Browse

You are looking at 21 - 30 of 11,631 items for :

  • Journal of Climate x
  • Refine by Access: All Content x
Clear All
Isla R. Simpson, Karen A. McKinnon, Frances V. Davenport, Martin Tingley, Flavio Lehner, Abdullah Al Fahad, and Di Chen

Abstract

An emergent constraint (EC) is a statistical relationship, across a model ensemble, between a measurable aspect of the present-day climate (the predictor) and an aspect of future projected climate change (the predictand). If such a relationship is robust and understood, it may provide constrained projections for the real world. Here, models from phase 6 of the Coupled Model Intercomparison Project (CMIP6) are used to revisit several ECs that were proposed in prior model intercomparisons with two aims: 1) to assess whether these ECs survive the partial out-of-sample test of CMIP6 and 2) to more rigorously quantify the constrained projected change than previous studies. To achieve the latter, methods are proposed whereby uncertainties can be appropriately accounted for, including the influence of internal variability, uncertainty on the linear relationship, and the uncertainty associated with model structural differences, aside from those described by the EC. Both least squares regression and a Bayesian hierarchical model are used. Three ECs are assessed: (i) the relationship between Southern Hemisphere jet latitude and projected jet shift, which is found to be a robust and quantitatively useful constraint on future projections; (ii) the relationship between stationary wave amplitude in the Pacific–North American sector and meridional wind changes over North America (with extensions to hydroclimate), which is found to be robust but improvements in the predictor in CMIP6 result in it no longer substantially constraining projected change in either circulation or hydroclimate; and (iii) the relationship between ENSO teleconnections to California and California precipitation change, which does not appear to be robust when using historical ENSO teleconnections as the predictor.

Restricted access
Mathew Barlow, Andrew Hoell, and Laurie Agel

Abstract

The ability of six CMIP6 models to reproduce the observed cold season teleconnection between tropical Indo-Pacific sea surface temperatures (SSTs) and precipitation in Southwest Asia, the coastal Middle East (CME), and northern Pakistan and India (NPI) is examined. The 1979–2014 period is analyzed to maximize observations over both the tropical ocean and the regions. Nine historical simulations for the same period are examined for each model to account for the internal variability of the coupled system. The teleconnection is examined in terms of SSTs, precipitation, 200-hPa geopotential heights, and derived quantities. All the models capture some of the broadest features of the teleconnections, but there is a wide range in the ability of the models to reproduce the magnitude and details. The differences appear related to both the models’ ability to capture the details of the tropical variability, including the position and strength of the precipitation anomalies in the Indo-west Pacific, and the models’ ability to accurately propagate the tropically forced response into the region. The teleconnections to the CME and NPI regions on the eastern and western margins, respectively, of the strongest signal are very similar in structure and have similar results, except that the models’ ability to reproduce the strength and details of the teleconnection is even more limited, consistent with their marginal locations and known influence of other modes of variability. For all three areas, the wide range in model ability to capture the leading teleconnection suggests caution in interpreting climate regional projections.

Open access
Yechul Shin, Sarah M. Kang, Ken Takahashi, Malte F. Stuecker, Yen-Ting Hwang, and Doyeon Kim

Abstract

This study examines the temporal evolution of the extratropically forced tropical response in an idealized aquaplanet model under equinox condition. We apply a surface thermal forcing in the northern extratropics that oscillates periodically in time. It is shown that tropical precipitation is unaltered by sufficiently high-frequency extratropical forcing. This sensitivity to the extratropical forcing periodicity arises from the critical time required for sea surface temperature (SST) adjustment. Low-frequency extratropical forcing grants sufficient time for atmospheric transient eddies to diffuse moist static energy to perturb the midlatitude SSTs outside the forcing region, as demonstrated by a one-dimensional energy balance model with a fixed diffusivity. As the transient eddies weaken in the subtropics, a further equatorward advection is accomplished by the Hadley circulation. The essential role of Hadley cell advection in connecting the subtropical signal to the equatorial region is supported by an idealized thermodynamical-advective model. Associated with the SST changes in the tropics is a meridional shift of the intertropical convergence zone. Since the time needed for SST adjustment increases with increasing mixed layer depth, the critical forcing period at which the extratropical forcing can affect the tropics scales linearly with the mixed layer depth. Our results highlight the important role of decadal-and-longer extratropical climate variability in shaping the tropical climate system. We also raise the possibility that the transient behavior of a tropical response forced by extratropical variability may be strongly dependent on cloud radiative effects.

Restricted access
Tianyi Wang and Tim Li

Abstract

The diversity of the Madden–Julian oscillation (MJO) in terms of its maximum intensity, zonal extent, and phase speed was explored using a cluster analysis method. The zonal extent is found to be significantly correlated to the phase speed. A longer zonal extent is often associated with a faster phase speed. The diversities of zonal extent and speed are connected with distinctive interannual sea surface temperature anomaly (SSTA) distributions and associated moisture and circulation patterns over the equatorial Pacific. An El Niño–like background SSTA leads to enhanced precipitation over the central Pacific, allowing a stronger vertically overturning circulation to the east of the MJO. This promotes both a larger east–west asymmetry of column-integrated moist static energy (MSE) tendency and a greater boundary layer moisture leading, serving as potential causes of the faster phase speed. The El Niño–like SSTA also favors the MJOs intruding farther into the Pacific, causing a larger zonal extent. The intensity diversity is associated with the interannual SSTA over the Maritime Continent and background moisture condition over the tropical Indian Ocean. An observed warm SSTA over the Maritime Continent excites a local Walker cell with a subsidence over the Indian Ocean, which could decrease the background moisture, weakening the MJO intensity. The intensity difference between strong and weak events would be amplified due to distinct intensity growth speed. The faster intensity growth of a strong MJO is attributed to a greater longwave radiative heating and a greater surface latent heat flux, both of which contribute to a greater total time change rate of the column-integrated MSE.

Restricted access
Jonathan D. Beverley, Matthew Collins, F. Hugo Lambert, and Robin Chadwick

Abstract

El Niño–Southern Oscillation (ENSO) is the leading mode of interannual climate variability, and it exerts a strong influence on many remote regions of the world, for example in northern North America. Here, we examine future changes to the positive-phase ENSO teleconnection to the North Pacific/North America sector and investigate the mechanisms involved. We find that the positive temperature anomalies over Alaska and northern North America that are associated with an El Niño event in the present day are much weaker, or of the opposite sign, in the CMIP6 abrupt 4×CO2 experiments for almost all models (22 out of 26, of which 15 have statistically significant differences). This is largely related to changes to the anomalous circulation over the North Pacific, rather than differences in the equator-to-pole temperature gradient. Using a barotropic model, run with different background circulation basic states and Rossby wave source forcing patterns from the individual CMIP6 models, we find that changes to the forcing from the equatorial central Pacific precipitation anomalies are more important than changes in the global basic state background circulation. By further decomposing this forcing change into changes associated with the longitude and magnitude of ENSO precipitation anomalies, we demonstrate that the projected overall eastward shift of ENSO precipitation is the main driver of the temperature teleconnection change, rather than the increase in magnitude of El Niño precipitation anomalies, which is nevertheless seen in the majority of models.

Restricted access
Arianna M. Varuolo-Clarke, Jason E. Smerdon, A. Park Williams, and Richard Seager

Abstract

Southeastern South America (SESA; encompassing Paraguay, southern Brazil, Uruguay, and northern Argentina) experienced a 27% increase in austral summer precipitation from 1902 to 2019, one of the largest observed trends in seasonal precipitation globally. Previous research identifies Atlantic multidecadal variability and anthropogenic forcing from stratospheric ozone depletion and greenhouse gas emissions as key factors contributing to the positive precipitation trends in SESA. We analyze multimodel ensemble simulations from phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP) and find that not only do Earth system models simulate positive SESA precipitation trends that are much weaker over the historical interval, but some models persistently simulate negative SESA precipitation trends under historical forcings. Similarly, 16-member ensembles from two atmospheric models forced with observed historical sea surface temperatures never simulate precipitation trends that even reach the lower bound of the observed trend’s range of uncertainty. Moreover, while future twenty-first-century projections from CMIP6 yield positive ensemble mean precipitation trends over SESA that grow with increasing greenhouse gas emissions, the mean forced response never exceeds the observed historical trend. Preindustrial control runs from CMIP6 indicate that some models do occasionally simulate centennial-scale trends in SESA that fall within the observational range, but most models do not. Results point to significant uncertainties in the attribution of anthropogenically forced influences on the observed increases in precipitation over SESA while also suggesting that internal decadal-to-centennial variability of unknown origin and not present in state-of-the-art models may have also played a large role in generating the twentieth-to-twenty-first-century SESA precipitation trend.

Open access
Hong-Chang Ren, Jinqing Zuo, and Weijing Li

Abstract

The interannual variability of boreal summer sea surface temperature (SST) in the tropical Atlantic displays two dominant modes, the Atlantic zonal mode highlighting SST variations in the equatorial–southern tropical Atlantic (ESTA) region and the northern tropical Atlantic (NTA) mode focusing on SST fluctuations in the NTA region except in the Gulf of Guinea. Observational evidence indicates that both the boreal summer ESTA and NTA warming are accompanied by a pair of anomalous low-level anticyclones over the western tropical Pacific, and the NTA-related anticyclone is more obvious than the ESTA-related one. Both atmosphere-only and partially coupled experiments conducted with the Community Earth System Model version 1.2 support the observed NTA–Pacific teleconnection. In contrast, the ESTA-induced atmospheric circulation response is negligible over the tropical Pacific in the atmosphere-only experiments, and although the response becomes stronger in the partially coupled experiments, obvious differences still exist between the simulations and observation. The ESTA-induced atmospheric circulation response features an anomalous low-level cyclone over the western tropical Pacific in the partially coupled experiments, opposite to its observed counterpart. It is found that the ESTA warming coincides with significantly La Niña–like SST anomalies in the central–eastern equatorial Pacific, the influence of which on the tropical atmospheric circulation is opposite to that of the ESTA warming, and therefore contributes to difference between the ESTA-related simulations and observation. Moreover, the cold climatological mean SST in the ESTA region is unfavorable to enhancing the ESTA–Pacific teleconnection during boreal summer.

Restricted access
Meilin Zhu, Lonnie G. Thompson, Huabiao Zhao, Tandong Yao, Wei Yang, and Shengqiang Jin

Abstract

Glacier changes on the Tibetan Plateau (TP) have been spatially heterogeneous in recent decades. The understanding of glacier mass changes in western Tibet, a transitional area between the monsoon-dominated region and the westerlies-dominated region, is still incomplete. For this study, we used an energy–mass balance model to reconstruct annual mass balances from October 1967 to September 2019 to explore the effects of local climate and large-scale atmospheric circulation on glacier mass changes in western Tibet. The results showed that Xiao Anglong Glacier is close to a balanced condition, with an average value of −53 ± 185 mm water equivalent (w.e.) yr−1 for 1968–2019. The interannual mass balance variability during 1968–2019 was primary driven by ablation-season precipitation, which determined changes in the snow accumulation and strongly influenced melt processes. The interannual mass balance variability during 1968–2019 was less affected by ablation-season air temperature, which only weakly affected snowfall and melt energy. Further analysis suggests that the southward (or northward) shift of the westerlies caused low (or high) ablation-season precipitation, and therefore low (or high) annual mass balance for glaciers in western Tibet. In addition, the average mass balance for Xiao Anglong Glacier was 83 ± 185, −210 ± 185, and −10 ± 185 mm w.e. yr−1 for 1968–90, 1991–2012, and 2013–19, respectively. These mass changes were associated with the variations in precipitation and air temperature during the ablation season on interdecadal time scales.

Restricted access
Samson Hagos, L. Ruby Leung, Oluwayemi Garuba, and Christina M. Patricola

Abstract

The frequency of North Pacific atmospheric rivers (ARs) affects water supply and flood risk over western North America. Thus, understanding factors that affect the variability of landfalling AR frequency is of scientific and societal importance. This study aims at identifying the sources of the moisture for North Pacific ARs and assessing how different modes of variability modulate these sources. To this end, the sources and variability of the background divergent component of the integrated moisture flux (DIVT) in ARs are identified using MERRA reanalysis. It is shown that in the boreal winter, this background DIVT in ARs is related to the outflow from the subsidence over the subtropics that transports moisture northward, while in summer it is related to the Asian monsoon and it transports moisture northwestward. This leads to a seasonal northwest–southeast movement of the AR frequency climatology. At the intraseasonal scale, propagation of the Madden–Julian oscillation introduces an anticlockwise rotation of the background DIVT, with northward transport in phases 1 and 2, westward in 3 and 4, southward in 5 and 6, and eastward in 7 and 8, making landfall over the west coast of North America most likely during the last two phases. Similarly, El Niño–Southern Oscillation variability also affects the frequency of ARs through modulation of the westerly background DIVT, favoring landfall over the U.S. West Coast during strong El Niño phases. It is shown that in general the likelihood of AR landfall over the western United States is correlated with the zonal background DIVT over northeastern Pacific.

Open access
Yonggang Liu, Peng Liu, Dawei Li, Yiran Peng, and Yongyun Hu

Abstract

It has been demonstrated previously that atmospheric dust loading during the Precambrian could have been an order of magnitude higher than in the present day and could have cooled the global climate by more than 10°C. Here, using the fully coupled atmosphere–ocean general circulation model CESM1.2.2, we determine whether such dust loading could have facilitated the formation of Neoproterozoic snowball Earth events. Our results indicate that global dust emission decreases as atmospheric CO2 concentration (pCO2) decreases due to increasing snow coverage, but atmospheric dust loading does not change or even increases due to decreasing precipitation and strengthening June–August (JJA) Hadley circulation. The latter lifts more dust particles to high altitude and thus increases the lifetime of these particles. As the climate becomes colder and the surface albedo higher, the cooling effect of dust becomes weaker; when the global mean surface temperature is approximately −13°C, dust has negligible cooling effect. The threshold pCO2 at which Earth enters a snowball state is between 280 to 140 ppmv when there is no dust, and is similar when there is relatively light dust loading (~4.4 times the present-day value). However, the threshold pCO2 decreases dramatically to between 70 and 35 ppmv when there is heavy dust loading (~33 times the present-day value), due to the decrease in planetary albedo, which increases the energy input into the climate system. Therefore, dust makes it more difficult for Earth to enter a snowball state.

Restricted access