Interactions among deep convection, sea surface temperature and radiation in the Asian monsoon region

The climatic interactions among deep convection, sea surface temperature and radiation in the Asian monsoon region have been examined using various satellite-derived data sets of the period 1983-90. Annual average Frequency of Deep Convection (FDC) is maximum over the equatorial east Indian ocean and adjoining west Pacific and Indonesian region. Maximum FDC zone shifts to Bay of Bengal during the monsoon (June-September) season. There is weak relationship between the variations in FDC and SST in the Indian ocean. Deep convective activity was suppressed over most of the tropical Indian ocean during El Nino of 1987 in spite of warmer SSTs. The pattern of inter-annual variation between FDC and SST behaves differently in the Indian ocean basin as compared to the Pacific ocean basin. Deep convective clouds interact with radiation very effectively in the Asian monsoon region to cause large net negative cloud radiative forcing. Variation in FDC explains more than 70% of the variation in surface short-wave cloud radiative forcing (SWCRF) and long wave cloud radiative forcing (LWCRF) in the atmosphere. On inter-annual scale, warmer SSTs may not necessarily increase deep convection in the Indian ocean. However, the inter-annual variation of deep convective clouds influences significantly the radiative budget of the surface-atmosphere system in the Asian monsoon region. The satellite observations suggest that warmer SSTs in the Indian ocean might have resulted from an increase in the absorbed solar radiation at the surface due to a reduction in deep convective cloud cover.

Mechanisms of hydrological responses to volcanic eruptions in the Asian monsoon and westerlies-dominated subregions

Explosive volcanic eruptions affect surface climate especially in monsoon regions, but responses vary in different regions and to volcanic aerosol injection (VAI) in different hemispheres. Here we use six ensemble members from last millennium experiment of the Coupled Model Intercomparison Project Phase 5, to investigate the mechanism of regional hydrological responses to different hemispheric VAI in the Asian monsoon region (AMR). It brings a significant drying effect over the AMR after northern hemisphere VAI (NHVAI), spatially, a distinct “wet get drier, dry gets wetter” response pattern emerges with significant drying effect in the wettest area (RWA) but significant wetting effect in the driest area (RDA) of the AMR. After southern hemisphere VAI (SHVAI), it shows a significant wetting effect over the AMR, but spatial response pattern is not that clear due to small aerosol magnitude. The mechanism of the hydrological impact relates to the indirect change of atmospheric circulation due to the direct radiative effect of volcanic aerosols. The decreased thermal contrast between the land and the ocean after NHVAI results in weakened EASM and SASM. This changes the moisture transport and cloud formation in the monsoon and westerlies-dominated subregions. The subsequent radiative effect and physical feedbacks of local clouds lead to different drying and wetting effects in different areas. Results here indicate that future volcanic eruptions may alleviate the uneven distribution of precipitation in the AMR, which should be considered in the near-term decadal prediction and future strategy of local adaptation to global warming. The local hydrological responses and mechanisms found here can also provide reference to stratospheric aerosol engineering.

Comparison of Reanalysis and Observational Precipitation Datasets Including ERA5 and WFDE5

Precipitation is a key component of the hydrological cycle and one of the most important variables in weather and climate studies. Accurate and reliable precipitation data are crucial for determining climate trends and variability. In this study, eleven different precipitation datasets are compared, six reanalysis and five observational datasets, including the reanalysis datasets ERA5 and WFDE5 from the ECMWF family, to quantify the differences between the widely used precipitation datasets and to identify their particular strengths and shortcomings. The comparisons are focused on the common time period 1983 through 2016 and on monthly, seasonal, and inter-annual times scales in regions representing different precipitation regimes, i.e., the Tropics, the Pacific Inter Tropical Convergence Zone (ITCZ), Central Europe, and the South Asian Monsoon region. For the analysis, satellite-gauge precipitation data from the Global Precipitation Climatology Project (GPCP-SG) are used as a reference. The comparison shows that ERA5 and ERA5-Land are a clear improvement over ERA-Interim and show in most cases smaller biases than the other reanalysis datasets (e.g., around 13% high bias in the Tropics compared to 17% for MERRA-2 and 36% for JRA-55). ERA5 agrees well with observations for Central Europe and the South Asian Monsoon region but underestimates very low precipitation rates in the Tropics. In particular, the tropical ocean remains challenging for reanalyses with three out of four products overestimating precipitation rates over the Atlantic and Indian Ocean.

A Paleoclimate Prognosis of the Future Asian Summer Monsoon Variability

In recent years, more and more record-breaking extreme weather/climate events have been reported from the Asian monsoon region, which have caused tremendous loss of property and lives. In this paper, we analyzed the Asian summer monsoon (ASM) variability during the Holocene and evaluated future climate extremes in monsoonal China from a paleoclimatic view. We found a significant regime transition to more chaotic fluctuations, with enhanced decadal variability of the ASM since 6.6 ka BP. We suggested the gradual intensification of ENSO was responsible for enhancing the ASM variability since the late mid-Holocene. If the observed relationship of monsoon mean intensity, ENSO and decadal variability of the ASM in the past 11.2 ka continue to exist, enhanced decadal variability of ASM in the future warming world will be expected. As a result, the intensification of daily precipitation extremes, superimposed on enhanced decadal variability of ASM, might make the record-breaking extremes more frequent in the future, increasing the risk of climate-related disasters in China.

Coupled Effects of Moisture Transport Pathway and Convection on Stable Isotopes in Precipitation across the East Asian Monsoon Region: Implications for Paleoclimate Reconstruction

This study investigated the variations in stable oxygen isotopes in daily precipitation (δ18Op) collected between 2010 and 2013 at four sites across the East Asian monsoon region to address the controversy whether local meteorological factors, moisture transport pathway or convection dominates the δ18Op changes. We found that the δ18Op time series exhibit opposite seasonal patterns between the southern and northern sites; however, relatively low δ18Op values occur at each site during summer. The opposite seasonal patterns are closely related to the proportional change in the contributions from oceanic (> 52% in the south) and continental (> 85% in the north) moisture sources. Moisture transport distances also influence the seasonal δ18Op fluctuations. In the south, the moisture transported over short distances from the middle of the western Pacific Ocean results in relatively high δ18Op values during the pre-monsoon season. In contrast, long-distance transport of moisture from the Indian and Equatorial Pacific Oceans during the monsoon season results in relatively low δ18Op values. In the north, relatively low δ18Op values during the monsoon season can be attributed to an increase in relatively distant moisture originated from the middle of the western Pacific Ocean. Convection only plays a role in affecting δ18Op values in the south during the monsoon season. Our study suggests that moisture transport pathway (moisture sources and moisture transport distances) is a major factor that governs seasonal variations in δ18Op across the East Asian monsoon region, which has implications for the interpretation of paleoclimate records from this region.

Ice microphysical processes exert a strong control on the simulated radiative energy budget in the tropics

Simulations of the global climate system at storm-resolving resolutions of 2 km are now becoming feasible and show promising realism in clouds and precipitation. However, shortcomings in their representation of microscale processes, like the interaction of cloud droplets and ice crystals with radiation, can still restrict their utility. Here, we illustrate how changes to the ice microphysics scheme dramatically alter both the vertical profile of cloud-radiative heating and top-of-atmosphere outgoing longwave radiation (terrestrial infrared cooling) in storm-resolving simulations over the Asian monsoon region. Poorly-constrained parameters in the ice nucleation scheme, overactive conversion of ice to snow, and inconsistent treatment of ice crystal effective radius between microphysics and radiation alter cloud-radiative heating by a factor of four and domain-mean infrared cooling by 30 W m−2. Vertical resolution, on the other hand, has a very limited impact. Even in state-of-the-art models then, uncertainties in microscale cloud properties exert a strong control on the radiative budget that propagates to both atmospheric circulation and regional climate. These uncertainties need to be reduced to realize the full potential of storm-resolving models.