Nonstationary westward translation of nonlinear frontal warm-core eddies

For the first time, an analytical theory and a very high-resolution, frontal numerical model, both based on the unsteady, nonlinear, reduced-gravity shallow water equations on a beta plane, have been used to investigate aspects of the migration of homogeneous surface, frontal warm-core eddies on a beta plane. Under the assumption that, initially, such vortices are surface circular anticyclones of paraboloidal shape and having both radial and azimuthal velocities that are linearly dependent on the radial coordinate (i.e., circular pulsons of the first order), approximate analytical expressions are found that describe the nonstationary trajectories of their centers of mass for an initial stage as well as for a mature stage of their westward migration. In particular, near-inertial oscillations are evident in the initial migration stage, whose amplitude linearly increases with time, as a result of the unbalanced vortex initial state on a beta plane. Such an initial amplification of the vortex oscillations is actually found in the first stage of the evolution of warm-core frontal eddies simulated numerically by means of a frontal numerical model initialized using the shape and velocity fields of circular pulsons of the first order. In the numerical simulations, this stage is followed by an adjusted, complex nonstationary state characterized by a noticeable asymmetry in the meridional component of the vortex's horizontal pressure gradient, which develops to compensate for the variations of the Coriolis parameter with latitude. Accordingly, the location of the simulated vortex's maximum depth is always found poleward of the location of the simulated vortex's center of mass. Moreover, during the adjusted stage, near-inertial oscillations emerge that largely deviate from the exactly inertial ones characterizing analytical circular pulsons: a superinertial and a subinertial oscillation in fact appear, and their frequency difference is found to be an increasing function of latitude. A comparison between vortex westward drifts simulated numerically at different latitudes for different vortex radii and pulsation strengths and the corresponding drifts obtained using existing formulas shows that, initially, the simulated vortex drifts correspond to the fastest predicted ones in many realistic cases. As time elapses, however, the development of a beta-adjusted vortex structure, together with the effects of numerical dissipation, tend to slow down the simulated vortex drift

Nonlinear transverse oscillations of a geostrophic front

A planar problem of nonlinear transverse oscillations of the surface (warm) front of a finite width is considered within the framework of a reduced-gravity model of the ocean. The source of oscillations is the departure of the front from its geostrophic equilibrium. When the current velocity is linear in the horizontal coordinate and the front's depth is quadratic in this coordinate, the problem is reduced to a system of four ordinary differential equations in time. As a result, the solution is obtained in a weakly nonlinear approximation and strongly nonlinear oscillations of the front are studied by numerically solving this system of equations by the Runge-Kutta method. The front's oscillations are always superinertial. Nonlinearity can lead to a decrease or increase in the oscillation frequency in comparison with the linear case. The oscillations are most intense when the current velocity is disturbed in the direction of the front's axis. A weakly nonlinear solution of the second order describes the oscillations very accurately even for initial velocity disturbances reaching 50% of its geostrophic value. An increase in the background-current shear leads to the damping of oscillations of the front's boundary. The amplitude of oscillations of the current velocity increases as the intensity of disturbances increases, and it is relatively small if background-current shears are small or large

1. Wochenbericht M68/2

Meteor 68/2 Der zweite Fahrtabschnitt der 68. Meteor-Expedition findet vom 6.6.2006 – 9.7.2006 unter der Fahrtleitung von PD Dr. Peter Brandt (IFM-GEOMAR) statt. Der Schwerpunkt des zweiten Fahrtabschnitts liegt auf der Bestimmung der Wassermassentransporte innerhalb der flachen tropisch-subtropischen Zelle im äquatorialen Bereich und insbesondere der Versorgungspfade zu den Auftriebsgebieten im äquatorialen und östlichen Atlantik. Der äquatoriale Auftrieb wird im wesentlichen durch den Äquatorialen Unterstrom (EUC) versorgt. Der außeräquatoriale Auftrieb, der innerhalb der zyklonal umströmten Dome (Angola Dome, Guinea Dome) stattfindet, scheint dagegen hauptsächlich durch die Süd- bzw. Nordäquatorialen Unterströme (SEUC, NEUC) versorgt zu werden. Die geplante Erfassung des äquatorialen Stromsystems wird ergänzt durch Mikrostrukturmessungen sowie Beobachtungen mit am Äquator bei 35°W, 23°W und 10°W verankerten Strömungsmessern, die im Rahmen eines DFG – Emmy Noether-Programms bzw. in Zusammenarbeit mit dem französischen EGEE und dem internationalen PIRATA Projekt durchgeführt werden. Als Vorarbeit zu einem geplanten Projekt innerhalb des BMBF Verbundvorhabens „Nordatlantik“ wird ein Verankerungsarray bei 23°W installiert, mit dem die Variabilität der Wassermassen und Zirkulation im zentralen äquatorialen Atlantik auf intrasaisonalen bis mehrjährigen Zeitskalen bestimmt werden kann. Die physikalischen Messungen werden von Tracermessungen (Helium) begleitet, die zusätzliche Informationen zum äquatorialen Auftrieb liefern sollen. Weiterhin enthält die Meteor-Reise M68/2 auch eine SOLAS-Komponente, die eine Fortführung und Erweiterung der auf den Meteor-Reisen M55 und M60/5 begonnenen biogeochemischen Arbeiten zum Stoffaustausch zwischen Ozean und Atmosphäre darstellt. 1. Wochenbericht M68/2, Recife-Mindelo 6.6.-11.6.200

4. Wochenbericht M80/1

Meteor-Reise 80/1 4. Wochenbericht M80/1, Mindelo-Mindelo 16.11.-22.11.200

1. Wochenbericht M80/1

Meteor-Reise 80/1 1. Wochenbericht M80/1, Mindelo-Mindelo 26.10.-1.11.200

Oxygen variability and tropical Atlantic circulation: Cruise No. M119

September 8 – October 12, 2015, Mindelo (Cape Verde) – Recife (Brazil