Nonlinear Zonal Propagation of Organized Convection in the Tropics

A wide range of the observed variability in the ITCZ is frequently explained in terms of equatorially trapped modes arising from Matsuno’s linear shallow-water model. Here, a series of zonally constant, meridionally symmetric aquachannel WRF simulations are used to study the propagation of tropical cloud clusters (CCs; patches of deep cloudiness and precipitation) in association with eastward-moving super cloud clusters (SCCs), also called convectively coupled Kelvin waves (CCKWs). Two independent but complementary methods are used: the first, from a local approach, involves a CC-tracking algorithm, while the second uses Lagrangian trajectories in a nonlocal framework. We show that the large-scale flow in low to midlevels advects the CCs either eastward or westward depending on model climatology, proximity to the CCKW axis, and latitude. Moreover, for most analyzed cases, sequences of CCs oscillate, describing qualitatively sinusoidal-like paths in longitude–time space, although with sharp transitions from westward to eastward motion due to westerly wind burst activity associated with the CCKWs. We also find that the discrete precipitation elements (CCs) are embedded in continuous tracks of positive moisture anomalies, which are parallel to the Lagrangian trajectories themselves. A conceptual model of the nonlinear SCC–CC interaction is presented.

Zonal Propagation of Near-Surface Zonal Currents in Relation to Surface Wind Forcing in the Equatorial Indian Ocean

Zonal propagation of zonal velocity along the equator in the Indian Ocean and its relationship with wind forcing are investigated with a focus on seasonal time scales using in situ observations from four acoustic Doppler current profilers (ADCPs) and an ocean reanalysis dataset. The results show that the zonal phase speed of zonal currents varies depending on season and depth in a very complicated way in relation to surface wind forcing. Surface layer zonal velocity propagates to the west in northern spring but to the east in fall in response to zonally propagating surface zonal winds, while in the pycnocline zonal phase speed is related to wind-forced ocean wave dynamics. In the western half of the analysis domain (78°–83°E), zonal phase speed in the pycnocline is eastward all year, which is attributed to the radiation of Kelvin waves forced in the western basin. In the eastern half of the domain (80°–90°E), zonal phase speed is westward at 50- to 100-m depths in northern fall, but eastward above and below, most likely due to Rossby waves generated at the eastern boundary.

The Relationship between Equatorial Mixed Rossby–Gravity and Eastward Inertio-Gravity Waves. Part II

Space–time spectral analysis of tropical cloudiness data shows strong evidence that convectively coupled n = 0 mixed Rossby–gravity waves (MRGs) and eastward inertio-gravity waves (EIGs) occur primarily within the western/central Pacific Ocean. Spectral filtering also shows that MRG and EIG cloudiness patterns are antisymmetric with respect to the equator, and they propagate coherently toward the west and east, respectively, with periods between 3 and 5 days, in agreement with Matsuno’s linear shallow-water theory. In contrast to the spectral approach, in a companion paper it has been shown that empirical orthogonal functions (EOFs) of 2–6-day-filtered cloudiness data within the tropical Pacific Ocean also suggest an antisymmetric pattern, but with the leading EOFs implying a zonally standing but poleward-propagating oscillation, along with the associated tropospheric flow moving to the west. In the present paper, these two views are reconciled by applying an independent approach based on a tracking method to assess tropical convection organization. It is shown that, on average, two-thirds of MRG and EIG events develop independently of one another, and one-third of the events overlap in space and time. This analysis also verifies that MRG and EIG cloudiness fields tend to propagate meridionally away from the equator. It is demonstrated that the lack of zonal propagation implied from the EOF analysis is likely due to the interference between eastward- and westward-propagating disturbances. In addition, it is shown that the westward-propagating circulation associated with the leading EOF is consistent with the expected theoretical behavior of an interference between MRGs and EIGs.

Latitudinal variability of the quasi-16-day wave in the middle atmosphere over Brazilian stations

A comparative study of the quasi-16-day wave (QSDW) in the middle atmosphere using meteor radar observations and reanalysis data from three Brazilian stations, Sao Joao do Cariri (7.4° S, 36.5° W) (CA), Cachoeira Paulista (22.7° S, 45° W) (CP), and Santa Maria (29.7° S, 53.7° W) (SM) has been carried out in the year 2005 to delineate its latitudinal variability characteristics. The broad spectral behavior around 16-day periodicity may indicate multiple modes of the concerned wave component. The wave amplitude shows a number of peaks over the year with the largest one in summer and winter in the case of mesosphere–lower thermosphere (MLT) and stratosphere, respectively. A potential coupling of the concerned wave with other short period planetary waves, especially at CA and CP is evinced. Although zonal propagation exhibits both eastward as well as westward waves there is a general preference of eastward waves at mid-latitude and westward waves at tropical latitudes. The prevailing westerly background wind in the middle atmosphere is conceived to favor the wave filtering of westward propagating Rossby waves at lower latitude.

The Longitudinal Variation of Equatorial Waves due to Propagation on a Varying Zonal Flow

The general 1D theory of waves propagating on a zonally varying flow is developed from basic wave theory, and equations are derived for the variation of wavenumber and energy along ray paths. Different categories of behavior are found, depending on the sign of the group velocity cg and a wave property B. For B positive, the wave energy and the wavenumber vary in the same sense, with maxima in relative easterlies or westerlies, depending on the sign of cg. Also the wave accumulation of Webster and Chang occurs where cg goes to zero. However, for B negative, they behave in opposite senses and wave accumulation does not occur. The zonal propagation of the gravest equatorial waves is analyzed in detail using the theory. For nondispersive Kelvin waves, B reduces to 2, and an analytic solution is possible. For all the waves considered, B is positive, except for the westward-moving mixed Rossby–gravity (WMRG) wave, which can have negative B as well as positive B. Comparison is made between the observed climatologies of the individual equatorial waves and the result of pure propagation on the climatological upper-tropospheric flow. The Kelvin wave distribution is in remarkable agreement, considering the approximations made. Some aspects of the WMRG and Rossby wave distributions are also in qualitative agreement. However, the observed maxima in these waves in the winter westerlies in the eastern Pacific and Atlantic Oceans are generally not in accord with the theory. This is consistent with the importance of the sources of equatorial waves in these westerly duct regions due to higher-latitude wave activity.

Multiplicative MJO Forcing of ENSO

The Madden–Julian oscillation (MJO) is parameterized to study the role of the feedback it receives from sea surface temperature (SST) in its influence on El Niño–Southern Oscillation (ENSO). The parameterization describes MJO surface westerlies in terms of a few basic parameters that include amplitude, zonal propagation extent, propagation speed, and the interval between adjacent events. It is used to drive a coupled ocean–atmosphere model of intermediate complexity tuned to a marginally stable regime. The MJO parameters acquire values either additively (i.e., based on observed estimates of most probable value and stochasticity) or multiplicatively (i.e., modulated by an evolving model ENSO SST, albeit with some stochasticity). Simulations reveal that ENSO variance increases with the stochasticity of MJO amplitude but is insensitive to the stochasticity of zonal extent and speed, except that ENSO vanishes completely when the propagation speed is zero. Likewise, ENSO strengthens linearly with the SST modulation of MJO amplitude, but not of speed and zonal extent—even though the two are known to be significantly influenced by SST. Ensemble comparisons between simulations with and without SST feedback demonstrate that SST feedback to the MJO acting in a stable regime can be responsible for the observed ENSO variance. The multiplicative case has a larger ensemble spread than the additive case, which manifests in a larger interdecadal variability of ENSO. The results emphasize that ENSO reproduction in coupled models depends on correctly representing the MJO, especially its amplitude and SST feedback.

Meridional Propagation of Planetary-Scale Waves in Vertical Shear: Implication for the Venus Atmosphere

It is shown that planetary-scale waves are inherently accompanied by latitudinal momentum transport when they propagate vertically in vertically sheared zonal flows. Because of the dependence of the wave's latitudinal scale on the intrinsic phase speed, positive (negative) vertical shear should force prograde (retrograde) waves to focus equatorward and retrograde (prograde) waves to expand poleward in the course of upward propagation. Consequently, Eliassen–Palm (EP) flux vectors are tilted from the vertical and nonzero latitudinal momentum fluxes occur. The direction of momentum transport should always be equatorward (poleward) in positive (negative) vertical shear irrespective of the zonal propagation direction. The idea was applied to upwardly propagating waves in the Venusian middle atmosphere, where vertical shear of strong midlatitude jets and equatorial superrotation exist. Numerical solutions showed that Kelvin and prograde inertio-gravity waves focus equatorward and mixed Rossby–gravity and Rossby waves expand poleward below the cloud top. The former is attributed primarily to the vertical shear of the superrotation, while the latter to the vertical shear beneath the midlatitude jets. Such characteristics of planetary-scale waves will cause angular momentum separation between high and low latitudes and, at least partly, contribute to the maintenance of the superrotation.

Warm season precipitation climatology: first European results

To date very low scores are associated to quantitative precipitation forecasts (QPF) of warm season precipitation, a fact mostly due to the little knowledge of the mechanisms driving these phenomena. The study aims to produce a five-year climatology (1999–2003) of warm season precipitation systems (MJJA) over Europe using Meteosat IR brightness temperatures as a contribution to a global study launched by the World Weather Research Programme (WWRP). Cold cloud persistence, span and duration of weather systems were determined to derive the zonal propagation speed and daily cycles.