The ocean’s response to stochastic atmospheric forcing

This thesis presents a series of investigations of the impact of high-frequency and small-scale atmospheric forcing on the ocean. Starting with a case study in the central Arabian Sea, a one-dimensional ocean mixed layer model is used to examine the ocean response to observed sub-daily atmospheric variability. Results show that including sub-daily atmospheric variability lowers the daily-mean sea surface temperature (SST) and damps its variability, but has little systematic effect on SST diurnal variability. This research is generalised by considering high-frequency variability via a stochastic component embedded in the atmospheric forcing and undertaking an ensemble of mixed-layer (ML) model simulations to explore the role of ML variation in generating a rectification effect on the SST. Different ML variations are engineered by tuning the solar penetration depth and comparing to a slab model where the ML depth is fixed. The SST rectification only emerges when the ML depth variation is included, and its magnitude is controlled by the background ML heat capacity that is associated with the solar penetration depth. Finally, we use a novel stochastic parameterization to represent spatially coherent mesoscale weather systems in atmospheric forcing fields. The stochastic parameterization adds high-frequency meso-scale variability to improve the ‘effective resolution’ of the atmospheric forcing fields and ‘fix’ the kinetic energy spectra. We conduct ocean model simulations with and without this realistic atmospheric forcing and find that the addition of the mesoscale forcing leads to coherent patterns of change in the SST and ML depth, which leads to statistically significant increases in transport in the subtropical and subpolar gyres in North Atlantic and in the Atlantic Meridional Overturning Circulation (AMOC). An elevated northward heat transport across the gyre boundary, driven by the enhanced subtropical gyre, slows down the subpolar gyre after about 10 years and slightly weakens the AMOC. This thesis illustrates that the high-frequency and mesoscale atmospheric variability can not only modulate local ocean status via one-dimensional response via the influence on the surface ML depth, but also excite non-local response of the circulation patterns that potentially influences the broader-scale ocean-atmosphere feedbacks