Radial and vertical velocity field in a steady state symmetric tropical storm

ABSTRACT. A method to construct a consistent structure of steady state symmetric tropical storms from a few known values of temperature anomaly in the centre and around it has been developed. The role of kinematic eddy coefficient of viscosity in producing the transverse circulation in a tropical storm has been tested and discussed. The well known features and characteristics of a tropical storm, such as, eyewall, sinking motion, inside the eyewall, low-level radial inflow and high level outflow are well produced in the model. The computation shows that there is an increase of transverse circulation with increase of the magnitude of eddy coefficient. In the boundary layer, the vertical eddy coefficient plays more important role than the radial eddy coefficient; while in the upper layer the latter is much more important than the former. It has also been found that in absence of radial exchange coefficient, there can be no sinking motion in the central region of the storm. The magnitude of radial and vertical wind in the eye region is more sensitive to the variation of radial eddy coefficient. In addition to the eddy coefficients, transverse circulations also depend upon the tangential wind distribution above the boundary layer.

On the Upper-Ocean Vertical Eddy Heat Transport in the Kuroshio Extension. Part II: Effects of Air-Sea Interactions

Encountering of energetic ocean eddies and atmosphere storms makes the winter Kuroshio extension a hotspot for air-sea interactions. This second part investigates the regulation of vertical eddy heat transport QT in the winter Kuroshio extension mixed layer by different types of air-sea interactions, including the atmosphere synoptic forcing, eddy thermal feedback resulting from eddy-induced surface heat flux anomalies, and eddy current feedback from eddy current’s imprint on wind stress.Atmosphere synoptic forcing modulates intra-seasonal variation of QT by boosting its component contributed by the turbulent thermal wind balance during strong cooling events associated with intense winds. In addition, the magnitude of QT is influenced by the direction of synoptic wind stress primarily via , with the latter exhibiting enhancement both in the downfront- and upfront-wind forcing. Enhanced by the downfront-wind forcing is attributed to increased turbulent vertical viscosity and front intensity caused by the destabilizing wind-driven Ekman buoyancy flux, whereas interaction of uniform wind stress with smaller turbulent vertical viscosity at the front center than periphery (a so-called internal Ekman pumping) accounts for the increased in the upfront-wind forcing. The eddy thermal feedback reduces QT significantly through weakening the fronts. In contrast, the eddy current feedback exerts negligible influences on QT, although it weakens eddy kinetic energy (EKE) evidently. This is due to the much reduced effect of eddy current feedback in damping the fronts compared to EKE and also due to the compensation from Ekman pumping induced by the eddy current feedback.

Wave-Modified Ekman Current Solutions for the Time-Dependent Vertical Eddy Viscosity

This study aimed to highlight a general lack of clarity regarding the scale of the temporal averaging implicit in Ekman-type models. Under the assumption of time and depth-dependent eddy viscosity, we present an analytical Fourier series solution for a wave-modified Ekman model. The depth dependence of eddy viscosity is based on the K-Profile Parameterization (KPP) scheme. The solution reproduces major characteristics of diurnal variation in ocean velocity and shear. Results show that the time variability in eddy viscosity leads to an enhanced mean current near-surface and a decrease in the effective eddy viscosity, which finally results in an intensified near-surface shear and wakes a low-level jet flow. Rectification values are dominated by the strength of diurnal mixing, and partly due to the nonlinear depth dependence of the eddy viscosity.

Does the interior of the ocean hide a major part of the eddy field?

<p>In ocean research, mesoscale eddies typically are detected through surface signatures based on satellite data. The assumption is that most eddies are surface intensified and have a vertical structure consistent with a surface intensified mode. However, in-situ eddy observations, especially in the tropical oceans, showed that the vertical eddy structure is often more complex than previously assumed (higher baroclinic modes), and a diverse subsurface eddy field is present, which does not show any surface signatures at all. Our objective here is a first step towards a quantification of the occurrence of subsurface relative to surface eddies. To do this, we use an actively eddying model to compare the subsurface eddy field to its surface signatures in order to be able to estimate which vertical eddy structures prevail and how much of the eddy field is hidden in the subsurface. In addition, the model results are compared against an unprecedented assemblage of observations of subsurface eddies in the tropical oceans. In a first step we focus on eddies in the model that are detectable at the surface for more than 120 days. We found that around 60 % of the detected eddies have a vertical structure associated with a surface intensified mode as previously assumed which are characterized by a strong surface signature. Around 40 % of the eddy field have a vertical structure associated to a higher baroclinic mode. They are often called “intrathermocline” eddies and are characterized by a rather weak surface signature. In a second step we track subsurface eddies (lifetime > 120 days) in the model by identifying density layer thickness anomalies and connect them with possible surface signatures. Around 30 % of the total eddy field of the model, are hidden in the subsurface with no detectable surface signature. In conclusion, our results show that subsurface eddies form a substantial contribution to the total eddy field. Consequently it is difficult to estimate the impact of the eddy field on the ocean when only working with surface based satellite data.</p>

On the Upper-Ocean Vertical Eddy Heat Transport in the Kuroshio Extension. Part I: Variability and Dynamics

Oceanic eddies play a crucial role in transporting heat from the subsurface to surface ocean. However, dynamics responsible for the vertical eddy heat transport QT have not been systematically understood, especially in the mixed layer of western boundary current extensions characterized by the coincidence of strong eddy activities and air–sea interactions. In this paper, the winter (December–March) QT in the Kuroshio Extension is simulated using a 1-km regional ocean model. An omega equation based on the geostrophic momentum approximation and generalized to include the viscous and diabatic effects is derived and used to decompose the contribution of QT from different dynamics. The simulated QT exhibits a pronounced positive peak around the center of the mixed layer (~60 m). The value of QT there exhibits multi-time-scale variations with irregularly occurring extreme events superimposed on a slowly varying seasonal cycle. The proposed omega equation shows good skills in reproducing QT, capturing its spatial and temporal variations. Geostrophic deformation and vertical mixing of momentum are found to be the two major processes generating QT in the mixed layer with the former and the latter accounting for its seasonal variation and extreme events, respectively. The mixed layer instability and the net effect of frontogenesis/frontolysis contribute comparably to the geostrophic deformation induced QT. The contribution of QT from vertical mixing of momentum can be understood on the basis of turbulent thermal wind balance.