Interannual Variability of the Overturning and Energy Transport in the Atmosphere and Ocean During the Late Twentieth Century with Implications for Precipitation and Sea Level

ABSTRACTThe overturning circulations in the atmosphere and ocean transport energy from the tropics to higher latitudes and thereby modulate Earth’s climate. The interannual variability in the overturning over the last 40 years is found to be dominated by two coupled atmosphere–ocean modes. The first is related to the meridional motion of the intertropical convergence zone and the second to El Niño. Both modes have a strong influence on the sea level variability in the tropical Indo-Pacific Ocean. The interannual variability of the cross-equatorial energy transport is dominated by the first mode, and the variability is larger in the Indo-Pacific Ocean than in the Atlantic Ocean or the atmosphere. Our results suggest an important role of oceanic energy transport in setting precipitation patterns in the tropics and a key role of the Indo-Pacific Ocean as a climate modulator.

Long-term variations in meridional motion of sunspot groups: comparison of DPD and SOON data

We have analyzed the Debrecen Photoheliographic Data (DPD) and the Solar Optical Observing Network (SOON) sunspot group data during the period 1977 – 2015 and find that during the maximum of solar cycle 23 there is a large difference in the mean meridional motion of sunspot groups determined from DPD and SOON data.

Meridional motions of the afternoon radar aurora, auroral electrojets, and absorption patches under variable IMF conditions

The meridional motions of the CUTLASS HF and STARE VHF coherent echoes, IMAGE equivalent electrojet currents, and IRIS absorption patches during the postnoon/early-evening event of 14 February 2000 are presented. The motions were found to be synchronous, to a first approximation, for all instruments. The temporal correlation between motions in the radar and magnetometer data was exceptionally good, although spatially the areas with the E-region backscatter and most intense equivalent currents were not coincident, with the HF (VHF) echoes being shifted 100–200km (20–50km) equatorward (poleward). The meridional motions of the radar echoes and electrojet currents appeared to be controlled by the IMF Bz changes; the meridional propagation direction was equatorward (poleward) during the intervals when the IMF was southward (northward), with one exception when the poleward progression continued after the IMF southward turning. We relate the observed meridional motion patterns to the polar cap expansion/contraction during variable IMF conditions and discuss the relative importance of two types of processes: the dayside reconnection and IMF-triggered substorms. We also investigate the irregularity Doppler velocity for the STARE (144MHz) and CUTLASS (12MHz) observations at large flow angles in the context of the eastward and westward electrojet systems. We show that the 144-MHz Doppler velocity is determined by a combination of two factors: the sense of electrojet currents and the aspect angle conditions within the STARE field of view. Finally, the behavior of small dayside enhancements of the IRIS absorption (up to 0.5dB at 38.2MHz) accompanying the radar echoes and electrojet currents is examined. Since the velocity of the meridional displacements was close to that of the poleward/equatorward progressing intense currents, it is suggested that the absorption patches observed during the event were related to the heating of the E-region plasma by the unstable plasma waves in the regions of enhanced electric fields. Key words. Ionosphere (auroral ionosphere; electric fields and currents; plasma convection)

Bifurcation of swirl in liquid cones

We show that rotation appears owing to bifurcation in primarily pure meridional steady motion of viscous incompressible fluid. This manifestation of the laminar axisymmetric 'swirl dynamo’ occurs in flows inside liquid conical menisci with the cone half-angle θc < 90°. The liquid flows towards the cone apex near the surface and moves away along the axis driven by (i) surface shear stresses (typical for electrosprays) or (ii) by body Lorentz forces (e.g. in the process of cathode eruption). When the motion intensity increases and passes a critical value, new swirling regimes appear resulting from the supercritical pitchfork bifurcation. This agrees with recent observations of swirl in Taylor cones. We find that when the swirl Reynolds number Γc reaches a threshold value, flow separation occurs and the meridional motion becomes two-cellular with inflows near both the surface and the axis, and an outflow near the cone θ = θs, 0 < θs < θc. In the limit of high Γc, the angular thickness of the near-surface cell tends to zero. In case (i) the swirl is concentrated near the surface while the motion inside the inner cell becomes purely meridional with the radial velocity being uniform. We also study the two-phase flow of a liquid inside and a gas outside the meniscus. Flow separation occurs in both media and then swirl is concentrated near the interface. In case (ii) we reveal another interesting effect: a cascade of flow separations near the axis. As the driving forces increase, meridional motion becomes multi-cellular although very slow in comparison with swirl. To cover all ranges of parameters we combine numerical calculations and asymptotic analyses.

A linear analysis of rotating stratified flow past a circular cylinder on a β-plane

A linear analysis of rotating stratified quasi-geostrophic flow past a circular cylinder on a β-plane is performed for moderate stratification, i.e. for σS = O(E½), covering effectively the range $E^{\frac{2}{3}} \ll \sigma \ll 1$, and for strong stratification such that σS = O(1). E [Lt ] 1 is the Ekman number and σS is the product of the Prandtl number and the inverse rotational Froude number. The parameter β measures the importance of the production of relative vorticity by meridional motion. The analysis pivots about a range of β which constrains the interior motion to follow geostrophic contours. For moderate stratification β = O(E1/4), covering effectively the range E½ [Lt ] β [Lt ] E⅛, while for strong stratification E [Lt ] β [Lt ] 1. The dominance of β in the interior is responsible for creating a narrow intense boundary layer along the eastern sector of the cylinder and an extensive blocked flow region surrounded by intense free shear layers west of the cylinder. These narrow regions which channel horizontally O(1) mass flux communicate through corner-like regions centred about the extreme meridional locations of the cylinder. The effect of stratification is to shear the flow vertically and to induce counter-flows laterally. When the stratification is strong the z-dependence is parametric. Nonlinear effects can be ignored when the Rossby number, ε, satisfies the constraints $\epsilon \ll E^{\frac{11}{12}}$ for moderate stratification and $\epsilon \ll \beta^{\frac{1}{7}} E^{\frac{6}{7}}$ for strong stratification. When expressed in terms of the Reynolds number, Re = ε/E, smallness of nonlinear effects can be assured also for high-Reynolds-number flows.