Kjell Arild Orvik

Interannual variability of the Atlantic inflow (AI) into the Nordic Seas is studied using moored current and temperature measurements in the Norwegian Atlantic Current during the 25‐year period 1995–2020. We show that the properties of the AI are strongly connected to the dynamics of the Northern Atlantic and demonstrate the robustness of the average winter wind field in driving the variability with 3‐month winter maxima in the range of (40–50) cms⁻¹. In that sense, the zero‐wind‐stress‐curl (ZWSC)‐lines aligned northeastward are crucial. Occasional events with slowdown of winter‐maxima are connected to a meridional and zonal shift of the ZWSC‐lines, and not the strength of the wind. This is manifested in a correlation of 0.3 versus the North Atlantic Oscillation (NAO) winter‐index, demonstrating that the NAO‐index is insufficient to explain the AI‐variability. The records show no long‐term‐trend over the 25‐year period 1995–2020, regarding volume flux, temperature, nor temperature flux.

An extraordinary warming of the Atlantic inflow to the Norwegian Sea toward the Arctic is observed over the last decade. In light of that we investigate the seasonal and interannual variations of the heat flux in the eastern Norwegian Atlantic Current (NwASC) using moored temperature (T)‐ and current (v) measurements in the Svinøy section (62°N); 1995–2005. By splitting the heat flux anomaly (vT)′ into v′ and T′ (, : 10‐year means; v′T′ negligible), we examine the relative contributions from variations in v and T. The dominating seasonal signal coincides almost completely with v′, whilst T′ has a minor modulating effect. On the interannual timescale the heat‐flux anomaly also coincides with v′, while T′ contributes significantly to the long term trend. There is a downward 10‐year trend in the velocity field of 3.9 cm s⁻¹ (12%), combined with a 1°C increase in temperature, resulting in a constant heat flux.

The Norwegian Atlantic Current (NwAC) as a poleward eastern boundary current is to be considered as the northern limb of the Meridional Overturning Circulation in the North Atlantic (MOC). It transports warm and saline Atlantic water (AW) northward toward the Arctic Ocean, before cooling and mixing with cold and low saline water masses return it to the North Atlantic to contribute in the MOC. In this study we focus on the AW in the Lofoten Basin (LB) of the Norwegian Sea (NS), where it occupies a wider and deeper domain than farther south and north. It comprises the major heat‐reservoir in the NS with contact with the atmosphere, and as such is an important area for cooling and buoyancy loss. We show that the deepening of the AW is mainly caused by the reduced northward flow in the LB, manifested in the deep counter‐current encountered by the Polar frontal jet. We demonstrate this effect by using an active reduced gravity model with a topographically‐steered deeper flow. To achieve maintenance of volume transport, the model shows that observed differences must be balanced by an oppositely‐directed deeper flow in the LB. The ocean appears to achieve maintenance of volume transport of AW due to buoyancy loss and varying deeper currents, by adjusting the vertical extension of AW, resulting in a substantial deepening in the LB. Based on the good fit between the model and observations, the suggested mechanism appears to be credible and feasible.

The Norwegian Atlantic Current (NwAC) through the Norwegian Sea serves as a conduit of warm and saline Atlantic water from the North Atlantic to the Arctic Ocean, an important factor for climate and ecology. In this study, we concentrate on how the North Atlantic wind stress curl (NAWSC) affects interannual variability on the major branch of the NwAC—the Norwegian Atlantic Slope Current (NwASC). Based on wind stress data from the NCEP reanalysis and estimated volume transport of the NwASC from current records during 1995–2003 in the Svinøy section (62°N), our analysis shows that the volume transport in the NwASC exhibits a maximum correlation of 0.88 with the zonally integrated NAWSC at 55°N 15 months earlier. Our findings reveal the NAWSC to be a major forcing for interannual variability of the NwASC in the range of 3.0‐ to 5.3 Sv (Sv = 10⁶ m³ s⁻¹). The 15 month time lag appears to be in accordance with a forced baroclinic Rossby wave in response to the local Ekman pumping changing the baroclinicity and strength of the North Atlantic Current (NAC). After being converted to a nearly barotropic shelf edge current along the Irish‐Scottish shelf through interaction with bottom topography, it appears as a barotropic response downstream in the Svinøy section. This result suggests the possibility of predicting conditions influenced by the NwASC more than a year in advance, using the NAWSC as a proxy.

The aim of the paper is to validate a particular model formulation in the study of wind-induced response in a wide, rotating channel with varying bottom topography. The model consists of a continuous stratified upper layer joined to a homogeneous lower layer. For this density profile, standard normal mode solution techniques can be extended to forced models with sloping bottoms in order to separate the vertical dependency. The governing equations are then reduced to a time dependent problem by expanding across channel variations in Fourier series and applying periodic boundary conditions along the channel. An initial-value problem is solved numerically, using an eigenmode solution technique starting from a state of rest, where a constant wind stress is suddenly imposed and then switched off after a certain period of time. The eigenmode solution technique is found to be applicable in the examination of the coupling between vertical modes and the analysis has resulted in a simplified coupling pattern for this specific two layer model. Modified baroclinic eigenmodes are introduced in a superposition of each baroclinic and the barotropic mode, according to the eigenvalue problem. For these modes the interaction amongst baroclinic modes is found to be insignificant while the barotropic eigenmodes are not found to be affected by the stratification. For varying bottom topography these modified baroclinic modes will reflect both stratification and topography. Model simulations show that the flow pattern is dominated by long period barotropic oscillations as topographic Rossby waves. The waves show a dominance in the second lateral mode, and they are less affected by the stratification. The barotropic response to the wind is mirrored in internal displacements and shows long periodic oscillations corresponding to the topographic waves. The use of a bottom Ekman layer has shown that bottom friction mainly influences on long-period barotropic modes. The baroclinic modes are damped by internal friction. DOI: 10.1034/j.1600-0870.1993.t01-1-00003.x

Topographic effects in stratified flows resolved by a spectral method