Wind limits on rain layers and diurnal warm layers

Elizabeth Jennifer Thompson, NOAA PSL, Physical Sciences Lab, Boulder, United States, Jim Moum, Oregon State Univ, Corvallis, OR, United States, Chris W Fairall, NOAA Boulder, Boulder, CO, United States and Steven A Rutledge, Colorado State University, Department of Atmospheric Science, Fort Collins, CO, United States
Abstract:
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 (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 h_S, stable layer depth, and U_S, the maximum U10 for which stratification should persist at h_S 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 suppressed MJO periods, (U10 ≤ 8 m/s with strong insolation), RLs and RL-DWLs were rare while DWLs occurred daily. During disturbed and active MJO periods, (U10 ≤ 8 m/s 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 with steady rain), RLs formed infrequently and DWLs were not detected.

Citation: Thompson, E. J., Moum, J. N., Fairall, C. W., & Rutledge, S. A. (2019). Wind limits on rain layers and diurnal warm layers. Journal of Geophysical Research: Oceans, 124, 897–924. https://doi.org/10.1029/ 2018JC014130