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Paul Edwin Curtis
and
Alexey V. Fedorov

Abstract

The present-day deep ocean global meridional overturning circulation is dominated by the Atlantic meridional overturning circulation (AMOC), with dense water sinking in the high-latitude North Atlantic Ocean. In contrast, deep-water formation in the subarctic North Pacific is inhibited by a strong upper-ocean halocline, which prevents the development of an analogous Pacific meridional overturning circulation (PMOC). Nevertheless, paleoclimate evidence suggests that a PMOC with deep-water formation in the North Pacific was active, for instance, during the warm Pliocene epoch and possibly during the most recent deglaciation. In the present study, we describe a spontaneous activation of the PMOC in a multimillennial abrupt 4 × CO2 experiment using one of the configurations of the Community Earth System Model (CESM1). Soon after the imposed CO2 increase, the model’s AMOC collapses and remains in a weakened state for several thousand years. The PMOC emerges after some 2500 years of integration, persists for about 1000 years, reaching nearly 10 Sv (1 Sv ≡ 106 m3 s−1), but eventually declines to about 5 Sv. The PMOC decline follows the AMOC recovery in the model, consistent with an Atlantic–Pacific interbasin seesaw. The PMOC activation relies on two factors: (i) gradual warming and freshening of the North Pacific deep ocean, which reduces ocean vertical stratification on millennial time scales, and (ii) upper-ocean salinity increase in the subarctic North Pacific over several centuries, followed by a rapid erosion of the pycnocline and activation of deep-water formation. Ultimately, our results provide insights on the characteristics of global ocean overturning in warm climates.

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Tyler Cox
,
Aaron Donohoe
,
Kyle C. Armour
,
Dargan M. W. Frierson
, and
Gerard H. Roe

Abstract

We investigate the linear trends in meridional atmospheric heat transport (AHT) since 1980 in atmospheric reanalysis datasets, coupled climate models, and atmosphere-only climate models forced with historical sea surface temperatures. Trends in AHT are decomposed into contributions from three components of circulation: (i) transient eddies, (ii) stationary eddies, and (iii) the mean meridional circulation. All reanalyses and models agree on the pattern of AHT trends in the Southern Ocean, providing confidence in the trends in this region. There are robust increases in transient-eddy AHT magnitude in the Southern Ocean in the reanalyses, which are well replicated by the atmosphere-only models, while coupled models show smaller magnitude trends. This suggests that the pattern of sea surface temperature trends contributes to the transient-eddy AHT trends in this region. In the tropics, we find large differences between mean-meridional circulation AHT trends in models and the reanalyses, which we connect to discrepancies in tropical precipitation trends. In the Northern Hemisphere, we find less evidence of large-scale trends and more uncertainty, but note several regions with mismatches between models and the reanalyses that have dynamical explanations. Throughout this work we find strong compensation between the different components of AHT, most notably in the Southern Ocean where transient-eddy AHT trends are well compensated by trends in the mean-meridional circulation AHT, resulting in relatively small total AHT trends. This highlights the importance of considering AHT changes holistically, rather than each AHT component individually.

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Tong Li
,
Xuebin Zhang
, and
Zhihong Jiang

Abstract

Weighting models according to their performance has been used to produce multimodel climate change projections. But the added value of model weighting for future projection is not always examined. Here we apply an imperfect model framework to evaluate the added value of model weighting in projecting summer temperature changes over China. Members of large-ensemble simulations by three climate models of different climate sensitivities are used as pseudo-observations for the past and the future. Performance of the models participating in the phase 6 of the Coupled Model Intercomparison Project (CMIP6) are evaluated against the pseudo-observations based on simulated historical climatology and trends in global, regional, and local temperatures to determine the model weights for future projection. The weighted projections are then compared with the pseudo-observations in the future period. We find that regional trend as a metric of model performance yields generally better skill for future projection, while past climatology as performance metric does not lead to a significant improvement to projection. Trend at the grid-box scale is also not a good performance indicator as small-scale trend is highly uncertain. For the model weighting to be effective, the metric for evaluating the model’s performance must be relatable to future changes, with the response signal separable from internal variability. Projected summer warming based on model weighting is similar to that of unweighted projection but the 5th–95th-percentile uncertainty range of the weighted projection is 38% smaller with the reduction mainly in the upper bound, with the largest reduction appearing in southeast China.

Open access
Chenyu Lv
,
Riyu Lu
, and
Wei Chen

Abstract

This study identifies a significantly positive relationship between summer surface air temperature (SAT) anomalies over two remote regions in the Eurasian continent and North America during the period 1979–2021 on the interannual time scale. The former region includes the East European Plain and the West Siberian Plain, and the latter region includes central and eastern North America. The regionally averaged summer SAT anomalies show a correlation coefficient of 0.66 between these two regions, which is significant at the 99% confidence level. This intercontinental SAT relationship can be explained by a wavelike pattern of circulation anomalies, which is the leading mode of upper-tropospheric circulation anomalies over the middle and high latitudes of the Northern Hemisphere in summer. Further analysis suggests that the sea surface temperature (SST) anomalies over the Pacific and North Atlantic in the preceding spring, being coupled with the leading mode of atmospheric circulation anomalies over the Pacific–Atlantic sector, persist into summer and affect the SATs in the two remote regions, resulting in the intercontinental SAT connection.

Significance Statement

Summer surface air temperature (SAT) has profound effects on public health and agricultural production. Here we find a significantly positive relationship between interannual variations of summer SATs over two remote regions, one in the Eurasian continent and the other in North America. This intercontinental relationship in SATs can be explained as a result of atmosphere–ocean coupling over the Pacific and North Atlantic in the preceding spring. The result is likely to be a critical implication for the seasonal forecast of SAT variations over the two regions. In addition, the concurrence of higher or lower temperatures in these two regions may have impacts on global grain production, since these two regions include many major grain-producing areas in the world.

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Zachary McGraw
,
Kevin DallaSanta
,
Lorenzo M. Polvani
,
Kostas Tsigaridis
,
Clara Orbe
, and
Susanne E. Bauer

Abstract

Volcanic super-eruptions have been theorized to cause severe global cooling, with the 74 kya Toba eruption purported to have driven humanity to near-extinction. However, this eruption left little physical evidence of its severity and models diverge greatly on the magnitude of post-eruption cooling. A key factor controlling the super-eruption climate response is the size of volcanic sulfate aerosol, a quantity that left no physical record and is poorly constrained by models. Here we show that this knowledge gap severely limits confidence in model-based estimates of super-volcanic cooling, and accounts for much of the disagreement among prior studies. By simulating super-eruptions over a range of aerosol sizes, we obtain global mean responses varying from extreme cooling all the way to the previously unexplored scenario of widespread warming. We also use an interactive aerosol model to evaluate the scaling between injected sulfur mass and aerosol size. Combining our model results with the available paleoclimate constraints applicable to large eruptions, we estimate that global volcanic cooling is unlikely to exceed 1.5°C no matter how massive the stratospheric injection. Super-eruptions, we conclude, may be incapable of altering global temperatures substantially more than the largest Common Era eruptions. This lack of exceptional cooling could explain why no single super-eruption event has resulted in firm evidence of widespread catastrophe for humans or ecosystems.

Significance Statement

Whether volcanic super-eruptions pose a threat to humanity remains a subject of debate, with climate models disagreeing on the magnitude of global post-eruption cooling. We demonstrate that this disagreement primarily stems from a lack of constraint on the size of volcanic sulfate aerosol particles. By evaluating the range of aerosol size scenarios, we demonstrate that eruptions may be incapable of causing more than 1.5°C cooling no matter how much sulfur they inject into the stratosphere. This could explain why archaeological records provide no evidence of increased human mortality following the Toba super-eruption. Further, we raise the unexplored possibility that the largest super-eruptions could cause global-scale warming.

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Zheng Wang
,
Lu Dong
,
Fengfei Song
,
Tianjun Zhou
, and
Xiaolong Chen

Abstract

The zonal sea surface temperature (SST) gradient across the tropical Pacific is a pacemaker of the variable rates of global warming. In both historical simulations and future projections, the current state-of-the-art climate models show evident spread in the changes of zonal SST gradient, but the reasons remain unknown. Here, we quantify the contributions of internal variability and model spread to the uncertainty of zonal SST gradient changes by analyzing 342 realizations from CMIP5 and CMIP6 models and several sets of large-ensemble simulations. We found that the internal variability dominates the total uncertainty at multidecadal time scales (∼31-yr trends). Although the ratio of internal uncertainty to the total uncertainty declines along with higher emission of greenhouse gases under global warming, it is still over 80% at the multidecadal time scales in the future. The Pacific decadal oscillation is identified as the key internal mode responsible for the multidecadal uncertainty. For the future projections at centurial time scales, the uncertainty of zonal SST gradient changes is mainly from the intermodel spread in response to external forcing, accounting for about 70% of the uncertainty based on the difference between 2070–99 and 1970–99. The model spread in the cloud–shortwave radiation–SST feedback over the tropical Pacific is important in the uncertainty of zonal SST gradient changes. In particular, the intensity of negative convective cloud feedback in the western Pacific dominates the spread in CMIP5 models, while the intensity of stratocumulus cloud feedback over the southeastern Pacific is the primary process influencing the uncertainty in CMIP6 models.

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Dylan Oldenburg
,
Young-Oh Kwon
,
Claude Frankignoul
,
Gokhan Danabasoglu
,
Stephen Yeager
, and
Who M. Kim

Abstract

Arctic Ocean warming and sea ice loss are closely linked to increased ocean heat transport (OHT) into the Arctic and changes in surface heat fluxes. To quantitatively assess their respective roles, we use the 100-member Community Earth System Model, version 2 (CESM2), Large Ensemble over the 1920–2100 period. We first examine the Arctic Ocean warming in a heat budget framework by calculating the contributions from heat exchanges with atmosphere and sea ice and OHT across the Arctic Ocean gateways. Then we quantify how much anomalous heat from the ocean directly translates to sea ice loss and how much is lost to the atmosphere. We find that Arctic Ocean warming is driven primarily by increased OHT through the Barents Sea Opening, with additional contributions from the Fram Strait and Bering Strait OHTs. These OHT changes are driven mainly by warmer inflowing water rather than changes in volume transports across the gateways. The Arctic Ocean warming driven by OHT is partially damped by increased heat loss through the sea surface. Although absorbed shortwave radiation increases due to reduced surface albedo, this increase is compensated by increasing upwelling longwave radiation and latent heat loss. We also explicitly calculate the contributions of ocean–ice and atmosphere–ice heat fluxes to sea ice heat budget changes. Throughout the entire twentieth century as well as the early twenty-first century, the atmosphere is the main contributor to ice heat gain in summer, though the ocean’s role is not negligible. Over time, the ocean progressively becomes the main heat source for the ice as the ocean warms.

Significance Statement

Arctic Ocean warming and sea ice loss are closely linked to increased ocean heat transport (OHT) into the Arctic and changes in surface heat fluxes. Here we use 100 simulations from the same climate model to analyze future warming and sea ice loss. We find that Arctic Ocean warming is primarily driven by increased OHT through the Barents Sea Opening, though the Fram and Bering Straits are also important. This increased OHT is primarily due to warmer inflowing water rather than changing ocean currents. This ocean heat gain is partially compensated by heat loss through the sea surface. During the twentieth century and early twenty-first century, sea ice loss is mainly linked to heat transferred from the atmosphere; however, over time, the ocean progressively becomes the most important contributor.

Open access
Xudong Wang
,
Dachao Jin
,
Zhaoyong Guan
,
Yu Zhang
, and
Qiuchang Han

Abstract

Low-level cross-equatorial flows (CEFs) over the Indian Ocean–western Maritime Continent (IO-wMC) play a crucial role in transporting energy, mass, and water vapor into the Northern Hemisphere during the Asian summer monsoon season (May–September). Utilizing ECMWF reanalysis data (ERA5), we investigate three CEFs over the IO-wMC: the Somali-CEF, Bay of Bengal CEF (BoB-CEF), and South China Sea CEF (SCS-CEF). The statistical independence of the Somali-CEF and BoB-CEF implies distinct formation processes on the interannual time scale. To examine the interannual variability of CEFs, we perform an empirical orthogonal function (EOF) analysis of vertically integrated meridional winds from surface pressure to 850 hPa over the equatorial IO-wMC. The first EOF mode reveals a weakening of the Somali-CEF and strengthening of the BoB-CEF and SCS-CEF, which is associated with the concurrent summer Indian Ocean dipole and quasi-biennial phase transition of El Niño events. The second EOF is related to the Indo-western Pacific Ocean capacitor (IPOC) mode that often emerges in post–El Niño summers. Despite the IPOC’s influence on meridional winds over the tropical southwestern Indian Ocean, three CEFs are uncorrelated with the EOF2. On the other hand, the Somali-CEF and BoB-CEF are significantly weakened in the EOF3. The EOF3 is an ENSO-unrelated internal IPOC mode. We further use a multilinear regression model based on preceding sea surface temperature (SST) over the Indo-Pacific and EOF3 to predict regional climate anomalies and compare the prediction skill with the SST-based models. Our results suggest that adding EOF2- and EOF3-related variability to the prediction model can improve Asian summer monsoon predictability.

Significance Statement

This study examines the interannual variability of low-level cross-equatorial flows (CEFs) over the Indian Ocean and western Maritime Continent, crucial components of the Asian summer monsoon. We identify three CEFs and investigate their relationships with interannual sea surface temperature modes in the Indo-Pacific region. The leading empirical orthogonal function (EOF) modes of equatorial meridional wind reveal diverse CEF configurations and post–El Niño summer wind variability. Also, the EOFs distinguishes between El Niño and El Niño–unrelated equatorial meridional wind variability. Utilizing a multilinear regression model based on both preceding Indo-Pacific SST modes and configurations of CEFs, we demonstrate the potential to enhance regional climate predictions, contributing to a more comprehensive understanding and forecasting of the Asian summer monsoon and its associated climate impacts.

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Ángel F. Adames Corraliza
and
Víctor C. Mayta

Abstract

Interactions between large-scale waves and the Hadley cell are examined using a linear two-layer model on an f plane. A linear meridional moisture gradient determines the strength of the idealized Hadley cell. The trade winds are in thermal wind balance with a weak temperature gradient (WTG). The mean meridional moisture gradient is unstable to synoptic-scale (horizontal scale of ∼1000 km) moisture modes that are advected westward by the trade winds, reminiscent of oceanic tropical depression–like waves. Meridional moisture advection causes the moisture modes to grow from “moisture-vortex instability” (MVI), resulting in a poleward eddy moisture flux that flattens the zonal-mean meridional moisture gradient, thereby weakening the Hadley cell. The amplification of waves at the expense of the zonal-mean meridional moisture gradient implies a downscale latent energy cascade. The eddy moisture flux is opposed by a regeneration of the meridional moisture gradient by the Hadley cell. These Hadley cell–moisture mode interactions are reminiscent of quasigeostrophic interactions, except that wave activity is due to column moisture variance rather than potential vorticity variance. The interactions can result in predator–prey cycles in moisture mode activity and Hadley cell strength that are akin to ITCZ breakdown. It is proposed that moisture modes are the tropical analog to midlatitude baroclinic waves. MVI is analogous to baroclinic instability, stirring latent energy in the same way that dry baroclinic eddies stir sensible heat. These results indicate that moisture modes stabilize the Hadley cell and may be as important as the latter in global energy transport.

Significance Statement

The tropics are characterized by steady circulations such as the Hadley cell as well as a menagerie of tropical weather systems. Despite progress in our understanding of both, little is known about how the mean circulations and the weather systems interact with one another. Here we show that tropical waves can grow by extracting moisture from the Hadley cell, thereby weakening it. They also transport moisture to higher latitudes. Our results challenge the notion that the Hadley cell is the sole transporter of energy out of the tropics and instead favor a view where tropical waves are also essential for the global energy balance. They dry the humid regions and moisten the drier regions via stirring.

Open access
Yong Liu
,
Zhongshi Zhang
,
Xiangyu Li
,
Hua Li
, and
Huopo Chen

Abstract

Winter precipitation is critical for glacier growth, snow accumulation, and terrestrial storage on the Tibetan Plateau (TP). However, less attention has been paid to the changes in winter precipitation on the TP. In this study, based on the newly developed high-accuracy precipitation dataset, our diagnosis illustrated a distinct precipitation deficit over the southern TP and northern Indian continent (STNI) in December. Together with the ERA5 reanalysis, CESM1.1 large ensemble experiments, and pacemaker experiments, the dynamic diagnosis revealed that the precipitation deficit was attributed to the decrease in dynamic effects. In particular, the enhanced descending motion, accompanied by the strengthened anticyclone and regional meridional circulation over the STNI, resulted in the precipitation deficit. The changes in circulation system related to the precipitation deficit could be attributed to the Pacific and Indian sea surface temperature (SST) variability. With the negative interdecadal Pacific oscillation (IPO), there were circumglobal alternative geopotential height anomalies and an anomalous anticyclone over the southern TP, and descending motion around the STNI associated with the enhanced Walker circulation over the Indo-Pacific, which exhibited a similar pattern to the precipitation deficit over the STNI in the observations. In addition, the synergistic effect of Pacific and Indian SST variability can improve the simulated precipitation deficit over the STNI relative to the Pacific SST variability alone. These results imply a contribution of the tropical SST variability to the precipitation trends over the STNI in early winter and provide an insight into the understanding of climate warming on the winter climate over the TP.

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