Synoptic-scale Waves Research Articles

The influence of El Niño on springtime synoptic-scale precipitation extremes in Southeastern China: insights from CMIP6 model simulations

This study focuses on El Niño impacts on springtime extreme precipitation in Southeastern China (SEC) by comparing observations with data from the Coupled Model Intercomparison Project phase 6 (CMIP6) historical runs. Observational and simulated results suggest that synoptic-scale temperature advection patterns over East Asia (EA) are closely associated with extreme precipitation in SEC, encompassing the Pearl River Basin (PRB), Yangtze River Basin (YRB), and Huaihe River Basin (HRB). Based on this, we introduce a temperature advection index (TAI) tailored to capture the cold-warm temperature advection dipole, which shows a significant positive correlation with SEC precipitation. Both observations and CMIP6 indicate that TAI-related circulations, characterized by upper-level synoptic-scale waves and a north–south oriented temperature gradient over EA, are conducive to extreme precipitation in northern PRB (NPRB)–YRB–HRB. However, the TAI-related synoptic-scale activities have a lesser impact on extreme precipitation in southern PRB (SPRB), as these disturbances mainly affect the mid-latitude weather. Further investigation reveals that during boreal spring following El Niño, 85% of extreme events in YRB–HRB are associated with positive TAI values, compared to 76% under climatological conditions. However, such a change in the association with TAI is not evident in CMIP6 simulations. From observations, atmospheric baroclinicity along the East Asian westerly jet is enhanced during El Niño, which promotes the development of TAI-related synoptic-scale disturbances. In contrast, CMIP6 models struggle to reproduce these observed baroclinicity signals during El Niño. This challenge arises from the background westerly jet bias and mean-state cold tongue bias in tropical Pacific temperature in models.

Simulating springtime extreme rainfall over Southern East Asia: unveiling the importance of synoptic-scale activities

This study investigates the influence of synoptic-scale activities on extreme precipitation during March–April–May (MAM) over Southern East Asia (SEA) using observational data and compares the results with the outputs from 20 Coupled Model Intercomparison Project phase 6 (CMIP6) historical runs. Observations show that SEA intense daily precipitation in MAM is linked to enhanced upper-level synoptic-scale waves; these disturbances are associated with significant anomalous temperature advection as well as moisture flux convergence, creating favorable conditions for extreme rainfall. Furthermore, it is found that a temperature advection index (TAI) can be utilized to characterize such synoptic-scale activities. Inspection of CMIP6 historical runs reveals that, among 20 models, 13 models perform well in accurately capturing the observed SEA rainfall pattern; such extreme events are also closely linked to TAI in the model environment. Overall, observed (simulated) results show that 78% (75%) of extreme events in the Yangtze River Basin–South Korea–south of Japan region can be attributed to positive TAI. Additionally, the related circulation anomalies such as the upper-level synoptic-scale wave feature, temperature advection, and moisture anomalies from these models closely resemble those observed during extreme precipitation days in SEA. Our findings suggest that TAI can effectively indicate both the frequency and intensity of extreme rainfall events in SEA, along with the associated synoptic-scale activities. Further study reveals a close lead-lag correlation between TAI and rainfall patterns over SEA. This correlation is characterized by eastward-propagating wave trains across the entire troposphere. Consequently, TAI not only acts as a benchmark for quantifying synoptic-scale extreme rainfall in SEA but also shows potential in predicting SEA rainfall linked to synoptic-scale disturbances.

Western North Pacific Monsoon Vorticity Distribution as a Potential Driver of Interannual Meridional Migration of the Boreal Summer Synoptic-Scale Wave Train

Abstract On interannual time scales, there is significant meridional migration of the boreal summer (May–October) synoptic-scale wave (SSW) train relative to the summer monsoon trough line over the western North Pacific (WNP) during 1979–2021. The associated plausible physical reasons for the SSW meridional migration are investigated by comparing analyses between two distinct groups: atypical SSW years where SSWs tend to prevail northward of the summer monsoon trough line and typical SSW years where SSWs largely occur along the summer monsoon trough line. During typical SSW years, SSWs originate primarily from equatorial mixed Rossby-gravity (MRG) waves and then develop into off-equatorial tropical depression (TD) waves in the lower troposphere of the monsoon region. During atypical SSW years, SSWs appear to be sourced from upper-level easterlies, propagating downward to the lower troposphere in the monsoon region, with a prevailing TD wave structure. A budget analysis of barotropic eddy kinetic energy suggests that interannual meridional SSW migration is closely related to changes in the vorticity distribution along the summer monsoon trough over the WNP, especially the western part of the summer monsoon trough. These changes cause low-frequency zonal convergence and shear differences, changing barotropic conversion around the monsoon trough and modulating interannual SSW meridional movement. In response to these changes, there are corresponding differences in SSW sources: a predominate MRG–TD wave pattern in typical SSW years and a predominate TD wave pattern in atypical SSW years. These results improve our understanding of the interannual variability of large-scale circulation and tropical cyclones.

Structure and Characteristics of Synoptic-Scale Waves in the Northern Eurasian Storm Track during Summer

Abstract This study examined the dominant structure and characteristics of synoptic-scale (2–8-day periods) waves over northern Eurasia during 40 summer seasons (June–August, 1979–2018). The synoptic-scale wave patterns are isolated using an extended empirical orthogonal function (EEOF) analysis on the 300-hPa geopotential height anomalies, and a composite based on atmospheric circulation fields and gridded precipitation product. The wave patterns are classified into two types from two pairs of EEOF modes. These two different wave types are defined as the polar frontal (PF) mode and Arctic frontal (AF) mode, respectively. The PF-mode waves are initiated in the North Atlantic sector to the west of the British Isles. They propagate eastward across Siberia into the North Pacific, and produce precipitation mainly over the Eurasian polar frontal zone. The AF-mode wave train arcs along the climatological Arctic frontal zone (AFZ). The AF-mode waves originate near the Scandinavian Peninsula. Their eastward passage brings precipitation along the AFZ. The development of the synoptic-scale waves is reflected by unique background conditions over northern Eurasia. The lower-tropospheric baroclinicity in southern Siberia and central Asia favored the baroclinic growth of the PF-mode waves. The AF-mode waves are trapped in the well-organized baroclinic zone along the north coast of the Eurasian continent. The baroclinic zone is coupled with a band of large meridional gradient of potential vorticity in the upper troposphere, suggesting that this band acts as a waveguide for the AF-mode waves. Significance Statement This study examines the synoptic-scale waves in the 2–8-day range of time scales over northern Eurasia during summer. The synoptic-scale waves are categorized into two distinct types at different latitude bands by the EEOF analysis on the 300-hPa z anomalies. They are defined as polar frontal (PF) mode and Arctic frontal (AF) mode. Then the EEOF-based composite analysis is conducted to detect the large-scale circulation anomalies associated with the propagation of different types of synoptic-scale waves. The structure and characteristics are examined. The roles of the mean background conditions in the development and propagation of the respective types are discussed. The behavior of these wave disturbances as rain-producing weather systems is also examined.

Toward a High-Resolution Wave Forecasting System for the Changjiang River Estuary

Based on a high-resolution unstructured SWAN model and GFS forecast wind, an operational wave forecasting system is conducted for the Changjiang River Estuary (CRE). The performance of the wave forecasting system is evaluated by comparing it with the altimeter observations and in situ wave buoys. The present operational system shows good accuracy in reproducing the seasonal and the synoptic-scale wave characteristics over the CRE. The forecasting capability in three different horizons, including 24 h, 48 h, and 72 h forecasts, is evaluated. Waves over the CRE exhibit distinct seasonal variability. Larger waves occur in both the summer and winter when typhoons and cold weather events affect the CRE. In contrast, waves with longer wave periods take place mainly in the wind transition seasons, i.e., the spring and fall, and the wave directions are more dispersed in these seasons. A seasonal varied forecasting capability is also revealed: better in the winter and spring than in the summer and fall and better during cold weather events than during typhoons. A cross comparison with the model analysis suggests that there is a systematic difference between wave measurements by Jason-3 and Sentinel-3A/3B. The significant wave height from Jason-3 compares best with the model analysis and forecasts and is systematically lower than Sentinel-3A/3B in lower wave conditions (<4 m) in the East China Sea. Substantial discrepancies exist among the three altimeters when the significant wave height exceeds 4 m, and further efforts are needed to discern their merits.

Effects of Wave-Mean Flow Interaction on the Multi-Time-Scale Variability of the AO Indices: A Case Study of Winters 2007/08 and 2009/10

Wave-mean flow interaction is usually regarded as accounting for the origin of the Arctic Oscillation/Northern Hemisphere Annular Mode (AO/NAM). It is inferred that the combination of the local wave-mean flow interactions at the AO/NAM’s three regional centers of action on three important time scales contributes to the main behavior of the AO/NAM index. To discuss the variations of the AO/NAM indices on the three prominent time scales, we take the 2007/08 and 2009/10 winters as two comparative examples to analyze the local wave-mean flow interactions at the AO/NAM’s three centers. The following three facets are identified: (1) Synoptic-scale wave breakings in the North Atlantic can explain the variances of the AO/NAM index on a time scale of 10–20 days. In the 2007/08 winter, there were both cyclonic and anticyclonic synoptic wave breakings, while in the 2009/10 winter, cyclonic synoptic wave breaking was dominant, and the flow characteristics were strikingly similar to the blocking. (2) In the 2007/08 and 2009/10 winters, the signals of the AO/NAM indices on the time scale of 30–60 days are mainly from the interactions between the upward propagating quasi-stationary waves and the polar vortex in the stratosphere. (3) This work also demonstrates that the AO/NAM is linked to the El Niño–Southern Oscillation (ENSO) by the Pacific–North American pattern (PNA) on the winter mean time scale. In the 2007/08 (2009/10) winter, La Niña (El Niño) forced the Pacific jet to shift poleward (equatorward), in favor of weakening (enhancing) the polar waveguide; thus, the polar vortex became stronger (weaker), corresponding to the positive (negative) winter mean AO/NAM index.

Influence of Synoptic‐Scale Waves on the Interdecadal Change in Tropical Cyclone Activity Over the Western North Pacific in the Early 2010s

AbstractIn this study, we investigated the interdecadal change in tropical cyclone (TC) activity over the western North Pacific (WNP) in the early 2010s. At the western boundary of the WNP, the interdecadal change in TC activity exhibited a meridional tripole pattern. In contrast to the reduced activity over the northern South China Sea (SCS) and Taiwan, TC activity increased over the southern SCS and to the north of Shanghai after the early 2010s. Herein, we focused on the northern WNP. Over the last decade, frequent TC occurrences have affected East China, Korea and Japan. In this work, we examined the influence of synoptic‐scale waves (SSWs) on the interdecadal change in TC activity. During the 2011–2021 period, SSWs tended to propagate northward, resulting in more TC tracks affecting the northern WNP and surrounding countries. In contrast, the westward‐propagating SSWs before the early 2010s were more likely to favor westward‐moving TCs.

Interannual snowfall variations in Central Asia and their association with ENSO and stratospheric polar vortex during winter

As a major source of moisture in Central Asia (CA), snowfall may significantly impact agriculture and economics in CA. The study has investigated the dominant modes of snowfall frequency during winter over CA and associated mechanisms. The first EOF mode (EOF1) of snowfall frequency corresponds to a homogeneous pattern over CA. In contrast, the second EOF mode (EOF2) is characterized by reversed anomalies over northern and southern CA. The mechanisms of the interannual variation of the two leading modes are different. EOF1 is influenced by the sea surface temperature anomalies (SSTA) over the North Atlantic and eastern tropical Pacific. Positive SSTA in the eastern tropical Pacific may stimulate a zonal wave train that propagates eastward and induce an anomalous cyclone in CA. The anomalous cyclone associated with ascending motions and water vapor transport convergence can contribute to increased snowfall frequency over CA. Besides, the interaction between the North Atlantic Oscillation and North Atlantic triple SSTA may also strengthen the zonal wave train. EOF2 is affected by the stratospheric polar vortex which is related to the wave reflections in winter. The wave reflections may strengthen the coupling of atmospheric circulation in the stratosphere and the troposphere, inducing a positive (negative) geopotential height anomaly over southern (northern) CA. These geopotential height anomalies may contribute to increased and decreased synoptic-scale wave activity over northern and southern CA which is conducive to the dipole mode of snowfall frequency over CA.

The Henan extreme rainfall in July 2021: Modulation of the northward-shift monsoon trough on the synoptic-scale wave train

The synoptic-scale wave train is a dominant pattern of the synoptic variability over the tropical western Pacific and usually affects the extreme weather over South China and Southeast Asia. Whether it could extend its influence and contribute to the Henan extreme rainfall in July 2021 still needs to be unraveled. We found that during the Henan extreme rainfall days a positively synoptic-scale vorticity disturbance dominated Henan province, China, which was embedded in the synoptic-scale wave train that originated from the western North Pacific. Moreover, the propagating pathway of this synoptic-scale wave train located northward and was likely modulated by the latitudinal location change of the monsoon trough over the western North Pacific. A northernmost displacement of the monsoon trough in July 2021 (∼23.2°N) would facilitate the synoptic-scale wave train to propagate farther northwestward via shifting the related barotropic conversion northward. Therefore, the synoptic-scale wave train from the tropics could reach Henan, provide the necessary lifting forcing, and supply abundant water vapor associated with the anomalous southerly for the occurrence of Henan extreme rainfall event. The results implicate that the pre-existing synoptic-scale wave train regulated by the location of the monsoon trough may be a potential precursor for heavy rainfalls in northern Central China.

Necessary conditions for the instability of quasigeostrophic waves induced by trace shortwave radiative absorbers

Necessary conditions for radiative–dynamical instability of quasigeostrophic waves induced by trace shortwave radiative absorbers are derived. The analysis pivots on a pseudomomentum conservation equation that is obtained by combining conservation equations for quasigeostrophic potential vorticity, thermodynamic energy, and trace absorber mixing ratio. Under the assumptions that the absorber-induced diabatic heating rate is small and the zonal-mean basic state is hydrodynamically neutral, a perturbation analysis of the pseudomomentum equation yields the conditions for instability. The conditions, which only require knowledge of the zonally averaged background distributions of wind and absorber, expose the physical processes involved in destabilization—processes not exposed in previous analytical and modeling studies of trace absorber-induced instabilities. The simplicity of instability conditions underscores their utility as a tool that is both interpretive and predictive. The conditions for instability, which have broad application to synoptic-scale waves in Earth's and other planetary atmospheres, are discussed in light of previous instability studies involving stratospheric ozone and Saharan mineral dust aerosols.

Impacts of Stratospheric Polar Vortex Shift on the East Asian Trough

Abstract Impacts of stratospheric polar vortex shift on the wintertime East Asian trough (EAT) on intraseasonal time scales are investigated using a reanalysis dataset and a climate model. The result based on composite analysis shows that the shift of the stratospheric polar vortex toward eastern Siberia (ES-shift event) is associated with higher geopotential height at 500 hPa than normal over East Asia, corresponding to the weakened EAT. Furthermore, the simulated EAT is also weakened when nudging the stratospheric state toward that during the ES-shift events. This study further found that there is no significant difference in the stratospheric polar vortex intensity between the ES-shift events and nonshift events, implying that the polar vortex strength change may have little influence on the possible connection between the polar vortex shift and the EAT change. The underlying mechanisms are listed as follows: First, the positive potential vorticity (PV) anomalies in the lower stratosphere associated with ES-shift events could explain approximately 40% of the local westerly anomalies in the upper troposphere to the north of East Asia via PV conservation, leading to the rise of the geopotential height over East Asia and the weakening of the EAT. Second, the shift of stratospheric polar vortex could modulate the synoptic-scale Rossby wave activity in the upper troposphere, favorable for the southward propagation of synoptic-scale waves and divergence of extended Eliassen-Palm flux in the upper troposphere. Finally, the transient wave feedback could enhance the tropospheric westerly anomalies in the north of East Asia and induce positive height anomalies to its south, further weakening the EAT. Our results revealed that the stratospheric polar vortex shift leads the EAT intensity variation by around 2–5 days, implying that the stratospheric polar vortex shift could be applicable to the prediction of the EAT intensity.

Close Linkage of the South China Sea Summer Monsoon Onset and Extreme Rainfall in May over Southeast Asia: Role of the Synoptic-Scale Systems

Abstract Based on multiple datasets and methods, this study reveals a close relation between the South China Sea summer monsoon (SCSSM) onset and the occurrence frequency of extreme rainfall over Southeast Asia (including the Indochina Peninsula, Hainan Island, and the Philippines) in May. Specifically, an early SCSSM onset tends to be accompanied by a higher chance of extreme rainfall over Southeast Asia, while a late SCSSM onset is likely to coincide with less heavy rainfall in May. It is suggested that the amplitude of tropical synoptic-scale systems may play an important role in the linkage between the SCSSM onset and the extreme rainfall over Southeast Asia. Accompanying an early SCSSM onset, anomalous cyclonic circulation appears over the SCS and Philippine Sea, which tends to increase the quantity of midtroposphere moisture via moisture advection and Ekman pumping. Besides, the barotropic energy conversion (from mean kinetic to eddy kinetic energy) associated with this anomalous cyclone will facilitate the development of the synoptic-scale systems. The vertical easterly wind shear accompanying an early SCSSM onset will confine the synoptic-scale wave train to the lower troposphere, which is also favorable for its growth. Once these synoptic-scale systems develop, the baroclinic process (from eddy available potential energy to eddy kinetic energy) will further promote their development. As such, the intensity of synoptic-scale systems is much stronger in early than late SCSSM onset years. The increased moisture and vigorous synoptic-scale systems further lead to an increase in the occurrence of extreme rainfall over Southeast Asia.

The impact of atmospheric Rossby waves and cyclones on the Arctic sea ice variability

The Arctic sea-ice extent has strongly declined over recent decades. A large inter-annual variability is superimposed on this negative trend. Previous studies have emphasised a significant warming effect associated with latent energy transport into the Arctic region, in particular due to an enhanced greenhouse effect associated with the convergence of the humidity transport over the Arctic. The atmospheric energy transport into the Arctic is mostly accomplished by waves such as Rossby waves and cyclones. Here we present a systematic study of the effect on Arctic sea ice of these atmospheric wave types. Through a regression analysis we investigate the coupling between transport anomalies of both latent and dry-static energy and sea-ice anomalies. From the state-of-the-art ERA5 reanalysis product the latent and dry-static transport over the Arctic boundary (70^{circ } N) is calculated. The transport is then split into transport by planetary and synoptic-scale waves using a Fourier decomposition. The results show that latent energy transport as compared to that of dry-static shows a much stronger potential to decrease sea ice concentration. However, taking into account that the variability of dry-static transport is of an order of magnitude larger than latent, the actual impact on the sea ice appears similar for the two components. In addition, the energy transport by planetary waves causes a strong decline of the sea ice concentration whereas the transport by synoptic-scale waves shows only little effect on the sea ice. The study emphasises the importance of the large-scale waves on the sea ice variability.

Convective Couplings with Equatorial Rossby Waves and Equatorial Kelvin Waves. Part I: Coupled Wave Structures

Abstract This study investigates precipitation amounts and apparent heat sources, which are coupled with equatorial Kelvin waves and equatorial Rossby waves, using TRMM PR level 2 data products. The synoptic structures of wave disturbances are also studied using the ERA5 dataset. We define the wave phase of equatorial waves based on FFT-filtered brightness temperature and conduct composite analyses. Rossby waves show a vertically upright structure and their upright vortices induce large-amplitude column water vapor (CWV) anomalies. Precipitation activity is almost in phase with CWV, and thus is consistent with a moisture mode. Kelvin waves, on the other hand, indicate a nearly quadrature phase relationship between temperature and vertical velocity, like gravity wave structure. Specific humidity develops from near the surface to the middle troposphere as the Kelvin wave progresses. A clear negative CWV anomaly also does not exist despite the existence of negative precipitation anomalies. Convective activity corresponds well with its tilting structure of moisture and modulates the phase relationship between temperature and vertical motion. For both wave cases, apparent heat sources can amplify available potential energy despite the difference of coupling mechanisms of these two waves; precipitation is driven by CWV fluctuation for the Rossby wave case, and by buoyancy-based fluctuations for the Kelvin wave case. These can be observational evidence of actual coupling processes that is comparable to previous idealized studies. Significance Statement A coupling mechanism between equatorial waves and convective activity is a significant issue in tropical meteorology. While many previous idealized studies suggested some instability mechanisms, their true roles are not yet clear because detailed precipitation characteristics are not well investigated. We aim to quantify precipitation and synoptic-scale wave disturbances, and compare equatorial Rossby waves and equatorial Kelvin waves, which should have different instability coupling modes between each other, in order to shed light on a convectively coupling mechanism. We found that precipitation is actually driven by column moisture in Rossby waves and by dynamical fluctuation in Kelvin waves. Despite these competing mechanisms, similar top-heavy heating can maintain convectively coupled disturbances. Our observational results will support and improve theoretical studies.

Influences of central Pacific warming on synoptic-scale wave intensity over the northwest Pacific

Influences of central Pacific warming on synoptic-scale wave intensity over the northwest Pacific

Structured design of Australia’s in situ wave observing network

ABSTRACT This work revisits and extends an analysis of the Australian wave observing network. The method is based on calculating correlations between modelled wave variables at observation sites and the Australian coastal domain and identifying areas of low correlation. This gives an indication of the areas where variability of the wave fields is poorly captured by existing observation locations, i.e. the network gaps. It is found that the gaps in the network that had been identified in previous work are to a large part reduced by the recent deployment of new infrastructure, but not eliminated. It is also found that the network does less well at observing the synoptic scale wave field than it does at observing the monthly wave climate. Lagged correlations are also considered but found to have only a modest impact on the ability of the network to observe the wave field variability.

Is our dynamical understanding of the circulation changes associated with the Antarctic ozone hole sensitive to the choice of reanalysis dataset?

Abstract. This study quantifies differences among four widely used atmospheric reanalysis datasets (ERA5, JRA-55, MERRA-2, and CFSR) in their representation of the dynamical changes induced by springtime polar stratospheric ozone depletion in the Southern Hemisphere from 1980 to 2001. The intercomparison is undertaken as part of the SPARC (Stratosphere–troposphere Processes and their Role in Climate) Reanalysis Intercomparison Project (S-RIP). The reanalyses are generally in good agreement in their representation of the strengthening of the lower stratospheric polar vortex during the austral spring–summer season, associated with reduced radiative heating due to ozone loss, as well as the descent of anomalously strong westerly winds into the troposphere during summer and the subsequent poleward displacement and intensification of the polar front jet. Differences in the trends in zonal wind between the reanalyses are generally small compared to the mean trends. The exception is CFSR, which exhibits greater disagreement compared to the other three reanalysis datasets, with stronger westerly winds in the lower stratosphere in spring and a larger poleward displacement of the tropospheric westerly jet in summer. The dynamical changes associated with the ozone hole are examined by investigating the momentum budget and then the eddy heat and momentum fluxes in terms of planetary- and synoptic-scale Rossby wave contributions. The dynamical changes are consistently represented across the reanalyses and support our dynamical understanding of the response of the coupled stratosphere–troposphere system to the ozone hole. Although our results suggest a high degree of consistency across the four reanalysis datasets in the representation of these dynamical changes, there are larger differences in the wave forcing, residual circulation, and eddy propagation changes compared to the zonal wind trends. In particular, there is a noticeable disparity in these trends in CFSR compared to the other three reanalyses, while the best agreement is found between ERA5 and JRA-55. Greater uncertainty in the components of the momentum budget, as opposed to mean circulation, suggests that the zonal wind is better constrained by the assimilation of observations compared to the wave forcing, residual circulation, and eddy momentum and heat fluxes, which are more dependent on the model-based forecasts that can differ between reanalyses. Looking forward, however, these findings give us confidence that reanalysis datasets can be used to assess changes associated with the ongoing recovery of stratospheric ozone.

Dynamic Control of the Dominant Modes of Interannual Variability of Snowfall Frequency in China

AbstractThis study investigates the first two leading modes of the interannual variability of frequency of snowfall events (FSE) over China in the winter during 1986–2018. The positive phase of the first leading mode (EOF1) is mainly characterized by positive FSE anomalies in northeastern–northwestern China and negative FSE anomalies in the three-river-source region. In contrast, the positive phase of the second leading mode (EOF2) is mainly characterized by positive FSE anomalies in central-eastern China (CEC). EOF1 is affected by the synoptic-scale wave activity over the midlatitudes of the East Asian continent, where active synoptic-scale wave activity over the midlatitudes may cause increased FSE over northeastern–northwestern China, and vice versa. In a winter of a negative phase of the North Atlantic Oscillation, an anomalous deep cold low may occur over Siberia, which may induce increased meridional air temperature gradient, increased atmospheric baroclinicity, and hence increased FSE over the midlatitudes of the East Asian continent. The EOF2 is affected by the interaction between anomalous northerly cold advection and anomalous southerly water vapor transport over CEC. The positive phase of EOF2 is associated with negative sea ice anomalies in the Barents Sea–Kara Sea region and negative sea surface temperature anomalies in the central-eastern tropical Pacific. Reduced sea ice in the Barents Sea–Kara Sea during January–February may cause increased northerly cold advection over CEC, while a La Niña–like condition during January may induce southerly water vapor transport anomalies over CEC.

Dynamic and thermodynamic modulations of the convectively coupled equatorial waves by the MJO

The convectively coupled equatorial waves (CCEWs), including the Kelvin, mixed-Rossby gravity (MRG), eastward inertio-gravity (EIG), and westward inertio-gravity (WIG) waves, are dominant synoptic-scale waves in the tropics. In this study, the modulation of the CCEWs by the MJO is examined with observational data from 1985–2005 over the Indian Ocean (IO), Maritime Continent (MC), and Western Pacific (WP). We find that the MRG wave is strengthened (weakened) to the west (east) of the MJO convection. In contrast, the Kelvin, WIG and EIG waves are mostly strengthened over the MJO convective center. As MJO modulates the background vertical wind shear and moisture fields, a further analysis was conducted to reveal the relationship between the background dynamic and thermodynamic field changes and the wave intensity change. A significant negative correlation between the MJO-scale vertical wind shear and the MRG intensity variation suggests that the MRG wave is primarily modulated by the dynamic field. The intensity changes of the WIG and EIG waves are significantly correlated to the MJO moisture field. The Kelvin wave, which has both quasi-geostrophic and gravity wave nature, is modulated by both the MJO-scale vertical wind shear and specific humidity.

The Lorenz energy cycle: trends and the impact of modes of climate variability

The atmospheric circulation is driven by heat transport from the tropics to the polar regions, embedding energy conversions between available potential and kinetic energy through various mechanisms. The processes of energy transformations related to the dynamics of the atmosphere can be quantitatively investigated through the Lorenz energy cycle formalism. Here we examine these variations and the impacts of modes of climate variability on the Lorenz energy cycle by using reanalysis data from the Japanese Meteorological Agency (JRA-55). We show that the atmospheric circulation is overall becoming more energetic and efficient. For instance, we find a statistically significant trend in the eddy available potential energy, especially in the transient eddy available potential energy in the Southern Hemisphere. We find significant trends in the conversion rates between zonal available potential and kinetic energy, consistent with an expansion of the Hadley cell, and in the conversion rates between eddy available potential and kinetic energy, suggesting an increase in mid-latitudinal baroclinic instability. We also show that planetary-scale waves dominate the stationary eddy energy, while synoptic-scale waves dominate the transient eddy energy with a significant increasing trend. Our results suggest that interannual variability of the Lorenz energy cycle is determined by modes of climate variability. We find that significant global and hemispheric energy fluctuations are caused by the El Nino-Southern Oscillation, the Arctic Oscillation, the Southern Annular Mode, and the meridional temperature gradient over the Southern Hemisphere.