Wind Limits on Rain Layers and Diurnal Warm Layers

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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 U 10 up to 9.8 m s −1 , while DWLs were only detected at U 10  < 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 (Formula presented.), stable layer depth, and (Formula presented.), the maximum U 10 for which stratification should persist at (Formula presented.) 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 U 10 ). This suggests a means to determine the presence of ocean stable layers at depths ≤ 5 m from U 10 and B. Near-surface stratification varied throughout two Madden-Julian Oscillation (MJO) cycles. In suppressed MJO periods, (U 10  ≤ 8 m s −1 with strong insolation), RLs and RL-DWLs were rare while DWLs occurred daily. During disturbed and active MJO periods, (U 10  ≤ 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, (U 10  = 7–17 m s −1 with steady rain), RLs formed infrequently and DWLs were not detected.

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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(2), 897–924. https://doi.org/10.1029/2018JC014130

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