Diurnal Signal in Turbulence During SW Monsoon in the Bay of Bengal
Diurnal Signal in Turbulence During SW Monsoon in the Bay of Bengal
Abstract:
A yearlong record of moored turbulence, current, temperature and conductivity in the northern Bay
of Bengal (18N, 89.5E) shows a strong, but intermittent, diurnal signal of turbulence with the turbulent
diffusivity (Kt) increasing by more than two orders of magnitude between 40 and 60 m. This diurnal
signal, which is observed only below the upper mixed layer, appears twice during the SW monsoon in May
and July with a ∼30-day persistence. The May period shows a peak in Kt (without a coincident increase
in temperature stratification) near local 1200, roughly 6-8 hours after a diurnal peak in local winds. In
contrast, the July period is characterized by 1.5-2 orders of magnitude variability in Kt with no consistent
correlation with variability in the winds. This signal also persists through a >80 km/h cyclonic storm.
This second diurnal signal is also not related to ancillary fields or internal tidal height, and hence may
be biogenic. The strong salinity stratification at this time of year suppresses turbulence below the mixed
layer. Hence, if this second diurnal signal is biological in origin, and provided it is not a localized effect
associated with the mooring, it could be an important albeit small source for mixing of upper ocean heat
into the deeper thermocline at this time of year.
of Bengal (18N, 89.5E) shows a strong, but intermittent, diurnal signal of turbulence with the turbulent
diffusivity (Kt) increasing by more than two orders of magnitude between 40 and 60 m. This diurnal
signal, which is observed only below the upper mixed layer, appears twice during the SW monsoon in May
and July with a ∼30-day persistence. The May period shows a peak in Kt (without a coincident increase
in temperature stratification) near local 1200, roughly 6-8 hours after a diurnal peak in local winds. In
contrast, the July period is characterized by 1.5-2 orders of magnitude variability in Kt with no consistent
correlation with variability in the winds. This signal also persists through a >80 km/h cyclonic storm.
This second diurnal signal is also not related to ancillary fields or internal tidal height, and hence may
be biogenic. The strong salinity stratification at this time of year suppresses turbulence below the mixed
layer. Hence, if this second diurnal signal is biological in origin, and provided it is not a localized effect
associated with the mooring, it could be an important albeit small source for mixing of upper ocean heat
into the deeper thermocline at this time of year.