability in the SCTR. Other coupled experiments are underway to study the predictability of the SCTR (heat content, SST, and climatic impacts).

Similar to what has recently been done in the southwestern Pacific, cyclone potential indices will be computed in the southwestern Indian Ocean and compared to actual cyclone occurrences. This will also be done for regional simulations with WRF. These tools will then be used to study: A) The sensitivity of cyclones to variability in the large-scale environment like the MJO or the ocean structure in the SCTR and B) the effect of the cyclone activity on the SCTR structure.

An analysis of the air-sea flux interannual variability in the Indian Ocean and of its feedback, similar to the approach described in Guilyardi et al (2008) will be developed for the Indian ocean.

4.2. Design of the oceanographic cruise The details of the oceanographic cruise will be finalised in 2008. We however indicate below the main features of

the cruise. The departure port of the cruise will most likely be Padang Bai in Bali (8°30’ S, 115°30’E; arrival point of INDOMIX cruise and commonly used during the INSTANT program) and the end port will be Victoria in Mahé, Seychelles (4°30’S, 55°30’E). In the meantime, one ATLAS mooring will be deployed in the NWAB (at 12°S, 107°E) in collaboration with PMEL (as a temporary process study, that may later be incorporated in the design of RAMA). The R/V Atalante will then sail towards Seychelles following 8°S (the I02 WOCE line, for which there is data in late 1995 and for which data from the 2012 Mirai cruise will also be available, giving some access to interannual variability). TRIO will deploy / service several ATLAS moorings from the RAMA array on the way (the exact number will depend on the state of the array at a time closer to the cruise). The 67°E section (already visited during Cirene, after an IOD) will be repeated and its RAMA moorings will be serviced/deployed.


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The cruise measurement strategy will be as follows:

• Oceanic ship measurements

o Classical continuous measurements (sounder, thermosalinograph, S-ADCP, ship meteorological station) will be performed during the cruise.

o CTD stations + VMP 5500 microstructure measurements: one CTD+L-ADCP station down to the bottom of the ocean plus microstructure measurements autonomous profiler (ASIP) will be deployed every ~1° all along the trajectory of the cruise (a total of 90 stations). The deep measurements and water sample collection will allow an exchange of data policy with the Japanese cruise CINDY one year later. Higher resolution sampling will be adapted in regions of strong bottom topography (to explore if extra mixing linked to tides is observed). The sampling above (CTDs + TSG + L-ADCP) will be enough to describe the vertical structures associated with the salinity front.

o The information collected every 1° by the CTD stations will be completed by a denser sampling with XBT and XCTD data. These data will give a more detailed view of the ocean structure perturbations associated with different atmospheric forcings. This will be also useful with respect to the AltiKa mission. XCTDs / XBTs will be deployed every 1° at alternate positions with XBTs (about 90 XCTDs/XBTs to be deployed).

• Atmospheric ship measurements

o Radiosondes will be deployed at 00, 06 and 12 and 18 UTC during the whole cruise to contribute to improved diurnal analysis and atmospheric processes. A possibility is also to adjust some radiosondes timing to improve the time match with overpasses of Megha Tropiques satellite.

o The INSU/Météo France flux mast (already used during Cirene, optimised for Atalante, and already used during NiuGini-Papua and INDOMIX) will collect continuous measurements of the flux perturbations associated with mesoscale, synoptic and large perturbations related to the large-scale dynamical response to organized convection at intraseasonal time scale.

o Additional in situ, sounding and radiative measurements could be also performed depending on the implication of different international teams.

o Aeroclippers could be deployed from the ship in order to initiate the Aeroclipper measurement trajectories in targeted regions of high scientific interest (cyclogenesis, active MJO phase) (TBC depending on the Aeroclipper development plan and on the result of feasibility studies). If feasible, this approach could significantly enlarge the application field of the Aeroclipper system in regard to the deployment from Island or continental coats.

• Buoy and floater measurements

o Meridional structure of the ocean mixed layer perturbations and surface flux perturbations associated with the intraseasonal variability will be given by the ATLAS mooring (the RAMA meridional line at 80°E is complete and the one at 67°E will be completed during Cirene). This will ensure long time series along 67°E and 80°E in the southern hemisphere that will be exploited following (Vialard et al., 2008a; Foltz et al., in prep) to study the heat budget in the SCTR at intraseasonal to interannual timescales.

o PROVOR will be deployed during the cruise in order to maintain the nominal coverage of the Argo Array. Past experiments with Cirene PROVOR (deployed in 2004 and then in 2007) show that even when they are deployed in groups, the PROVOR disperse quickly and provide good coverage.

o Gliders could be used to performed measurements around selected ATLAS mooring in order to help in determining the perturbation of the heat balance related to different atmospheric forcings (cyclogenesis, MJO).

The TRIO cruise and the ATLAS mooring will give additional useful information on the ocean thermal structure and on the atmospheric profiles in the cyclogenesis region south of the equator. The supplementary radiosondes given by TRIO will be assimilated in the GTS and will participate in the radiosonde network reinforcement of SWICE, giving improved meteorological analysis and forecast in the region.

Rossby Waves: with the help of Altika data, a denser sampling will be performed in one of these structures to obtain the in situ structure of its physical / biogeochemical properties, up to now observed from sttelites only.

The overall duration of the cruise is estimated as follows. The total distance to be covered (Bali-NWAB mooring-section along 8°S-section at 67°E-Sychelles) is around 5200 nautical miles (i.e. 22 days at 10 knots). The maximum


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number of ATLAS moorings deployments/retrievals is 10. It takes about 12 hours to deploy or service an ATLAS mooring, and hence a maximum of 5 additional days. The ~90 CTD+L-ADCP stations to the bottom with microstructure profiler deployment every 1° should take ~3 hours each (11 days). About 5 longer stations will be performed (12h to 36hours; monitoring of the tidal cycle and longer mixing time series; study of the diurnal cycle): 8 days are provisioned for that. The total expected duration of the cruise is 46 days.

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