The surface diurnal warm layer in the Indian Ocean during CINDY/DYNAMO

Adrian Matthews, Dariusz Baranowski, Karen Heywood, Piotr Flatau, Sunke Schmidtko

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A surface diurnal warm layer is diagnosed from Seaglider observations, and develops on half the days in the CINDY/DYNAMO Indian Ocean experiment. The diurnal warm layer occurs on days of high solar radiation flux (>80 W m-2) and low wind speed (<6 m s-1), and preferentially in the inactive stage of the Madden-Julian Oscillation. Its diurnal harmonic has an exponential vertical structure with a depth scale of 4-5 m (dependent on chlorophyll concentration), consistent with forcing by absorption of solar radiation. The effective sea surface temperature (SST) anomaly due to the diurnal warm layer often reaches 0.8°C in the afternoon, with a daily mean of 0.2°C, rectifying the diurnal cycle onto longer time scales. This SST anomaly drives an anomalous flux of 4 W m-2 that cools the ocean. Alternatively, in a climate model where this process is unresolved, this represents an erroneous flux that warms the ocean. A simple model predicts a diurnal warm layer to occur on 30-50% of days across the tropical warm pool. On the remaining days, with low solar radiation and high wind speeds, a residual diurnal cycle is observed by the Seaglider, with a diurnal harmonic of temperature that decreases linearly with depth. As wind speed increases, this already weak temperature gradient decreases further, tending towards isothermal conditions.
Original languageEnglish
Pages (from-to)9101–9122
Number of pages22
JournalJournal of Climate
Issue number24
Publication statusPublished - Dec 2014

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