The interannual variability of the tropical divergence tilt

<p>The large-scale divergence field tilts eastward with latitude moving away from its near-equatorial maximum in the summer hemisphere. This tilt, observed for all hemispheres and seasons, is also apparent in a hierarchy of models of varying complexity, including the simple Gill model. Previous theoretical work has shown that the divergence tilt determines the sign of the divergent momentum flux in the deep tropics, suggesting a possible connection to wave propagation.</p><p>In this presentation,  we show that changes in the divergence tilt are one of two primary drivers of the interannual eddy momentum flux variability in the tropics. We also show that interannual changes in the divergence tilt are strongly correlated with the West Pacific Oscillation, with an associated large extratropical impact. The dynamical mechanisms behind this association are also discussed.</p>

The Interannual Variability of the Tropical Divergence Tilt and Its Connection with the Extratropical Circulation

Previous theoretical work has suggested that the strength of the divergent eddy momentum flux in the deep tropics, due to correlations between rotational zonal velocities and divergent meridional velocities, increases with the meridional tilt of the large-scale divergence field. To test that idea, this work investigates the interannual variability of the divergent eddy momentum flux in reanalysis data. Consistent with the theory, it is found that the eddy momentum flux variability is driven by two main parameters: the amplitude of the tropical stationary wave and the tilt of the divergence field. Together, these two parameters account for 80% (90%) of the interannual eddy momentum flux variance during boreal (austral) winter. The interannual variability of these parameters is governed by the internal atmospheric dynamics. During boreal winter, interannual changes in MJO variability explain nearly half of the interannual variance in the stationary wave amplitude, depending on whether on average MJO anomalies interfere constructively or destructively with the stationary wave. The interannual variability of the divergence phase tilt is modulated by tropical–extratropical interactions in the Pacific. The tilt increases during the negative phase of the west Pacific Oscillation associated with a dipole of upper-level divergence (convergence) on the northern (southern) side of the Pacific jet exit region.

Analysis of variability in divergence and turn-over induced by three idealized convective systems with a 3D cloud resolving model

The sensitivity of upper tropospheric and lower stratospheric convective outflows and related divergence fields is analysed using an ensemble of cloud resolving model (CM1) simulations in LES-mode including various physically manipulated simulations for three different convective systems initialized with an idealized trigger. The main goal of this study is to assess to what extend the divergence field depends on cloud microphysical processes, the mode of convection and on the processes of convective momentum transport and moist static energy redistribution. We find that latent heat release (representing the microphysical uncertainty) plays an essential role by explaining much of magnitude of the divergence field that will be formed. Convective organisation explains another important fraction of the variability in the divergence field that is formed by a convective system and behaves non-linearly, likely partly via condensation and subsequent (re-)evaporation/sublimation. The detrainment of stratospheric air also shows large sensitivity among the experiments.

Superclusters from velocity divergence fields

ABSTRACT Superclusters are a convenient way to partition and characterize the large-scale structure of the Universe. In this Letter, we explore the advantages of defining superclusters as watershed basins in the divergence velocity field. We apply this definition on diverse data sets generated from linear theory and N-body simulations, with different grid sizes, smoothing scales, and types of tracers. From this framework emerges a linear scaling relation between the average supercluster size and the autocorrelation length in the divergence field, a result that holds for one order of magnitude from 10 up to 100 Mpc h−1. These results suggest that the divergence-based definition provides a robust context to quantitatively compare results across different observational or computational frameworks. Through its connection with linear theory, it can also facilitate the exploration of how supercluster properties depend on cosmological parameters, paving the way to use superclusters as cosmological probes.

The large-scale tilt of the eddy divergence in the tropics

<p>This work is concerned with the large-scale structure of the upper-level divergence/precipitation field in the deep tropics. Once the fine ITCZ structure is filtered out, the coarse-grained eddy divergence field is found to tilt eastward moving away from its maximum near the equator in the summer hemisphere. This robust tilt (observed for both hemispheres and seasons) is also present in the classical Gill solution.</p><p>In this presentation we show that the sign of the tilt is intimately linked to the direction of the eddy momentum flux. The observed eastward tilt is such that the momentum flux is directed towards the wave source, suggesting that the observed tilt is determined by wave propagation.</p><p>We also discuss the determination of the tilt in the simple Gill model and its sensitivity to the meridional Hadley flow. We show that the increase in the cross-equatorial momentum flux when the Hadley cell strengthens is associated with an increased tilt of the divergence field in the downstream direction of the flow, supporting the conjecture that the tilt is associated with propagation. </p>

The Role of the Divergent Circulation for Large-Scale Eddy Momentum Transport in the Tropics. Part II: Dynamical Determinants of the Momentum Flux

This paper investigates the coupling between the rotational and divergent circulations aiming to explain the observations that show that the tropical eddy momentum flux is due to correlations between divergent eddy meridional velocities and rotational eddy zonal velocities. A simple linear model in which the observed eddy divergence field is used to force the vorticity equation can reproduce quite well the observed tropical eddy momentum flux. The eddy momentum flux in the model shows little sensitivity to the basic-state winds and is mainly determined by the characteristics of the divergent forcing. Vortex stretching and divergent advection of planetary vorticity produce eddy momentum flux contributions with the same sign but the former forcing dominates. It is shown that the main factor affecting the direction of the eddy momentum flux response to both forcings is the meridional tilt of the divergence phase lines, albeit with an opposite sign to the classical relation between rotational momentum flux and streamfunction phase tilt. How this divergent structure is determined remains an open question.

Accurate fitting functions for peculiar velocity spectra in standard and massive-neutrino cosmologies

We estimate the velocity field in a large set of N-body simulations including massive neutrino particles, and measure the auto-power spectrum of the velocity divergence field as well as the cross-power spectrum between the cold dark matter density and the velocity divergence. We perform these measurements at four different redshifts and within four different cosmological scenarios, covering a wide range in neutrino masses. We find that the nonlinear correction to the velocity power spectra largely depends on the degree of nonlinear evolution with no specific dependence on the value of neutrino mass. We provide a fitting formula based on the value of the rms of the matter fluctuations in spheres of 8 h−1 Mpc, describing the nonlinear corrections with 3% accuracy on scales below k = 0.7 h Mpc−1.

Comments on “The Gulf Stream Convergence Zone in the Time-Mean Winds”

In a recent study, O’Neill et al. analyzed the divergence of surface winds above the northwest Atlantic. In the time mean, a band of convergence is found, overlying the southern flank of the Gulf Stream. To quantify the impact of synoptic storms, the authors proposed to compare the time-mean divergence with the divergence averaged in the absence of rain. In the resulting conditional-average field, divergence was found to be positive nearly everywhere. O'Neill et al. concluded that this absence of convergence precludes the Ekman-balanced mass adjustment to be responsible for the atmospheric response above the Gulf Stream. Using a simplistic toy model as well as a numerical simulation representative of a storm track, we show that the absence of negative divergence values purely results from the correlation between rain and convergence: the conditional average based on the absence of rain necessarily implies a shift toward positive divergence values. In consequence, we argue that conditional statistics (based on the absence of rain or removing extreme values in the divergence field), as produced by O’Neill et al., do not allow conclusions on the mechanisms underlying the atmospheric response to the Gulf Stream. They nevertheless highlight the essential role of synoptic storms in shaping the divergence field in instantaneous fields.

Characterizing the Diurnal Cycle of Low-Level Circulation and Convergence Using CFSR Data in Southeastern South America

The focus of this study is the characterization of the diurnal cycle of low-level wind and divergence field (under two different synoptic situations observed during the South American Low-Level Jet Experiment) within the South American domain encompassed between 20° and 35°S east of the Andes, using Climate Forecast System Reanalysis (CFSR). The objective is to highlight the existence of a spatial variation of these quantities and differences in the strength of their diurnal cycle between the two synoptic situations. Inertial oscillations and thermally driven circulations as well as convection-related contributions to mesoscale convergence and their implications for deep convection initiation/maintenance are addressed in each selected subregion. Prevalence of synoptic-scale forcing over the diurnally forced circulations, or vice versa, is also analyzed. Both mesoscale wind diurnal cycle and related divergence fields are sensitive to varying synoptic conditions and display regional variability. Thermal circulations related to topographical features superpose on the diurnal inertial oscillation that, while present in the whole domain, dominates the central plain subregions. The most evident diurnal cycle in the divergence field is restricted to sloped areas just to the east of the Andes and the Sierras de Córdoba where the mesoscale component of the divergence field is responsible for modulating the total divergence. CFSR provides a broad perspective of low-level circulation over southeastern South America (SESA) during the specific 15-day period. Results from this study might stimulate future research on a relationship between low-level circulation and the initiation of convection in SESA using CFSR to perform high-resolution simulations.

Typhoon-Position-Oriented Sensitivity Analysis. Part I: Theory and Verification

A new sensitivity analysis method is proposed for the ensemble prediction system in which a tropical cyclone (TC) position is taken as a metric. Sensitivity is defined as a slope of linear regression (or its approximation) between state variable and a scalar representing the TC position based on ensemble simulation. The experiment results illustrate important regions for ensemble TC track forecast. The typhoon-position-oriented sensitivity analysis (TyPOS) is applied to Typhoon Shanshan (2006) for the verification time of up to 48 h. The sensitivity field of the TC central latitude with respect to the vorticity field obtained from large-scale random initial perturbation is characterized by a horizontally tilted pattern centered at the initial TC position. These sensitivity signals are generally maximized in the middle troposphere and are far more significant than those with respect to the divergence field. The results are consistent with the sensitivity signals obtained from existing methods. The verification experiments indicate that the signals from TyPOS quantitatively reflect an ensemble-mean position change as a response to the initial perturbation. Another experiment with Typhoon Dolphin (2008) demonstrates the long-term analysis of forecast sensitivity up to 96 h. Several additional tests have also been carried out to investigate the dependency among ensemble members, the impacts of using different horizontal grid spacing, and the effectiveness of ensemble-Kalman-filter-based perturbations.