MJO variations in salinity and barrier-layer thickness in the eastern equatorial Indian Ocean

Data from Argo profiling floats in the tropical Indian Ocean and a mooring at 90E,0N are used to examine subseasonal variations in upper ocean salinity and barrier-layer thickness (BLT) during boreal winter. In the eastern equatorial Indian Ocean, subseasonal variations in BLT are energetic. However, composites used to isolate the Madden-Julian Oscillation (MJO) component of the subseasonal signal reveal that, on average, the MJO anomaly in BLT is negligible despite large swings in both the mixed-layer depth and the isothermal-layer depth. This discrepancy is likely due to the diversity of individual MJO events: BLT is shown to respond rapidly (within a day) to strong wind or rain forcing, so even subtle differences in the phasing and strength of MJO-related atmospheric anomalies can have a very different effect on the barrier layer.

We also evaluate the effect of the barrier layer on the upper ocean response to MJO forcing. When the barrier layer is thick, entrainment cooling during the MJO is reduced, so the MJO drives a weaker sea surface temperature anomaly. This suggests that modulation of BLT can have significant consequences for the response of the upper ocean to the MJO, and hence, potentially, for feedbacks of the ocean onto the atmosphere on MJO timescales.