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Martin P. Hoerling
,
Jon K. Eischeid
,
Henry F. Diaz
,
Balaji Rajagopolan
, and
Eric Kuhn

Abstract

Of concern to Colorado River management, as operating guidelines post-2026 are being considered, is whether water resource recovery from low flows during 2000–2020 is possible. Here we analyze new simulations from the sixth generation of the Coupled Model Intercomparison Project (CMIP6) to determine plausible climate impacts on Colorado River flows for 2026–2050 when revised guidelines would operate. We constrain projected flows for Lee Ferry, the gauge through which 85% of the river flow passes, using its estimated sensitivity to meteorological variability together with CMIP6 projected precipitation and temperature changes. The critical importance of precipitation, especially its natural variability, is emphasized. Model projections indicate increased precipitation in the Upper Colorado River basin due to climate change, which alone increases river flows 5%–7% (relative to a 2000–2020 climatology). Depending on the river’s temperature sensitivity, this wet signal compensates some, if not all, of the depleting effects from basin warming. Considerable internal decadal precipitation variability (~5% of the climatological mean) is demonstrated, driving a greater range of plausible Colorado River flow changes for 2026–2050 than previously surmised from treatment of temperature impacts alone: the overall precipitation-induced Lee Ferry flow changes span −25% to +40% contrasting with a −30% to −5% range from expected warming effects only. Consequently, extreme low and high flows are more likely. Lee Ferry flow projections, conditioned on initial drought states akin to 2000–2020, reveal substantial recovery odds for water resources, albeit with elevated risks of even further flow declines than in recent decades.

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Arshdeep Singh
,
Sanjiv Kumar
,
Liang Chen
,
Montasir Maruf
,
Peter Lawrence
, and
Min-Hui Lo

Abstract

This study examines the effects of land use (LU) change on regional climate, comparing historical and future scenarios using seven climate models from Coupled Model Intercomparison Phase 6 – Land Use Model Intercomparison Project experiments. LU changes are evaluated relative to land use conditions during the pre-industrial climate. Using the Community Earth System Model version 2 Large Ensemble (CESM2-LE) experiment, we distinguish LU impacts from natural climate variability. We assess LU impact locally by comparing the impacts of climate change in neighboring areas with and without LU changes. Further, we conduct CESM2 experiments with and without LU changes to investigate LU-related climate processes.

A multi-model analysis reveals a shift in LU-induced climate impacts, from cooling in the past to warming in the future climate across mid-latitude regions. For instance, in North America, LU's effect on air temperature changes from −0.24±0.18°C historically to 0.62±0.27°C in the future during the boreal summer. The CESM2-LE shows a decrease in LU-driven cooling from −0.92±0.09°C in the past to −0.09±0.09°C in future boreal summers in North America.

A hydroclimatic perspective linking LU and climate feedback indicates LU changes causing soil moisture drying in the mid-latitude regions. This contrasts with hydrology-only views showing wetter soil conditions due to LU changes. Furthermore, global warming causes widespread drying of soil moisture across various regions. Mid-latitude regions shift from a historically wet regime to a water limited transitional regime in the future climate. This results in reduced evapotranspiration, weakening LU-driven cooling in future climate projections. A strong linear relationship exists between soil moisture and evaporative fraction in mid-latitudes.

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Teryn J. Mueller
,
Christina M. Patricola
, and
Emily Bercos-Hickey

Abstract

The El Niño–Southern Oscillation (ENSO) influences seasonal Atlantic tropical cyclone (TC) activity by impacting environmental conditions important for TC genesis. However, the influence of future climate change on the teleconnection between ENSO and Atlantic TCs is uncertain, as climate change is expected to impact both ENSO and the mean climate state. We used the Weather Research and Forecasting model on a tropical channel domain to simulate 5-member ensembles of Atlantic TC seasons in historical and future climates under different ENSO conditions. Experiments were forced with idealized sea-surface temperature configurations based on the Community Earth System Model (CESM) Large Ensemble representing: a monthly-varying climatology, Eastern Pacific El Niño, Central Pacific El Niño, and La Niña. The historical simulations produced fewer Atlantic TCs during Eastern Pacific El Niño compared to Central Pacific El Niño, consistent with observations and other modeling studies. For each ENSO state, the future simulations produced a similar teleconnection with Atlantic TCs as in the historical simulations. Specifically, La Niña continues to enhance Atlantic TC activity, and El Niño continues to suppress Atlantic TCs, with greater suppression during Eastern Pacific El Niño compared to Central Pacific El Niño. In addition, we found a decrease in Atlantic TC frequency in the future relative to historical regardless of ENSO state, which was associated with a future increase in northern tropical Atlantic vertical wind shear and a future decrease in the zonal tropical Pacific SST gradient, corresponding to a more El Niño-like mean climate state. Our results indicate that ENSO will remain useful for seasonal Atlantic TC prediction in the future.

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Matthew Patterson
,
Christopher O’Reilly
,
Jon Robson
, and
Tim Woollings

Abstract

The coupled nature of the ocean-atmosphere system frequently makes understanding the direction of causality difficult in ocean-atmosphere interactions. This study presents a method to decompose turbulent surface heat fluxes into a component which is directly forced by atmospheric circulation, and a residual which is assumed to be primarily ‘ocean-forced’. This method is applied to the North Atlantic in a 500-year pre-industrial control run using the Met Office’s HadGEM3-GC3.1-MM model. The method shows that atmospheric circulation dominates interannual to decadal heat flux variability in the Labrador Sea, in contrast to the Gulf Stream where the Ocean primarily drives the variability. An empirical orthogonal function analysis identifies several residual heat flux modes associated with variations in ocean circulation. The first of these modes is characterised by the ocean warming the atmosphere along the Gulf Stream and North Atlantic Current and the second by a dipole of cooling in the western subtropical North Atlantic and warming in the sub-polar North Atlantic. Lead-lag regression analysis suggests that atmospheric circulation anomalies in prior years partly drive the ocean heat flux modes, however there is no significant atmospheric circulation response in years following the peaks of the modes. Overall, the heat flux dynamical decomposition method provides a useful way to separate the effects of the ocean and atmosphere on heat flux and could be applied to other ocean basins and to either models or reanalysis datasets.

Open access
Arthur Coquereau
,
Florian Sévellec
,
Thierry Huck
,
Joël J.-M. Hirschi
, and
Antoine Hochet

Abstract

As well as having an impact on the background state of the climate, global warming due to human activities could affect its natural oscillations and internal variability. In this study, we use four initial-condition ensembles from the CMIP6 framework to investigate the potential evolution of internal climate variability under different warming pathways for the 21st century. Our results suggest significant changes in natural climate variability, and point to two distinct regimes driving these changes. First, a decrease of internal variability of surface air temperature at high latitudes and all frequencies, associated with a poleward shift and the gradual disappearance of sea-ice edges, which we show to be an important component of internal variability. Second, an intensification of the interannual variability of surface air temperature and precipitation at low latitudes, which appears to be associated with the El Niño–Southern Oscillation (ENSO). This second regime is particularly alarming because it may contribute to making the climate more unstable and less predictable, with a significant impact on human societies and ecosystems.

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Verónica Martín-Gómez
,
Belén Rodríguez-Fonseca
,
Irene Polo
, and
Marta Martín-Rey

Abstract

In the last decades, many efforts have been made to understand how different tropical oceanic basins are able to impact El Niño Southern Oscillation (ENSO). However, the collective connectivity among the tropical oceans and their associated influence on ENSO is less understood. Using a complex network methodology, the degree of collective connectivity among the tropical oceans is analyzed focusing on the detection of periods when the tropical basins collectively interact and Atlantic and Indian basins influence the equatorial Pacific sea surface temperatures (SST). The background state for the periods of strong collective connectivity is also investigated.

Our results show a marked multidecadal variability in the tropical interbasin connection, with periods of stronger and weaker collective connectivity. These changes seem to be modulated by changes in the North Atlantic ocean mean state a decade in advance. In particular, strong connectivity occurs in periods with colder than average tropical north Atlantic surface ocean. Associated with this cooling an anomalous convergence of the vertical integral of total energy flux (VIEF) takes place over the tropical north-west Atlantic, associated with anomalous divergence of VIEF over the equatorial eastern Pacific. In turn, an anomalous zonal surface pressure gradient over the tropical Pacific weakens the trades over the western equatorial Pacific. Consequently, a shallower thermocline emerges over the western equatorial Pacific, which can enhance thermocline feedbacks, the triggering of ENSO events, and therefore, ENSO variability. By construction, our results put forward opposite conditions for periods of weak tropical basins connectivity. These results have important implications for seasonal to decadal predictions.

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Tingting Zhu
and
Jin-Yi Yu

Abstract

Utilizing a 2200-year CESM1 pre-industrial simulation, this study examines the influence of property distinctions between single-year (SY) and multi-year (MY) La Niñas on their respective impacts on winter surface air temperatures across mid-to-high latitude continents in the model, focusing on specific teleconnection mechanisms. Distinct impacts were identified in four continent sectors: North America, Europe, Western Siberia (W-Siberia), and Eastern Siberia (E-Siberia). The typical impacts of simulated SY La Niña events are featured with anomalous warming over Europe and W&E-Siberia and anomalous cooling over North America. Simulated MY La Niña events reduce the typical anomalous cooling over North America and the typical anomalous warming over W&E-Siberia but intensify the typical anomalous warming over Europe. The distinct impacts of simulated MY La Niñas are more prominent during their first winter than during the second winter, except over W-Siberia, where the distinct impact is more pronounced during the second winter. These overall distinct impacts in the CESM1 simulation can be attributed to the varying sensitivities of these continent sectors to the differences between MY and SY La Niñas in their intensity, location, and induced sea surface temperature anomalies in the Atlantic Ocean. These property differences were linked to the distinct climate impacts through the Pacific North America, North Atlantic Oscillation, Indian Ocean-induced wave train, and Tropical North Atlantic-induced wave train mechanisms. The modeling results are then validated against observations from 1900 to 2022 to identify disparities in the CESM1 simulation.

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Fang Zhou
,
Siseho Christonette Siseho
,
Minghong Liu
,
Dapeng Zhang
, and
Haoxin Zhang

Abstract

Focusing on summer precipitation over the Tibetan Plateau (TP), this study mainly investigates the joint impacts of the North African and the Western Pacific subtropical highs (i.e., NASH and WPSH) through examining circulation and moisture anomalies. Results show that there are several boundary combination types of the two subtropical highs. The anomalous vertical motion with sufficient moisture transport under different boundary types plays the dominant role in TP precipitation anomaly. When the WPSH strengthens westward approaching to the TP, it can transport water vapor northward from Northwest Pacific and North Indian Oceans to the south edge of the TP and induce ascending motion over the southeastern TP, contributing to more precipitation there. When the NASH enhances and extends eastward, it can transport water vapor eastward from North Atlantic Ocean to the southwest eastern TP and give rise to ascending motion there, inducing positive precipitation anomaly over the southwest eastern TP. When the two subtropical highs simultaneously intensify and extend to the TP, water vapor can be transported to the TP widely from the North Atlantic Ocean, the North Indian Ocean and the northwest Pacific Ocean with the strengthening of the westerly, resulting in the location of the ascending motion and rain belt shifting obviously northward. Further analyses indicate that the pre-winter ENSO and summer North Atlantic air–sea interaction are two indispensable possible modulation factors for the joint impact of the two subtropical highs on TP precipitation.

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F. Guo
,
S. C. Clemens
,
X. Du
,
X. Liu
,
Y. Liu
,
J. Sun
,
H. Fan
,
T. Wang
, and
Y. Sun

Abstract

Millennial-scale climate change is thought to be synchronous throughout the northern hemisphere and has been demonstrated to be strongly modulated by longer-term glacial-interglacial and orbital scale processes. However, processes that modulate the magnitude of millennial-scale variability (MMV) at the glacial-interglacial timescale remain unclear. We present multi-proxy evidence showing out-of-phase relationships between the MMV of East Asian and North Atlantic climate proxies at the eccentricity band. During most late Pleistocene glacial intervals, the MMV in North Atlantic SST and East Asian Monsoon proxies show a gradual weakening trend from glacial inceptions into glacial maxima, inversely proportional to that of North Atlantic ice rafted detritus record. The inverse glacial-age trends apply to both summer- and winter-monsoon proxies across the loess, speleothem, and marine archives, indicating fundamental linkages between MMV records of the North Atlantic and East Asia. We infer that intensified glacial-age iceberg discharge is accompanied by weakened Atlantic meridional overturning circulation via changes in freshwater input and water-column stability, leading to reduction in North Atlantic SST and wind anomalies, subsequently propagating dampened millennial-scale variability into the mid-latitude East Asian Monsoon region via the westerlies. Our results indicate that the impact of North Atlantic iceberg discharge and the associated variability in water-column stability at the millennial-scale is a primary influence on hydroclimate instability in East Asia.

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Ying Dai
,
Peter Hitchcock
, and
Isla R. Simpson

Abstract

This study evaluates the representation of the composite-mean surface response to Sudden Stratospheric Warmings (SSWs) in 28 CMIP6 models. Most models can reproduce the magnitude of the SLP response over the Arctic, although the simulated Arctic SLP response varies from model to model. Regarding the structure of the SLP response, most models exhibit a basin-symmetric negative NAM-like response with a cyclonic Pacific SLP response, whereas the reanalysis shows a highly basin-asymmetric negative NAO-like response without a robust Pacific center.

We then explore the drivers of these model biases and spread by applying a multiple linear regression. The results show that the polar-cap temperature anomalies at 100 hPa (ΔT 100) modulate both the magnitude of the Arctic SLP response and the cyclonic Pacific SLP response. Apart from ΔT 100, the intensity and latitudinal location of the climatological eddy-driven jet in the troposphere also affect the magnitude of the Arctic SLP response. The compensation of model biases in these two tropospheric metrics and the good model representation of ΔT 100 explains the good agreement between the ensemble mean and the reanalysis on the magnitude of the Arctic SLP response, as indicated by the fact that the ensemble mean lies well within the reanalysis uncertainty range and that the reanalysis mean sits well within the model distribution. The Niño-3.4 SST anomalies and North Pacific SST dipole anomalies together with ΔT 100 modulate the cyclonic Pacific SLP response. In this case, biases in both oceanic drivers work in the same direction and lead to the cyclonic Pacific SLP response in models that is not present in the reanalysis.

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