Exploring the mechanism behind successive droughts: Intensification of continental droughts due to positive feedbacks between subsurface water storage anomalies and atmospheric process

<p>In the year 2018, Central Europe experienced a meteorological drought and a heatwave, which led to a subsequent evolution of a hydrological drought that is still detectable in subsurface water storage anomalies today. Most likely, the drought also led to significant changes in the energy balance between solar radiation, latent and sensible heat fluxes. In conjunction with water scarcity in the subsurface, these changes may lead to feedbacks that mitigate or enforce drought conditions in the context of land-atmosphere coupling. Understanding these feedbacks is of great interest, especially under various large-scale weather patterns that strongly influence the water and energy budgets over Europe at the interannual time scale. We improve our understanding by applying the Terrestrial Systems Modeling Platform (TSMP) over the 12km resolution pan-European CORDEX model domain simulating the water and energy cycles from the groundwater to the top of the atmosphere. TSMP couples a hydrological, land-surface and atmospheric model, facilitating studies of feedbacks between total water storage anomalies, the energy budget and atmospheric processes. To investigate the feedbacks, we performed TSMP ensemble simulations of three anomalously dry water years (September to August) over Central Europe. The ensembles were initialized with the surface and subsurface states of the end of August of the drought years 2011, 2018 and 2019 from an ERA-Interim driven climatology simulated continuously with TSMP from 1989 to 2019. Every ensemble consists of 22 members, each representing a full subsequent water year, sampled from ERA-Interim reanalysis meteorological boundary conditions from 1996 to 2019, thereby simulating the influence of drought conditions over a wide range of large-scale weather patterns that occurred in Europe since 1996. In addition, to illustrate the potential range of feedbacks we also ran idealized experiments with a completely dry or wet subsurface. The results show that drought conditions may have a significant impact on cloud water and solar radiation at interannual timescales. Effects in winter are negligible, while in summer, an impact of the drought conditions of the previous year on cloud water and solar radiation is detectable in all three ensembles. The results suggest that positive feedbacks between dry subsurface water storage anomalies and atmosphere processes are not negligible and may intensify drought conditions also at the interannual time scale.</p>

North Atlantic Oscillation Effect on Interannual Variability in Winter Precipitation over the Gulf Stream

In winter, the warm water of the Gulf Stream anchors a salient precipitation band. Previous studies suggested a close relationship between the sea surface temperature (SST) front and the precipitation band through sea level pressure (SLP) adjustment. This study uses 17 years of high-resolution precipitation observations to reveal that the variation in wintertime precipitation over the Gulf Stream is related to the North Atlantic Oscillation (NAO) at the interannual time scale. The moisture budget analysis shows that the climatological precipitation band is supported by the large evaporation from the Florida Current, mean flow, and synoptic moisture convergence within the boundary layer, with a negative contribution from mean-flow moisture advection by the prevailing northwesterlies. For interannual variability, by contrast, the negative contribution of mean-flow moisture advection significantly decreases due to anomalous southeasterlies west of the intensified Azores high at the positive NAO phase. The contributions from mean-flow moisture advection and mean and synoptic convergence vary greatly along the Gulf Stream. In addition, mean-flow and synoptic moisture convergences positively contribute to the precipitation band both in climatology and at the interannual time scale, indicative of a positive feedback between precipitation and boundary layer convergence. Our analysis suggests that the SLP adjustment mechanism across the SST front is still at work in interannual variability, and the variation of synoptic activities over the Gulf Stream plays an important role in modulating the frontal precipitation. By relating the frontal precipitation to the NAO, this study bridges small-scale air–sea interaction and large-scale atmospheric circulation.

Comparison of Moisture Transport between Siberia and Northeast Asia on Annual and Interannual Time Scales

The moisture supplies over Siberia and Northeast Asia are investigated by comparing their similarities and differences, enlightened by the seesaw pattern in their summer precipitation. Based on the rotated empirical orthogonal functions in the 3-month standardized precipitation evapotranspiration index (SPEI_03), Siberia and Northeast Asia are defined as the regions within 55°–70°N, 80°–115°E and 40°–55°N, 90°–115°E, respectively. Our results show that over both regions, evaporation contributes the most to the precipitation amount at the annual time scale, and moisture convergence contributes the most on the interannual time scale. For moisture convergence, both the stationary and transient terms are subject to impacts of the midlatitude westerlies. For the annual cycle, the net moisture supply over both Siberia and Northeast Asia is closely associated with both stationary and transient moisture transport. However, on the interannual time scale, the net moisture convergence is closely related to the stationary term only. The examination of the boundary moisture transport shows that in addition to the zonal component, the meridional stationary moisture transport plays a key role in the net moisture convergence. The transient moisture transport mainly depends on moisture transport through the western and southern boundaries, with a comparable magnitude to that of the stationary one, further confirming the importance of the stationary and transient terms on the moisture supply for the annual cycle. In addition, the circulations responsible for moisture transport anomalies indicate that the stationary moisture circulation is the key factor for the moisture supply anomalies over both Siberia and Northeast Asia, with limited impacts from the transient moisture circulation.

Local Oceanic Precursors for the Summer Monsoon Onset over the Bay of Bengal and the Underlying Processes

Local sea surface temperature (SST) plays an important role in the onset of the Bay of Bengal (BoB) summer monsoon (BoBSM). Previous study indicated that the meridionally warmest SST axis (WSSTA) appears in mid-April in the central BoB, which may be a precursor for the BoBSM onset. In this study, it is found that a warm but not the meridionally warmest center, which is defined as the secondary WSSTA (SWSSTA), occurs in early April in the central BoB, leading the BoBSM onset by five pentads. Dates of the SWSSTA occurrence are significantly positively correlated with dates of the WSSTA occurrence in the central BoB and the BoBSM onset on an interannual time scale. The SWSSTA is an earlier precursor for the BoBSM onset. The formation of the oceanic precursor and its impact on the BoBSM onset are as follows. Before the BoBSM onset, resulting from more surface heat input and shallower mixed layer affected by the low-level anticyclone and subtropical high in the central BoB, local SST shows the most rapid increase. Meanwhile, the situation is adverse to the rapid increase of SST in the equatorial BoB. For this reason, the SWSSTA occurs, and the WSSTA subsequently appears in the central BoB. The WSSTA in turn enhances local convection, eliminates the low-level anticyclone, and moves the subtropical high outward away from the BoB by inducing atmospheric instability, thus developing a heating center. Convectional heating further strengthens southwesterlies in the BoB by exciting mixed planetary–gravity waves, resulting in the BoBSM onset.

Interannual Shift of the Tropical Upper-Tropospheric Trough and Its Influence on Tropical Cyclone Formation over the Western North Pacific

The east–west migration of the tropical upper-tropospheric trough (TUTT) on the interannual time scale and its influence on tropical cyclone (TC) formation over the western North Pacific (WNP) is investigated in this study. Climatologically, the TUTT can be identified from 100 to 400 hPa with a relative vorticity maximum between 150 and 200 hPa. In addition to the strong westerly vertical wind shear in the south flank of the TUTT, this study shows that the cold-core system is associated with low relative humidity and subsidence to the east of the trough axis. As a result, the TC formation is enhanced (suppressed) in the eastern portion of the WNP when the TUTT shifts eastward (westward) on the interannual time scale. The interannual TUTT shift is closely associated with the SST anomalies in the central and eastern tropical Pacific or ENSO phases. The warm (cold) phase of ENSO corresponds to the eastward (westward) shift of the TUTT. The physical factors found to be responsible for the influence of ENSO on TC formation can be associated with the east–west shift of the TUTT. It is shown that the interannual variations of TC formation in the eastern part of the WNP basin are closely associated with the east–west shift of the TUTT due to the associated environmental conditions that are generally not favorable for TC formation.

Impacts of Autumn Arctic Sea Ice Concentration Changes on the East Asian Winter Monsoon Variability

The present study investigated the impacts of autumn Arctic sea ice concentration (SIC) changes on the East Asian winter monsoon (EAWM) and associated climate and circulation on the interannual time scale. It is found that the Arctic SIC anomalies have little impact on the southern mode of EAWM, but the northern mode is significantly associated with both western and eastern Arctic SIC anomalies. When there is less (more) SIC in eastern (western) Arctic, the EAWM tends to be stronger. The concurrent surface air temperature anomalies are induced both locally due to the direct effect of ice cover and in remote regions through anomalous wind advection. Analysis showed that eastern Arctic SIC anomalies have a larger effect on local atmospheric stability of the lower troposphere than western Arctic SIC anomalies. Winter temperature over the midlatitudes of East Asia is lower when there is more (less) SIC in the western (eastern) Arctic. The atmospheric response to the Arctic SIC anomalies is dominantly barotropic in autumn, and changes to baroclinic over the midlatitudes of Asia, but remains barotropic in other regions in winter. The mid- to high-latitude circulation systems, including the Siberian high, the East Asian trough, and the East Asian westerly jet stream, play important roles in connecting autumn Arctic SIC anomalies and the northern mode of the EAWM variability. No obvious concurrent sea surface temperature anomalies accompany Arctic SIC variations on the interannual time scale, indicating that the Arctic SIC anomalies have independent impacts on the East Asian winter climate.