How spice is stirred in the Bay of Bengal

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Spiro Jaeger, Gualtiero
MacKinnon, Jennifer A.
Lucas, Andrew J.
Shroyer, Emily L.
Nash, Jonathan D.
Tandon, Amit
Farrar, J. Thomas
Mahadevan, Amala
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Ocean dynamics
Water masses/storage
In situ oceanic observations
Spectral analysis/models/distribution
The scale-dependent variance of tracer properties in the ocean bears the imprint of the oceanic eddy field. Anomalies in spice (which combines anomalies in temperature T and salinity S on isopycnal surfaces) act as passive tracers beneath the surface mixed layer (ML). We present an analysis of spice distributions along isopycnals in the upper 200 m of the ocean, calculated with over 9000 vertical profiles of T and S measured along ~4800 km of ship tracks in the Bay of Bengal. The data are from three separate research cruises—in the winter monsoon season of 2013 and in the late and early summer monsoon seasons of 2015 and 2018. We present a spectral analysis of horizontal tracer variance statistics on scales ranging from the submesoscale (~1 km) to the mesoscale (~100 km). Isopycnal layers that are closer to the ML-base exhibit redder spectra of tracer variance at scales ≲10 km than is predicted by theories of quasigeostrophic turbulence or frontogenesis. Two plausible explanations are postulated. The first is that stirring by submesoscale motions and shear dispersion by near-inertial waves enhance effective horizontal mixing and deplete tracer variance at horizontal scales ≲10 km in this region. The second is that the spice anomalies are coherent with dynamical properties such as potential vorticity, and not interpretable as passively stirred.
Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 50(9), (2020): 2669-2688, doi:10.1175/JPO-D-19-0077.1
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Jaeger, G. S., MacKinnon, J. A., Lucas, A. J., Shroyer, E., Nash, J., Tandon, A., Farrar, J. T., & Mahadevan, A. (2020). How spice is stirred in the Bay of Bengal. Journal of Physical Oceanography, 50(9), 2669-2688.
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