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The prediction skill for precipitation anomalies in late spring and summer months—a significant component of extreme climate events—has remained stubbornly low for years. This paper presents a new idea that utilizes information on boreal spring land surface temperature/subsurface temperature (LST/SUBT) anomalies over the Tibetan Plateau (TP) to improve prediction of subsequent summer droughts/floods over several regions over the world, East Asia and North America in particular. The work was performed in the framework of the GEWEX/LS4P Phase I (LS4P-I) experiment, which focused on whether the TP LST/SUBT provides an additional source for subseasonal-to-seasonal (S2S) predictability. The summer 2003, when there were severe drought/flood over the southern/northern part of the Yangtze River basin, respectively, has been selected as the focus case. With the newly developed LST/SUBT initialization method, the observed surface temperature anomaly over the TP has been partially produced by the LS4P-I model ensemble mean, and 8 hotspot regions in the world were identified where June precipitation is significantly associated with anomalies of May TP land temperature. Consideration of the TP LST/SUBT effect has produced about 25–50% of observed precipitation anomalies in most hotspot regions. The multiple models have shown more consistency in the hotspot regions along the Tibetan Plateau-Rocky Mountain Circumglobal (TRC) wave train. The mechanisms for the LST/SUBT effect on the 2003 drought over the southern part of the Yangtze River Basin are discussed. For comparison, the global SST effect has also been tested and 6 regions with significant SST effects were identified in the 2003 case, explaining about 25–50% of precipitation anomalies over most of these regions. This study suggests that the TP LST/SUBT effect is a first-order source of S2S precipitation predictability, and hence it is comparable to that of the SST effect. With the completion of the LS4P-I, the LS4P-II has been launched and the LS4P-II protocol is briefly presented. [ABSTRACT FROM AUTHOR]

Previous studies have reported significant uncertainty in the relationship between El Niño-Southern Oscillation (ENSO) and the winter surface air temperature (SAT) over central North America (NA). This paper uses reanalysis data and statistical methods to investigate the possible modulation effect of the Victoria mode (VM) during the preceding spring on the winter ENSO–SAT relationship. The correlation between ENSO and the winter SAT over central NA was found to be significantly positive during the positive VM phase. However, this significant correlation is almost absent during the negative VM phase. Further analysis revealed that the zonal location of the sea surface temperature anomalies (SSTA) over the tropical Pacific is an essential aspect of this VM modulation. During the positive VM phase, the dominant positive SSTA in the central-eastern tropical Pacific contribute to the formation of the negative phase of the North Pacific Oscillation (NPO)-like teleconnection, which further enhances the anomalous southerly winds and SAT over central NA. In contrast, during the negative VM phase, the Aleutian low-like atmospheric response induced by the central tropical Pacific SSTA is limited to the central North Pacific, which has a limited effect on the variation of the central NA SAT. The VM modulation mechanism is shown to be well reproduced by historical simulations of the Coupled Model Intercomparison Project Phase 6. Our results suggest that, despite the uncertainty associated with the ENSO–SAT connection, ENSO may provide prediction skills for the winter SAT over central NA during the positive VM phase. [ABSTRACT FROM AUTHOR]

Air temperature variability and prediction are important considering the potential implications on human health, water resource management, agriculture and land-atmosphere interaction. In this paper, we investigate the delayed influence of El Niño on spring Surface Air Temperature (SAT) variability over India using observations and assess its predictability in coupled climate models. Analysis suggests that El Niño, at its decaying phase, exerts a strong influence on the spring SAT over Southern Peninsular India (SPI). This warming is mainly induced by an anomalous anticyclone, which extends from the Western North Pacific (WNP) region to the SPI. The anomalous anticyclone-induced downdraft reduces the cloud cover and increases incoming shortwave radiation at surface, which in turn causes a dry atmospheric column and dry soil moisture over this region. As a result of this decreased soil moisture, latent heat flux into atmosphere reduces and enhances sensible heat transfer, further contributing to dry soil moisture. Both dry soil moisture and anomalous anticyclone contributes to enhanced SAT over SPI region. It is found that 6 years out of the top 10 warmest years over the SPI region correspond to the decaying phase of El Niño, suggesting the importance of the present study. It is also found that the intense SAT warming over SPI aids in increasing frequencies of Discomfort Index hours (exceeding 28 and 30 °C in a day) and the Universal Thermal Climate Index during El Niño decaying spring years. Further, we have examined the predictability of the spring SAT during decaying El Niño years based on the Asia-Pacific Economic Cooperation Climate Center (APCC) models. It is noted that most of the APCC models reasonably predicted the spatial distribution of warm SAT over India during the El Niño decaying spring season at 1 and 3 month lead. This study highlights the predictability of spring SAT a season in advance over India, which would help in planning better adaptation and mitigation strategies. [ABSTRACT FROM AUTHOR]

Recent observational and modeling studies have demonstrated the substantial influence of the Tibetan Plateau (TP) spring land surface temperature (LST) and subsurface temperature (SUBT) on downstream summer droughts/floods events in East Asia, highlighting the potential application of LST/SUBT on sub-seasonal to seasonal prediction (S2S). In this study, we employ the National Centers for Environment Prediction—Global Forecast System/Simplified Simple Biosphere model version 2 (GFS/SSiB2) to investigate the potential role of the late spring warm LST anomaly over the TP on the extraordinary June 1998 flood in the south of the Yangtze River region. Numerical experiments indicate that the warmer (above normal) May LST over the TP may contribute to the extreme flood of 1998 over the south of the Yangtze River region, with the LST reproducing about 57% and 64% of observed above-normal rainfall anomaly over the south of the Yangtze River region and southeastern China, respectively. Further analyses reveal a possible effect of springtime TP's LST on summer southern and eastern Asian rainfall and identify some hot spots, suggesting that the TP's spring LST effect is not only limited to the Yangtze River region, but to a much larger scale. The imposed warm LST/SUBT over the TP triggers a strong wave activities propagating eastward along the upper-level westerly jet, associated with an increase of the atmospheric baroclinic instability as well as a strengthening and southeastward movement of the South Asian high, leading to intensified moisture convergence and convective instability favorable to the excessive rainfall in the downstream region of East Asia. The results of the 1998 case have also been compared with the results from year of 2003, which had a very cold spring LST anomaly over the TP and a severe downstream June 2003 drought (flood) in southern (northern) of the Yangtze River Basin area. Simulation results provide further evidence of the great importance of the TP spring land surface temperature anomaly in regulating summer extreme hydroclimatic events (e.g. droughts and floods) in South and East Asia. The present study suggests that consideration of LST/SUBT anomalies has a strong potential for more skillful S2S prediction of extreme hydroclimatic events such as floods, droughts and heatwaves over both Southern and Eastern Asia. [ABSTRACT FROM AUTHOR]

Flash droughts feature rapid onsets of soil moisture drought events and result in severe impacts and damages, especially on agricultural and ecological systems. How the flash drought regime across China varies on multitemporal scales with climate change is not fully clear yet. In this study, we extended the flash drought definition to apply to arid regions by adding an absolute soil moisture variation criterion. Then, we detected flash drought events across China during 1981–2021 and characterized their frequency, duration, and affected area changes on seasonal, annual, and decadal scales, using soil moisture data from the European Center for Medium-Range Weather Forecasts climate reanalysis-Land. Results show that flash drought hotspots appeared in North China and the Yangtze River Basin. During 1981–2021, the hotspots, even nationwide, underwent significant increases in frequencies, durations, and affected areas of flash droughts. The increases held in the extremely high values of the frequencies and durations in the decadal comparisons. Especially, North China saw the most extensive and rapid increases. Seasonally, flash drought frequencies and durations intensified more during spring and autumn, and seasonal hotspots in eastern China shifted in phase with spatial patterns of soil moisture loss balanced by precipitation and evapotranspiration. Thus, flash droughts tended to amplify atmospheric aridity. These findings on the hotspot regions and the spatiotemporal evolutions of flash droughts across China would pinpoint soil moisture responses to climate change and prepare for climate change impacts on local ecosystems. [ABSTRACT FROM AUTHOR]

Westerly wind bursts (WWBs), aperiodic burst-like atmospheric disturbances in the tropics, play an important role during the development stage of El Niño–Southern Oscillation (ENSO) events and in turn can be strongly modulated by ENSO itself. Previous work argued that WWBs exhibit a pronounced seasonality at the equator during an El Niño evolution, i.e., being more frequent and vigorous during the developing boreal autumn compared to the decaying spring. Here we show that this seasonal difference in WWB activity at the equator is a segmentary manifestation of a meridional migration of WWB activity, which is closely linked to the southward displacement of the El Niño-related deep convection from the developing autumn to decaying spring. The highest climatological SSTs over the western-central Pacific are located mainly to the north of the equator in boreal autumn and south of it during boreal spring; hence the equatorially quasi-symmetric El Niño-related SST anomalies tend to induce deep convection and generate more WWBs in the Northern Hemisphere during the developing autumn and more in the Southern Hemisphere during the decaying spring. Despite some differences in the zonal distribution and intensity, there appears a close resemblance between the seasonal-varying features of WWBs during eastern Pacific (EP) and central Pacific (CP) El Niño events regarding the north–south displacement, due to similar meridional shifts of deep convection anomalies. This study provides a more comprehensive picture of the spatiotemporal characteristics of WWBs during El Niño events and improves our understanding of the relationship between ENSO and high-frequency atmospheric variability. [ABSTRACT FROM AUTHOR]

The preferred spring development of the Pacific Meridional Mode (PMM) is investigated through a combined observational analysis and modeling approach. A mixed-layer heat budget analysis shows that the PMM experiences its strongest growth two months prior to its mature phase, and the growth is primarily attributed to the surface latent heat flux (LHF) anomaly, via the wind-evaporation-SST (WES) feedback. The spring-fall difference in the LHF anomaly is caused by both the anomalous and seasonal mean wind fields. Idealized atmospheric general circulation model experiments (GCM) show that given the same PMM heating, atmospheric anomalous wind and LHF responses are much stronger in boreal spring than in boreal fall, which favors a greater WES feedback. Experiments with an intermediate air-sea coupled model with specified SST and surface wind mean state demonstrate that a PMM-like SSTA perturbation grows fastest in boreal spring among all seasons, indicating that the tropical mean wind and SST fields in spring are most favorable for the PMM development. Finally, we show that the atmospheric teleconnection between mid-latitude North Pacific and PMM is much more pronounced during the development of PMM in boreal spring than in boreal fall. [ABSTRACT FROM AUTHOR]

Twenty years of IMERG precipitation estimates are used to evaluate the contributions of mesoscale convective system (MCS) rainfall to total rainfall in the Congo Basin. Studying these systems advances our basic understanding of Congo Basin rainfall on all time scales. The seasonality of MCS rainfall in the Congo Basin follows the seasonality of total rainfall with high rainfall in spring, summer, and fall and a winter dry season in each hemisphere. In the equinoctial seasons, MCS rainfall accounts for ≥ 80% of total rainfall within 5° of the equator with the highest rainfall rates occurring along the eastern and western boundaries of the basin. In boreal summer, MCS rainfall maxima occur near the Cameroon Highlands (9°–18°E) and in boreal winter, they occur along the eastern orography (22°–28°E). The 80% percent contribution is sustained in the continental interior (15°–25°E, 5°S–5°N) throughout the year. The diurnal cycle of MCS rainfall is similar to that of total rainfall. Diurnal cycles are unimodal in the equinoctial seasons but are regionally and seasonally inhomogeneous in the solstitial seasons. Regardless of modality, MCS rainfall is highest at 15Z (1600/1700 LT) and lowest at 10Z. MCS percent contribution changes little throughout the diurnal cycle but is highest (≥ 90%) at 04Z close to the continental interior. Larger MCSs contribute their greatest percentage of MCS rainfall (83–92%) between 04Z and 07Z, while more-intensely precipitating MCSs have no seasonally or regionally consistent diurnal cycle. Seasonal and diurnal MCS rainfall maxima are associated with unstable MSE profiles in the lower troposphere. Changes in moisture drive the seasonal cycle of MSE while changes in temperature drive its diurnal cycle. [ABSTRACT FROM AUTHOR]