Effects of anthropogenic aerosol and greenhouse gas emissions on Northern Hemisphere monsoon precipitation: mechanisms and uncertainty

Northern Hemisphere Land monsoon precipitation (NHLM) exhibits multidecadal variability, decreasing over the second half of the 20st century and increasing after the 1980s. We use a novel combination of CMIP6 simulations and several large ensembles to assess the relative roles of drivers of monsoon precipitation trends, analyzing the effects of anthropogenic aerosol (AA), greenhouse gas (GHG) emissions and natural forcing. We decomposed summer global monsoon precipitation anomalies into dynamic and thermodynamic terms to assess the drivers of precipitation trends. We show that the drying trends are likely to be mainly due to increased AA emissions, which cause shifts of the atmospheric circulation and a decrease in moisture advection. Increases in GHG emissions cause monsoon precipitation to increase due to strengthened moisture advection. The uncertainty in summer monsoon precipitation trends is explored using three initial condition large ensembles. AA emissions have strong controls on monsoon precipitation trends, exceeding the effects of internal climate variability. However, uncertainties in the effects of external forcings on monsoon precipitation are high for specific periods and monsoon domains, and due to differences in how models simulate shifts in atmospheric circulation. The effect of AA emissions is uncertain over the northern African monsoon domain, due to differences among climate models in simulating the effects of AA emissions on net shortwave radiation over the North Atlantic Ocean.

Seasonal Extreme Rainfall Over Indian Monsoon Region: A Moisture Budget Analysis to Distinguish the Role of ENSO and non-ENSO Forcing

Indian summer monsoon rainfall (ISMR) variability of ±10% of its long-term mean leads to flood and drought, affecting the life and economic situation of the country. It is already established that the interannual variability of ISMR is influenced by large scale boundary forcing such as SST anomalies of tropical Pacific, Indian and Atlantic Oceans. The ISMR association between Pacific SST anomalies in the form of El Nino Southern Oscillation (ENSO) is only studied in detail. Meanwhile, the present and previous studies show that the ENSO accounts for around 50% of the extreme years, while the other half is associated with other processes. A differentiation between extremes induced by ENSO and non-ENSO processes are attempted here with the help of moisture and moist static energy budget. The significant contribution to the rainfall extremes comes from moisture advection induced by anomalous winds generated by the boundary forcing and the secondary contribution from moisture convergence. For the non-ENSO cases, there is a contribution from local fluxes, which are not prominent in the cases of ENSO induced cases. In the ENSO cases, anomalous winds are from the equatorial central Pacific, while EQWIN/IOD cases influence extremes through the local evaporation and moisture advection from the Indian Ocean. Extreme years independent of ENSO/IOD/ EQWIN have moisture advection from the anomalous winds across Africa and the Atlantic and are associated with moisture advection toward the northern parts of India. These differences in moisture processes are responsible for the difference in rainfall distribution over India also.

The role of large-scale moistening by adiabatic lifting in the Madden-Julian Oscillation convective onset

The initiation of the Madden-Julian Oscillation over the Indian Ocean is examined through the use of a moisture budget that applies a version of the weak temperature gradient (WTG) approximation that does not neglect dry adiabatic vertical motions. Examination of this budget in ERA-Interim reveals that horizontal moisture advection and vertical advection by adiabatic lifting govern the moistening of the troposphere for both primary and successive MJO initiation events. For both types of initiation events, horizontal moisture advection peaks prior to the maximum moisture tendency, while adiabatic lifting peaks after the maximum moisture tendency. Once convection initiates, moisture is maintained by anomalous radiative and adiabatic lifting. Adiabatic lifting during successive MJO initiation is attributed to the return of the circumnavigating circulation from a previous MJO event, while in primary events the planetary-scale circulation appears to originate over South America. Examination of the same budget with data from the DYNAMO northern sounding array shows that adiabatic lifting contributes significantly to MJO maintenance, with a contribution that is comparable to that of surface heat fluxes. However, results from the DYNAMO data disagree with those from ERA-Interim over the importance of adiabatic lifting to the moistening of the troposphere prior to the onset of convection. In spite of these differences, the results from the two data sets show that small departures from WTG balance in the form of dry adiabatic motions cannot be neglected when considering MJO convective onset.

Surface And Atmospheric Patterns For Early And Late Rainy Season Onset Years In South America

The biosphere-atmosphere interactions associated with the rainy season onset in South America (SA) are not well understood. This study aimed to analyse the atmospheric and surface patterns associated with early, neutral and late rainy season onset years in SA. The years 2006, 2004 and 2008 were characterized as having early, neutral and late rainy season onsets, respectively, in comparison to the climatological mean (1998–2016). Distinct atmospheric conditions were identified in the early and late rainy season onset years. In the early onset year, the northwesterly moisture flux and moisture advection were higher than average over SA’s centre-east, promoting precipitation. In the late onset year, precipitation was enhanced in SA’s northwest and the configuration of multiple atmospheric blocking episodes contributed to the rainy season onset delay. Surface conditions also contributed to both the early/late rainy season onset. In the early onset year, there were wetter and cooler pre-onset conditions over centre-east SA. In the late onset year, atmospheric conditions were dry and warm prior to onset. Despite the atmospheric instability promoted by the increase in sensible heating, dry atmospheric conditions were not favourable to convection, thus delaying rainy season onset. These findings highlight the importance of the land-surface as well as atmospheric conditions for the rainy season onset in SA, and also how the onset variability promotes different atmospheric and surface patterns. The results will help develop modelling capability for better prediction of rainy season onset focusing on biosphere-atmosphere processes improvements.

Local and remote atmospheric responses to soil moisture anomalies in Australia

Three sets of model experiments are performed with the Community Earth System Model to study the role of soil moisture anomalies as a boundary forcing for the formation of upper-level Rossby wave patterns during Southern Hemisphere summer. In the experiments, soil moisture over Australia is set to ±1STD of an ERA-Interim reanalysis derived soil moisture reconstruction for the years 2009 to 2016 and 50 ensemble members are run. The local response is a positive heating anomaly in the dry simulations that results in a thermal low-like circulation anomaly with an anomalous surface low and upper-level anticyclone. Significant differences in convective rainfall over Australia are related to differences in convective instability and associated with changes in near surface moisture and moisture advection patterns. A circum-hemispheric flow response is identified both in the upper-level flow and in the surface storm tracks that overall resembles a positive Southern Annular Mode-like flow anomaly in the dry simulations. The structure of this atmospheric response strongly depends on the background flow. The results point to a modulation of the hemispheric flow response to the forcing over Australia by the El Niño Southern Oscillation. Significant changes of precipitation over the Maritime continent and South Africa are found and significant differences in the frequency of surface cyclones are present all along the storm tracks.

The MJO on the equatorial beta-plane: an eastward propagating Rossby wave induced by meridional moisture advection

Linearized wave solutions on the equatorial beta-plane are examined in the presence of a background meridional moisture gradient. Of interest is a slow, eastward propagating n = 1 mode that is unstable at planetary scales and only exists for a small range of zonal wavenumbers (≲ 6). The mode dispersion curve appears as an eastward extension of the westward propagating equatorial Rossby wave solution. This mode is therefore termed the eastward propagating equatorial Rossby wave (ERW). The zonal wavenumber 2 ERW horizontal structure consists of a low-level equatorial convergence center flanked by quadrupole off-equatorial gyres, and resembles the horizontal structure of the observed MJO. An analytic, leading order dispersion relationship for the ERW shows that meridional moisture advection imparts eastward propagation, and that the smallness of a gross moist stability like parameter contributes to the slow phase speed. The ERW is unstable near planetary scales when low-level easterlies moisten the column. This moistening could come from either zonal moisture advection or surface fluxes or a combination thereof. When westerlies instead moisten the column, the ERW is damped and the westward propagating long Rossby wave is unstable. The ERW does not exist when the meridional moisture gradient is too weak. A moist static energy budget analysis shows that the ERW scale selection is partly due to finite timescale convective adjustment and less effective zonal wind-induced moistening at smaller scales. Similarities in the phase speed, preferred scale and horizontal structure suggest that the ERW is a beta-plane analog of the MJO.

A dynamic and thermodynamic analysis of the 11 December 2017 tornadic supercell in the Highveld of South Africa

On 11 December 2017, a tornadic supercell initiated and moved through the northern Highveld region of South Africa for 7 h. A tornado from this supercell led to extensive damage to infrastructure and caused injury to and displacement of over 1000 people in Vaal Marina, a town located in the extreme south of the Gauteng Province. In this study we conducted an analysis in order to understand the conditions that led to the severity of this supercell, including the formation of a tornado. The dynamics and thermodynamics of two configurations of the Unified Model (UM) were also analysed to assess their performance in predicting this tornadic supercell. It was found that this supercell initiated as part of a cluster of multicellular thunderstorms over a dry line, with three ingredients being important in strengthening and maintaining it for 7 h: significant surface to mid-level vertical shear, an abundance of low-level warm moisture influx from the tropics and Mozambique Channel, and steep mid-level lapse rates. It was also found that the 4.4 km grid spacing configuration of the model (SA4.4) performed better than the 1.5 km grid spacing version. SA1.5 underestimated the low-level warm moisture advection and convergence, and missed the storm initiation. SA4.4 captured the supercell; however, the mid-level vorticity was found to be 1 order of magnitude smaller than that of a typical mesocyclone. A grid length of 4.4 km is too coarse to fully capture the details of a mesocyclone, which may also explain why the model underestimated the surface to mid-level wind shear and low-level horizontal mass and moisture flux convergence. Future investigations will involve experimental research over the Highveld region of South Africa to understand mesoscale and local dynamics processes responsible for tornadogenesis in some severe storms. Such a study, to the best of our knowledge, has never been conducted.

Circulation patterns linked to extreme wet and dry conditions in Mozambique, relationship with climatic modes, and changes since 1961

This study investigates circulation types (CTs) in Africa south of the equator that can be associated with extreme wet and dry conditions in Mozambique; the relationship between the CTs and climatic modes in the south Indian Ocean; and changes in the frequency of occurrence of the CTs since 1961. Obliquely rotated principal component analysis applied to sea level pressure field from NCEP-NACR for the 1961-2020 period was used to derive physically interpretable CTs. At the synoptic scale, widespread rainfall in Mozambique was found to be related to widespread cyclonic activity on the Mozambique landmass and in the southwest Indian Ocean, coupled with abundant onshore moisture advection by southeast winds. On average, this circulation type (CT) was found to be significantly related to the positive phase of the Indian Ocean Dipole and the El Niño climatic modes, which both favor anomalous warming of the (western) tropical Indian Ocean. The aforementioned climatic modes equally constrain the annual frequency of occurrence of the CT, possibly due to the anomalous deepening of cyclonic activity in the southwest Indian Ocean during their active periods. Since 1961 the frequency of occurrence and days of persistence of the aforementioned CT has increased, suggesting a possible increase in the vulnerability of the hydroclimate of Mozambique. Extreme dry conditions in Mozambique can be related to widespread anticyclonic activity on the Mozambique landmass and the southwest Indian Ocean, coupled with the weakening of onshore moisture advection.

Synoptic situations in Africa south of the equator linked to wet events in Namibia; a case study with the February 2008 flood episode

Namibia is one of the water stressed regions in sub-Saharan Africa, with an erratic rainfall pattern. This study investigates synoptic situations that can be favorable for wet events in Namibia. Obliquely rotated principal component analysis applied to the T-mode matrix (variable is time series and observation is grid points) of sea level pressure data set from NCEP-NCAR was used to characterize the modes of large-scale atmospheric circulation variability in Africa south of the equator, in the form of circulation types (CTs). 18 CTs were classified and the linkage of the CTs to wet events in Namibia showed that during austral summer and early austral autumn when sea surface temperature (SST) is warm at the southwest Indian ocean and continental heating is active on the southern African landmasses, stronger (weaker) anticyclonic circulation at the South Indian Ocean high-pressure (South Atlantic Ocean high-pressure) can be associated with enhanced low-level moisture advection by southeast (southwest) winds to Namibia, resulting in wet events in most regions in Namibia. Also, enhanced moisture uptake in the Mozambique Channel might compensate for a relatively weaker moisture advection rate by the South Indian Ocean high-pressure, so that enhanced rainfall can still be expected in Namibia under this scenario. During the early February 2008 flood episode in parts of Namibia, enhanced moisture uptake in the Mozambique Channel coupled with strong southeast winds advecting abundant moisture to Namibia was found to have contributed to the flood.

Mean States and Future Projections of Precipitation Over the Monsoon Transitional Zone in China in CMIP5 and CMIP6 Models

The mean states and future projections of precipitation over the monsoon transitional zone (MTZ) in China are examined based on the historical and climate change projection simulations from phase 5 and phase 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6, respectively). Ensemble means of CMIP6 models exhibit a clear improvement in capturing the annual mean and seasonal cycle of the precipitation over the MTZ, both in its spatial pattern and magnitude, compared to the counterparts of CMIP5 models. In addition, both CMIP5&6 models project a significant increase in the annual total precipitation amount and annual precipitation range, but with slightly stronger changes in CMIP6. For the climatological mean precipitation amount, the two versions’ model ensembles show high consistency in the substantial role played by local evaporation in the supply of moisture in both the present-day and future-projection scenarios, with little contribution from the horizontal and vertical advection of moisture. The precipitation amount is projected to increase in all seasons, but with the strongest signals in summer. An analysis of the moisture budget indicates that the increase in summer precipitation is mainly due to evaporation and vertical moisture advection changes in both CMIP5&6 models. However, the change in vertical moisture advection in CMIP5 is primarily attributable to the thermodynamic effects associated with the humidity changes. By contrast, the dynamic effects induced by the atmospheric circulation changes play a dominant role for CMIP6, which is likely related to the stronger warming gradient between the mid–high latitudes and the tropics.