Title: Response of the Indonesian Seas and its potential feedback to the Madden Julian Oscillation
Description: The impact of the Madden Julian Oscillation (MJO), a major source of intraseasonal variability in the tropical atmosphere, on the Indonesia Seas is investigated using satellite-derived, reanalysis and mooring data. The MJO footprint on the Indonesian Seas is evident from the surface layer into the pycnocline. In the surface, MJO air-sea heat fluxes govern the intraseasonal sea surface temperature (SST) variations. Within the pycnocline, the MJO reduces the transfer of the Pacific water to the Indian Ocean, the Indonesian Throughflow (ITF). In addition to the ocean’s response, the oceanic feedback to the MJO is also examined. Warmer SST in the Indonesian Seas during the suppressed phase of the MJO promotes the MJO convective phase to propagate eastward over the maritime continent (MC). Intraseasonal SST variation accounts for 55 - 60% of the total non-seasonal SST variance across the Indonesian Seas. It is most energetic in Banda and Timor Seas, with its standard deviation varying between 0.4 – 0.5°C.

Coupled to the MJO surface fluxes, the intraseasonal SST exhibits stronger variation in boreal winter than in summer. A slab ocean model indicates that MJO surface heat fluxes account for 69-78% of the intraseasonal SST variability. The SST increases by 1.1° - 2°C, on average, in response to intense surface heating and weak winds over the suppressed (dry) MJO phase, and then decreases by 1.8° - 2.1°C over the course of the ensuing MJO active phase that is characterized by enhanced convective cooling and westerly wind bursts. Intraseasonal variability is also significant in the Sulawesi Sea SST, but it is mostly derived from eddies and local winds. Over the period 1980 - 2012, we observe 86 significant MJO (Real-time Multi variate MJO index > 1) events occurring in the Indian Ocean, of which 51 events achieve eastward propagation (EP) over the MC, while 35 events attentuate in the eastern Indian Ocean, or show no propagation (NP) over the MC. Eastward propagation (EP) MJO events occur more frequently during La Niña years than during El Niño years.

Analyses of SST across the Indonesian Seas during the suppressed phase of the MJO events indicate that the SST in Java, Banda, and Timor Seas attributed to the EP MJO events is warmer by 0.5oC that associated with the NP MJO events. The warmer SST corresponds with enhanced surface latent heat flux, sensible heat flux, and low-level moisture in the atmospheric boundary layer, driven by diurnal activity. The EP MJO events are more frequent during La Niña, as the SST response to MJO events is influenced by the thermocline depth: shallower thermocline during El Niño enables cooler subsurface water under the MJO forcing to reduce SST that then attenuates MJO activity, with deeper thermocline of La Niña having the opposite outcome. Moored velocity data in Makassar Strait between 2004 – August 2011 and August 2013 – August 2015 document substantial direct impacts of the MJO on the ITF, particularly with the surface layer (< 80 m ). A composite of the along-strait velocity within the surface layer for 10 MJO events observed during the observational period exhibits strong northward velocity within days, following the peak of MJO wind stress.

The MJO forces both northward along-strait pressure gradient and the resultant of northward wind stress and turbulent stress at the base of the surface layer that, together with the seasonal forcing, maintain the reduction or even reversal of the ITF southward transport on timescales of 1-3 months during boreal winter.