Inter-annual and intra-seasonal variation of some characteristics of monsoon disturbances formed over the Bay

The characteristics of monsoon disturbances during drought and flood years for the period 1971-96 are studied to find out their inter-annual variations. Variations of some of the characteristics of monsoon disturbances formed over Bay during 1979-88, with respect to different monsoon conditions such as strong, weak and break monsoons, are also studied. The results show that monsoon disturbance days are higher during flood years than during drought years. Drought years are associated with higher chances of low pressure areas to intensity into depressions, less westward movement, more horizontal extent, intense pressure departure from normal in comparison with flood years. However, more monsoon disturbances tilt significantly during flood years. The rainfall associated with these disturbances is highly variable and does not depend on the density, horizontal and vertical extent of the individual system. More number of lows intensify into depressions during strong monsoon conditions compared to those of weak monsoon conditions. Lows and depressions during strong monsoons have more westward movement and longer life period. Generally, very few lows form during break monsoon and none of them intensify into depression. Hence, the presence of mid-tropospheric heating during strong and weak monsoons is essential for the formation of depression. Synoptic systems which abate break monsoon condition and re-establish normal monsoon are also discussed.

Scale sensitivity of the Gill circulation, Part II: Off-equatorial case

We investigate the steady dynamical response of the atmosphere on the equatorial β-plane to a steady, localized, mid-tropospheric heating source. Following Part I which investigates the case of an equatorial diabatic heating, we explore the sensitivity of the Gill circulation to the latitudinal location of the heating, together with the sensitivity to its horizontal scale. Again, we focus on characteristics of the response which would be particularly important if the circulation interacted with the hydrologic and energy cycles: overturning circulation and low-level wind. In the off-equatorial case, the intensity of the overturning circulation has the same limit as in the equatorial case for small horizontal extent of the diabatic heating, which is also the limit in the f-plane case. The decrease in this intensity with increasing horizontal scale of the diabatic heating is slightly faster in the off-equatorial case than in the equatorial case, which is due to the increase of rotational winds at the expense of divergent winds. The low-level westerly jet is more intense than in the equatorial case, with larger maximum wind and eastward mass transport that tend to infinity for small horizontal extent of the diabatic heating. In terms of spatial characteristics, this jet has a similar latitudinal extent as in the equatorial case but, unlike in the equatorial case, it extends further equatorward than poleward of the diabatic-heating center. It also extends further eastward than in the equatorial case.

Explaining Thermal Tides in the Upper Atmosphere During the 2015 El Ni�o

Increased tropospheric heating and reduced dissipation combine to explain an anomalously large thermal tide.

New Connections Between Tropical Dynamics and Lower Stratospheric Chemistry

<p>Matsuno-Gill circulations arising from tropospheric heating have been widely studied in tropical meteorology, but their impact on stratospheric chemistry and composition has seldom been explicitly evaluated. We show how anticyclonic Rossby wave gyres that form near the tropopause due to equatorially-symmetric Matsuno-Gill heating in near-equinox months provide a mechanism to influence chemistry in the tropical and subtropical upper-troposphere/lower-stratosphere (UTLS). This heating both generates anticyclonic flow in the lower stratosphere, which entrains extratropical air from higher latitudes deeper into the tropics of both hemispheres, and induces cooling in this already cold region. These two aspects of the circulation combine to allow heterogeneous chlorine activation on the surface of sulfuric acid aerosols to proceed rapidly. We use reanalysis to show that these Matsuno-Gill heating and wind response patterns are present in the months of interest, and then demonstrate that, in the WACCM model, they can substantially influence the distributions of species related to chlorine activation such as ClO and NO<sub>2</sub>. This provides a potential target for future tropical UTLS observation campaigns, and demonstrates a previously unrecognized mechanism in near-equinox months for dynamical influences on the spatial structures of atmospheric composition changes in this region. </p>

Polar vortex regimes in a simple general circulation model

<p>A dry dynamical-core model is used to investigate the regime behavior of the polar vortex under the influence of tropical upper-tropospheric warming. Up to 5 K temperature increase in this region, the polar vortex strength and variability hardly changes. Only for temperature increases above 8 K the polar night jet speeds up by approximately 20 m s<sup>−1</sup> and the probability of sudden stratospheric warmings is strongly reduced.</p><p>A comparison of climatological-mean differences of the zonal-mean zonal winds between the two regimes and the first empirical orthogonal function of the zonal-mean zonal wind closest to the regime transition at around 7.5 K temperature increase reveals that the system oscillates between both regimes at the regime transition. Every regime is present for a long time accounting for the peaked autocorrelation time scale being distinctive of a regime transition. From a dynamical point of view the strong polar vortex regime is characterized by less negative Eliassen-Palm (EP) flux divergence in the stratosphere and an equatorward refraction of EP flux in the midlatitudes compared to the weak polar vortex regime.</p><p>In order to quantify the influence of the polar vortex on the tropospheric circulation during tropospheric warming, another set of tropical upper-tropospheric heating simulations without a polar vortex is performed. This reveals that the latitudes of the tropospheric jets in both sets of simulations coincide for tropical upper-tropospheric warmings up to 5 K, or equivalently, when the polar vortex is in its weak regime. However, when the polar vortex starts to transition to the strong regime, i.e. for tropospheric warmings above 5 K, the poleward contraction of the tropospheric jet is strongly enhanced compared to the set of simulations without polar vortex.</p>

Importance of the non-linearity of the convective response to surface temperature for eastern Pacific El Niños

<p> Understanding the key physical processes involved in the development and diversity of El Niño events is essential to anticipate their multiple impacts. Current El Niño theories generally assume that wind stress responds linearly to El Niño Sea Surface Temperature (SST) anomalies. Yet, the deep atmospheric convection that energizes this wind stress response has obvious nonlinear features. Observations indeed indicate that rainfall (a proxy of the tropospheric heating) increases slowly with SST up to 26.5<sup>O</sup>C, followed by a sharp increase of rainfall at higher SSTs. In this study, we use that mean observed relation to derive a nonlinear relation between SST and rainfall anomalies, that depends on the background climatological SST. This relation performs much better to explain rainfall anomalies in the eastern Pacific (Niño3 region) than a linear relation, which underestimates rainfall during most extreme El Niños events. On the other hand, it underestimates rainfall anomalies in the western Pacific (Nino4), because it only considers the local SST forcing and neglects the atmospheric convergence feedback. Our observational results are in line with previous modeling studies, who have also underlined the importance of the nonlinearity of the convective response to SST anomalies for large El Niño events in coupled models. We end by discussing other possible sources of nonlinearity in the wind stress and heat flux responses to SST, which play a strong role in the most essential El Niño feedbacks.</p>

The role of wave–wave interactions in sudden stratospheric warming formation

The effects of wave–wave interactions on sudden stratospheric warming formation are investigated using an idealized atmospheric general circulation model, in which tropospheric heating perturbations of zonal wave numbers 1 and 2 are used to produce planetary-scale wave activity. Zonal wave–wave interactions are removed at different vertical extents of the atmosphere in order to examine the sensitivity of stratospheric circulation to local changes in wave–wave interactions. We show that the effects of wave–wave interactions on sudden warming formation, including sudden warming frequencies, are strongly dependent on the wave number of the tropospheric forcing and the vertical levels where wave–wave interactions are removed. Significant changes in sudden warming frequencies are evident when wave–wave interactions are removed even when the lower-stratospheric wave forcing does not change, highlighting the fact that the upper stratosphere is not a passive recipient of wave forcing from below. We find that while wave–wave interactions are required in the troposphere and lower stratosphere to produce displacements when wave number 2 heating is used, both splits and displacements can be produced without wave–wave interactions in the troposphere and lower stratosphere when the model is forced by wave number 1 heating. We suggest that the relative strengths of wave number 1 and 2 vertical wave flux entering the stratosphere largely determine the split and displacement ratios when wave number 2 forcing is used but not wave number 1.

The Role of Eddy-Eddy Interactions in Sudden Stratospheric Warming Formation

The effects of eddy-eddy interactions on sudden stratospheric warming formation are investigated using an idealized atmospheric general circulation model, in which tropospheric heating perturbations of zonal wave numbers 1 and 2 are used to produce planetary scale wave activity. Eddy-eddy interactions are removed at different vertical extents of the atmosphere in order to examine the sensitivity of stratospheric circulation to local changes in eddy-eddy interactions. We show that the effects of eddy-eddy interactions on sudden warming formation, including sudden warming frequencies, are strongly dependent on the wave number of the tropospheric forcing and the vertical levels where eddy-eddy interactions are removed. Significant changes in sudden warming frequencies are evident when eddy-eddy interactions are removed even when the lower stratospheric wave forcing does not change, highlighting the fact that the upper stratosphere is not a passive recipient of wave forcing from below. We find that while eddy-eddy interactions are required in the troposphere and lower stratosphere to produce displacements when wave number 2 heating is used, both splits and displacements can be produced without eddy-eddy interactions in the troposphere and lower stratosphere when the model is forced by wave number 1 heating. We suggest that the relative strengths of wave numbers 1 and 2 vertical wave flux entering the stratosphere largely determine the split and displacement ratios when wave number 2 forcing is used, but not wave number 1.

Beyond the Rigid Lid: Baroclinic Modes in a Structured Atmosphere

The baroclinic-mode decomposition is a fixture of the tropical-dynamics literature because of its simplicity and apparent usefulness in understanding a wide range of atmospheric phenomena. However, its derivation relies on the assumption that the tropopause is a rigid lid that artificially restricts the vertical propagation of wave energy. This causes tropospheric buoyancy anomalies of a single vertical mode to remain coherent for all time in the absence of dissipation. Here, the authors derive the Green’s functions for these baroclinic modes in a two-dimensional troposphere (or, equivalently, a three-dimensional troposphere with one translational symmetry) that is overlain by a stratosphere. These Green’s functions quantify the propagation and spreading of gravity waves generated by a horizontally localized heating, and they can be used to reconstruct the evolution of any tropospheric heating. For a first-baroclinic two-dimensional right-moving or left-moving gravity wave with a characteristic width of 100 km, its initial horizontal shape becomes unrecognizable after 4 h, at which point its initial amplitude has also been reduced by a factor of 1/π. After this time, the gravity wave assumes a universal shape that widens linearly in time. For gravity waves on a periodic domain the length of Earth’s circumference, it takes only 10 days for the gravity waves to spread their buoyancy throughout the entire domain.