The Northeast US has faced the most rapidly increasing occurrences of extreme rainfall within the US in the past few decades. The latest fully-coupled 25-km GFDL SPEAR simulation, possessing 10 ensemble members, presents a good opportunity to study changes in regional extreme rainfall and relevant physical processes in both current and future climates. The surge in extreme rainfall over the Northeast US since the 1990s is primarily linked to events associated with tropical cyclones (TCs). In a future warming climate, the 25-km GFDL SPEAR SSP5-8.5 simulations project unprecedented rainfall events over the Northeast US, driven by increasing anthropogenic radiative forcing and distinguishable from natural variability, by the mid-21st century. Also, the occurrences of extreme rainfall related to both atmospheric rivers and TCs are projected to increase, even though the number of TC in the North Atlantic is projected to decrease in the 25-km GFDL SPEAR SSP5-8.5 simulations. Factors such as enhancing TC intensity, strengthening TC-related rainfall, or/and westward shift in TC tracks may offset the influence of declining TC numbers in the model projections, leading to more frequent TC-related extreme rainfall over the Northeast US in the future. On the other hand, the increase in extreme rainfall linked to atmospheric rivers is projected to outpace that associated with TCs. Given the distinct spatial patterns of rainfall resulting from atmospheric rivers and TCs, shifts in their relative contributions carry profound implications for risk prevention and mitigation strategies.

The Maritime Continent (MC) has experienced significant anthropogenic land use changes, mainly deforestation, which has led to local surface warming and marked convergence in the lower troposphere and divergence in the upper. The remote consequences of this deforestation remain unclear and present considerable uncertainties. In this study, we employ a fully coupled climate model and a linear baroclinic model to explore the effects of altered land-atmosphere interactions due to MC deforestation on high-latitude climates. Our series of idealized experiments demonstrates that MC deforestation can induce upper-level diabatic heating. This generates a barotropic Rossby wave that moves poleward, drawing energy from the subtropical jet across the Central to Eastern Pacific regions via eddy-mean flow interactions. Such interactions amplify the Aleutian Low, promoting the northward transport of warm air, leading to notable warming anomalies. This influx of warmth contributes to sea ice melt, initiating a positive ice-albedo feedback. A lapse-rate feedback is also observed in adjacent high-latitude land areas, amplifying terrestrial warming. These reinforcing feedbacks, combined with the direct temperature transport enabled by the strengthened Aleutian Low, cumulatively result in pronounced high-latitude warming originally due to the tropical land use changes. Deforestation in the Maritime Continent (MC) has far-reaching implications, extending to remote climatic areas. This study explores the mechanisms by which deforestation in the MC impacts climates in middle-to-high latitudes, especially during the boreal winter. The deforestation effect originates from the MC, traveling along the subtropical jet. The interactions between localized and larger-scale atmospheric flows play a pivotal role in this transmission. These interactions bolster the Aleutian Low during winter, resulting in the warming of the Bering Sea. This warming results from the direct transport of warm air, facilitated by the intensified Aleutian Low and feedback loops enhanced by the ice albedo feedback and changes in radiations. Our idealized experiments show that MC deforestation can strengthen the Aleutian Low and lead to a warmer Bering Sea in the winter. Deforestation in the Maritime Continent triggers a chain reaction in winter Rossby wave dynamics and strengthens the Aleutian Low The intensified Aleutian Low transports warm air from lower latitudes to the Bering Sea region, resulting in significant low-level warming Local lapse rate changes and ice-albedo feedback jointly enhance low-level warming

AbstractThe Northeast United States (NEUS) has faced the most rapidly increasing occurrences of extreme precipitation within the US in the past few decades. Understanding the physics leading to long‐term trends in regional extreme precipitation is essential but the progress is limited partially by the horizontal resolution of climate models. The latest fully coupled 25‐km GFDL (Geophysical Fluid Dynamics Laboratory) SPEAR (Seamless system for Prediction and EArth system Research) simulations provide a good opportunity to study changes in regional extreme precipitation and the relevant physical processes. Here, we focus on the contributions of changes in synoptic‐scale events, including atmospheric rivers (AR) and tropical cyclone (TC)‐related events, to the trend of extreme precipitation in the fall season over the Northeast US in both the recent past and future projections using the 25‐km GFDL‐SPEAR. In observations, increasing extreme precipitation over the NEUS since the 1990s is mainly linked to TC‐related events, especially those undergoing extratropical transitions. In the future, both AR‐related and TC‐related extreme precipitation over the NEUS are projected to increase, even though the numbers of TCs in the North Atlantic are projected to decrease in the SPEAR simulations using the SSP5‐8.5 projection of future radiative forcing. Factors such as enhancing TC intensity, strengthening TC‐related precipitation, and/or westward shift in Atlantic TC tracks may offset the influence of declining Atlantic TC numbers in the model projections, leading to more frequent TC‐related extreme precipitation over the NEUS.

Extreme precipitation is among the most destructive natural disasters. Simulating changes in regional extreme precipitation remains challenging, partially limited by climate models’ horizontal resolution. Here, we use an ensemble of high-resolution global climate model simulations to study September–November extreme precipitation over the Northeastern United States, where extremes have increased rapidly since the mid-1990s. We show that a model with 25 km horizontal resolution simulates much more realistic extreme precipitation than comparable models with 50 or 100 km resolution, including frequency, amplitude, and temporal variability. The 25 km model simulated trends are quantitatively consistent with observed trends over recent decades. We use the same model for future projections. By the mid-21st century, the model projects unprecedented rainfall events over the region, driven by increasing anthropogenic radiative forcing and distinguishable from natural variability. Very extreme events (>150 mm/day) may be six times more likely by 2100 than in the early 21st century.

Zenodo repository for: Jong, B.-T., H. Murakami, T. L. Delworth, and W. F. Cooke: Synoptic-scale contributions to extreme precipitation trends over the Northeast U.S. in 25-km GFDL SPEAR (to be submitted to Earth's Future) Please also cite and refer to: Jong, B.-T., Delworth, T. L., Cooke, W. F., Tseng, K.-C., & Murakami, H. (2023). Increases in extreme precipitation over the Northeast United States using high-resolution climate model simulations. Npj Climate and Atmospheric Science, 6(1), 18. https://doi.org/10.1038/s41612-023-00347-w Delworth, T. L., Cooke, W. F., Adcroft, A., Bushuk, M., Chen, J., Dunne, K. A., et al. (2020). SPEAR: The Next Generation GFDL Modeling System for Seasonal to Multidecadal Prediction and Projection. Journal of Advances in Modeling Earth Systems, 12(3), e2019MS00189. https://doi.org/10.1029/2019ms001895 For more details about SPEAR_HI simulations, please see https://www.gfdl.noaa.gov/spear/

<p><span>Extreme precipitation, both the occurrence and intensity, over the Northeast United States has significantly increased since the 1990s, evidenced by observations. The most salient increase has happened in the fall season (September to November). Understanding the attribution and projection of long-term trends in regional extreme precipitation is essential to adaptationion planning such as infrastructure upgrade. However, such work is challenging due to uncertainties caused by internal climate variability and the requirement of medium-to-high model resolution as well as ensemble size. In this work, we leverage the newly-developed GFDL (Geophysical Fluid Dynamics Laboratory) SPEAR (<strong>S</strong>eamless System for <strong>P</strong>rediction and <strong>EA</strong>rth System <strong>R</strong>esearch) models which generate 25-km high-resolution simulations (ten members; SPEAR-HI) and 50-km large-ensemble simulations (30 members; SPEAR-MED) for both historical simulations from 1921 to 2014 and projections for the Shared Socioeconomic Pathway 5-8.5 (SSP585) from 2014 to 2100. We aim to address two related scientific questions using GFDL-SPEAR: (1) what are the factors that have contributed to the increasing autumn extreme precipitation over the Northeast US since 1990s? How much of the increase could be attributed to anthropogenic forcing? (2) when would the increased extreme precipitation in response to forced climate change emerge from the noise of internal climate variability? </span></p><p><span>Our preliminary results first suggest that higher atmospheric resolution in climate models is critical to facilitate the simulations of regional extreme precipitation. For example, SPEAR-HI can simulate comparable frequency of extreme precipitation over the Northeast US (rain rate > 50 mm/day), compared to the observation; while SPEAR-MED underestimates the frequency. Second, the recent increasing Northeast US extreme precipitation is unlikely due to the warming North Atlantic sea surface temperature, even though the timing of the abrupt increase in extreme precipitation coincided with the timing when the Atlantic Multidecadal Oscillation shifted from a cold to warm phase in the mid-1990s. Our ongoing work focuses on evaluating the attributions from other factors including internal variability, aerosols, and greenhouse gas. Last, we analyze SPEAR-HI SSP585 projections and extended control simulations starting from the year 1850. We estimate that the anthropogenically forced increase in the Northeast US autumn extreme precipitation would emerge from the noise of internal climate variability around the 2040s. However, ongoing work will employ more systematic methods to estimate the time of emergence. </span></p>

Diagnosis of rapidly developing springtime droughts in the central United States has mostly been made via numerous individual case studies rather than in an aggregate sense. This study investigates common aspects of subseasonal "meteorological drought" evolution, here defined as persistent precipitation minus evapotranspiration (P - ET) deficits, revealed in early (1 April-15 May) and late (16 May-30 June) spring composites of 5-day running mean JRA-55 reanalysis data for three different central U.S. regions during 1958-2018. On average, these droughts are initiated by a quasi-stationary Rossby wave packet (RWP), propagating from the western North Pacific, which arises about a week prior to drought onset. The RWP is related to a persistent ridge west of the incipient drought region and strong subsidence over it. This subsidence is associated with low-level divergent flow that dries the atmosphere and suppresses precipitation for roughly 1-2 weeks, and generally has a greater impact on the local moisture budget than does reduced poleward moisture transport. The resulting "dynamically driven" evaporative demand corresponds to a rapid drying of the root-zone soil moisture, which decreases around 40 percentiles within about 10 days. Anomalous near-surface warmth develops only after the P - ET deficit onset, as does anomalously low soil moisture that then lingers a month or more, especially in late spring. The horizontal scale of the RWPs, and of the related drought anomalies, decreases from early to late spring, consistent with the climatological change in the Pacific Rossby waveguide. Finally, while this composite analysis is based upon strong, persistent P - ET deficits, it still appears to capture much of the springtime development of "flash droughts" as well.

During the summer when an El Niño event is transitioning to a La Niña event, the extratropical teleconnections exert robust warming anomalies over the U.S. Midwest threatening agricultural production. This study assesses the performance of current climate models in capturing the prominent observed extratropical responses over North America during the transitioning La Niña summer, based on atmospheric general circulation model experiments and coupled models from the North American Multimodel Ensemble (NMME). The ensemble mean of the SST-forced experiments across the transitioning La Niña summers does not capture the robust warming in the Midwest. The SST-forced experiments do not produce consistent subtropical western Pacific (WP) negative precipitation anomalies and this leads to the poor simulations of extratropical teleconnections over North America. In the NMME models, with active air–sea interaction, the negative WP precipitation anomalies show better agreement across the models and with observations. However, the downstream wave train pattern and the resulting extratropical responses over North America exhibit large disagreement across the models and are consistently weaker than in observations. Furthermore, in these climate models, an anomalous anticyclone does not robustly translate into a warm anomaly over the Midwest, in disagreement with observations. This work suggests that, during the El Niño to La Niña transitioning summer, active air–sea interaction is important in simulating tropical precipitation over the WP. Nevertheless, skillful representations of the Rossby wave propagation and land–atmosphere processes in climate models are also essential for skillful simulations of extratropical responses over North America.

El Niño–Southern Oscillation (ENSO) teleconnections have been recognized as possible negative influences on crop yields in the United States during the summer growing season, especially in a developing La Niña summer. This study examines the physical processes of the ENSO summer teleconnections and remote impacts on the United States during a multiyear La Niña life cycle. Since 1950, a developing La Niña summer is either when an El Niño is transitioning to a La Niña or when a La Niña is persisting. Due to the distinct prior ENSO conditions, the oceanic and atmospheric characteristics in the tropics are dissimilar in these two different La Niña summers, leading to different teleconnection patterns. During the transitioning summer, the decaying El Niño and the developing La Niña induce suppressed deep convection over both the subtropical western Pacific (WP) and the tropical central Pacific (CP). Both of these two suppressed convection regions induce Rossby wave propagation extending toward North America, resulting in a statistically significant anomalous anticyclone over northeastern North America and, therefore, a robust warming signal over the Midwest. In contrast, during the persisting summer, only one suppressed convection region is present over the tropical CP induced by the La Niña SST forcing, resulting in a weak and insignificant extratropical teleconnection. Experiments from a stationary wave model confirm that the suppressed convection over the subtropical WP during the transitioning summer not only contributes substantially to the robust warming over the Midwest but also causes the teleconnections to be different from those in the persisting summer.

During the strong 2015/16 El Niño, only normal to below-average precipitation fell across California in the late winter. This disagrees with both predictions by the ensemble mean of forecast models and expectations for strong El Niños. The authors examine one of the possible reasons why this event did not bring expected precipitation to California in the late winter. The maximum equatorial Pacific sea surface temperature anomalies (SSTAs) were located, compared to the 1982/83 and 1997/98 strong El Niños, farther to the west in the 2015/16 winter, which possibly caused less convection in the eastern tropical Pacific and shifted the teleconnection patterns westward in the North Pacific, thus weakening the influences on California. The SSTA and precipitation forecast for February–April 2016, based on the North American Multimodel Ensemble, showed large discrepancies from observations, with the ensemble mean of most of the models overestimating SSTAs in the eastern tropical Pacific and California precipitation. Atmospheric general circulation model (AGCM) experiments were conducted to test the hypothesis that the warmer eastern tropical Pacific SSTA forecast may have caused the wetter forecast in California in 2015/16 compared to observations. The AGCM experiments suggest it is difficult to assert that the eastern tropical Pacific SSTAs caused the too-wet California precipitation forecast, especially in Southern California, given that the models disagree. Results indicate forecast error can be influenced by atmosphere-model sensitivity to forecast SSTs, but they also indicate atmospheric internal variability may have been responsible for the combination of a strong El Niño and near-normal California precipitation.

California has experienced severe drought in recent years posing great challenges to agricultural production, water resources, and land management. El Nino, as the prime source of seasonal to interannual climate predictability, offers the potential of amelioration of drought in California. Here El Nino's impacts on California winter precipitation are examined, focusing on variations by season, region, and the strength of El Nino using observational data for the period 1901-2010. The El Nino influence on California precipitation strengthens from early to late winter and is stronger in the south than the north. Eight of ten moderate-to-strong El Ninos in the late winter put southern California in the wettest tercile and none of these ten events put northern California in the driest tercile. The early to late winter strengthening of the El Nino impact on precipitation occurs even as El Nino weakens and is associated with a strengthening and eastward extending tropical deep convection anomaly allowed by the late winter warming of the climatological mean sea surface temperature over the tropical eastern Pacific.