Wind Limits on Rain Layers and Diurnal Warm Layers. Issue 2 (7th February 2019)

• Record Type: Journal Article
• Authors: Thompson, Elizabeth J.; Moum, James N.; Fairall, Christopher W.; Rutledge, Steven A.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 2(2019)
• Page Start: 897
• Page End: 924
• Publication Date: 2019-02-07
• Subjects: ocean boundary layer -- tropical meteorology -- Madden‐Julian Oscillation (MJO) -- air‐sea interaction -- ocean mixed layer -- rain; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2018JC014130
• Languages: English
• ISSNs: 2169-9275

Abstract

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 h ̂ S, stable layer depth, and U ̂ S, the maximum U 10 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 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. 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, we more precisely determined the wind speed below which 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. Key Points: Rain layers were detected at values of U 10 up to 9.8 m s −1, while diurnal warm layers were only detected at U 10 < 7.6 m s −1 (99th percentile values) The formation and persistence of stable layers at depths ≤ 5 m were well estimated by U 10 and the surface buoyancy flux, B Rain layers (and their combinations with diurnal warm layers) formed often in disturbed and active MJO periods before westerly wind bursts