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Extreme precipitation events (EPEs) are among the most pervasive weather hazards in the western Himalayan region (WHR), posing widespread damage to life, infrastructure, and agriculture. This study investigates the synoptic and large‐scale characteristics linked to winter precipitation extremes over the WHR. EPEs are identified as events surpassing the 95th percentile threshold. A composite analysis is employed using two reanalyses—the fifth‐generation European Centre for Medium‐Range Weather Forecasts Reanalysis (ERA5) and the Indian Monsoon Data Assimilation and Analysis (IMDAA)—to elucidate the synoptic conditions conducive to EPEs. Our findings suggest that EPEs in the WHR are linked to an intensified subtropical westerly jet, characteristically shifted to south than normal. Enhanced kinetic energy in the upper troposphere, attributed to increased baroclinic instability, reinforces moisture convergence and strengthens synoptic‐scale circulation, triggering deep convection and supporting EPEs. Notably, the interplay of pronounced Rossby waves sinking over the WHR and regional orography significantly modulates the intensity of western disturbances (WDs). Employing clustering analysis, we observed that the strongest EPEs are linked to anomalous vorticity in the upper to middle troposphere, together with deep convection via strengthened WDs, suggesting the potential role of large‐scale influences. Using Lagrangian method, we identify that the Arabian Sea is the primary moisture source for EPEs in the WHR. We further delved into the role of large‐scale connections and EPEs through quasi‐resonant amplification (QRA) analysis. The findings unveil distinct QRA fingerprints in meridional temperature gradients along with notably magnified, quasi‐stationary midlatitude planetary waves characterized by zonal wave numbers 6/7/8 (baroclinic waves) contributing to EPEs. Overall, our results highlight the underlying physical mechanisms for winter precipitation extremes, emphasizing QRA's role in amplifying planetary waves and promoting EPEs, underscoring the WHR's vulnerability to evolving climatic conditions. [ABSTRACT FROM AUTHOR]

The characteristics of cold core eddies and their effects on the hydrodynamics and biological production in the Andaman seas were studied using in-situ and satellite observations. The existence and spatial extent of the eddy are revealed by the precise structure and patterns of the temperature-salinity (T–S) profiles, nutrients, and chl a. A good illustration of this is the anomaly in sea surface height (SSHA). The Cyclonic Eddies (CE1 and CE2) are monitored using the Okubo-Weiss parameter (− 2 × 10–11/s2) calculated from the satellite SSHA and a geostrophic current centered at 8°N and 92°E and 13°N and 93°E, respectively. At the eastern flank of CE1, measurements are taken in-situ along 8°N and 92.5°–93.5°E. Vertical currents measured with Acoustic Doppler Current Profiles (ADCP) at 0.3 m/s show northward flow along the track. A significant northward flow (0.3 m/s) can be seen in the vertical currents recorded with ADCP, yet a weak southern flow can be seen over the western margin. In addition, the SSHA acquired from altimetry demonstrates the spatial extent, supporting the occurrence of cyclonic eddies. The vertical shear in the horizontal flow is the main contributor to baroclinic instability (Ri 0.0001) in the water column, according to an analysis of the factors that could result in the formation of an eddy. The T–S profiles show that the area contains Bay of Bengal (BoB) water, and there are semi-annual Rossby waves there as well. However, the wind stress curl did not provide a reliable indication of divergence in the region. The region's biological production (chl a) and nutrient distribution (NO2, NO3, PO4, and SiO4) were impacted by the eddy. CE2 is connected to convective mixing processes that take place along the northwest coast of the Andaman Islands as a result of the primarily cold, dry continental air flowing from the north east. [ABSTRACT FROM AUTHOR]

The quasi 5‐day wave (Q5DW) with zonal wavenumber 1 is a dominant planetary wave (PW) oscillation in the polar summer mesospheric temperature and polar mesospheric cloud (PMC) fields. In this paper, the Q5DW signal derived from 16 years (2007–2022) of Microwave Limb Sounder temperature observations is used to investigate the role of this PW mode on the onset of PMC seasons in the northern hemisphere (NH). PMC data from the Cloud Imaging and Particle Size (CIPS) instrument during this time indicates that NH PMC season onsets ranged from 15 to 28 May, with earliest onsets in 2013, 2015, 2019, 2020, 2021, and 2022. Except 2013 and 2022, the other four earlier onsets were also characterized by enhanced Q5DW activity. The wave amplification appears to be driven by baroclinic instability arising from the negative meridional gradient of potential vorticity in the high‐latitude summer mesosphere. CIPS data show that when the Q5DW was present at the beginning of the season, clouds formed preferentially in the cold troughs of the wave. We thus propose that the much colder troughs due to enhanced Q5DW activity in mid‐May of 2015, 2019, 2020, and 2021 influenced the timing of PMC onset in these years. While the 11‐year solar cycle, inter‐ and intra‐hemispheric coupling due to gravity wave and PW activity have been shown to contribute to earlier onset of PMC seasons in the NH, our analysis suggests that enhanced Q5DW activity also plays a major role. Plain Language Summary: The quasi 5‐day wave (Q5DW), a wave that takes ∼5 days to travel around a latitude circle, is a dominant planetary scale oscillation in the polar summer mesospheric temperature and polar mesospheric cloud (PMC) fields. We present the first study of the Q5DW impact on the northern hemisphere (NH) PMC season onset. The Q5DW activity is derived from 16 years (2007–2022) of satellite temperature observations. This activity is then compared to the onset of NH PMC seasons determined from the Cloud Imaging and Particle Size (CIPS) observations. The season onsets ranged from 15 to 28 May, with earliest onsets in 2013, 2015, 2019, 2020, 2021, and 2022. Earlier onsets in 2015, 2019, 2020, and 2021 were characterized by increased Q5DW power presumably driven by instability due to large vertical shear in the zonal winds. PMCs are found to form in the cold troughs of the Q5DW. We thus propose that the much colder troughs due to enhanced Q5DW activity in mid‐May of 2015, 2019, 2020, and 2021 influenced the timing of PMC onset in these years. While several other factors contribute to earlier onset of PMC seasons in the NH, our analysis suggests that enhanced Q5DW activity also plays a major role. Key Points: There is year‐to‐year variability in the quasi 5‐day wave (Q5DW) activity in the northern hemisphere summerObservations from 2007 to 2022 indicate that four of the six earliest polar mesospheric cloud (PMC) season onsets are characterized by higher Q5DW activityPMC first form in the longitude sector coincident with the cold trough of the Q5DW [ABSTRACT FROM AUTHOR]

The Antarctic Slope Current (ASC), coupled with the Antarctic Slope Front (ASF), encircles Antarctica and is a roughly alongshore flow. Yet, the instabilities of the ASC/ASF can induce cross-slope exchanges. The ASC/ASF instabilities have been studied extensively in previous literature. However, the corresponding responsible mechanisms and influences still need to be further clarified. Based on an idealized eddy-resolving model, this study focuses on the instabilities of the ASC/ASF with constant open boundary forcing. The ASC/ASF is classified into three types: Fresh Shelf, Warm Shelf, and Dense Shelf. Focused on the Fresh Shelf and Dense Shelf cases, two high-resolution process-oriented numerical experiments are conducted to reveal the typical characteristics, the dynamic mechanisms, and the influences of instabilities. In the Fresh Shelf case, the instabilities are characterized by a submesoscale vortex train over the middle-lower slope, associated with the Topographic Rossby waves. In the Dense Shelf case, a mesoscale vortex train is present over the lower slope, and abundant filaments and jets can flow across the shelf break. The baroclinic instability greatly contributes to the generation of instabilities in the two cases, yet the barotropic instability contributes less to the instabilities in the Fresh Shelf case. Coherent eddies have been identified and significantly favor the hydrographic anomalies by the advection of water boluses retaining source water rather than the polarities of coherent eddies. Instabilities only contribute to exchanges across the continental rise in the Fresh Shelf case but effectively result in cross-slope exchanges in the Dense Shelf case. • Instabilities of the Antarctic Slope Current are very important. • Baroclinic instability greatly contributes to intrinsic instabilities. • Barotropic instability contributes less in the Fresh Shelf regions. • Eddy-induced transport cannot cross the shelf break in the Fresh Shelf regions. • Eddy-induced transport favors cross-slope exchanges in the Dense Shelf regions. [ABSTRACT FROM AUTHOR]

In the real atmosphere, the development of large-scale motion is often related to the baroclinic properties of the atmosphere. So, it is necessary to discuss the stability condition of baroclinic flow. It is advantageous to use a layered model to discuss baroclinic instability, not only to apply the potential vortex equation directly, but also to deal with shear of basic flow. The stability and oscillatory shear ability of Rossby waves are studied based on the two-layer Phillips model in the β plane; then, we summarize the baroclinic instability of time-dependent zonal shear flows. The multiscale method is used to eliminate some terms of natural frequency oscillations of nonlinear operators in the third-order expansion, thus generating an equation about the amplitude of the lowest-order Rossby wave in the long-time variable. The large amplitude perturbation begins to decrease, which produces the desired behavior. After the amplitude decreases for some time, the amplitude of Rossby waves can still be found to oscillate periodically with the time variable. [ABSTRACT FROM AUTHOR]

Intra‐seasonal variations have been frequently observed in the North Equatorial Undercurrent (NEUC) jets, yet their dynamical nature remains elusive. Based on field observations and model results, we identify three flavors of the ISVs: Wave (accounting for 10%), wave‐vortex dipole (WVD) (20%) and vortex (70%). The wave flavor refers to a Rossby wave, the WVD flavor consists of a pair of counter‐rotating vortices, and the vortex flavor refers to a sub‐thermocline monopole eddy. To our knowledge, the WVD is identified for the first time and is found characterized by a unique dynamical feature: It manifests both as a second baroclinic mode‐like Rossby wave and as a pair of dipole‐like sub‐thermocline eddies. It is further identified as a second baroclinic mode‐like Rossby wave‐initiated instability wave in an equilibrium being fueled by baroclinic conversion. These results indicate that mesoscale instability waves are an important component of subthermocline intra‐seasonal and mesoscale variations. Plain Language Summary: At the depths of 200–1,000 m of the Pacific Ocean between 10°N $10{}^{\circ}\mathrm{N}$ and 18°N $18{}^{\circ}\mathrm{N}$, the North Equatorial Undercurrent consisting of several jets flows eastward with mean velocity of ∼5 cm s−1. But it is not simply a steady current; instead, eastward and westward perturbations interlace, with an oscillation period of 70–120 days, which is called intra‐seasonal variations (ISVs). These variations are known to exist from previous field observations, but their dynamical nature remains elusive. Here we present that they are possibly associated with three distinct flavors of subsurface mesoscale activities with a length scale of O(100 km). They are called, "vortex wave", "wave‐vortex dipole" and "vortex". Among them, the wavevortex dipole flavor is an instability wave, dynamically intermediate between vortex wave and vortex, which might be an important component of oceanic intra‐seasonal and mesoscale variations. Key Points: Three flavors of the sub‐thermocline variability—wave, vortex (mesoscale eddy) and wave‐vortex dipole (WVD)—of the North Equatorial Undercurrent are identifiedThe WVD consists of a pair of antisymmetric vortices and is characterized by lens‐like temperature anomaliesThe WVD is identified as a second baroclinic mode‐like Rossby wave‐initiated instability wave fueled by baroclinic conversion [ABSTRACT FROM AUTHOR]

Interaction between CO2 and atmospheric radiation has a significant part in changing horizontal and vertical temperature distributions, through which it can affect the midlatitude atmospheric dynamics. Baroclinic instability, which is the energy source of large-scale eddy formation in midlatitudes, depends on meridional and vertical eddy fluxes of heat. While several studies have been devoted to the impact of CO2 concentration present in the atmosphere on baroclinic eddies in climate time-scales, the current work aims to determine the corresponding impact over a period of 10 to 20 days relevant to the duration of baroclinic life cycles when the flow is deterministically predictable. To this end, the radiative parametrization scheme called RRTMG (Rapid Radiative Transfer Model for General Circulation Models) has been coupled with the Diabatic Contour-Advective Semi-Lagrangian (DCASL) dynamical core. Except for parametrization of radiation as well as surface processes to represent radiative interaction with the surface, no other physics parametrizations are included in the model. The idealized life-cycle experiments are carried out for both the predominantly anticyclonic (LC1) and cyclonic (LC2) Rossby wave breaking with the same initialization but five different CO2 concentrations (0, 250, 500, 750 and 1,000 ppmv). The impacts of different CO2 concentrations on eddy growth, mean flow and eddy–mean flow interaction are discussed. Results show that the presence of CO2 has a damping effect on instability in the baroclinic growth stage, amounting to about 7% and 4% reduction in the peak value of domain area-average eddy kinetic energy for LC1 and LC2, respectively. In addition, increase in CO2 concentration has a small negative impact on the growth rate, vertical eddy propagation and jet intensification. [ABSTRACT FROM AUTHOR]

Baroclinic instability of a flow with a constant vertical shear in a zonal channel has been studied. The maximally truncated SQG model was used to describe the dynamics of flow disturbances. A dynamic system describing nonlinear interactions between unstable counter‐propagating Rossby waves with one zonal wave number and a neutral mode independent of zonal coordinate has been formulated within the framework of this model. Both analytical and numerical solutions of the system show that exponentially increasing disturbances at the linear stage of instability development give way to nonlinear oscillations (vacillations). The occurrence of such oscillations is caused by the law of conservation of surface potential energy. According to numerical estimates, the period of oscillations is of the order of a month, which is in agreement with measurement data obtained for the winter atmosphere. [ABSTRACT FROM AUTHOR]

Edge baroclinic waves are generated in a geostrophic flow with a vertical shear near a solid surface. The study investigates a new class of baroclinic waves in flows with horizontal and vertical shears and a linear distribution of potential vorticity. It is shown that taking account of the horizontal shear leads to the appearance of new features of wave dynamics. These include the nonmodal growth of energy in the initial stage of development, the time dependence of the vertical wave scale, and the possibility of generation of stationary or blocked waves. The horizontal shear makes the mechanism of generation of baroclinic waves by initial vortex perturbations more efficient. One important feature is associated with vortex paths, which are formed by the superposition of a baroclinic wave on the flow with horizontal shear. [ABSTRACT FROM AUTHOR]

A two-dimensional regime diagram for geostrophic turbulence in the ocean is constructed by plotting observation-based estimates of the non-dimensional eddy length-scale against a nonlinearity parameter equal to the ratio of the root-mean-square eddy velocity and baroclinic Rossby phase speed. Two estimates of the eddy length-scale are compared: the equivalent eddy radius inferred from the area enclosed by contours of sea-surface height, and the 'unsuppressed' mixing length, based on an estimate of the eddy diffusivity with mean flow effects removed. For weak nonlinearity, as found in the Tropics, the mixing length mostly corresponds to the stability threshold for baroclinic instability whereas the eddy radius corresponds to the Rhines scale; it is suggested that this mismatch is indicative of the inverse energy cascade that occurs at low latitudes in the ocean and the zonal elongation of eddies. At larger values of nonlinearity, as found at mid- and high latitudes, the eddy length-scales are much shorter than the stability threshold, within a factor of 2.5 of the Rossby deformation radius. [ABSTRACT FROM AUTHOR]

We identify the dynamical drivers of systematic changes in persistent quasi-stationary states (regimes) of the Southern Hemisphere troposphere and their secular trends. We apply a purely data-driven approach, whereby a multiscale approximation to nonstationary dynamical processes is achieved through optimal sequences of locally stationary fast vector autoregressive factor processes, to examine a high resolution atmospheric reanalysis over the period encompassing 1958-2013. This approach identifies regimes and their secular trends in terms of the predictability of the flow and is Granger causal. A comprehensive set of diagnostics on both isentropic and isobaric surfaces is employed to examine teleconnections over the full hemisphere and for a set of regional domains. Composite states for the hemisphere obtained from nonstationary nonparametric cluster analysis reveal patterns consistent with a circumglobal wave 3 (polar)-wave 5 (subtropical) pattern, while regional composites reveal the Pacific South American pattern and blocking modes. The respective roles of potential vorticity sources, stationary Rossby waves and baroclinic instability on the dynamics of these circulation modes are shown to be reflected by the seasonal variations of the waveguides, where Rossby wave sources and baroclinic disturbances are largely contained within the waveguides and with little direct evidence of sustained remote tropical influences on persistent synoptic features. Warm surface temperature anomalies are strongly connected with regions of upper level divergence and anticyclonic Rossby wave sources. The persistent states identified reveal significant variability on interannual to decadal time scales with large secular trends identified in all sectors apart from a region close to South America. [ABSTRACT FROM AUTHOR]

Tropopause folding is believed to be connected to the development of cyclonic systems in midlatitudes as well as the larger-scale, lower-frequency processes in the lower latitudes. To investigate such connections, a statistical and dynamical analysis of tropopause folds occurred in Southwest Asia is carried out for the period of 1989–2018. Tropopause folds are identified using the Era-Interim reanalysis dataset from the European Centre for Medium-Range Weather Forecasts. The spatial–temporal distributions of tropopause folding show that the frequency of folding is higher in the winter hemisphere than in the summer hemisphere and greater values of the frequency are found mostly within an elongated zonal band which is situated between 20 and 40 degrees latitude. Southwest Asia also has strong positive anomalous values of tropopause folding frequency and achieves the values more than five times the average of the Northern Hemisphere in the summer, especially in the northern latitudes of this region including Afghanistan, Iran and the eastern Mediterranean. This can be attributed mainly to the formation of summer monsoon in the subtropical latitudes of the Indian Ocean. In this regard, the examination of the effects of El Niño and La Niña on the spatial–temporal distributions of tropopause folding reveals that the values of 3-month running average of tropopause folding frequency coincident with La Niña tend to be larger than those with El Nino in both the hemispheres and Southwest Asia. This result is consistent with the previous findings on the tendency for higher occurrence of anticyclonic (cyclonic) baroclinic wave life cycles during La Niña (El Niño). Dynamically, the folds occurred in the study area are associated with the formation of waves at 500 hPa level and subsequently the creation of surface cyclones. In areas of high frequency of folding events, strong meridional shear of the zonal winds and jet stream amplification are observed at 200 hPa level. The analysis of wave activity flux and its divergence for the selected month of June 2015 shows two regions of wave propagation, one with a wave-packet like signature of flux divergence/convergence centers in the upper troposphere and one over the lower troposphere. The upper-tropospheric wave packet ends with an area of flux convergence related with the receipt and decay of Rossby waves, while in the lower troposphere, a flux divergence region as a source of stationary waves associated with the Indian monsoon activity is identified. • Frequency of folding is higher in winter than in summer and greater values exist mostly between 20-40 degrees latitude. • Frequency of folding in Southwest Asia can exceed five times the average frequency of the Northern Hemisphere in summer. • Tropopause fold frequency coincident with La Niña is larger than that with El Niño in both the hemispheres and southwest Asia. • In June 2015 with the maximum folding frequency, two regions of wave propagation are identified in the troposphere. • The frequencies of moderate and deep folds are maximized in the peak of the eddy-driven midlatitude cell activity in winter. [ABSTRACT FROM AUTHOR]

The Kuroshio, one of the most energetic western boundary currents in the world, shows variations in its mesoscale features and recirculation gyres, providing an excellent test case of interactions between the mesoscale field and Kuroshio Extension (KE) states. A three-layer quasi-geostrophic model was used to reconstruct flow fields continuous in time and the horizontal plane from the TOPEX/POSEIDON altimeter data based on the variational method. Compared with the solutions obtained by the nudging method, the present results proved that the variational solution was closer to the real field. In the assimilation period, 1993-1997, the baroclinic instability index (BII) was defined to be the phase shift from the uppermost layer to the lowest layer with mesoscale features. In the first half of the assimilation period, the KE took the transition from the elongated to contracted states, in which BII decreased gradually, as a consequence of the KE state shift. In the second half period, BII increased in the downstream region just west of the Shatsky Rise, in which baroclinic instability contributed to the final stage of the contracted state, and was followed by rapidly weakened instability as a trigger of the opposite transition from the contracted to elongated states. The wind-driven recirculation gyre played an active role on the KE transition in the first half period, although not in the second half. [ABSTRACT FROM AUTHOR]

The linear baroclinic and parametric instabilities of boundary currents with piecewise-constant potential vorticity are studied in a two-layer quasi-geostrophic model. The growth rates of both the exponential modes and of the optimal perturbations are calculated for the baroclinic instability of steady coastal currents. We show that the growth rates of the exponential modes are maximal for a vertically symmetric flow. Furthermore, the vertical asymmetries induced by different layer thicknesses, the presence of a barotropic potential vorticity or bottom topography, all act to dampen the growth rates and favor growth at shorter wavelengths. It is shown that this behavior can be predicted from the conditions for vertical resonance of Rossby waves on the two potential vorticity fronts. Also, the baroclinic instability of the optimal perturbations has larger growth rates at shorter wavelengths and shorter time scales. As well, the presence of a sloping bottom of moderate amplitude favors the growth of these optimal perturbations. Finally, we compute the growth rates of parametric instability of oscillatory coastal flows. We show that subharmonic resonance is the most unstable mode of growth. In addition, a second region of parametric instability is found (for the first time) away from marginality of exponential-mode baroclinic instability. It is shown that the functional dependency of the growth rates of parametric instability, for optimal excitation, are similar to that of the optimal perturbations of baroclinic instability. To explain this a mechanism for parametric instability, involving the rapid growth of short-wave optimal perturbations, is proposed. [ABSTRACT FROM AUTHOR]

Based on instability theory and some former studies, the Simple Ocean Data Assimilation (SODA) data are analyzed to further study the difference between the propagation of the ENSO-related oceanic anomaly in the off-equatorial North Pacific Ocean before and after 1976. The investigation shows that after 1976 in the off-equatorial North Pacific Ocean, there is a larger area where the necessary conditions for baroclinic and/or barotropic instability are satisfied, which may help oceanic anomaly signals propagating in the form of Rossby waves to absorb energy from the mean currents so that they can grow and intensify. The baroclinic energy conversion rate in the North Pacific after 1976 is much higher than before 1976, which indicates that the baroclinic instability has intensified since 1976. From another perspective, the instability analysis gives an explanation of the phenomena that the ENSO-related oceanic anomaly signal in the North Pacific has intensified since 1976. [ABSTRACT FROM AUTHOR]

A stability analysis of the coupled ocean–atmosphere is presented which shows that the potential energy (PE) of the upper layer of the ocean is available to generate coupled growing planetary waves. An independent analysis suggests that the growth of these waves would be maintained in the presence of oceanic friction. The growing waves are a consequence of relaxing the rigid lid approximation on the ocean, thus allowing an upward transfer of energy across the sea surface. Using a two and a half layer model consisting of an atmospheric planetary boundary layer, coupled with a two layer ocean comprising an active upper layer and a lower layer in which the velocity perturbation is vanishingly small, it is shown that coupled unstable waves are generated, which extract PE from the main thermocline. The instability analysis is an extension of earlier work [Tellus 44A (1992) 67], which considered the coupled instability of an atmospheric planetary boundary layer coupled with an oceanic mixed layer, in which unstable waves were generated which extract PE from the seasonal thermocline. The unstable wave is an atmospheric divergent barotropic Rossby wave, which is steered by the zonal wind velocity, and has a wavelength of about 6000 km, and propagates eastward at the speed of the deep ocean current. It is argued that this instability, which has a multidecadal growth time constant, may be generated in the Southern Ocean, and that its properties are similar to observations of the Antarctic Circumpolar Wave (ACW). [Copyright &y& Elsevier]