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Laura Paccini, Cathy Hohenegger, and Bjorn Stevens

Abstract

This study investigates whether the representation of explicit and parameterized convection influences the response to the Atlantic meridional mode (AMM). The main focus is on the precipitation response to the AMM-SST pattern, but possible implications for the atmospheric feedback on SST are also examined by considering differences in the circulation response between explicit and parameterized convection. On the basis of analysis from observations, SST composites are built to represent the positive and negative AMM. These SST patterns, in addition to the March–May climatology, are prescribed to the atmospheric ICON model. High-resolution simulations with explicit convection (E-CON) and coarse-resolution simulations with parameterized convection (P-CON) are used over a nested tropical Atlantic Ocean domain and a global domain, respectively. Our results show that a meridional shift of about 1° in the precipitation climatology explains most of the response to the AMM-SST pattern in simulations both with explicit convection and with parameterized convection. Our results also indicate a linearity in the precipitation response to the positive and negative AMM in E-CON, in contrast to P-CON. Further analysis of the atmospheric response to the AMM reveals that anomalies in the wind-driven enthalpy fluxes are generally stronger in E-CON than in P-CON. This result suggests that SST anomalies would be amplified more strongly in coupled simulations using an explicit representation of convection.

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Anne Felsberg, Gabriëlle J. M. De Lannoy, Manuela Girotto, Jean Poesen, Rolf H. Reichle, and Thomas Stanley

Abstract

This global feasibility study assesses the potential of coarse-scale, gridded soil water estimates for the probabilistic modeling of hydrologically triggered landslides, using Soil Moisture Ocean Salinity (SMOS), Soil Moisture Active Passive (SMAP), and Gravity Recovery and Climate Experiment (GRACE) remote sensing data; Catchment Land Surface Model (CLSM) simulations; and six data products based on the assimilation of SMOS, SMAP, and/or GRACE observations into CLSM. SMOS or SMAP observations (~40-km resolution) are only available for less than 20% of the globally reported landslide events, because they are intermittent and uncertain in regions with complex terrain. GRACE terrestrial water storage estimates include 75% of the reported landslides but have coarse spatial and temporal resolutions (monthly, ~300 km). CLSM soil water simulations have the added advantage of complete spatial and temporal coverage, and are found to be able to distinguish between “stable slope” (no landslide) conditions and landslide-inducing conditions in a probabilistic way. Assimilating SMOS and/or GRACE data increases the landslide probability estimates based on soil water percentiles for the reported landslides, relative to model-only estimates at 36-km resolution for the period 2011–16, unless the CLSM model-only soil water content is already high (≥50th percentile). The SMAP Level 4 data assimilation product (at 9-km resolution, period 2015–19) more generally updates the soil water conditions toward higher landslide probabilities for the reported landslides, but is similar to model-only estimates for the majority of landslides where SMAP data cannot easily be converted to soil moisture owing to complex terrain.

Open access
Shuai Hu, Tianjun Zhou, and Bo Wu

Abstract

The year-to-year variations of Tibetan Plateau (TP) summer rainfall have tremendous climate impacts on the adjoining and even global climate, attracting extensive research attention in recent decades to understand the underlying mechanism. In this study, we investigate an open question of how El Niño–Southern Oscillation (ENSO) influences the TP precipitation. We show that the developing ENSO has significant impacts on the summer rainfall over the southwestern TP (SWTP), which is the second EOF mode of the interannual variability of summer rainfall over the TP. The moisture budget indicates that both the suppressed vertical motion and the deficit of moisture contribute to the reduction of SWTP rainfall during El Niño’s developing summer, with the former contribution 4 times larger than the latter. Moist static energy analyses indicate that the anomalous advection of climatological moist enthalpy by anomalous zonal wind is responsible for the anomalous descending motions over the SWTP. The El Niño–related southward displacements of the South Asian high and the upper-level cyclonic anomalies over the west of TP stimulated by the suppressed Indian summer monsoon precipitation are two key processes dominating the anomalous zonal moist enthalpy advection over SWTP. Meanwhile, the India–Burma monsoon trough is strengthened during El Niño developing summer, which prevents the advection of water vapor into the SWTP, and thus contributes to the deficit of summer SWTP rainfall. Our results help to understand the complicated ENSO-related air–sea interaction responsible for the variability of TP precipitation and have implications for seasonal prediction of the TP climate.

Open access
Zhang Yue, W. Zhou, and Tim Li

Abstract

The complex interaction between the Indian Ocean dipole (IOD) and El Niño–Southern Oscillation (ENSO) is further investigated in this study, with a focus on the impacts of the IOD on ENSO in the subsequent year [ENSO(+1)]. The interaction between the IOD and the concurrent ENSO [ENSO(0)] can be summarized as follows: ENSO(0) can trigger and enhance the IOD, while the IOD can enhance ENSO(0) and accelerate its demise. Regarding the impacts of IOD(0) on the subsequent ENSO(+1), it is revealed that the IOD can lead to anomalous SST cooling patterns over the equatorial Pacific after the winter following the IOD, indicating the formation of a La Niña–like pattern in the subsequent year. While the SST cooling tendency associated with a positive IOD is attributable primarily to net heat flux (thermodynamic processes) from autumn to the ensuing spring, after the ensuing spring the dominant contribution comes from oceanic processes (dynamic processes) instead. From autumn to the ensuing spring, the downward shortwave flux response contributes the most to SST cooling over the central and eastern Pacific, due to the cloud–radiation–SST feedback. From the ensuing winter to the ensuing summer, changes in latent heat flux (LHF) are important for SST cooling, indicating that the release of LHF from the ocean into the atmosphere increases due to strong evaporation and leads to SST cooling through the wind–evaporation–SST feedback. The wind stress response and thermocline shoaling verify that local Bjerknes feedback is crucial for the initiation of La Niña in the later stage.

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Hyung-Gyu Lim, Jong-Yeon Park, John P. Dunne, Charles A. Stock, Sung-Ho Kang, and Jong-Seong Kug

Abstract

Human activities such as fossil fuel combustion, land-use change, nitrogen (N) fertilizer use, emission of livestock, and waste excretion accelerate the transformation of reactive N and its impact on the marine environment. This study elucidates that anthropogenic N fluxes (ANFs) from atmospheric and river deposition exacerbate Arctic warming and sea ice loss via physical–biological feedback. The impact of physical–biological feedback is quantified through a suite of experiments using a coupled climate–ocean–biogeochemical model (GFDL-CM2.1-TOPAZ) by prescribing the preindustrial and contemporary amounts of riverine and atmospheric N fluxes into the Arctic Ocean. The experiment forced by ANFs represents the increase in ocean N inventory and chlorophyll concentrations in present and projected future Arctic Ocean relative to the experiment forced by preindustrial N flux inputs. The enhanced chlorophyll concentrations by ANFs reinforce shortwave attenuation in the upper ocean, generating additional warming in the Arctic Ocean. The strongest responses are simulated in the Eurasian shelf seas (Kara, Barents, and Laptev Seas; 65°–90°N, 20°–160°E) due to increased N fluxes, where the annual mean surface temperature increase by 12% and the annual mean sea ice concentration decrease by 17% relative to the future projection, forced by preindustrial N inputs.

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Yanyi He, Kaicun Wang, and Fei Feng

Abstract

Surface incident solar radiation (R s) is important for providing essential information on climate change. Existing studies have shown that the R s values from current reanalyses are significantly overestimated throughout China. The European Centre for Medium-Range Weather Forecasts (ECMWF) recently released the fifth generation of atmospheric reanalysis (i.e., ERA5) with a much higher spatiotemporal resolution and a major upgrade compared to its predecessor, ERA-Interim. This study is to verify whether ERA5 can improve the R s simulation using sunshine duration–derived R s values at ~2200 stations over China from 1979 to 2014 as reference data. Compared with the observed multiyear national mean, the R s overestimation is reduced from 15.88 W m−2 in ERA-Interim to 10.07 W m−2 in ERA5. From 1979 to 1993, ERA-Interim (−1.99 W m−2 decade−1; p < 0.05) and ERA5 (−2.42 W m−2 decade−1; p < 0.05) estimates of R s in China continued to decrease and the decline of the latter is closer to the observed. After 1993, they both show a strong brightening (i.e., 2.26 W m−2 decade−1 in ERA-Interim and 1.49 W m−2 decade−1 in ERA5) but observations show a nonsignificant increase by 0.30 W m−2 decade−1. Due to the improvement of total cloud cover (TCC) simulation by ERA5, its R s trend bias induced by the TCC trend bias is smaller than that in ERA-Interim. In addition, the reason why the simulation trend in ERA5 remains biased might be that ERA5 still ignores aerosol changes on interannual or decadal time scales. Therefore, subsequent reanalysis products still need to improve their simulation of clouds, water vapor, and aerosol, especially in aerosol direct and indirect effects on R s.

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Zhenhai Zhang and F. Martin Ralph

Abstract

Some extratropical cyclones (ETC) begin their development in close proximity to a preexisting atmospheric river (AR). This study investigates the differences in the cyclogenesis stage between these cyclogenesis events and those that begin without an AR nearby. Well-established ETC and AR detection methods are applied to reanalysis over the North Pacific during the 1979–2009 cool seasons (November–March). Of the 3137 cyclogenesis cases detected, 35% are associated with a nearby AR at the time of initial cyclogenesis. Of all 448 cyclones that deepened explosively in the 24 h after their initiation, 60% began with a preexisting AR nearby. The roles of both dry and diabatic processes that contribute to cyclogenesis are examined, specifically, low-level baroclinicity, upper-level forcing, water vapor inflow, and latent heating. ETCs that develop associated with a preexisting AR receive nearly 80% more water vapor inflow on average, enhancing latent heating and intensifying cyclone deepening in the genesis stage. In contrast, neither low-level baroclinicity nor upper-level potential vorticity exhibit statistically significant differences between cyclogenesis events with and without an AR. Cyclogenesis events associated with an exceptionally strong AR at the ETC initial time deepen even more rapidly in the genesis stage, indicating that the intensity of an antecedent AR can modulate cyclogenesis. About half of the cyclogenesis cases off the U.S. West Coast are associated with ARs at their initial time. These results imply that errors in initial conditions related to ARs can contribute to errors in both AR and ETC predictions, as well as their concomitant impacts.

Open access
Abdullah A. Fahad, Natalie J. Burls, Erik T. Swenson, and David M. Straus

Abstract

Subtropical anticyclones and midlatitude storm tracks are key components of the large-scale atmospheric circulation. Focusing on the Southern Hemisphere, the seasonality of the three dominant subtropical anticyclones, situated over the South Pacific, South Atlantic, and south Indian Ocean basins, has a large influence on local weather and climate within South America, southern Africa, and Australia, respectively. Generally speaking, sea level pressure within the Southern Hemisphere subtropics reaches its seasonal maximum during the winter season when the Southern Hemisphere Hadley cell is at its strongest. One exception to this is the seasonal evolution of the South Pacific subtropical anticyclone. While winter maxima are seen in the South Atlantic and south Indian subtropical anticyclones, the South Pacific subtropical anticyclone reaches its seasonal maximum during local spring with elevated values extending into summer. In this study, we investigate the hypothesis that the strength of the austral summer South Pacific subtropical anticyclone is largely due to heating over the South Pacific convergence zone. Using added-cooling and added-heating atmospheric general circulation model experiments to artificially change the strength of austral summer diabatic heating over the South Pacific convergence zone, our results show that increased heating, through increased upper-level divergence, triggers a Rossby wave train that extends into the Southern Hemisphere midlatitudes. This propagating Rossby wave train creates a high and low sea level pressure pattern that projects onto the center of the South Pacific subtropical anticyclone to intensify its area and strength.

Open access
Zizhen Dong, Lin Wang, Peiqiang Xu, Sittichai Pimonsree, Atsamon Limsakul, and Patama Singhruck

Abstract

Based on several observational and reanalysis datasets for the winters 1901–2017, this study investigates the interdecadal (ID) variation of the Southeast Asian rainfall (SEAR) and its potential drivers. The dominant mode of the wintertime SEAR on the ID time scale features enhanced precipitation over the eastern Maritime Continent and the Philippines and a slight decrease of precipitation over the western Maritime Continent, or the opposite sign. The ID SEAR variability peaks at the 8–16-yr period and explains more than 20% of the total variance regardless of the datasets and period considered, highlighting the importance of the ID variability of the SEAR. The atmospheric circulation that facilitates abundant ID SEAR is characterized by enhanced lower-tropospheric wind convergence and cyclonic anomalies over the South China Sea and the Philippines. On the one hand, this wind convergence is attributed to the enhanced Walker circulation induced by the negative phase of the interdecadal Pacific oscillation (IPO). On the other hand, it is attributed to the enhanced northerly anomalies along the coast of East Asia induced by a strong East Asian winter monsoon (EAWM) and reduced autumn Arctic sea ice in the Barents–Kara Seas. These mechanisms are further confirmed by model experiments from phase 5 of the Coupled Model Intercomparison Project (CMIP5). The effects of the IPO, EAWM, and Arctic sea ice on the SEAR are mostly independent. They together explain approximately 70% of the SEAR variance on the ID time scale.

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Ellen Dyer and Richard Washington

Abstract

The interannual variability, trends, and the mean climatology of East African long rains are difficult for models to simulate. This is in part because long rains do not respond in a simple way to large-scale modes of variability such as ENSO and because of interactions with complex topography. Here we focus on the Kenyan regional climate in the ERA-Interim dataset during the long rains to create a set of atmospheric diagnostics that can be applied to the evaluation of climate models. Subseasonal observed rainfall and reanalysis reveal that very wet seasons and very dry seasons develop differently at the beginning of the season. Subseasonal aggregation periods (days 60–80, 80–100, 90–120, 120–150) highlight local (e.g., midtropospheric ascent, moisture flux convergence in the lower to midtroposphere, and midtropospheric moisture) and large-scale (e.g., midtropospheric zonal winds over central Africa, upper-tropospheric velocity potential) diagnostics that are useful to evaluate model atmospheric circulation affecting Kenyan rainfall in mean and wet or dry extremes.

Open access