Moisture Budget Research Articles

Sensitivity of Air‐Sea Heat Exchange in Cold‐Air Outbreaks to Model Resolution and Sea‐Ice Distribution

AbstractModeling air‐sea interactions during cold air outbreaks poses a major challenge because of the vast range of scales and physical processes involved. Using the Polar WRF model, we investigate the sensitivity of downstream air mass properties to (a) model resolution, (b) the sharpness of the marginal‐ice zone (MIZ), and (c) the geometry of the sea ice edge. The resolved sharpness of the MIZ strongly affects peak heat fluxes and the atmospheric water cycle. For sharper MIZs, roll convection is initiated closer to the sea ice edge, increasing both evaporation and precipitation. This yields an increased heat transfer into the atmosphere while the net effect on the atmospheric moisture budget is small. Overall, higher atmospheric resolution increases both the peak and net heat extracted from the ocean. The geometry of the sea ice edge can induce convergence or divergence zones that affect the air‐sea exchange.

The Relationship between Precipitation and Precipitable Water in CMIP6 Simulations and Implications for Tropical Climatology and Change

AbstractIt is well documented that over the tropical oceans, column-integrated precipitable water (pw) and precipitation (P) have a nonlinear relationship. In this study moisture budget analysis is used to examine this P–pw relationship in a normalized precipitable water framework. It is shown that the parameters of the nonlinear relationship depend on the vertical structure of moisture convergence. Specifically, the precipitable water values at which precipitation is balanced independently by evaporation versus by moisture convergence define a critical normalized precipitable water, pwnc. This is a measure of convective inhibition that separates tropical precipitation into two regimes: a local evaporation-controlled regime with widespread drizzle and a precipitable water–controlled regime. Most of the 17 CMIP6 historical simulations examined here have higher pwnc compared to ERA5, and more frequently they operate in the drizzle regime. When compared to observations, they overestimate precipitation over the high-evaporation oceanic regions off the equator, thereby producing a “double ITCZ” feature, while underestimating precipitation over the large tropical landmasses and over the climatologically moist oceanic regions near the equator. The responses to warming under the SSP585 scenario are also examined using the normalized precipitable water framework. It is shown that the critical normalized precipitable water value at which evaporation versus moisture convergence balance precipitation decreases as a result of the competing dynamic and thermodynamic responses to warming, resulting in an increase in drizzle and total precipitation. Statistically significant historical trends corresponding to the thermodynamic and dynamic changes are detected in ERA5 and in low-intensity drizzle precipitation in the PERSIANN precipitation dataset.

Cyclostratigraphy and paleoenvironmental inference from downhole logging of sediments in tropical Lake Towuti, Indonesia

Lake Towuti is located on central Sulawesi/Indonesia, within the Indo Pacific Warm Pool, a globally important region for atmospheric heat and moisture budgets. In 2015 the Towuti Drilling Project recovered more than 1000 m of drill core from the lake, along with downhole geophysical logging data from two drilling sites. The cores constitute the longest continuous lacustrine sediment succession from the Indo Pacific Warm Pool. We combined lithological descriptions with borehole logging data and used multivariate statistics to better understand the cyclic sequence, paleoenvironments, and geochronology of these sediments. Accurate chronologies are crucial to analyze and interpret paleoclimate records. Astronomical tuning can help build age-depth models and fill gaps between age control points. Cyclostratigraphic investigations were conducted on a downhole magnetic susceptibility log from the lacustrine facies (10–98 m below lake floor) from a continuous record of sediments in Lake Towuti. This study provides insights into the sedimentary history of the basin between radiometric ages derived from dating a tephra layer (~ 797 ka) and C14-ages (~ 45 ka) in the cores. We derived an age model that spans from late marine isotope stage (MIS) 23 to late MIS 6 (903 ± 11 to 131 ± 67 ka). Although uncertainties caused by the relatively short record and the small differences in the physical properties of sediments limited the efficacy of our approach, we suggest that eccentricity cycles and/or global glacial-interglacial climate variability were the main drivers of local variations in hydroclimate in central Indonesia. We generated the first nearly complete age-depth model for the lacustrine facies of Lake Towuti and examined the potential of geophysical downhole logging for time estimation and lithological description. Future lake drilling projects will benefit from this approach, since logging data are available just after the drilling campaign, whereas core descriptions, though more resolved, only become available months to years later.

Model Uncertainty in the Projected Indian Summer Monsoon Precipitation Change under Low-Emission Scenarios

The projected ISM precipitation changes under low-emission scenarios, Representative Concentration Pathway 2.6 (RCP2.6) and Shared Socioeconomic Pathway 1-2.6 (SSP1-2.6), are investigated by outputs from models participating in phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6). Based on the high-emission scenarios like RCP8.5, the Intergovernmental Panel on Climate Change Fifth Assessment Report suggests a wetter Indian summer monsoon (ISM) by the end of 21st century. Although the multi-model ensemble mean (MME) ISM precipitation under RCP2.6 and SSP1-2.6 is still projected to increase over 2050–2099 referenced to 1900–1949, the intermodel spread of the ISM precipitation change is tremendous in both CMIPs. Indeed, the signal-to-noise ratio (SNR) of ISM precipitation change, defined as the MME divided by its intermodel standard deviation, is even below 1 under the low-emission scenarios. This casts doubts on a future wetter ISM in a warmer climate. Moisture budget analyses further show that most of the model uncertainty in ISM precipitation change is caused by its dynamical component from the atmospheric circulation change. As expected, the interhemispheric surface warming contrast is essential in causing the intermodel differences in ISM circulation and precipitation changes under low-emission scenarios. In addition, the projected wetter ISM is prominently enhanced from CMIP5 to CMIP6, along with reduced model uncertainty. However, the resultant increased SNR in CMIP6 is still low in most ISM regions. The results imply that ISM precipitation change is highly uncertain under low-emission scenarios, which greatly challenges the decisions-making in adaptation policies for the densely populated South Asian countries.

Satellite Perspectives of Sea Surface Temperature Diurnal Warming on Atmospheric Moistening and Radiative Heating during MJO

AbstractDiurnal air–sea coupling affects climate modes such as the Madden–Julian oscillation (MJO) via the regional moist static energy budget. Prior to MJO initiation, large-scale subsidence increases (decreases) surface shortwave insolation (winds). These act in concert to significantly warm the uppermost layer of the ocean over the course of a single day and the ocean mixed layer over the course of 1–2 weeks. Here, we provide an integrated analysis of multiple surface, top-of-atmosphere, and atmospheric column observations to assess the covariability related to regions of strong diurnal sea surface temperature (dSST) warming over 44 MJO events between 2000 and 2018 to assess their role in MJO initiation. Combining satellite observations of evaporation and precipitation with reanalysis moisture budget terms, we find 30%–50% enhanced moistening over high-dSST regions during late afternoon using either ERA5 or MERRA-2 despite large model biases. Diurnally developing moisture convergence, only modestly weaker evaporation, and diurnal minimum precipitation act to locally enhance moistening over broad regions of enhanced diurnal warming, which rectifies onto the larger scale. Field campaign ship and sounding data corroborate that strong dSST periods are associated with reduced middle-tropospheric humidity and larger diurnal amplitudes of surface warming, evaporation, instability, and column moistening. Further, we find greater daytime increases in low cloud cover and evidence of enhanced radiative destabilization for the top 50th dSST percentile. Together, these results support that dSST warming acts in concert with large-scale dynamics to enhance moist static energy during the suppressed to active phase transition of the MJO.

Characteristics of Large-Scale Circulation Affecting the Inter-Annual Precipitation Variability in Northern Sumatra Island during Boreal Summer

This study aims to explore the contrasting characteristics of large-scale circulation that led to the precipitation anomalies over the northern parts of Sumatra Island. Further, the impact of varying the Asian–Australian Monsoon (AAM) was investigated for triggering the precipitation variability over the study area. The moisture budget analysis was applied to quantify the most dominant component that induces precipitation variability during the JJA (June, July, and August) period. Then, the composite analysis and statistical approach were applied to confirm the result of the moisture budget. Using the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Anaysis Interim (ERA-Interim) from 1981 to 2016, we identified 9 (nine) dry and 6 (six) wet years based on precipitation anomalies, respectively. The dry years (wet years) anomalies over the study area were mostly supported by downward (upward) vertical velocity anomaly instead of other variables such as specific humidity, horizontal velocity, and evaporation. In the dry years (wet years), there is a strengthening (weakening) of the descent motion, which triggers a reduction (increase) of convection over the study area. The overall downward (upward) motion of westerly (easterly) winds appears to suppress (support) the convection and lead to negative (positive) precipitation anomaly in the whole region but with the largest anomaly over northern parts of Sumatra. The AAM variability proven has a significant role in the precipitation variability over the study area. A teleconnection between the AAM and other global circulations implies the precipitation variability over the northern part of Sumatra Island as a regional phenomenon. The large-scale tropical circulation is possibly related to the PWC modulation (Pacific Walker Circulation).

Evaluating the performances of cloud microphysical parameterizations in WRF for the heavy rainfall event of Kerala (2018)

This study evaluates the performances of four different cloud microphysical parameterization (CMP) schemes of the Weather Research and Forecasting (WRF) model at 3 km horizontal resolution (lead time up to 96 h) for the Heavy Rainfall Event (HRE) over Kerala in August 2018. The goal is to identify the major drivers for rain making mechanism and evaluate the ability of CMPs to accurately simulate the event with special emphasis on rainfall. It is found that the choice of CMP has a considerable impact on the rainfall forecast characteristics and associated convection. Results are validated against the India Meteorological Department (IMD) station data and Global Precipitation Measurement (GPM) observations and found that, among the four CMP schemes, viz., Milbrandt (MIL), Thompson Aerosol Aware (TAA), WRF double-moment 6-class scheme (WDM6) and WRF single-moment 6-class scheme (WSM6); WDM6 is the best performing scheme in terms of rainfall. It is noted that mixed phase processes are dominant in this scenario and the inability (ability) of MIL and TAA (WDM6 and WSM6) to predict the frozen hydrometeors, and thus simulating the cold rain processes realistically led to large (small) errors in the rainfall forecast. The moisture convergence was prominent in the foothills of the Western Ghats and highly influential in facilitating orography driven lifting of moisture. The moisture budget results suggest that horizontal moisture flux convergence (MFC) was the major driver of convection with WDM6 predicting the peaks of MFC most consistently with the observed Tropical Rainfall Measuring Mission (TRMM) rainfall product. Additionally, from Contiguous Rain Area analysis it is also found that the WDM6 has the least volumetric error. This is to highlight that hydrometeor distributions are strongly modulated by MFC, which further impacts the latent heat generation and rainfall over the region. Overall results infer the substantial influence of CMPs on the forecast of the Heavy Rainfall Event. The findings of this study will be highly useful for operational forecasting agencies and disaster management authorities for mitigation of damages caused by this kind of severe HREs in the future.

Propagating Mechanisms of the 2016 Summer BSISO Event: Air‐Sea Coupling, Vorticity, and Moisture

AbstractDuring the summer of 2016, a boreal summer intraseasonal oscillation (BSISO) event was observed over Southeast Asia and the South China and Philippine seas. Precipitation anomalies associated with this event propagated northward at a speed of 0.5–1° per day from July to August. To understand the mechanisms, a regional atmosphere‐ocean coupled system with the Weather Research and Forecasting (WRF) model and the Hybrid Coordinate Ocean Model (HYCOM) is used to study this BSISO event. The 50‐day‐long coupled simulations reasonably capture large‐scale northward propagation of the event. Coupled simulations with altered air‐sea interaction and atmosphere‐only simulations with prescribed sea surface temperature illuminate the insignificant role of air‐sea interaction within the computation domain in the northward propagation the for this event. Diagnostics of the coupled simulation as well as the ECMWF‐Interim reanalyses indicate that convection and barotropic vorticity are largely in phase north of 5°N during the event. The BSISO convection is accompanied by moisture anomalies whose magnitudes increase as the BSISO propagates northward. Analysis of the moisture budget shows that positive horizontal advection leads positive moisture anomalies on the intraseasonal time scale north of 10°N. The vorticity‐convection relationship, the lead‐lag relationship between moisture and its horizontal advection, and the latitude dependence of each for this BSISO event are consistent with general features of BSISO events composited with ECWMF‐Interim reanalysis and satellite precipitation data sets.

Equatorial Moisture Dynamics of the Quasi‐Biweekly Oscillation in the Tropical Northwestern Pacific During Boreal Summer

AbstractThe pronounced summertime quasi‐biweekly oscillation (QBWO) in the tropical northwestern Pacific usually originates from equatorial convective anomalies (ECAs) in the western Pacific. In this study, ECAs in relation to the QBWO are investigated in terms of the equatorial moisture dynamics. The results show that the development of ECAs is preconditioned by significant moisture anomalies associated with zonal moisture advection along the equator. Prior to initiation, an equatorially westward‐moving moisture precursor at a speed of approximately 5° longitude per day could be traced back to 140°W. A moisture budget analysis indicates that the successive westward movement of the moisture precursor is primarily due to the interaction between the zonal gradient of moisture anomalies and the mean easterly trade winds. The equatorial moisture dynamics likely maintain the quasi‐biweekly variability of ECAs regarding the QBWO in the tropical northwestern Pacific.

Dynamic and thermodynamic contributions to Northern China dryness in El Niño developing summer

AbstractIn El Niño developing summer, Northern China shows reduced precipitation. The underlying mechanism is investigated through an analysis on the regional moisture and moist static energy budget. It is shown that the thermodynamic effect in relation to water vapour changes increases the regional wetness, but it is much smaller than the dynamic effect in relation to anomalies of atmospheric circulation. The anomalous vertical descent term is particularly pronounced. This anomalous vertical motion is mainly constrained by the horizontal advection of climatological moist enthalpy by anomalous wind, with a deficit of moist static energy in the region. A descent is then produced to compensate the energy lose and leads to dryness. Further investigation shows that an El Niño‐induced anomalous cyclone over Northeast Asia is the main ingredient in controlling the dynamic effect for both water vapour budget and moist static energy budget.

Dominant modes of interannual variability in precipitation over the Hengduan Mountains during rainy seasons

AbstractPresent study investigates the dominant modes of interannual variability in precipitation over the Hengduan Mountains (HM) during rainy seasons. The leading two empirical orthogonal function (EOF) modes of HM precipitation explain 28.4 and 13.9% of the total variance, respectively. Positive EOF1 phase features enhanced precipitation anomalies over the southern HM, and a dipole structure with wet anomalies occupying large areas from the YunGui plateau to Tibetan Plateau is observed in positive EOF2 phase. Analysis of moisture budget and omega equation indicate the importance of leading EOF modes related circulation anomalies, which bring the horizontal warm advection to the HM, favouring the anomalous upward motions and precipitation increase there. For EOF1, the HM is controlled by an anomalous quasi‐barotropic cyclone. This cyclone is one part of a zonally oriented barotropic wave‐like pattern highly resembling the Silk Road pattern. Besides, the SST anomalies over the Indo‐Pacific sector are similar to the decaying phase of La Niña, which induces the wet anomalies and further triggers the anomalous lower‐level cyclone as a Rossby wave response. For EOF2, an anomalous cyclone appears over the Northeast Asia from lower to upper levels, and southerly wind anomalies prevail over the HM. The North Atlantic Oscillation and its coupled tripole SST pattern trigger a zonal wave‐like pattern at middle and high latitudes emanating from the Central Europe, across Mongolia to Northeast Asia. Moreover, the SST anomalies over the Indo‐Pacific sector resemble the developing phase of El Niño, which induces the meridional Pacific‐Japan/East Asia‐Pacific teleconnection from tropical NWP to Northeast Asia.

Reevaluating the impacts of oceanic vertical resolution on the simulation of Madden–Julian Oscillation eastward propagation in a climate system model

The upper ocean plays a critical role in determining the Madden–Julian Oscillation (MJO) characteristics through modulating the tropical atmosphere–ocean interaction. By increasing the oceanic vertical resolution, its impacts on the MJO eastward propagation are discussed in this study by using a climate system model. With a refined vertical resolution in the upper ocean, warmer surface ocean and shallower mixed layer depth are produced in the tropics, which induces associated atmospheric changes as the response to the ocean feedbacks. Enhanced November–April-mean vertically-integrated specific humidity is found around the equatorial region with the increased vertical resolution, which strengthens the zonal and meridional moisture gradients. The lead-lag correlation of MJO precipitation demonstrates that the simulated MJO eastward propagation is improved with increased oceanic vertical resolution by improving the simulations of convective instability at the east of the MJO convective center, the boundary layer moisture convergence, the low- and upper-level circulation, and the vertical structure of equivalent potential temperature and diabatic heating. Moreover, the zonal asymmetry of the tendency of specific humidity is also improved by increasing the oceanic vertical resolution. The vertically-integrated moisture budget analysis is applied to further investigate the dominance of the moistening and drying processes. Results reveal that the drying processes are successfully reproduced over the central Indian Ocean in the case of increased oceanic vertical resolution, whilst the moistening processes are not well captured over the Maritime Continent and the MJO “detour” region. It suggests that additional modifications are needed to further improve the MJO simulation.

Large‐scale dynamic, heat and moisture structures of monsoon‐influenced precipitation in the East Asian monsoon rainy area

AbstractThis study investigated large‐scale dynamic structures along with heat and moisture budgets associated with monsoon‐influenced precipitation over a typical rainy domain in the East Asian monsoon region. Large‐scale dynamically and thermodynamically consistent forcing data based on multiple measurements were produced by a one‐dimensional constrained variational analysis method. Using the forcing data, distinct characteristics of large‐scale states were documented for cases of notable pre‐monsoon rainfall before the South China Sea monsoon onset (SCSMO), extreme rainfall during SCSMO, and persistent strong rainfall after SCSMO. The pre‐monsoon period mainly resulted from weak and discrete cloud regimes. The extreme‐rain period was associated with a severe deep convective system moving from the west, and the persistent‐rain period was related to moderate convective cells separated from the strong convective system over the northern South China Sea. Large‐scale features including vertical velocity, wind convergence, and diabatic heating and drying were the strongest (weakest) during the extreme‐rain (pre‐monsoon) period. For the pre‐monsoon and persistent‐rain periods, the period‐averaged profiles of vertical velocity and diabatic heating and drying showed a one‐peak structure. However, during the extreme‐rain period, a leap‐forward mutation was seen in the vertical structure and magnitude of these large‐scale states. The multiple peaks shown in the vertical profiles of vertical velocity and diabatic heating during the extreme‐rain period may indicate various convective cloud systems co‐existed. The diabatic heating profiles of extreme rainfall indicated that the first rainfall peak was related to the successive occurrence of stratiform anvils and convective clouds, while the second rainfall peak, which was most intense, was associated with shallow convective clouds, severe deep convective clouds, and detraining stratiform anvils.

Response of summer extreme precipitation over East Asia during the mid-Holocene versus future global warming

Changes in extreme precipitation have significant impacts on society and the environment. In recent years, the response of extreme precipitation over East Asia to global warming has received considerable attention; however, few studies have focused on the extreme precipitation in geological warm periods, especially the mid-Holocene—the most recent warm period on the orbital timescale. In this study, the outputs of mid-Holocene, pre-industrial and CO2-induced warming experiments using a coupled climate model (MPI-ESM-P) were employed to examine the changes in extreme precipitation over East Asia in the mid-Holocene and under future warming scenario, respectively. The results show that, during the mid-Holocene, summer extreme precipitation increases over the east of China but decreases over Japan and the surrounding oceans. However, under CO2-induced warming, summer extreme precipitation increases over the southeast of China, Japan, and the surrounding oceans, but decreases over the north of East Asia. Moisture budget analysis shows that these distinct responses come from different changes in moisture and circulation during the two periods. During the mid-Holocene, higher insolation over the Northern Hemisphere leads to a larger ocean–land thermal contrast and a stronger East Asian summer monsoon (EASM). The strengthened EASM brings more moisture to continental East Asia, resulting in an increase in both mean and extreme precipitation. In contrast, the CO2-induced warming facilitates an anomalous heating center in the upper troposphere over the tropical part of Northwestern Pacific ocean, which weakens the EASM. However, enhanced atmospheric moisture counterbalances the effect of the weakened EASM, resulting in more extreme precipitation over the south of East Asia.

Projection of Future Summer Precipitation over the Yellow River Basin: A Moisture Budget Perspective

The projection of future precipitation over the Yellow River Basin (YRB) is of great importance to regional climate change adaptation and mitigation. Using the historical simulations and projections under the four combined scenarios of the shared socioeconomic pathways and the forcing levels of the Representative Concentration Pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5) provided by the multimodel ensemble mean of 10 models in phase six of the Coupled Model Intercomparison Project (CMIP6), the projected spatial and temporal changes of future summer precipitation over the YRB and the possible physical mechanisms underlying future summer precipitation changes are investigated. Large discrepancies in precipitation exist among the four scenarios during the latter half period of the 21st century, with precipitation under SSP5-8.5 being the largest. Nevertheless, the precipitation under each of the four scenarios shows a similar spatial pattern over the YRB, with an east–west-oriented gradient. A comparison of projected moisture transport into the YRB among the four scenarios reveals two channels (westerlies and monsoon flow) under SSP5-8.5, whereas the monsoon flow from adjacent oceans is important under the other three scenarios. Further analysis of the unique features of the projected moisture flux and substantial increase in summer precipitation under SSP5-8.5 indicates that the future summer precipitation trend over the YRB can be mainly attributed to an increase in evaporation and moisture advection.

Effects of the South Asian summer monsoon anomaly on interannual variations in precipitation over the South-Central Tibetan Plateau

An anomalous South Asian summer monsoon (SASM) system could generate a large anomaly in precipitation and hydrological disasters in the SASM-prevailing area, as widely reported for the Indian Peninsula. However, how the SASM system influences the precipitation anomaly over the South-Central Tibetan Plateau (SCTP) is largely unknown. In this study, we (a) analyze the influences of the early and late onset (demise) of the SASM on the interannual variations in precipitation over the SCTP during 1979–2015; and (b) illuminate the underlying mechanisms and asymmetric effects with regard to the onset and demise of the SASM by analyzing the characteristics of water vapor transport and moisture budgets in this region. Results indicate that the precipitation anomaly over the SCTP is dominated by the cyclonic and anticyclonic water vapor transport associated with the anomalous SASM activities, causing moisture convergence and divergence in this region. The topographic effect in the southeastern Tibetan Plateau (SETP) and southwestern Tibetan Plateau (SWTP) further strengthens the anomaly in water vapor transport in vertical direction and contributes to the precipitation anomaly through moisture convergence and divergence. The anomalous SASM and topography exhibit asymmetric effects between the onset and demise as well as between the early and late onset (demise) of the SASM. They cause 23.41%, 15.91%, and 1.96% difference in precipitation between the early and late SASM-onset years, and 13.05%, 21.50%, and 29.86% difference in precipitation between the early and late SASM-demise years in the SETP, central Tibetan Plateau (CETP), and SWTP, by regulating the horizontal and vertical thermodynamic and dynamic processes. The results help improve our understanding of the SASM-precipitation relationship over the SCTP and guide the prediction of precipitation and alleviation of water-related disasters in the region and its surroundings that are home to billions of people in Asia.

Future Antarctic snow accumulation trend is dominated by atmospheric synoptic-scale events

Over the last century, the increase in snow accumulation has partly mitigated the total dynamic Antarctic Ice Sheet mass loss. However, the mechanisms behind this increase are poorly understood. Here we analyze the Antarctic Ice Sheet atmospheric moisture budget based on climate reanalysis and model simulations to reveal that the interannual variability of regional snow accumulation is controlled by both the large-scale atmospheric circulation and short-lived synoptic-scale events (i.e. storm systems). Yet, when considering the entire continent at the multi-decadal scale, only the synoptic-scale events can explain the recent and expected future snow accumulation increase. In a warmer climate induced by climate change, these synoptic-scale events transport air that can contain more humidity due to the increasing temperatures leading to more precipitation on the continent. Our findings highlight that the multi-decadal and interannual snow accumulation variability is governed by different processes, and that we thus cannot rely directly on the mechanisms driving interannual variations to predict long-term changes in snow accumulation in the future.

Modulation of Madden-Julian Oscillation Activity by the Tropical Pacific-Indian Ocean Associated Mode

In this study, the impacts of the tropical Pacific-Indian Ocean associated mode (PIOAM) on Madden-Julian Oscillation (MJO) activity were investigated using reanalysis data. In the positive (negative) phase of the PIOAM, the amplitudes of MJO zonal wind and outgoing longwave radiation are significantly weakened (enhanced) over the Indian Ocean, while they are enhanced (weakened) over the central and eastern Pacific. The eastward propagation of the MJO can extend to the central Pacific in the positive phase of the PIOAM, whereas it is mainly confined to west of 160°E in the negative phase. The PIOAM impacts MJO activity by modifying the atmospheric circulation and moisture budget. Anomalous ascending (descending) motion and positive (negative) moisture anomalies occur over the western Indian Ocean and central-eastern Pacific (Maritime Continent and western Pacific) during the positive phase of the PIOAM. The anomalous circulation is almost the opposite in the negative phases of the PIOAM. This anomalous circulation and moisture can modulate the activity of the MJO. The stronger moistening over the Indian Ocean induced by zonal and vertical moisture advection leads to the stronger MJO activity over the Indian Ocean in the negative phase of the PIOAM. During the positive phase of the PIOAM, the MJO propagates farther east over the central Pacific owing to the stronger moistening there, which is mainly attributable to the meridional and vertical moisture advection, especially low-frequency background state moisture advection by the MJO’s meridional and vertical velocities.

The Modulation of Daily Southern Africa Precipitation by El Niño–Southern Oscillation across the Summertime Wet Season

AbstractThe spatiotemporal evolution of daily southern Africa precipitation characteristics, and associated atmospheric circulation, related to El Niño and La Niña is examined across the region’s November–April wet season. Precipitation characteristics are examined in terms of monthly changes in daily average precipitation, the number of precipitation days, and the number of heavy precipitation days in three independently constructed estimates of observed precipitation during 1983–2018. Mechanisms related to precipitation changes, including contributions from mass divergence, water vapor transports, and transient eddies, are diagnosed using the atmospheric moisture budget based on the ERA5 reanalysis. El Niño is related to precipitation anomalies that build during December–March, the core of the rainy season, culminating in significantly below average values stretching across a semiarid region from central Mozambique to southeastern Angola. A broad anticyclone centered over Botswana drives these precipitation anomalies primarily through anomalous mass divergence, with moisture advection and transient eddies playing secondary roles. La Niña is related to significantly above average daily precipitation characteristics over all Africa south of 20°S in February and much less so during the other five months. February precipitation anomalies are primarily driven through mass divergence due to a strong anomalous cyclonic circulation, whereas a similar circulation is more diffuse during the other months. The spatiotemporal evolutions of anomalies in daily precipitation characteristics across southern Africa related to El Niño and La Niña are not equal and opposite. The robustness of an asymmetric evolution, which could have implications for subseasonal forecasts, needs to be confirmed with analysis of additional empirical data and established with climate model experimentation.

Features and Possible Causes of the Extreme Precipitation Anomaly in China During Winter 2019/2020

In this paper, the features and possible causes of extreme precipitation in the winter of 2019/2020 in China are investigated based on the observational station daily dataset provided by the China Meteorological Administration and the ECMWF ERA5 reanalysis dataset. Record-breaking precipitations are found in most areas of China in the winter of 2019/2020, particularly for the north of China. The magnitude of precipitation in south (north) of China in 2019/2020 increase 0.39mm/day (0.15mm/day), about 39.0% (105.5%) compared with the climatological mean. Moisture budget analysis shows that contributions of atmospheric circulation anomaly on horizontal moisture advection and total moisture flux convergence are primarily responsible for the positive anomalies of precipitation in China. Further analysis suggests that the weakened East Asian Trough and the anti-cyclone circulation anomaly over the Sea of Japan favor the transportation of low-level warm and humid airflow from the Northwest Pacific and the East China Sea to the north of China. Meanwhile, the westward movement of the Subtropical High and the Southern Branch Trough anomalies transport low-level warm and humid air from the Bay of Bengal and the South China Sea to south of China. The warm moist air flows northeastward from the East China Sea, Bay of Bengal and the South China Sea while the climatological cold wind flows southeastward, which causes the increase of local precipitation in the winter of 2019/2020. In consideration of the fact that stratospheric polar vortex anomaly has a process of downward propagation to the troposphere, it is of great significance to study the changes of the stratosphere and the relationships between stratosphere and troposphere.