Flux Convergence Research Articles

Physical mechanisms for the dominant summertime high-latitude atmospheric teleconnection pattern and the related Northern Eurasian climates

Summertime atmospheric teleconnection patterns over Eurasia have a significant influence on regional weather and climate. Despite extensive studies on the subtropical patterns, the high-latitude counterpart has received relatively less attention. This study proposes physical mechanisms for the formation and maintenance of the dominant high-latitude teleconnection pattern. The formation of the pattern is associated with variability in synoptic-scale eddy activity due to the meridional gradient of sea surface temperature anomalies in the vicinity of the Gulf Stream, causing a meridional shift of the central axis of storm track at the exit of Atlantic jet. The resultant convergence of transient vorticity fluxes to the west of the British Isles induces low-frequency cyclonic circulation anomalies and continued propagation of Rossby waves downstream along northern Eurasia. Once these circulation anomalies are formed, the subsequent latent heat-related diabatic anomalies over the northern Eurasian landmass act as another source of Rossby waves to maintain the teleconnection pattern. Regional temperature and precipitation variability is closely linked to the wave pattern along a route through northern Eurasia, and even precipitation over the East Asian summer monsoon region is influenced by the teleconnection pattern.

Analysis of Mutriku's OWC performance

An analysis of oscillating water columns (OWC) at the Mutriku Wave Power Plant to understand their efficiency in transforming ocean energy into electricity. The study aims to estimate the rate of convergence of wave energy flux to kilowatts hour of each OWC and determine whether it is more accurate to estimate for each OWC independently or as part of a system of equations. The dataset includes electricity production data from 7 turbines in the MWPP during 2019, along with ocean measurements such as significant wave height, wave period, wave energy flux, swell direction, and wind direction. All models in the paper have varying coefficients which change with the significant wave height. The methodology includes the use of generalized least squares and local polynomial kernel methodologies for coefficient estimation.

Adverse impact of terrain steepness on thermally driven initiation of orographic convection

Abstract. Diurnal mountain winds precondition the environment for deep moist convection through horizontal and vertical transport of heat and moisture. They also play a key role in convection initiation, especially in strongly inhibited environments, by lifting air parcels above the level of free convection. Despite its relevance, the impact of these thermally driven circulations on convection initiation has yet to be examined systematically. Using idealized large-eddy simulations (Δx=50 m) with the Weather Research and Forecasting (WRF) model, we study the effect of cross-valley circulations on convection initiation under synoptically undisturbed and convectively inhibited conditions, considering quasi-2D mountain ranges of different heights and widths. In particular, we contrast convection initiation over relatively steep mountains (20 % average slope) and less steep ones (10 %). One distinctive finding is that, under identical environmental conditions, relatively steep mountain ranges lead to a delayed onset and lower intensity of deep moist convection, although they cause stronger thermal updrafts at ridge tops. The temporal evolution of convective indices, such as convective inhibition and convective available potential energy, shows that destabilization over the steeper mountains is slower, presumably due to lower low-level moisture. Analysis of the ridgetop moisture budget reveals the competing effects of moisture advection by the mean thermally driven circulation and turbulent moisture transport. In general, at mountaintops, the divergence of the turbulent moisture flux offsets the convergence of the advective moisture flux almost entirely. Due to the stronger ridgetop updraft, the mean advective moistening over the steeper mountains is higher; nevertheless, the total moistening is lower and the width of the updraft zone is narrower on average. Thus, buoyant updrafts over the steeper mountains are more strongly affected by the turbulent entrainment of environmental air, which depletes their moisture and cloud water content and makes them less effective at initiating deep convection. Saturated updrafts over less steep mountains, on the other hand, gain more moisture from the vapor flux at cloud base, leading to significantly higher moisture accumulation. The lower entrainment rates in these simulations are revealed by the fact that equivalent potential temperature in the cloud decreases less strongly with height than over steeper terrain. The precipitation efficiency, a measure of how much of the condensed water eventually precipitates, is considerably larger over the less steep mountains, also due to lower total condensation compared with the steeper simulations. The relationship between mountain size and precipitation amount depends on the thermodynamic profile. It is nearly linear in cases with low initial convective inhibition but more complex otherwise. The weaker convection over steeper mountains is a robust finding, valid over a range of background environmental stability and mountain sizes.

Trends in Warm Season Mesoscale Convective Systems Over Asia in 2001–2020

AbstractMesoscale convective systems (MCSs) frequently occur over Asia during the warm season, often producing intense precipitation with associated socioeconomic impacts. Here we reveal significant trends in MCS occurrence frequency and related precipitation in Asia during the warm season (March–September) in 2001–2020, using a tracking method that combines cloud and precipitation criteria with high‐resolution satellite data from the Global Precipitation Measurement mission. To examine whether there are differences between MCSs of different scales, both meso‐α scales (MαCSs) and meso‐β scales (MβCSs), with horizontal scales of 200–2,000 km and 20–200 km, are tracked. The distribution pattern of frequency and related precipitation of both MαCSs and MβCSs are quite similar and manifest positive trends over East Asia (EA) and Northeast Asia, and negative trend over Southeast Asia (SEA). The MCS precipitation trend contributes significantly to total precipitation trend, with MαCSs contributing the most. Our analysis indicates the trend in lower‐tropospheric water vapor flux convergence has a similar spatial pattern to the MCS frequency and related precipitation trend. Based on an atmospheric moisture flux decomposition analysis, the water vapor flux convergence trend can largely be explained by the change in horizontal wind convergence, while the specific humidity trend driven largely by temperature change plays a minor role. The trend in wind convergence in EA and SEA is possibly related to the evident trend in the lower‐tropospheric anticyclone over the western North Pacific and SEA, which might be due to the relatively stronger warming in the Indian Ocean during the past two decades.

Interannual Variability of the Mass-Weighted Isentropic Zonal Mean Meridional Circulation in the Northern Hemisphere Winter

Abstract The atmospheric general circulation diagnosed by the mass-weighted isentropic zonal mean exhibits an extratropical direct (ETD) circulation driven by eddy momentum transport. We investigated regional atmospheric modulations associated with the year-to-year variability of ETD circulation in boreal winter using a reanalysis dataset. Composite analyses showed that the interannual variability of ETD circulation accompanies a seesaw-like variation between the Aleutian low (AL) and Icelandic low (IL), which coincides with the Pacific–North American teleconnection pattern and North Atlantic dipole anomaly. A diagnosis using isentropic airmass fluxes representing the geographical Lagrangian mean motion indicated that the significant intensification of upper-tropospheric poleward warm airflow over the eastern North Pacific is responsible for the enhanced ETD circulation, together with the intensified equatorward lower cold airflow over East Asia and eastern North America. The resulting enhanced upward propagation of the stationary planetary waves corresponds to the intensification of the divergence of the Eliassen–Palm flux, which explains the modulated ETD circulation in view of the balance between the Coriolis force and eddy momentum flux convergence. These results suggest that the AL–IL seesaw promotes variations in the global general circulation, and the anomalous ETD circulation interacts with several teleconnection patterns involving the modulation of planetary waves through anomalous meridional heat transport by the basinwide-scale eddies.

What Determines the Leading Mode of Wintertime Northern Hemisphere Snow-Cover Interannual Variability?

Abstract The Northern Hemisphere snow-cover (NHSC) interannual variability can serve as a crucial source of climate predictability. However, the spatiotemporal characteristics of the NHSC interannual variability and associated formation mechanism lack a thorough investigation using the latest reliable datasets. In this observationally based study, we identify the leading mode of the wintertime NHSC interannual variability and reveal the major determinants and mechanisms for forming the mode. Results show that the leading mode exhibits a hemispheric dipole pattern characterized by snow-cover anomalies in the central-eastern United States and Europe, contrary to those in the Tibetan Plateau (TP). Importantly, the leading mode of TP-only snow-cover variability is essentially a regional manifestation of the dipole NHSC anomaly pattern. The wintertime hemispheric atmospheric circulation anomaly mode, that is, the Arctic Oscillation (AO), primarily determines the dipole NHSC anomaly pattern via altering snowfall and snowmelt. During a positive phase of AO, an anomalous atmospheric anticyclone over the midlatitude North Atlantic favors increased warm-air advection over the central-eastern United States and Europe, reducing the snowfall and the snow cover there. Meanwhile, the anomalously warm atmospheric thermal condition in Europe accompanied by enhanced westerly moisture flux convergence tends to heat the underlying land surface via reducing upward sensible heat flux and increasing downward longwave radiation, thereby causing increased snowmelt and decreased snow cover in northeastern Europe. Associated with the aforementioned atmospheric circulation anomalies, an anomalous atmospheric cyclone prevailing over the TP tends to cause increased southeasterly moisture flux convergence, favoring snowfall and snow increase in the TP.

ENSO‐Related Southern Hemisphere Blocking Dynamics

AbstractThis study establishes a new blocking climatology in the Southern Hemisphere (SH) by using the improved detection method with the reanalysis data from 1979 to 2016. Furthermore, it investigates the dynamical linkage between SH blocking and ENSO phases. It is found that the ENSO warm phase is more conducive to the blocking formation than the cold phase while the blocking intensity is not affected by the ENSO phases. In addition, blocking in ENSO warm phase is characterized with upper‐level divergence anomaly and the cold phase with convergence anomaly equatorward of blocking onset region. Through the diagnostic analysis of tropopause divergence for the composite blocking events via an improved divergence tendency equation, the study shows that the ageostrophic effect exhibits dual effects, namely, it induces anticyclonic ageostrophic vorticity favoring the formation of blocking whereas it results in convergence anomaly close to the blocking onset region thus inhibiting blocking locally. In addition, the nonlinear term in the divergence tendency equation is found to be a dominant forcing for favoring blocking onset. Nevertheless, despite the primary contributions from the ageostrophic effect and nonlinear term to blocking formation, they could barely explain the SH blocking onset preference in ENSO warm phase. Finally, through the localized Eliassen‐Palm flux this study reveals that in ENSO warm phase the composite blocking onset region and the immediate upstream region are characterized by more localized EP flux convergence, thereby leading to more effective westerly wind weakening and favoring the blocking formation.

Factors affecting interannual variation in late summer rainfall in the Red River Delta of Vietnam

Based on station monthly observational statistics, the major rainfall in the Red River Delta (RRD) of Vietnam occurs in late summer (July–September) with conspicuous year-to-year variation. Using the ± 0.8 of the seasonal total rainfall standard deviation as criteria, seven wet and six dry years are identified over the period 1983–2015. In addition to the 70.5% of the seasonal total rainfall contributed by the heavy rainfall days, the distinct heavy rainfall accumulation difference between wet and dry years seems to fundamentally establish these two separated extreme wet and dry groups. As revealed from further analyses, the large variability in rainfall is attributed to the influence of tropical cyclones (TCs) and 7–24- and 30–60-day intraseasonal oscillations (ISOs); in particular, the number of TCs affecting the RRD and rainfall produced by TCs are more (less) during the wet (dry) years, and the amplitudes of ISOs are also enhanced (reduced). In many cases, heavy rainfall days are induced by the combined effect of both ISOs and TCs, while some heavy rainfall events are mainly triggered by ISOs. It is found from the water vapor budget analyses that an anomalous cyclone (anticyclone) dominates over the Indochina Peninsula in wet (dry) years, resulting in more (less) water vapor being transported to the RRD, whereas the anomalous convergence (divergence) of water vapor flux leads to the maintenance of excessive (insufficient) rainfall over the RRD. However, the El Niño–Southern Oscillation (ENSO) forcing shows minor effects on the interannual variation in rainfall in the RRD.

Vertical Convergence of Turbulent and Double-Diffusive Heat Flux Drives Warming and Erosion of Antarctic Winter Water in Summer

Abstract The seasonal warming of Antarctic Winter Water (WW) is a key process that occurs along the path of deep water transformation to intermediate waters. These intermediate waters then enter the upper branch of the circumpolar overturning circulation. Despite its importance, the driving mechanisms that mediate the warming of Antarctic WW remain unknown, and their quantitative evaluation is lacking. Using 38 days of glider measurements of microstructure shear, we characterize the rate of turbulent dissipation and its drivers over a summer season in the northern Weddell Sea. Observed dissipation rates in the surface layer are mainly forced by winds and explained by the stress scaling (r2 = 0.84). However, mixing to the base of the mixed layer during strong wind events is suppressed by vertical stratification from sea ice melt. Between the WW layer and the warm and saline circumpolar deep water, a subsurface layer of enhanced dissipation is maintained by double-diffusive convection (DDC). We develop a WW layer temperature budget and show that a warming trend (0.2°C over 28 days) is driven by a convergence of heat flux through mechanically driven mixing at the base of the mixed layer and DDC at the base of the WW layer. Notably, excluding the contribution from DDC results in an underestimation of WW warming by 23%, highlighting the importance of adequately representing DDC in ocean models. These results further suggest that an increase in storm intensity and frequency during summer could increase the rate of warming of WW with implications for rates of upper-ocean water mass transformation. Significance Statement Around Antarctica, the summer warming of the subsurface cold Antarctic Winter Water feeds the upper layer of the overturning circulation. This study aims to quantify the mechanisms that mediate the warming of Antarctic Winter Water. Our results reveal that the observed warming of this layer can be explained by both surface wind-driven mixing processes as well as double-diffusive convection occurring beneath the Winter Water layer. Understanding the role of these mechanisms is important for understanding the regions upper-ocean heat distribution, the rates of water mass transformation and how they might respond to changes in sea ice, stratification, and the overlying large-scale winds.

Diurnal Variation Characteristics of Clouds and Precipitation during the Summer Season in Two Typical Climate Regions of the Tibetan Plateau

Mêdog and Nagqu are two typical climate regions of the Tibetan Plateau, with different atmospheric conditions and local orography. This may lead to different diurnal variation patterns of clouds and precipitation. This paper investigates the diurnal variations of clouds and precipitation in Mêdog and Nagqu, using ground-based measurements from Ka-band cloud radar and a Particle Size and Velocity (PARSIVEL) disdrometer. High frequencies of cloud cover and precipitation occur from 23:00 local solar time (LST) to 05:00 LST in Mêdog, while low frequencies appear from 11:00 LST to 17:00 LST. The occurrence frequencies in Nagqu maintain high values from 13:00 LST to 21:00 LST. In terms of mean rain rate, heavier rainfall appears in the evening and at night in Mêdog, with peaks at 00:00 LST and 18:00 LST, respectively. In Nagqu, the heaviest rainfall occurs at 12:00 LST. In addition, the afternoon convective rainfall in Nagqu is characterized by a much higher concentration of large drops, which can be classified as continental-like. The morning rainfall has the lowest concentration of large drops and can be classified as maritime-like. Finally, the mechanisms of diurnal variations in the two regions are discussed. The diurnal cycle of clouds and precipitation in Mêdog may be associated with the nocturnal convergence of moisture flux and mountain–valley wind circulation. Diurnal variations in Nagqu have a high correlation with the diurnal cycle of solar radiation. The high nocturnal frequency of clouds and precipitation in the two regions at night is closely related to the convergence of moisture flux.

Nanoscale contact state design for nodal energy transfer at crossed Ag nanowires

The thermal conduction properties of the cross-junction formed by one pair of Ag nanowires were characterized using experiments and simulations. By laser-sintering crossed nanowires on a substrate on which the Ag ion complex ink was dispersed, we succeeded in visualizing the heat flow trajectory of the junction according to the application of energy. To theoretically understand the energy transfer behavior at the junctions, Ag nanowires with penta-twinned cross-sections were modeled by all-atom molecular dynamics simulations. The equilibrium and non-equilibrium simulation results showed the structural stability at the interface junction, convergence of heat flux in an unsteady state, and effective thermal conductivity of the entire system in a steady state. In particular, we focused on the dependence of the diameter on the nanoscale region, which is the most important design variable for Ag nanowires. Accordingly, a structure–property relationship governed by the nanoscale geometry of the junction was identified.

A new approach for the Modified Local Green’s Function Method applied to solid mechanics problems

The Modified Local Green’s Function Method (MLGFM) is an integral method that does not need a previous explicit fundamental solution or a Green’s function, since projections of Green’s functions, determined by Finite Element Method, are used as fundamental solution to solve problems. As demonstrated in previous works, the MLGFM presents good convergence for potential, displacements, boundary flux and tractions, although the obtaining of the Green’s function projections can require a high computational effort. This paper proposes an alternative formulation to the MLGFM that avoids the need to obtain these projections and consequently reduces the number of numerical operations. The new formulation presents the same accuracy as the previous one but with less computational effort, and it is applied to some problems in solid mechanics and compared with the standard Finite Element Method and the Boundary Element Method.

Impact of Arctic Sea Ice Interannual Variation on Nonmonsoonal Winter Precipitation over the Eurasian Continent

Abstract The present study analyzes the impact of interannual variation in autumn (September–November) Arctic sea ice concentration (SIC) on early winter (November–January) precipitation over nonmonsoonal Eurasian (NME) regions based on both observations and numerical model experiments. An energy budget analysis shows that negative autumn SIC anomalies in the Beaufort–Chukchi–East Siberian Seas (BCES) induce heating in the overlying atmospheric column, which excites a Rossby wave that propagates from the BCES through the Atlantic Ocean to the mid- and high-latitude Eurasian continent. This Rossby wave obtains energy from the mean flow by both baroclinic and barotropic energy conversions. In comparison, the baroclinic energy conversion is more important than the barotropic energy conversion. The low-level anomalous cyclone and anticyclone of the Rossby wave dominate the eastern North Atlantic Ocean–southern Europe and western Russia, respectively. Anomalous westerly wind along the south flank of the anomalous cyclone transports moisture from the North Atlantic Ocean to the continent, resulting in a water vapor flux convergence and positive precipitation anomaly over southern Europe in early winter. Pronounced anomalous northerly winds in the eastern part of the western Russian anticyclone cause negative precipitation anomalies over vast regions of central Asia and the west Siberian plain in early winter. The Rossby wave ray tracing experiment and numerical sensitivity experiments support the above BCES SIC–NME precipitation connection.

The role of external and local atmospheric moisture fluxes in the interannual variability of peak summer precipitation over the northern fringe of the East Asian summer monsoon

On the northern fringe of the East Asian summer monsoon (NFEASM), both external atmospheric moisture transport and local evaporation are important moisture sources of moisture that feed peak summer precipitation, but their relative importance in regulating the precipitation variability remains unclear. This study uses the atmospheric water budget analysis to investigate the contributions of external and local atmospheric moisture fluxes to the interannual variability of peak summer (July and August) precipitation over the NFEASM between 1980 and 2015. The results show that, compared with the net atmospheric moisture flux (i.e., moisture flux convergence, MFC), although climatologically evaporation is of the same magnitude as precipitation, its standard deviation is much smaller than that of precipitation and MFC. With respect to moisture conservation, the atmospheric moisture budget indicates that the interannual variability in summer precipitation is derived primarily from changes in MFC. Moreover, the closer correlation between MFC and precipitation than between MFC and evaporation also implies that the variability of summer precipitation over the NFEASM is controlled mainly by fluctuations in the external atmospheric moisture fluxes, particularly by the moisture transport from the southwest, which is closely linked to the anticyclonic anomalous circulation over eastern China. In addition, although the westerly does not bring substantial moisture, its meridional movement can change the upper-level zonal wind divergence pattern, thereby affecting peak summer precipitation over the NFEASM.

Analysis of a multiple non-supercell tornado event in Mexico

Multiple tornado events frequently occur in different parts of the world. A region that seems favorable for recurring multiple tornado events due to its geographical location and topography is the eastern sector of the Trans-Mexican Volcanic Belt. A notable multiple tornado event that occurred in this region is investigated. It consisted of five simultaneous tornadoes during the convergence of three air masses: a cold air mass displaced from the north with moderately high relative humidity and specific humidity, a comparatively warm air mass with moderate relative humidity from the southeast, and a hotter air mass with low relative humidity from the southwest. This situation generated a line of intense horizontal wind shear through which the five tornadoes occurred. Numerical simulations revealed three cyclonic vortices and other minor disturbances along the shear line. Vertical cross-sections show rising air in these disturbances suggesting a cyclonic character. The results indicate that the relatively warm mass from the southeast moves over the one associated with a cold front coming from the north, generating multiple cyclonic rotation zones. Experiments with Lagrangian particles revealed vertical movements in the cyclonic rotation zones implying that the maximum heights of the tornadoes varied between 1000 and 1500 m. Moisture flux convergence calculations display structures resembling Kelvin-Helmholtz instabilities. This research proposes a conceptual model where the convergence of masses induced by topography can lead to multiple tornado generation.

Parametric subharmonic instability of inertial shear at ocean fronts

The two-dimensional stability of vertically sheared inertial oscillations at ocean fronts is explored through a linear stability analysis and nonlinear simulations. Baroclinic effects reduce the minimum frequency of inertia-gravity waves to an extent determined by the balanced Richardson number ${{Ri}}$ of the front. Below a critical value of ${{Ri}}$ , which depends on the strength of the inertial shear, the inertial oscillations become unstable to parametric subharmonic instability (PSI) resulting in growing perturbations that oscillate at half the inertial frequency $f$ . Since the critical value is always greater than 1, PSI can occur at fronts stable to symmetric instability. Although modest in weak inertial shear, growth rates exceeding $f/2$ can be achieved for inertial shear greater than or equal to the thermal wind shear. Our formulation allows for non-hydrostatic perturbations and can be applied to initially unstratified geostrophic adjustment problems. We find that PSI will almost totally damp the transient oscillations that arise during geostrophic adjustment. The perturbations gain energy at the expense of the inertial oscillations through ageostrophic shear production. The perturbations then themselves become unstable to secondary Kelvin–Helmholtz instabilities creating a pathway by which the inertial oscillations can be dissipated rapidly. In contrast to symmetric and baroclinic instabilities that draw on a front's kinetic or potential energy, PSI acts to increase the energy stored in the balanced front as the convergence and divergence of the eddy-momentum fluxes set up a secondary circulation in the sense to stand up the front.

Urbanization-Induced Diurnal Variation in Short-Duration Rainfall Events in Wuhan, China

Increasing of evidence suggests that rapid urbanization modifies precipitation and increases atmospheric instability in urban areas mainly due to urban heat island (UHI)-induced thermal effects and urban building-induced dynamic effects. However, few studies focus on the impact of urbanization on the diurnal variations in urban rainfall in terms of initiating time, frequency, amount, and intensity. Here, six years of hourly data from a relatively dense meteorological observation network in the Wuhan area in central China are used to quantify the issue of urbanization-induced diurnal variations in hourly rainfall and short-duration rainfall (SDR) events by comparing urban and surrounding rural stations during the summer dry period (13 July to 4 August), corresponding to hot weather with little rainfall and high evaporation. The results indicate that: (i) A higher frequency of hourly rainfall appears during night-time and afternoon over urban areas as compared to rural areas. The urban rainfall is more concentrated during the night-time; (ii) More and stronger urban SDR events are triggered during the night-time over urban areas, especially in terms of amount and intensity of rainfall events. A greater diurnal urban–rural difference in SDR events is detected than that in hourly mean rainfall; (iii) There is an enhancement in the night-time and afternoon urban SDR events when UHI intensity increases before they are initiated, and this is detectable in the frequency, amount, and intensity of SDR events. It is also found that the UHI-induced thermal effects are the main reason for the stronger nocturnal SDR events in Wuhan during the summer dry period, which is caused by increases in the convection current and water vapor flux convergence in the urban areas.

The Forecast Skill of the Summer Precipitation Over Tibetan Plateau Improved by the Adoption of a 3D Sub‐Grid Terrain Solar Radiative Effect Scheme in a Convection‐Permitting Model

AbstractWe have successfully incorporated a 3‐dimensional sub‐grid terrain solar radiative effect (3D STSRE) parameterization scheme into a convection‐permitting Weather Research and Forecasting model (WRF_CPM) in this study. Impacts of 3D STSRE scheme on the ability of WRF_CPM in forecasting the precipitation in summer over the Tibetan Plateau (TP) and nearby regions with complex terrain have been systematically addressed by conducting experiments without and with the 3D STSRE scheme. Results show that the application of 3D STSRE scheme can obviously mitigate the overestimation of surface solar radiation (SSR) and rainfall over TP and nearby regions, especially over the areas with much more rugged terrain (i.e., southern TP) in the WRF_CPM without 3D STSRE scheme. Further mechanism analyses indicate that the decreased surface heating induced by the reduction of SSR reduces the intensity of the thermal‐low pressure over the TP, which leads to the diminished strength of southwesterly winds and thereafter the weaker convergence of moisture flux over the southern TP. Moreover, the weakened surface thermal forcing makes the local atmosphere more stable, suppressing the vertical water vapor transport and local convection. These effects greatly alleviate the overestimation of precipitation over the southern TP produced by the WRF_CPM without the 3D STSRE scheme.

Climatological characteristics of the monsoon breaks during the southwest monsoon season of the Philippines

AbstractThis study investigates the climatological characteristics of the monsoon breaks during the southwest monsoon season (June–September) of the Philippines from 1979 to 2020. The monsoon break is defined as a period when the average rainfall across the eight western coast stations of the Philippines decreases below 5 mm·day−1 for at least three consecutive days. About 208 monsoon break events were identified, with 131 short‐ (3–4 days; 63% of the total monsoon break events), 71 medium‐ (5–9 days; 34%) and 6 long‐duration (≥10 days; 3%) monsoon break events. The short‐ and medium‐duration monsoon breaks were found to be more frequent during June and less frequent during August. Lag composite analyses of the time series of rainfall, low level zonal wind (UWND850hPa), vertically integrated moisture flux convergence (VIMFC) and mid‐tropospheric geopotential height (HGT500hPa) relative to the starting date of the monsoon break events show that the monsoon break is characterized by decrease in rainfall, weakening of westerly winds, increase in HGT500hPa and suppression of VIMFC over Luzon Island. Further analyses of the large‐scale circulation features show the appearance of an anticyclonic circulation over Luzon Island that suppresses the rainfall in the region and westward extension of the western North Pacific Subtropical High (WNPSH). The occurrence of the monsoon break events appears to be modulated by the 30–60‐day mode of the boreal summer intraseasonal oscillation (BSISO). About 39%, 55% and 82% of the short‐, medium‐ and long‐duration monsoon break events occur between Phases 1 and 4 during significant suppressed phases of the BSISO (i.e., amplitude >1), respectively. Convective activities and tropical cyclone activities are generally suppressed in these phases. The suppression of convective activities near the Philippines may facilitate the westward expansion of WNPSH, which further suppresses the rainfall and weaken the southwest monsoon.

Land–Atmosphere Interactions Partially Offset the Accelerated Tibetan Plateau Water Cycle through Dynamical Processes

Abstract The Tibetan Plateau (TP) has become wetter and warmer during the past four decades, which leads to an adjustment in the surface energy budget, characterized by enhanced surface latent heat and weakened surface sensible heat. However, the impacts of these surface energy changes on climate are unclear. In this study, we investigate the atmospheric response to the altered surface energy budget in the monsoon season over the TP using regional climate simulations. The inhibited surface sensible heating weakens the thermal effect of the TP, which further suppresses low-level convergence and upper-level divergence, thereby weakening the water vapor flux convergence over the plateau. The weakening of low-level air humidity by this dynamical response exceeds the supply from the enhanced surface evaporation, causing decreased precipitation (decreasing more in the wet eastern plateau and less in the dry west). Further analyses show that the precipitation frequency increases mainly for light precipitation while decreasing for heavy precipitation. It is thus demonstrated that on the TP, land surface energy–atmosphere interactions can mitigate the rate of precipitation increase, suppress the increase in frequency of heavy precipitation, and weaken the east–west contrast in precipitation amount, through a dynamical mechanism. Overall, land–atmosphere interactions on the TP exert negative feedback to partially offset the accelerated plateau water cycle under a changing climate.