Wind Limits on Rain Layers and Diurnal Warm Layers

Stratification of the upper few meters of the ocean limits the penetration depth of wind mixing and the vertical distribution of atmospheric fluxes. Significant density stratification at depths ≤ 5 m was observed in 38% of a 2-month data set from the central Indian Ocean collected during the DYNAMO experiment (Dynamics of the MJO, Madden-Julian Oscillation). Diurnal warm layers (DWLs) formed by solar heating populated 30% of the data set and rain layers (RLs) populated 16%. Combined contributions from rain and insolation formed RL-DWLs in 9% of the data set. RLs were detected at values of U10 up to 9.8 m s , while DWLs were only detected at U10 < 7.6 m s 1 (99th percentile values), symptomatic of the greater buoyancy flux provided by moderate to high rain rate compared to insolation. From the ocean friction velocity, u*w, and surface buoyancy flux, B, we derived estimates of bhS, stable layer depth, and b US, the maximum U10 for which stratification should persist at bhS for fixed B. These estimates predicted (1) 36 out of 44 observed stratification events (88% success rate) and (2) the wind limits of these events, which are considered to be the 99th percentile values of U10). This suggests a means to determine the presence of ocean stable layers at depths ≤ 5 m from U10 and B. Near-surface stratification varied throughout two Madden-Julian Oscillation (MJO) cycles. In suppressedMJO periods, (U10 ≤ 8m s 1 with strong insolation), RLs and RL-DWLs were rare while DWLs occurred daily. During disturbed and active MJO periods, (U10 ≤ 8 m s 1 with increased rain and cloudiness), multiple RLs and RL-DWLs formed per day and DWLs became less common. When westerly wind bursts occurred, (U10 = 7–17 m s 1 with steady rain), RLs formed infrequently and DWLs were not detected. Plain Language Summary We found that rainfall and clear skies often led to stabilization of the upper 5 m of the central Indian Ocean, except during strong winds. Near-surface stable layers impact the density and mixing of the ocean because rain water and near-surface water heated by the Sun are lighter than typical ocean water, which is relatively cooler and saltier. Stable layers are important to understand because they influence sea surface temperature and ocean heat content, which impact weather and climate. Compared to previous studies, wemore precisely determined the wind speed belowwhich stable layers form and above which they do not. This will hopefully aid researchers and forecasters tasked with predicting tropical weather, climate, and ocean processes. Prior to this study, it was unclear how often ocean stable layers formed during each phase of the Madden-Julian Oscillation (MJO), a major tropical phenomenon that impacts weather and climate around the world. We found that rain layers (and their combinations with diurnal warm layers) occurred most often during disturbed and active MJO periods prior to wind bursts. Diurnal warm layers tended to form in earlier stages of the MJO, in its suppressed and disturbed periods. 1. Background and Motivation Near-surface ocean stabilization can be achieved by either heat or freshwater inputs, which suppress subsurface mixing and confine subsequent surface inputs of heat, momentum, and freshwater to a shallow nearsurface layer (Anderson et al., 1996; Asher et al., 2014; Drushka et al., 2016; Fairall, Bradley, Godfrey, et al., 1996; Fairall et al., 1997, 2006; Miller, 1976; Price, 1979; Smyth et al., 1996a, 1996b; Soloviev & Lukas, 1996; Soloviev & Vershinsky, 1982; Wijesekera & Gregg, 1996; Wijesekera et al., 1999; You, 1995). Near-surface stable layers can in turn impact sea surface temperature and salinity (SST and SSS). Thermal stratification produces diurnal warm layers (DWLs), and freshwater stratification produces rain layers (RLs). RLs occur often in tropical oceans where precipitation is high and wind speeds are relatively low (Drushka et al., 2016; Lukas & Lindstrom, 1991). However, the wind limits on RLs and the prevalence of RLs throughout the life cycle of the Madden-Julian Oscillation (MJO) are unreported. On 30to 60-day time scales, the MJO explains the majority THOMPSON ET AL. 1 Journal of Geophysical Research: Oceans