Moisture Flux Divergence Research Articles

A moisture budget perspective on Australian rainfall variability

AbstractRainfall variability over Australia is revisited from the viewpoint of the atmospheric moisture budgets in three regions: the extratropics, Subtropics, and Tropics. The budgets are calculated using three‐hourly European Centre for Medium‐Range Weather Forecasts Reanalysis v5 (ERA5) and ERA5‐Land data between 1979 and 2022. The use of the moisture budget at short time‐scales enables the investigation of the relationship between synoptic weather‐scale processes and the longer term variability of the rainfall climate. The total variability in the vertically integrated moisture flux divergence (VIMD) is significantly larger than the evaporation minus precipitation (E − P), to a large extent due to the sub‐daily time‐scales. E − P is related more closely to moisture flux convergence in winter (summer) over south (north) Australia, suggesting a clear seasonality in the relationship between the two budget terms. The E − P–VIMD relationship is nearly in phase in the Tropics, whereas VIMD leads E − P by 9–15 hr with eastward‐propagating signals in the extratropics and Subtropics. Such seasonal and regional discrepancies in the relationship are attributed to the background state of moisture availability and temperature as represented by relative humidity and lifting condensation levels. The variability of the budget imbalance and its seasonality are dominated by the variability in VIMD. The imbalance reduces rapidly with temporal smoothing, with the storage term approaching zero at approximately 20 days, which can be thought of as making a transition time‐scale from high‐frequency weather‐related variability into slow‐varying background conditions. Weather‐related variability (cyclones, fronts, and thunderstorms) dominates the overall E − P variability in the extratropics and Subtropics, whereas slow‐varying background conditions contribute equally to the total variability in the Tropics.

Impacts of El Niño diversity on East Asian summertime precipitation extremes

This study examines the impacts of Eastern Pacific (EP) and Central Pacific (CP) El Niño on summertime extreme precipitation over East Asia during the El Niño decay phase. The findings reveal distinct patterns of extreme precipitation for the two El Niño types. During EP El Niño, more intense extreme precipitation occurs over south of the Yangtze River (SYR), while suppressed extremes are observed over Mei-Yu rainband in China, Baiu in Japan, and Changma in South Korea (MBC). Conversely, CP El Niño leads to weaker (stronger) extreme precipitation over SYR (MBC). This study also differentiates between tropical cyclone (TC) and non-TC related precipitation, showing that TCs have minimal influence on the overall extreme precipitation compared to non-TC related events. Further investigation reveals that EP and CP El Niño distinctly influence atmospheric circulation patterns, thereby causing different distributions of non-TC extreme precipitation. Specifically, during EP El Niño, sea surface temperature (SST) warming signals in the tropical Indian Ocean induce the southward displacement of the South Asia High (SAH), westerly jet (WJ), and Western Pacific subtropical high (WPSH). This amplifies moisture flux convergence, elevating the likelihood of intense extreme precipitation over SYR. Conversely, MBC experiences moisture flux divergence, resulting in fewer precipitation extremes. During CP El Niño, SST warming signals in the Maritime Continent prompt the northward shift of SAH, WJ, and WPSH by modulating local Hadley circulations. These anomalies lead to strengthened moisture convergence (divergence) over MBC (SYR), consequently resulting in a higher (lower) likelihood of intense extreme precipitation over MBC (SYR).

Intraseasonal variations of precipitation over southern China during boreal winter and their two primary influencing factors

AbstractThis paper utilizes reanalysis data and statistical methods to elucidate the physical mechanisms of intraseasonal variations of precipitation over southern China during the boreal winter, as well as the two primary factors influencing these variations. Anomalous southerly (northerly) wind over southern China will result in the strong moisture convergence (divergence) in enhanced (suppressed) scenarios, and the stronger positive (negative) horizontal advection of absolute vorticity at upper troposphere induces the anomalous ascending (descending) motion in enhanced (suppressed) scenarios. The Madden–Julian oscillation (MJO) in the tropics and a wave train over the North Atlantic and Eurasia with four activity centres in the mid–high latitudes (referred as NAE‐wave train) are the two primary factors that modulate the intraseasoanl precipitation over southern China. The intraseasonal circulation induced by MJO and NAE‐wave train exhibits distinct features, resulting in different physical mechanisms of their influences. The moisture flux divergence anomalies over southern China associated with MJO mainly stem from the meridional moisture convergence, whereas the meridional moisture advection is the first contributor in the influence of NAE‐wave train. The anomalous vorticity advection at upper troposphere associated with MJO is primarily influenced by the meridional advection, but that associated with NAE‐wave train is dominated by the zonal advection. Anomalous precipitation induced by MJO is concentrated mainly over South China, while that caused by NAE‐wave train is primarily located in the mid–lower Yangtze River basin. Therefore, different phases locking of MJO and NAE‐wave train could lead to various precipitation anomalies over southern China.

Decadal Change in the Linkage between QBO and the Leading Mode of Southeast China Winter Precipitation

Abstract In this study, observational and model datasets are used to analyze winter precipitation and its leading empirical orthogonal function (EOF1) mode over Southeast China. EOF1 displays a dominant monosign pattern during the last 60 years; however, its major impacting factors have a decadal transition near the mid-1990s. The first principal component (PC1) is related to El Niño–Southern Oscillation (ENSO) after the mid-1990s and to the quasi-biennial oscillation (QBO) before the mid-1990s. An enhanced ENSO–precipitation relationship is associated with stronger ENSO-induced tropical zonal circulation and the westward shift of ENSO-induced SST over the tropical Pacific after the mid-1990s compared to before the mid-1990s. Negative correlation coefficients between QBO and precipitation are evident before the mid-1990s but are no longer statistically significant after the mid-1990s. This change originates from the interaction between QBO’s subtropical influences and the Holton–Tan effect. The QBO’s subtropical influence and the Holton–Tan effect lead to a zonal pressure gradient and meridional wind anomalies over East Asia before the mid-1990s, which further influence the meridional transport of water vapor and precipitation over Southeast China. However, the Holton–Tan effect is enhanced after the mid-1990s. Downward stratospheric polar vortex signals and the QBO’s subtropical influence cause a meridional pressure gradient over East Asia, and thus the relevant moisture flux divergence lacks statistical significance. The above results indicate that the subtropical response to QBO and the Holton–Tan effect should be considered together when using the QBO signal to improve forecasts of winter precipitation over East Asia. Significance Statement Southeast China winter precipitation (SCWP) is often attributed to variation in lower-atmospheric dynamics and sea surface temperature, such as El Niño–Southern Oscillation (ENSO). Few studies focus on the role of the mid- to upper atmosphere. In this study, we diagnose the influence of the stratospheric quasi-biennial oscillation (QBO) on SCWP. Both observational and modeling analyses indicate a strong decadal change in the QBO–SCWP relationship, which limits the use of the QBO in seasonal forecasts of SCWP. This decadal change originates from the strength of the QBO’s modulation of the stratospheric polar vortex. Our results provide a new perspective on the use of the mid- to upper atmosphere in seasonal forecasts of SCWP.

Long-term changes in the diurnal cycle of total cloud cover over the Tibetan Plateau

The diurnal cycle of total cloud cover (DCTCC) plays an important role in climate change over the Tibetan Plateau (TP). This study used the International Satellite Cloud Climate Project-H (ISCCP-H) observations, the fifth generation of reanalysis of the European Centre for Medium Range Weather Forecasts (ERA5), the second-generation Modern-ERA Retrospective analysis for Research and Applications (MERRA-2) reanalysis data, and 21 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) AMIP simulations from 1984 to 2014 to investigate the climatology and long-term variations in the DCTCC over the TP. The total cloud cover (TCC) over the TP exhibits sinusoidal-like unimodal diurnal cycles and is quantified with four metrics: the daily mean (μ), daily peak time (tmax), daily coefficient of variation (cv), and full width at half-maximum of DCTCC (half width); the 30-year means of these four indices of TCC averaged over the TP were approximately 65.45%, 14:00 LST, 0.16 and 10 h, respectively. From 1984 to 2014, the TP-average μ of TCC decreased by −2.64%/decade, which was closely associated with the abrupt change in high cloud (clouds with cloud top pressure <440 hPa, defined by the ISCCP) cover (HCC) from 1998 to 1999. The tmax was postponing by approximately 0.12 h/decade, and the DCTCC was narrowing and becoming steeper. These climatology observations and trends were dominated by the middle cloud (clouds with cloud top pressure between 440 and 680 hPa, defined by the ISCCP) cover and contributed to by the HCC. There were gross deviations in the climatological DCTCC from the reanalysis datasets and models, and they can hardly reproduce the long-term trends of the DCTCC. The climatology and long-term variations of the diurnal cycles of the 2 m temperature (T2m), 10 m wind speed (U10m), vertical integral of divergence of moisture flux (DMF) and relative humidity (RH) at 250 hPa and 500 hPa exhibited significant correlations with those of the DCTCC over the TP. Over most of the TP region, the secular variations in μ of TCC were negatively correlated with those of T2m, which were dominated by the correlations between TCC and T2m during the daytime. The secular variations in the nighttime-averaged HCC have a positive association with those in the T2m over the northern TP. The spatial pattern of correlations between μ of TCC and μ of U10m and DMF were mainly reflected in the HCC. The secular variations in μ of RH were positively correlated with those of the cloud cover. The lags in the tmax of the TCC were negatively correlated with the variations in the tmax of T2m and positively associated with those of RH at 250 hPa over parts of the eastern TP. The long-term changes in the cv of the TCC were mainly positively correlated with those of T2m over most of the TP region and were partly correlated with the other factors. The decadal narrowing of the diurnal cycles of HCC was positively contributed to by the long-term trends in the half width of RH at 250 hPa over the central TP.

Effects of Urban Expansion and Anthropogenic Heat Enhancement on Tropical Cyclone Precipitation in the Greater Bay Area of China

AbstractThe impact of urban expansion and anthropogenic heat (AH) enhancement on tropical cyclone precipitation (TCP) in the Guangdong–Hong Kong–Macau Greater Bay Area (GBA) of China is investigated by using the Weather Research and Forecasting (WRF) model. Sensitivity experiments are conducted during the landfall periods of two tropical cyclones, Hato (2017) and Mangkhut (2018), by artificially varying the surface AH flux from 0 to 600 W/m2 and the urban land surface from 1985 to 2017, respectively. Results show that the TCP in the GBA's urban region increases with both urban expansion and AH enhancement. However, in the urban downstream region, only AH enhancement causes an increase in TCP, whereas urban expansion has no significant influence on TCP. AH enhancement and urban expansion affect surface temperature, surface sensible heat flux, and surface latent heat flux, leading to changes in atmospheric instability and surface water evaporation, resulting in changes in low‐level moisture convergence. The change patterns of the vertically integrated moisture flux divergence between 850 and 910 hPa in the urban and urban downstream regions of the GBA are consistent with those of the TCP changes in this area, indicating that low‐level moisture convergence is the key factor determining the TCP changes in the region.

A Precipitation Isotopic Response in 2014–2015 to Moisture Transport Changes in the Central Himalayas

AbstractThe impact of moisture transport and sources on precipitation stable isotopes (δ18O and d‐excess) in the central Himalayas are crucial to understanding the climatic archives. However, this is still unclear due to the lack of in‐situ observations. Here we present measurements of stable isotopes in precipitation at two stations (Yadong and Pali) in the central Himalayas during 2014–2015. Combined with simulations from the dispersion model FLEXPART, we investigate effects on precipitation stable isotopes related to changes in moisture sources and convections in the region, and possible influence by El Niño. Our results suggest that the moisture supplies related to evaporation over northeastern India and moisture losses related to convective activities over the Bay of Bengal (BoB) and Bangladesh region play important roles in changes in δ18O and d‐excess in precipitation in the Yadong Valley. Outgoing longwave radiation and moisture flux divergence analysis further confirm that the contribution from continental evaporation dominates the moisture supply in the central Himalayas with a lesser contribution from convection over the BoB during the 2015 monsoon season compared with 2014. A change in the altitude effect is observed in 2015, which is more significant than the temperature and precipitation amount effect during the observation period. These findings provide valuable insights into climatic interpretations of paleo‐isotopic archives with an isotopic response to changes in moisture transport to the central Himalayas.

Sensitivity of Precipitation Displacement of a Simulated MCS to Changes in Land Surface Conditions

AbstractThis study investigates the role of the land surface on the precipitation produced by an elevated mesoscale convective system (MCS) in Iowa between 24–25 June 2015 during the Plains Elevated Convection at Night (PECAN) field campaign. Previous studies have shown a strong effect of low‐level atmospheric moisture on the location of this MCS. A series of semi‐idealized and realistic simulations with irrigation are conducted to understand the effect of moisture perturbations on the MCS precipitation displacement. In general, numerical simulations place the MCS east of the observed location. Adding moisture directly in the low‐level atmosphere in the semi‐idealized experiments reduces this displacement error. However, experiments with perturbed soil moisture result in drying over Iowa induced by moisture flux divergence from cooler low‐level temperatures and higher surface pressure, causing the MCS to move further to the east. The irrigation impact on low‐level moisture is highly dependent on the length of simulation period. Shorter simulations on the order of days generate similar drying over Iowa but the opposite is found for month‐long simulations. Despite the lack of low‐level moistening in the perturbed soil moisture and short‐term irrigation experiments, the sensitivity to low‐level moisture is similar in all runs. More low‐level moisture generates a more convectively unstable environment with less inhibition and a lower level of free convection that leads to more rapid MCS development and a change in MCS location.

On the Geomorphic, Meteorological, and Hydroclimatic Drivers of the Unusual 2018 Early Summer Salt Dust Storms in Central Asia

AbstractA series of synoptically forced dust events, including an extreme salt storm from the Aralkum desert, stuck large parts of Central Asia during early summer 2018, causing flight cancellation, widespread haze, and salt deposition on croplands. This manuscript explores the geomorphic, meteorological, and hydroclimatic drivers of the unusual dust activity based on the three prerequisite factors of dust outbreak: sediment supply, sediment availability, and dust‐uplifting winds. Dry lake beds and lower river floodplains are identified as the source area of frequent dust episodes likely due to the abundant supply of fine‐grain sediments. Whereas, minimal dust activity was detected at the region's major sandy deserts. The salt dust storms were triggered by cold intrusion and strong postfrontal winds at the southeast flank of a rapidly developing anticyclone over western Russia which was enhanced by the Scandinavian blocking upstream. The 2018 early summer dust events in Central Asia were identified as part of spatially compounding weather hazards linked to a slow‐moving synoptic wave pattern across the Northern Hemisphere midlatitudes. Finally, the aeolian sediment availability at the dust source areas was enhanced by below‐average soil moisture and vegetation during summer 2018 linked to reduced precipitation in the preceding winter. The 2017/18 winter precipitation deficit was driven by a double‐dip, Central Pacific La Niña episode associated with large‐scale precipitation‐suppressing subsidence and enhanced moisture flux divergence over Central Asia.

The bimodality of South Asia High and its relationship with Asian rainfall in September

AbstractThis study examines daily longitudinal locations of South Asia High (SAH) in September and its connection with Asian September rainfall variation. Results suggest that daily longitudinal locations of September SAH exhibits as bimodality, which is featured by two preferable locations with one high‐frequency location lying over southern Tibetan Plateau and the other over southern Iranian Plateau. Referred to the two preferred locations, the SAH is classified into east and west SAH mode. The two modes may be attributed to a warm column over central eastern Tibetan or eastern Iranian Plateau, which warms the above troposphere and maintains a SAH at upper troposphere over corresponding location. In addition, bimodality of the SAH has a close relationship with Asian September rainfall. The east mode of SAH is associated with an increasing rainfall over western China and decreasing rainfall over northern Indian Peninsula, southwestern Tibetan Plateau, Indochina Peninsula, eastern China and Korea Peninsula, while the west mode of SAH is linked with the reverse pattern. Specifically, the atmospheric circulation at upper troposphere is featured by one large anticyclonic circulation above eastern Tibetan Plateau and two cyclonic circulations over northern Iranian Plateau and eastern Korea. In the lower troposphere, southerly and northerly anomalies form a convergence zone over western China. Meantime, a cyclonic moisture circulation with warm and wet southerly vapour flow results in above‐normal rainfall over western China. An anticyclone over northern Iranian Plateau at lower troposphere results in an anticyclonic moisture divergence and deficient rainfall over southwestern Tibetan Plateau and northern India. Moreover, a cyclonic circulation over eastern Korea Peninsula with marked northwesterly and moisture flux divergence results in below‐normal rainfall over eastern China and Korea Peninsula.

Assessment of Model-Derived Mass Flux Relative to Satellite Gravimetry via a Water Balance in North America

Abstract GRACE and GRACE Follow-On (GRACE-FO) mission data are utilized to assess mass flux derived from the North American Regional Reanalysis (NARR) and the NLDAS-2 Noah land surface model via multiple water balance formulations. Water balances are computed for 18 medium size basins in North America at the USGS Watershed Boundary Dataset HU2 level over the span of the GRACE and GRACE-FO missions (2002–21). Performance of model-derived mass flux is presented in the context of statistical agreement to changes in terrestrial water storage (ΔTWS) derived from Center for Space Research (CSR) GRACE RL06 mass concentrations (mascons), and GRACE and NARR uncertainty is estimated against comparable datasets. The land surface water balance method utilizing NLDAS-2 Noah consistently outperforms the total column method utilizing NARR, which is likely due to enhanced precipitation forcing and an updated Noah model version used in NLDAS-2. The surface approach to the calculation of atmospheric moisture flux divergence is carried through the presented analyses and is demonstrated to be comparable in performance to the more common volume approach. Mass balance methodology, basin characteristics, and ΔTWS signal characteristics are assessed to quantify effects on model performance and while factors such as basin size, basin average topography gradient, and ΔTWS annual amplitude are shown to have a measurable effect on model performance, no single factor exhibited a dominant or consistent effect. Drought conditions are shown to have a significant temporally localized effect on model-derived mass flux accuracy, with NARR being particularly susceptible to this effect. Significance Statement Measurements of Earth’s gravity field from the GRACE and GRACE-FO satellite missions are utilized to create estimates of water storage changes in 18 North American river basins that are compared to changes in water storage calculated from an atmospheric model reanalysis (NARR) and a land surface model (NLDAS-2 Noah). The resulting comparison demonstrates that certain basin characteristics can have a slight effect on model accuracy, while climatic conditions such as drought can have a major impact on model accuracy. This work provides useful quantification of when and where modeled water transport loses accuracy, which is integral to our understanding of the present and future distribution of this crucial resource and the natural processes that affect it.

Assessing precipitation variations in the Yangtze River Basin during 1979–2019 by vertically integrated moisture flux divergence

Assessing precipitation variations in the Yangtze River Basin during 1979–2019 by vertically integrated moisture flux divergence

Atmospheric circulation types controlling rainfall in the Central American Isthmus

AbstractRainfall mechanisms in the Central American Isthmus are controlled by complex physical interactions across spatial and temporal scales, which are reflected on the dynamics of atmospheric circulation patterns affecting the region. However, physical mechanisms and their relationships with thermodynamic distributions connected to overturning circulations remain elusive. Here, a set of six recurrent daily atmospheric patterns, or weather types (WT), is defined using a k‐means++ clustering algorithm on standardized fields of Convective Available Potential Energy (CAPE) and winds at 925, 850, and 200 hPa. The relationships between these weather types, their temporal characteristics, and anomalous distributions of moisture flux divergence, equivalent potential temperature (saturated and unsaturated), and observed rainfall are used to describe physical processes controlling the latter, for all seasons. Regional observed rainfall is analysed from a set of 174 automatic stations from all countries from Mexico to Panama. By modulating vertically integrated moisture fluxes, these weather types, and the different climate drivers linked to them, control the temporal and spatial rainfall characteristics in the region, especially over the Pacific side of the isthmus. During some stages of the regional rainy season, described by two weather types, thermal anomalies in convective quasi‐equilibrium characteristic of the upward branch of the Hadley cell force westerly flow over Central America, enhancing rainfall. While during other stages, the enhancement of the trades and the displacement of convection to the ITCZ area over the eastern tropical Pacific, characteristic of the midsummer drought, diminishes rainfall. This study sets the stage for a better understanding of the mechanistic relationship between these weather types and rainfall characteristics in general, like onset, demise, and duration of rainy seasons. Hence, these results can inform process‐based model diagnostics aiming at bias‐correcting climate predictions at multiple timescales.

Investigation of raindrop size distribution and its potential influential factors during warm season over China

The nation-wide feature of raindrop size distribution (hereafter referred to as DSD) remains largely unknown across China. We here analyzed the DSD using high-resolution disdrometer measurements collected throughout China during the warm season (May through August) of 2020. Overall, pronounced unimodal structures were found for the monthly mean DSD, while little differences were revealed in the monthly variation of DSD. Spatially, the regions with a high value of mass-weighted mean diameter Dm generally witnessed a low value of intercept parameter Nw. The DSD parameters were further investigated by six regions of interest: Northeast China Plain (ECP), North China Plain (NCP), Yangtze River Delta (YRD), Pearl River Delta (PRD), Sichuan Basin (SCB), and Qinghai-Tibet Plateau (QTP). Specially, Dm (log10Nw) over ECP, NCP, YRD, PRD and SCB was found to have a larger (smaller) value than the surrounding areas, whereas the Dm (log10Nw) in the QTP had a smaller (larger) value. Furthermore, explicit analyses were performed in attempt to better understand the influential factors accounting for the regional disparity of DSD by two categories: stratiform and convective precipitation. Dm, log10Nw, spectral width and peak concentration all showed larger values in the convective precipitation than those in the stratiform precipitation. Interestingly, the Dm-log10Nw pairs for convective precipitation resembled roughly the “maritime” cluster, indicating a maritime nature of convective precipitation throughout the warm season in China. The PRD had the largest Dm and the smallest log10Nw, while the QTP had the smallest Dm and the largest log10Nw in the stratiform precipitation. In addition, several widely used empirical equations of Z-R relations tended to underestimate R compared with our actual fitting. The impacts of environmental variables were further studied, including convection available potential energy (CAPE), surface temperature, wind shear and vertically integrated moisture flux divergence (MFD). The dependence of DSD on environmental variable was revealed to varying degrees. It is noteworthy that larger CAPE but lower MFD was found to be accompanied with heavy rain produced by convective precipitation than by stratiform precipitation. The invaluable DSD dataset hereby will lay a solid foundation for potentially improved quantitative precipitation estimation from weather radar in the future in China.

Recent rainfall conditions in the Congo Basin

In the Congo Basin, a drying trend in the April–May–June rains prevailed between 1979 and 2014, accompanied by a decline in forest productivity. This article examines the subsequent years, in order to determine whether rainfall conditions have improved and to examine meteorological factors governing conditions in those years. It is shown that a wetter period, comparable to that of 1979–1993, spanned the years 2016–2020. However, the meteorological factors responsible for the wetter conditions appear to be significantly different from those related to the earlier wet period. The wetter conditions of 1979–1993 were associated with changes in the tropical Walker circulation, in moisture flux and flux divergence, and in Pacific sea-surface temperatures (SST), namely a warmer central and eastern Pacific and a cooler western Pacific, compared to the dry phase in 2000–2014. This resulted in a lower-than-average trans-Pacific SST gradient. In contrast, SSTs were almost ubiquitously higher in the 2016–2020 period than in either prior period. However, there was some reduction in the trans-Pacific gradient. The Walker circulation and moisture flux/flux divergence were not factors in this episode. The major factors provoking the return to wetter years appear to be an increase in convective available potential energy and in total column water vapor. This could be related to the general warming of the oceans and land.

The Orinoco Low-Level Jet and Its Association with the Hydroclimatology of Northern South America

Abstract We investigated the relationship between the frequency of occurrence of the Orinoco low-level jet (OLLJ) and hydroclimatic variables over northern South America. We use data from the ERA5 atmospheric reanalysis to characterize the spatial and temporal variability of the OLLJ in light of the low-level jet (LLJ) classification criteria available in the literature. An index for the frequency of occurrence of an LLJ was used, based on the hourly maxima of wind speed. The linkages among the OLLJ, water vapor flux, and precipitation were analyzed using a composite analysis. Our results show that during December–February (DJF), the OLLJ exhibits its maximum wind speed, with values around 8–10 m s−1. During DJF, the analysis shows how the OLLJ transports atmospheric moisture from the tropical North Atlantic Ocean. During this season, the predominant pathway of the OLLJ is associated with an area of moisture flux divergence located over northeastern South America. During June–August (JJA), an area of moisture flux convergence associated with the northernmost location of the ITCZ inhibits the entrance of moisture from northerlies. We also show that the occurrence of the OLLJ is associated with the so-called cross-equatorial flow. During DJF, the period of strongest activity of the OLLJ is associated with the northerly cross-equatorial flow and dry season, whereas during JJA the southerly cross-equatorial flow from the Amazon River basin predominates and contributes to the rainy season over the Orinoco region.

Diagnostic evaluation of river discharge into the Arctic Ocean and its impact on oceanic volume transports

Abstract. This study analyses river discharge into the Arctic Ocean using state-of-the-art reanalyses such as the fifth-generation European Reanalysis (ERA5) and the reanalysis component from the Global Flood Awareness System (GloFAS). GloFAS, in its operational version 2.1, combines the land surface model (Hydrology Tiled European Centre for Medium-Range Weather Forecasts – ECMWF – Scheme for Surface Exchanges over Land, HTESSEL) from ECMWF’s ERA5 with a hydrological and channel routing model (LISFLOOD). Furthermore, we analyse GloFAS' most recent version 3.1, which is not coupled to HTESSEL but uses the full configuration of LISFLOOD. Seasonal cycles as well as annual runoff trends are analysed for the major Arctic watersheds – Yenisei, Ob, Lena, and Mackenzie – where reanalysis-based runoff can be compared to available observed river discharge records. Furthermore, we calculate river discharge over the whole pan-Arctic region and, by combination with atmospheric inputs, storage changes from the Gravity Recovery and Climate Experiment (GRACE) and oceanic volume transports from ocean reanalyses, we assess closure of the non-steric water volume budget. Finally, we provide best estimates for every budget equation term using a variational adjustment scheme. Runoff from ERA5 and GloFAS v2.1 features pronounced declining trends induced by two temporal inhomogeneities in ERA5's data assimilation system, and seasonal river discharge peaks are underestimated by up to 50 % compared to observations. The new GloFAS v3.1 product exhibits distinct improvements and performs best in terms of seasonality and long-term means; however, in contrast to gauge observations, it also features declining runoff trends. Calculating runoff indirectly through the divergence of moisture flux is the only reanalysis-based estimate that is able to reproduce the river discharge increases measured by gauge observations (pan-Arctic increase of 2 % per decade). In addition, we examine Greenlandic discharge, which contributes about 10 % of the total pan-Arctic discharge and features strong increases mainly due to glacial melting. The variational adjustment yields reliable estimates of the volume budget terms on an annual scale, requiring only moderate adjustments of less than 3 % for each individual term. Approximately 6583±84 km3 of freshwater leaves the Arctic Ocean per year through its boundaries. About two-thirds of this is contributed by runoff from the surrounding land areas to the Arctic Ocean (4379±25 km3 yr−1), and about one-third is supplied by the atmosphere. However, on a seasonal scale budget residuals of some calendar months were too large to be eliminated within the a priori spreads of the individual terms. This suggests that systematical errors are present in the reanalyses and ocean reanalysis data sets, which are not considered in our uncertainty estimation.

Determination of climatic conditions related to precipitation anomalies in the Tropical Andes by means of the random forest algorithm and novel climate indices

AbstractUnderstanding precipitation and its relation with atmospheric and oceanic conditions is vital in the face of climate change. This is crucial in the Tropical Andes (TA) because millions of people depend on water originated in the cordillera. Unfortunately, the paucity of meteorological monitoring that exists in mountainous regions is accentuated in the tropics. In this context, climate indices, remotely sensed, and gridded datasets, are useful tools to study climate and precipitation in the TA, and additional climate indices can be calculated from reanalysis datasets. The combination of this information with traditional indices has the potential to improve our understanding of precipitation. Our objective was to use the k‐means algorithm to regionalize precipitation in the TA (different regions have different climate), and then use the random forest algorithm to study the variables related to precipitation in each of these regions in seasonal timescales. Here, we show the suitability of the random forest algorithm to reveal climate processes and the high potential of the novel climate indices to improve the regressions. We found that convective available potential energy was the most important variable for precipitation in the northern TA, except for the Chocó, where v at 850 hPa was the most important one. Meanwhile, vertical integral of divergence of moisture flux was the most important one in the southern TA. Interestingly, in the DJF season when the South American low‐level jet (SALLJ) is more active, u and v at 850 hPa showed their lowest relative importance and the total column of water vapour showed its maximum, this could indicate that precipitation anomalies are controlled by atmospheric moisture availability rather than by the speed of the SALLJ during DJF. These results deepen our understanding of precipitation anomalies in the TA and the related oceanic and atmospheric conditions. The proposed methodology was proven to be suitable and it could be used in the future to test and formulate new hypotheses, and to forecast seasonal precipitation.

Daily Atmospheric Circulation Patterns and Their Influence on Dry/Wet Events in Iran

Analyzing atmospheric circulation patterns characterize prevailing weather in a region. The method of principal component analysis and clustering was used to classify daily atmospheric circulation patterns. The average daily geopotential height of 500 hPa with 0.5° resolution of the ECMWF (1990–2019) were extracted from the Middle East. The S array was used to identify air types, and k-means clustering was used to classify daily air types. All days were divided into eighteen groups. Then, the surface maps and moisture flux divergence at the 850-hPa level of each pattern were studied. The, the connection between circulation patterns and precipitation occurrence is investigated by the PI index. The existence of a variety of precipitation and temperature regimes and consequent dry/wet periods is related to the type and frequency of the circulation patterns. In patterns with south to southwesterly currents, the low-pressure surface center extends from the south of the Red Sea to southern Turkey and is associated with the mid-level trough, where the moisture fluxes converge in the south of the Red Sea, southwest/south of Iran, and east of the Mediterranean Sea. Therefore, according to the intensity of the patterns, the most precipitation falls in the country’s western half, and the Zagros Mountain’s wind side. With the eastward movement of the Cyclonic patterns, the rainfall area extends to the eastern half of the country. With the pattern that the thermal low surface pressure extends to 35 °N latitude and is associated with the mid-level subtropical high, almost no rain occurs in the country.

Meteorological causes of the catastrophic rains of October/November 2019 in equatorial Africa

The rains of October and November 2019 brought disaster to much of equatorial Africa. In East Africa tremendous rains triggered flooding and landslides in Kenya, causing over 100 deaths and the displacement of some 18,000 people. The situation was exacerbated by an unprecedented locust plague made possible by the intense rains. Floods in the Democratic Republic of the Congo displaced some 40,000 people. The level of the Mono River jumped a meter in four days, producing floods that affected some 50,000 residents of Benin and Togo. This article places the October and November 2019 rainfall extremes in historic context and analyzes the juxtaposition of forcings required to explain these unprecedented hydro-climatic extremes in equatorial Africa. The meteorological factors considered include the Dipole Mode Index, zonal winds over the central Indian Ocean, the Walker circulation, moisture flux and divergence, ENSO and tropical sea-surface temperatures in the Atlantic and Indian Oceans, and zonal circulation. The Dipole Mode Index reached record levels and anomalously high values persisted for five months. This was clearly a major factor in eastern Africa. The impact there was enhanced by increased moisture flux from the Indian Ocean and a marked reduction of the descending branch of the Indian Ocean Walker cell, which strongly controls October–November rainfall. The factors in other regions included extremely warm SSTs in the tropical eastern Atlantic Ocean, the Walker circulation, anomalous moisture flux and flux divergence, and changes in the zonal winds. • Unprecedented rainfall in Oct/Nov 2019 brought disaster to much of equatorial Africa. • Rainfall was 150–400% above normal over most of East Africa and the Guinea Coast. • Rainfall was more than 400% above normal in northern Somalia. • The strong Indian Ocean dipole event could only explain anomalies in some regions. • SSTs, zonal winds, and equatorial waves and the MJO were also important factors.