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 Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). In 2015, a Super El Niño occurred in the tropical Pacific alongside a positive IOD in the Indian Ocean and was followed in 2016 by a simultaneous La Niña 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.

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 hydrological events, such as droughts. Proxy data, speleothem records, lake sediment sequences and tree ring chronologies were used to reconstruct paleo drought conditions. These trends were compared with modelled and historic El Niño Southern Oscillation (ENSO) data to assess if the ENSO climate phenomena is causing droughts in Thailand. Drought periods were found to occur both during El Niño events and ENSO neutral conditions. This indicates droughts are not a product of one climate pattern, but likely the result of numerous patterns interacting. There is uncertainty regarding how climate patterns will evolve under climate change, but changes in amplitude and variability could potentially lead to more frequent and wider reaching hydrological disasters. It is vital that policies are implemented to cope with the resulting social and economic repercussions, including diversification of crops and reorganisation of water consumption behaviour in Thailand.

Significant multi-centennial climate variability with a clear peak at approximately 200 years is found in a pre-industrial control simulation conducted with the EC-Earth3 climate model. The oscillation mainly emerges from the North Atlantic and appears to be closely associated with the Atlantic Meridional Overturning Circulation (AMOC). By examining the salinity advection feedback, we find that the perturbation flow of mean subtropical-subpolar salinity gradients in the subpolar area governs as positive feedback to the AMOC anomaly. Meanwhile, the mean advection of salinity anomalies and the vertical mixing or convection acts as negative feedback to restrain the AMOC anomaly. In a warmer climate, although the AMOC becomes weaker, such low-frequency variability still exists, indicating the robustness of the salinity advection feedback mechanism.

<p>A distinct multi-centennial climate variability signal is apparent in the EC-Earth3 model 2000-year pre-industrial control simulation. This variability arises primarily in the North Atlantic basin and appears to be closely associated with Atlantic Meridional Overturning Circulation (AMOC). It is mainly modulated by the ocean heat transport and freshwater exchange between the Arctic Ocean and the North Atlantic. When a stronger AMOC occurs, it is coherent with anomalous anticyclonic surface currents in the Arctic and cyclonic surface currents in Greenland, Iceland, Norwegian Seas and Labrador Seas. The increased heat in the subpolar gyre region strengthens the oceanic surface evaporation, resulting in a saltier deep convection region and hence strengthens the deep-water formation. Meanwhile, stronger AMOC transports more ocean heat into the Arctic and melts the sea-ice, causing more freshwater to enter the Arctic. The AMOC strength and freshwater accumulation in the Arctic both reach their peaks in about 50 years. Then, in the following 50 years, the freshwater in Arctic slowly pours into the Greenland-Iceland-Labrador Seas, weakens the subpolar gyre, inhibits deep-water formation and eventually weakens the AMOC. Finally, the oscillation shifts to the opposite phase. These physical processes sustain a 160-200 year variability of AMOC, which is considered as the main driver of the multi-centennial climate variability signal in our simulation.</p> <p> </p> <p>In high CO2 forced climates, here simulated with climate sensitivity experiments with alternate CO2 levels of 400 and 560 ppm respectively, the multi-centennial variability of AMOC is present but has a suppressed amplitude. AMOC variability under 400 ppm CO2 forcing shows a similar frequency band as that in the pre-industrial simulation, with enhanced Arctic-Atlantic salinity anomaly exchange. Under 560 ppm CO2 forcing, the AMOC variability shows a lower frequency band. Here, alongside the Arctic-Atlantic salinity exchange, there are also salinity anomalies propagating from the south Atlantic to the north Atlantic. This leads to a longer maintaining of meridional inter-basin exchanges in the entire Atlantic and Arctic. The decrease of Arctic sea-ice under stronger radiative forcing will cause more freshwater to enter the North Atlantic, slow down the deep-water flow, and thus suppress the AMOC strength. Meanwhile the mechanism that sustains AMOC variability, which was inferred from the pre-industrial simulation, will also change as less sea ice in the North Atlantic and Arctic lead to a more well mixed Arctic-Atlantic salinity anomaly exchange.</p> <p>These experiments indicate that the dynamics of the meridional inter-basin exchange in the north Atlantic and its influence on the salinity are essential components to the centennial climate variability and should be considered when assessing future North Atlantic climate.</p>