Multiscale Variability of the Atmospheric Mixed Layer during DYNAMO

Observations taken during the 1992-93 TOGA COARE have shown that the atmospheric mixed layer in the tropical western Pacific undergoes considerable variation on time scales ranging from a month (the time scale of the Madden-Julian Oscillation or MJO) down to the diurnal time scale. While the mean mixed-layer depth during COARE was 512 m, there were large excursions from this mean as it ranged from several hundred meters in recovering downdraft wakes to nearly 1000 m during the westerly wind burst period following the convectively active phase of the MJO. Since there is mounting evidence that accurate prediction of the MJO is dependent upon realistic representation of cloud populations, and convective clouds emanate from the top of the boundary layer, it is important to understand how the mixed layer varies over the range of convective conditions observed within the MJO life cycle.

The 2011-12 DYNAMO field campaign, along with its companion experiments CINDY and AMIE, has offered unique opportunity to explore the atmospheric mixed layer variation in association with the MJO in a different ocean basin. Since the Indian Ocean is frequently the initiation site for the MJO and air-sea interaction is regarded as an important factor in the initiation process, it is of great interest to know how the mixed layer varies in this region over the MJO time scale.

Efforts are underway to investigate the multiscale variability of the atmospheric mixed layer at all oceanic sites during DYNAMO. Preliminary analysis of soundings from Leg 3 of the R/V Revelle for the period 10 November-3 December reveals a mean mixed-layer depth of 510 m, consistent with COARE results. This cruise experienced a strong MJO event in mid-to-late November. It was accompanied by shallow mixed layers in recovering precipitation downdraft wakes followed by very deep (~1000 m) mixed layers during the period of strong surface winds following the heaviest precipitation, also consistent with COARE findings. Further work is ongoing to investigate the behavior of the mixed layer over a longer period at the Revelle and at other DYNAMO sites.