Wind Limits on Rain Layers and Diurnal Warm Layers
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Wind Limits on Rain Layers and Diurnal Warm Layers
  • Published Date:

    2019

  • Source:
    Journal of Geophysical Research: Oceans,
    124, 897–924
Filetype[PDF-4.49 MB]


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  • Description:
    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 urn:x-wiley:21699275:media:jgrc23188:jgrc23188-math-0001, stable layer depth, and urn:x-wiley:21699275:media:jgrc23188:jgrc23188-math-0002, the maximum U 10 for which stratification should persist at urn:x-wiley:21699275:media:jgrc23188:jgrc23188-math-0003 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 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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