A numerical modelling study of the interannual variability in the Indian Ocean

A simple reductA1 gravity wind-driven ocean circulation model is used to study the interannual variability in the upper layer of the Indian Ocean (24°S-23°N and 3S°E-IIS0E). The monthly mean wind stress for the period 1977-1986 are used as a forcing in the model. The model reproduces most of the observed features of the annual cycle of the upper layer circulation in the Indian Ocean when was forced with the ten-year average monthly mean wind. The circulation features and the model upper layer thickness show considerable interannual variability in most part of the basin; in particular, the Somali Current, the basin wide southern hemisphere gyre, the Equatorial Currents and the gyres in the Bay of Bengal. Six consecutive years starting from 1978 to 1983 which include two bad monsoon years of 1979 and 1982 are chosen to study the interannual variability. February circulation field shows stronger Equatorial Counter Currents in bad monsoon years, whereas. the cunents north of Madagascar flowing up to the African coast are found to be stronger in good monsoon years. The southward return flow from the Southern Gyre in August is strong and more to southern latitudes in the bad monsoon years. The flow circulated eastward to form another eddy east of Southern Gyre. The basin wide gyre of the southern hemisphere (SH) shows less variability in two consecutive normal years than in contrasting years.

Numerical investigation of coastal circulation around India

A simple wind driven ocean circulation model with one active layer is used to simulate the coastal circulation around India. The close agreement of numerical results to that of the observed fields ind1cate the influence of wind on the coastal circulation. The northward currents along the west coast of India during winter months are dominated by remote forcing from Bay of Bengal; however the southward currents during summer months are less influenced by the remote forcing. The coastaly trapped Kelvin waves which give rise to the remote forcing response are found to be produced by the annual cycle in the local wind of the Bay of Bengal. Equatorial waves do not provide the correct phase of west coast circulation. The island chains of Maldive and Laccadive do not affect the model circulation significantly. But the exclusion of Sri Lanka from the model geometry significantly alters the circulation of southwestern Bay of Bengal during summer months. Some of these findings are already shown by sophisticated multilayer models, e.g., McCreary et al. 1993. However, some of these results are again reproduced here in order to highlight the significance of such simple model and hence the simple model is used for detail study.

A multi-resolution ocean simulation of the anthropogenic radiocarbon transient

<p>We have implemented <sup>14</sup>C and further abiotic tracers (<sup>39</sup>Ar, CFC-12, and SF<sub>6</sub>) into the state-of-the-art ocean circulation model FESOM2. Different to other global ocean circulation models, FESOM2 employs unstructured meshes with variable horizontal resolution. This approach allows for improvements in areas which are commonly poorly resolved in global ocean modelling studies such as upwelling regions, while keeping the overall computational costs still sufficiently moderate. Here, we present results of a transient simulation running from 1850-2015 CE tracing the evolution of the bomb radiocarbon pulse with a focus on the evolution of marine radiocarbon ages. In addition we explore the potential of <sup>39</sup>Argon to complement <sup>14</sup>C dating of marine waters.</p>

Effects of Wave-Induced Processes in a Coupled Wave–Ocean Model on Particle Transport Simulations

This study investigates the effects of wind–wave processes in a coupled wave–ocean circulation model on Lagrangian transport simulations. Drifters deployed in the southern North Sea from May to June 2015 are used. The Eulerian currents are obtained by simulation from the coupled circulation model (NEMO) and the wave model (WAM), as well as a stand-alone NEMO circulation model. The wave–current interaction processes are the momentum and energy sea state dependent fluxes, wave-induced mixing and Stokes–Coriolis forcing. The Lagrangian transport model sensitivity to these wave-induced processes in NEMO is quantified using a particle drift model. Wind waves act as a reservoir for energy and momentum. In the coupled wave–ocean circulation model, the momentum that is transferred into the ocean model is considered as a fraction of the total flux that goes directly to the currents plus the momentum lost from wave dissipation. Additional sensitivity studies are performed to assess the potential contribution of windage on the Lagrangian model performance. Wave-induced drift is found to significantly affect the particle transport in the upper ocean. The skill of particle transport simulations depends on wave–ocean circulation interaction processes. The model simulations were assessed using drifter and high-frequency (HF) radar observations. The analysis of the model reveals that Eulerian currents produced by introducing wave-induced parameterization into the ocean model are essential for improving particle transport simulations. The results show that coupled wave–circulation models may improve transport simulations of marine litter, oil spills, larval drift or transport of biological materials.

Convection and heat transfer in island (warm) wakes

The interaction between the incoming winds with high mountainous islands produces a wind-sheltered area in the leeward side, known as the atmospheric wake. In addition to weaker winds, the wake is also characterized by a clearing of clouds, resulting in intense solar radiation reaching the sea surface. As a consequence, a warm oceanic wake forms on the leeward side. This phenomenon detectable from space can extend 100 km offshore of Madeira, where the sea surface temperature can be 4⁰C higher than the surrounding oceanic waters. This study considers in-situ, remote sensing, and ocean circulation model data, to investigate the effects of the warm wake in the vertical structure of the upper ocean. To characterize the convective layer (25-70m) developing within the oceanic wake, 200 vertical profiles of temperature, salinity and turbulence were considered, together with the computation of the Density Ratio and Turner-angle. In comparison to the open-ocean water column, wake waters are strongly stratified with respect to temperature although highly unstable. The vertical profiles of salinity show distinct water parcels that sink and/or rise as a response to the intense heat fluxes. During the night, the ocean surface cools, leading to the stretching of the mixed layer which was replicated by the ocean circulation model. In exposed, non-wake regions however, particularly in the southeast and north coast of the island, the stretching of the mixed layer is not detectable.