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Wind Limits on Rain Layers and Diurnal Warm Layers
Author(s) -
Thompson Elizabeth J.,
Moum James N.,
Fairall Christopher W.,
Rutledge Steven A.
Publication year - 2019
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1029/2018jc014130
Subject(s) - stratification (seeds) , environmental science , madden–julian oscillation , atmospheric sciences , mixed layer , climatology , buoyancy , geology , convection , meteorology , physics , seed dormancy , botany , germination , quantum mechanics , dormancy , biology
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 ofh ̂ S , stable layer depth, andU ̂ S , the maximum U 10 for which stratification should persist ath ̂ 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 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.