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Ren-Jie Wu, Min-Hui Lo, and Bridget R. Scanlon

Abstract

Terrestrial water storage anomaly (TWSA) is a critical component of the global water cycle where improved spatiotemporal dynamics would enhance exploration of weather and climate-linked processes. Thus, correctly simulating TWSA is essential not only for water-resource management but also for assessing feedbacks to climate through land-atmosphere interactions. Here we evaluate simulated TWSA from 25 climate models (from the Climate Model Intercomparison Project 6) through comparison with TWSA from GRACE satellite data (2003 – 2014) in 14 river basins globally and assess causes of discrepancies by examining precipitation (P), evapotranspiration (ET), and runoff (Roff) fluxes during recharge (increasing TWS) and discharge (decreasing TWS) cycles. Most models show consistent biases in seasonal amplitudes of TWS anomalies relative to GRACE output: higher modeled amplitudes in river basins in high northern latitudes, Parana, and Congo, and lower amplitudes in most mid-latitude basins and other tropical basins. This TWSA systematic bias also exists in the previous CMIP5 simulations. Models overestimate P compared to observed P datasets in 7 out of 14 basins, which increases (decreases) seasonal storage amplitude relative to GRACE in the recharge (discharge) cycle. Overestimation (underestimation) of runoff is another common contributing factor in the discharge phase that increases (decreases) TWSA amplitudes relative to GRACE in 5 river basins. The results provide a comprehensive assessment of the reliability of the simulated annual range in TWSA through comparison with GRACE data that can be used to guide future model development.

Open access
Ruhua Zhang, Wenshou Tian, Xin He, Kai Qie, Di Liu, and Hongying Tian

Abstract

Using observation, reanalysis and model datasets, the impact of the El Niño-Southern Oscillation (ENSO) on winter precipitation in southern China is re-examined. The results show that positive correlation between ENSO and winter precipitation in southern China after 1995 is significantly higher than that before 1995. Significant positive correlation is located mainly over the southern coastal areas of China before 1995, whereas the positive correlation extends northward to the Yangtze River basin after 1995. These changes in the relationship between ENSO and winter precipitation are related to the ENSO pattern and Philippine anticyclone changes. An increasing trend is observed in the ENSO amplitude, while the area with cooler SST in the Philippine seas extends westward after 1995 compared with that before 1995, leading to an extension of the anticyclone from the east side to the west side of the Philippines. The westward extension of anticyclone after 1995 could enhance the winter precipitation over southern China through modifying water vapour fluxes and vertical motion. Model results support the observation analyses of the changes in ENSO-precipitation relationship and the corresponding mechanism. The mean SST changes could also modify the ENSO-precipitation relationship.

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Maialen Martija-Díez, Belén Rodríguez-Fonseca, and Jorge López-Parages

Abstract

In certain regions, such as Europe, the increase in global air-temperatures in the world is translated into more frequent extreme events. Recent studies suggest that the increasing intensity in heatwaves seems to be related to the interannual variability of the mean temperature, a finding that motivates the search for its possible predictability. El Niño Southern Oscillation (ENSO) is the principal predictor of global climate variability at interannual timescales. Its impact on the European climate has been deeply studied in relation to rainfall variability, but only a few studies exist that focus on its impact on temperature. In this work, we focus on the analysis of the interannual variability of maximum and minimum temperature in order to find some predictability and trends. To that end, we choose the western European region, which has experienced intense heatwaves and is also the main region of air temperature interannual variability in Europe. Our results indicate that the ENSO impact on temperatures over this region is non-linear and non-stationary. We have found the way in which, during the decades prior to 1980s, the increase in temperatures is related to La Niña in summer and to El Niño in fall during the decades after the 1980s, which shows a change in the seasonality of the impact. We study the dynamical mechanisms involved, which suggest a circumglobal response for summer and an arching-like teleconnection pattern in fall. The aforementioned warmer conditions in western European temperatures are found to be significantly correlated to ENSO characteristics of previous seasons, which suggests a potential source for improving the seasonal forecast.

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Elizabeth Carter, Dimitris A. Herrera, and Scott Steinschneider

Abstract

The literature has established dozens of potential predictive indices (PIs) of anomalous warm season precipitation in the Midwestern US that could have utility in subseasonal to seasonal (S2S) forecasts This analysis posits that these predictive indices relate to one of three “modes of action” that work in tandem to drive anomalous hydroclimatic circulation into the continental interior. These include contributions from the (1) geostrophic mass flux, (2) ageostrophic mass flux, and (3) atmospheric moisture supply, and represent semi-independent, interactive forcings on S2S precipitation variability. This study aggregates 24 PIs from the literature that are related to the three modes of action. Using an interpretable machine learning algorithm that accounts for non-linear and interactive responses in a noisy predictive space, we evaluate the relative importance of PIs in predicting S2S precipitation anomalies from March-September. Physical mechanisms driving PI skill are confirmed using composite analysis of atmospheric fields related to the three modes of action. In general, PIs associated with ageostrophic mass flux anomalies are important in early summer, while PIs associated with Atlantic-sourced atmospheric moisture supply are important in late summer. At a two month lead, PIs associated with continental-scale thermodynamic processes are more important relative to PIs associated with local convective phenomena. PIs representing geostrophic mass flux anomalies are also critical throughout the warm season, in real-time and at a 1-2 month lag, but particularly in during transitional months (spring/fall). Several new PIs describing zonal and meridional asymmetry in hemispherical thermal gradients emerge as highly important, with implications for both S2S forecasting and climate change.

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Zhibo Li, Ying Sun, Tim Li, Wen Chen, and Yihui Ding

Abstract

The South Asian summer monsoon (SASM) is one of the most crucial climate components in boreal summer. The future potential changes in the SASM have great importance for climate change adaption and policy setting in this populous region. To understand the SASM changes and its link with the global warming of 1.5°C to 5°C above the preindustrial level, we investigate the changes in the SASM circulation and precipitation based on a large-ensemble simulation conducted with Canadian Earth System Model version 2 (CanESM2). With the global mean surface temperature (GMST) increase, the large-ensemble mean of SASM circulation is projected to weaken almost linearly while the precipitation and precipitable water to enhance quasi-linearly. A double anticyclone along the tropical Indian Ocean is a major anomalous circulation pattern for each additional degree of warming and is responsible for the weakening of the lower-level westerlies. The decreased upper-level land-sea thermal contrast (TCupper) is the main thermal driver for the weakening of the SASM circulation while the lower-level thermal contrast contributes little. The non-linearly decreased TCupper is mainly related to the temperature response to the increased CO2 forcing and convection induced latent heat release in the tropics. The increase in the SASM precipitation is mainly due to the quasi-linearly increased positive contribution of the thermodynamic component, while the dynamic component has a negative impact. Both horizontal moisture advection and moisture convergence contribute to the precipitation increase, and moisture convergence plays a dominant role. These results provide new insight that the SASM changes can be roughly scaled by the GMST changes.

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Zhaoxiangrui He, Aiguo Dai, and Mathias Vuille

Abstract

South American climate is influenced by both Atlantic multidecadal variability (AMV) and Pacific multidecadal variability (PMV). But how they jointly affect South American precipitation and surface air temperature is not well understood. Here we analyze composite anomalies to quantify their combined impacts using observations and reanalysis data. During an AMV warm (cold) phase, PMV-induced JJA precipitation anomalies are more positive (negative) over 0°-10°S and southeastern South America, but more negative (positive) over the northern Amazon and central Brazil. PMV-induced precipitation anomalies in DJF are more positive (negative) over Northeast Brazil and southeastern South America during the warm (cold) AMV phase, but more negative (positive) over the central Amazon Basin and central-eastern Brazil. PMV’s impact on AMV-induced precipitation anomalies shows similar dipole patterns. The precipitation changes result from perturbations of the local Hadley and Walker Circulations. In JJA, PMV- and AMV-induced temperature anomalies are more positive (negative) over entire South America when the other basin is in a warm (cold) phase, but in DJF temperature anomalies are more positive (negative) only over the central Andes and central-eastern Brazil and more negative (positive) over southeastern South America and Patagonia. Over central Brazil in JJA and southern Bolivia and northern Argentina in DJF, the temperature and precipitation anomalies are negatively correlated. Our results show that the influence of Pacific and Atlantic multidecadal variability need to be considered jointly, as significant departures from the mean AMV or PMV fingerprint can occur during a cold or warm phase of the other basin’s mode.

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YUE WU, XIAO-TONG ZHENG, QI-WEI SUN, YU ZHANG, YAN DU, and LIN LIU

Abstract

Ocean salinity plays a crucial role in the upper ocean stratification and local marine ecosystem. This study reveals that ocean salinity presents notable decadal variability in upper 200 meters over the southeast Indian Ocean (SEIO). Previous studies linked this salinity variability with precipitation anomalies over the Indo-Pacific region modulated by the tropical Pacific decadal variability. Here we conduct a quantitative salinity budget analysis and show that, in contrast, oceanic advection, especially the anomalous meridional advection, plays a dominant role in modulating the SEIO salinity on the decadal timescale. The anomalous meridional advection is mainly associated with a zonal dipole pattern of sea level anomaly (SLA) in the South Indian Ocean (SIO). Specifically, positive and negative SLAs in the east and west of the SIO correspond to anomalous southward oceanic current, which transports much fresher seawater from the warm pool into the SEIO and thereby decreases the local upper ocean salinity, and vice versa. Further investigation reveals that the local anomalous wind stress curl associated with tropical Pacific forcing is responsible for generating the sea level dipole pattern via oceanic Rossby wave adjustment on decadal timescale. This study highlights that the local ocean-atmosphere dynamical adjustment is critical for the decadal salinity variability in the SEIO.

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Xinfeng Liang, Chao Liu, Rui M. Ponte, and Don P. Chambers

Abstract

Ocean heat content (OHC) is key to estimating the energy imbalance of the earth system. Over the past two decades, an increasing number of OHC studies were conducted using oceanic objective analysis (OA) products. Here we perform an intercomparison of OHC from eight OA products with a focus on their robust features and significant differences over the Argo period (2005-2019), when the most reliable global scale oceanic measurements are available. For the global ocean, robust warming in the upper 2000 m is confirmed. The 0-300 m layer shows the highest warming rate but is heavily modulated by interannual variability, particularly the El Niño–Southern Oscillation. The 300-700 m and 700-2000 m layers, on the other hand, show unabated warming. Regionally, the Southern Ocean and mid-latitude North Atlantic show a substantial OHC increase, and the subpolar North Atlantic displays an OHC decrease. A few apparent differences in OHC among the examined OA products were identified. In particular, temporal means of a few OA products that incorporated other ocean measurements besides Argo show a global-scale cooling difference, which is likely related to the baseline climatology fields used to generate those products. Large differences also appear in the interannual variability in the Southern Ocean and in the long-term trends in the subpolar North Atlantic. These differences remind us of the possibility of product-dependent conclusions on OHC variations. Caution is therefore warranted when using merely one OA product to conduct OHC studies, particularly in regions and on timescales that display significant differences.

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Qiao Liu, Tim Li, and Weican Zhou

Abstract

Relative impacts of the climatological annual mean, the climatological annual variation, the synoptic, the intra-seasonal and the inter-annual flows on meridional moisture transport were investigated based on reanalysis data. Due to an in-phase relationship between the poleward wind and specific humidity, the synoptic and intra-seasonal motions contribute about 50% and 30% of the maximum zonal and annual mean poleward moisture transport in the middle latitudes, respectively. The preferred latitudinal location (40°N or S) of the maximum zonal mean moisture transport by the synoptic motion is attributed to the combined effect of the maximum wind variability poleward of 40°N or S in association with atmospheric baroclinic instability and the maximum moisture variability equatorward of 40°N or S in association with the anomalous advection of the mean moisture. While the MJO and ENSO have a small contribution to the long-term mean transport, they may strongly affect regional moisture transport through interaction with the mean moisture and through the modulation to higher-frequency modes. A statistical relationship between tropical cyclone (TC) moisture and intensity was constructed based on a large number of high-resolution Weather Research and Forecasting (WRF) model simulations, and the so-derived relationship was further applied to estimate TC moisture transport. It is found that TC transport accounts for about 30% (53%) of the climatological seasonal mean total moisture transport over key northern (southern) hemispheric TC track regions in the northern (southern) hemispheric TC season.

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Ty A. Dickinson, Michael B. Richman, and Jason C. Furtado

Abstract

Extreme precipitation across multiple timescales is a natural hazard that creates a significant risk to life, with a commensurately large cost through property loss. We devise a method to create 14-day extreme event windows that characterize precipitation events in the contiguous United States (CONUS) for the years 1915 through 2018. Our algorithm imposes thresholds for both total precipitation and the duration of the precipitation to identify events with sufficient length to accentuate the synoptic and longer time scale contribution to the precipitation event. Kernel density estimation is employed to create extreme event polygons which are formed into a database spanning from 1915 through 2018. Using the developed database, we clustered events into regions using a k-means algorithm. We define the “Hybrid Index”, a weighted composite of silhouette score and number of clustered events, to show the optimal number of clusters is 14. We also show that 14-day extreme precipitation events are increasing in the CONUS, specifically in the Dakotas and much of New England. The algorithm presented in this work is designed to be sufficiently flexible to be extended to any desired number of days on the subseasonal-to-seasonal (S2S) timescale (e.g., 30 days). Additional databases generated using this framework are available for download from our GitHub. Consequently, these S2S databases can be analyzed in future works to determine the climatology of S2S extreme precipitation events and be used for predictability studies for identified events.

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