. Palaeoclimate simulations improve our understanding of the climate, inform us about the performance of climate models in a different climate scenario, and help to identify robust features of the climate system. Here, we analyse Arctic warming in an ensemble of 16 simulations of the mid-Pliocene Warm Period (mPWP), derived from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2). The PlioMIP2 ensemble simulates Arctic (60–90° N) annual mean surface air temperature (SAT) increases of 3.7 to 11.6 °C compared to the pre-industrial, with a multi-model mean (MMM) increase of 7.2 °C. The Arctic warming amplification ratio relative to global SAT anomalies in the ensemble ranges from 1.8 to 3.1 (MMM is 2.3). Sea ice extent anomalies range from −3.0 to −10.4 × 06 km2 with a MMM anomaly of −5.6 × 106 km2, which constitutes a decrease of 53 % compared to the pre-industrial. The majority (11 out of 16) models simulate summer sea ice-free conditions (≤ 1 × 06 km2) in their mPWP simulation. The ensemble tends to underestimate SAT in the Arctic when compared to available reconstructions. The simulations with the highest Arctic SAT anomalies tend to match the proxy dataset in its current form better. The ensemble shows some agreement with reconstructions of sea ice, particularly with regards to seasonal sea ice. Large uncertainties limit the confidence that can be placed in the findings and the compatibility of the different proxy datasets. We show that, while reducing uncertainties in the reconstructions could decrease the SAT data-model discord substantially, further improvements are likely to be found in enhanced boundary conditions or model physics. Lastly, we compare the Arctic warming in the mPWP to projections of future Arctic warming and find that the PlioMIP2 ensemble simulates greater Arctic amplification, an increase instead of a decrease in AMOC strength compared to pre-industrial, and a lesser strengthening of northern modes of variability than CMIP5 future climate simulations. The results highlight the importance of slow feedbacks in equilibrium climate simulations, and that caution must be taken when using simulations of the mPWP as an analogue for future climate change.

/pdf/evaluation-of-arctic-warming-in-mid-pliocene-climate-18gwzpjir6.pdf

Evaluation of Arctic warming in mid-Pliocene climate simulations

Despite tectonic conditions and atmospheric CO2 levels (pCO2) similar to those of present-day, geological reconstructions from the mid-Pliocene (3.3-3.0 Ma) document high lake levels in the Sahel and mesic conditions in subtropical Eurasia, suggesting drastic reorganizations of subtropical terrestrial hydroclimate during this interval. Here, using a compilation of proxy data and multi-model paleoclimate simulations, we show that the mid-Pliocene hydroclimate state is not driven by direct CO2 radiative forcing but by a loss of northern high-latitude ice sheets and continental greening. These ice sheet and vegetation changes are long-term Earth system feedbacks to elevated pCO2. Further, the moist conditions in the Sahel and subtropical Eurasia during the mid-Pliocene are a product of enhanced tropospheric humidity and a stationary wave response to the surface warming pattern, which varies strongly with land cover changes. These findings highlight the potential for amplified terrestrial hydroclimate responses over long timescales to a sustained CO2 forcing. 

/pdf/past-terrestrial-hydroclimate-sensitivity-controlled-by-1tqme35l.pdf

Past terrestrial hydroclimate sensitivity controlled by Earth system feedbacks

In this study, the Atlantic Multidecadal Oscillation (AMO) under different boundary
conditions during the Holocene, i.e. orbital change, greenhouse gas concentration, the Laurentide ice sheet
and its melting, are examined in several long-term simulations using the Earth system model COSMOS.
The simulated AMO indices exhibit a quasi-multidecadal periodicity, consistent with the typical 50-80 year
cycle of the AMO indicated by the observation data. A warm phase of the AMO accompanies a hemispheric
scale warming in the NH, with the maximum warming over the North Atlantic and part of the Arctic. Such a
warming favors more evaporation and thus more precipitation over most part of the North Atlantic,
especially enhancing the Atlantic intertropical convergence zone.
Concerning the forcing mechanism of the AMO, the most common believed physical process involves the
variation of the Atlantic meridional overturning circulation (AMOC). Although a fully understanding of such a
process is beyond the scope of this study, our results show the strong correlation between the AMO index
and AMOC index on multidecadal timescales and, during a warm phase of the AMO, the AMOC is
intensified significantly.
The climate influence of the AMO during the Holocene demonstrates that there is no remarkable change in
its spatial pattern under different climate background conditions, which further reveals that the AMO is an
internal variability of the climate system. However, we imply that this influence can be distinguished
regarding the regional scale feature and its magnitude. It has been supported by a previous study, which
suggests that the regional response to the AMO can be modulated by orbitally induced shifts in large-scale
ocean-atmosphere circulation. Moreover, we argue that the climate influence of the AMO might be
amplified by a vigorous background climate condition, in particular during a cold period.

Simulated Atlantic Multidecadal Oscillation during the Holocene

Northwestward shift of the northern boundary of the East Asian summer monsoon during the mid-Holocene caused by orbital forcing and vegetation feedbacks

Mid-Holocene European climate revisited: New high-resolution regional climate model simulations using pollen-based land-cover

. As global warming is proceeding due to rising greenhouse
gas concentrations, the Earth system moves towards climate states that
challenge adaptation. Past Earth system states are offering possible
modelling systems for the global warming of the coming decades. These
include the climate of the mid-Pliocene ( ∼  3 Ma), the last
interglacial ( ∼  129–116 ka) and the mid-Holocene
( ∼  6 ka). The simulations for these past warm periods are the
key experiments in the Paleoclimate Model Intercomparison Project (PMIP)
phase 4, contributing to phase 6 of the Coupled Model Intercomparison Project (CMIP6).
Paleoclimate modelling has long been regarded as a robust out-of-sample
test bed of the climate models used to project future climate changes. Here,
we document the model setup for PMIP4 experiments with EC-Earth3-LR and
present the large-scale features from the simulations for the mid-Holocene,
the last interglacial and the mid-Pliocene. Using the pre-industrial
climate as a reference state, we show global temperature changes,
large-scale Hadley circulation and Walker circulation, polar warming, global
monsoons and the climate variability modes – El Nino–Southern Oscillation
(ENSO), the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO).
EC-Earth3-LR simulates reasonable climate responses during past warm
periods, as shown in the other PMIP4-CMIP6 model ensemble. The systematic
comparison of these climate changes in past three warm periods in an
individual model demonstrates the model's ability to capture the climate
response under different climate forcings, providing potential implications
for confidence in future projections with the EC-Earth model.

/pdf/simulating-the-mid-holocene-last-interglacial-and-mid-32i1udjs40.pdf

Simulating the mid-Holocene, last interglacial and mid-Pliocene climate with EC-Earth3-LR

The Indian Summer Monsoon (ISM) plays a vital role in the livelihoods and economy of those living on the Indian subcontinent, including the small, mountainous country of Bhutan. The ISM fluctuates over varying temporal scales and its variability is related to many internal and external factors including the El Nino Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). In 2015, a Super El Nino occurred in the tropical Pacific alongside a positive IOD in the Indian Ocean and was followed in 2016 by a simultaneous La Nina and negative IOD. These events had worldwide repercussions. However, it is unclear how the ISM was affected during this time, both at a regional scale over the whole ISM area and at a local scale over Bhutan. First, an evaluation of data products comparing ERA5 reanalysis, TRMM and GPM satellite, and GPCC precipitation products against weather station measurements from Bhutan, indicated that ERA5 reanalysis was suitable to investigate ISM change in these two years. The reanalysis datasets showed that there was disruption to the ISM during this period, with a late onset of the monsoon in 2015, a shifted monsoon flow in July 2015 and in August 2016, and a late withdrawal in 2016. However, this resulted in neither a monsoon surplus nor a deficit across both years but instead large spatial-temporal variability. It is possible to attribute some of the regional scale changes to the ENSO and IOD events, but the expected impact of a simultaneous ENSO and IOD events are not recognizable. It is likely that 2015/16 monsoon disruption was driven by a combination of factors alongside ENSO and the IOD, including varying boundary conditions, the Pacific Decadal Oscillation, the Atlantic Multi-decadal Oscillation, and more. At a local scale, the intricate topography and orographic processes ongoing within Bhutan further amplified or dampened the already altered ISM.

https://www.mdpi.com/2073-4433/12/8/954/pdf

Regional and Local Impacts of the ENSO and IOD Events of 2015 and 2016 on the Indian Summer Monsoon—A Bhutan Case Study

Irregular climate events frequently occur in Southeast Asia due to the numerous climate patterns combining. Thailand sits at the confluence of these interactions, and consequently experiences major ...

https://www.mdpi.com/2571-550X/3/2/18/pdf

The Role of El Niño in Driving Drought Conditions over the Last 2000 Years in Thailand

Glacier mass balance is heavily influenced by climate, with responses of individual glaciers to various climate parameters varying greatly. In northern Sweden, Rabots Glaciar’s mass balance has decreased since it started being monitored in 1982. To relate Rabots Glaciar’s mass balance to changes in climate, the sensitivity to a range of parameters is computed. Through linear regression of mass balance with temperature, precipitation, humidity, wind speed and incoming radiation the climate sensitivity is established and projections for future summer mass balance are made. Summer mass balance is primarily sensitive to temperature at −0.31 m w.e. per °C change, while winter mass balance is mainly sensitive to precipitation at 0.94 m w.e. per % change. An estimate using summer temperature sensitivity projects a dramatic decrease in summer mass balance to −3.89 m w.e. for the 2091–2100 period under climate scenario RCP8.5. With large increases in temperature anticipated for the next century, more complex modelling studies of the relationship between climate and glacier mass balance is key to understanding the future development of Rabots Glaciar.

Mass Balance Sensitivity and Future Projections of Rabots Glaciär, Sweden

The impact of moisture transport and sources on precipitation stable isotopes (δ18O and d-excess) in the central Himalayas are crucial to understanding the climatic archives. However, this is still unclear due to the lack of in-situ observations. Here we present measurements of stable isotopes in precipitation at two stations (Yadong and Pali) in the central Himalayas during 2014-2015. Combined with simulations from the dispersion model FLEXPART, we investigate effects on precipitation stable isotopes related to changes in moisture sources and convections in the region, and possible influence by El Niño. Our results suggest that the moisture supplies related to evaporation over northeastern India and moisture losses related to convective activities over the Bay of Bengal (BoB) and Bangladesh region play important roles in changes in δ18O and d-excess in precipitation in the Yadong valley. Outgoing longwave radiation and moisture flux divergence analysis further confirm that the contribution from continental evaporation dominates the moisture supply in the central Himalayas with a lesser contribution from convection over the BoB during the 2015 monsoon season compared with 2014. A change in the altitude effect is observed in 2015, which is more significant than the temperature and precipitation amount effect during the observation period. These findings provide valuable insights into climatic interpretations of paleo-isotopic archives with an isotopic response to changes in moisture transport to the central Himalayas. 

Josefine Axelsson

Papers

Simulating the mid-Holocene, last interglacial and mid-Pliocene climate with EC-Earth3-LR

Regional and Local Impacts of the ENSO and IOD Events of 2015 and 2016 on the Indian Summer Monsoon—A Bhutan Case Study

The Role of El Niño in Driving Drought Conditions over the Last 2000 Years in Thailand

Mass Balance Sensitivity and Future Projections of Rabots Glaciär, Sweden

A precipitation isotopic response in 2014‐2015 to moisture transport changes in the central Himalayas