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1We present the first crossdated tree ring record from central Vietnam, derived from the growth 2 rings of the rare cypress Fokienia hodginsii from the mountains of Quang Nam Province near the 3 Laos border. The Quang Nam Fokienia hodginsii time series (QNFH), based on the crossdated 4 sequences of 71 increment core samples from 37 mature trees, is the third published 5 dendrochronological record from this species. The record extends 667 years from AD 1347 to 6 2013 and exhibits a mean series intercorrelation of 0.526, similarly significant with the first two 7published Fokienia hodginsii records: 0.474 for Mu Cang Chai (MCFH) and 0.578 for Bidoup -8Nui Ba National Park (BDFH) in the north and south of Vietnam, respectively. The Expressed 9Population Signal (EPS) for the QNFH record exceeds the generally accepted threshold of 0.85 10 back to AD 1567, but remains above 0.8 back to 1550. Similar to the MCFH and BDFH records, 11 QNFH expresses statistically significant linkages to regional hydroclimate metrics and the El 12 Niño-Southern Oscillation (ENSO). Here we present a reconstruction of the Standardized 13Precipitation Evapotranspiration Index (SPEI) for the month of April, averaged over a large 14 region of Southeast Asia. As with prior studies we demonstrate that cool phase (La Niña) and 15 warm phase (El Niño) events are linked to regional wet and dry conditions, respectively, with 16 linkages to modulation of the surface water temperature over the adjacent sea to the east of 17Vietnam as well as the Indian Ocean. A late 18 th century megadrought that is expressed widely 18 across South and Southeast Asia, and notably from the MCFH and BDFH records described 19 above, is not as pronounced in Central Vietnam and we explore the reasons why. 20 21

The timing of active-phase East Asian summer monsoon (Meiyu) undergoes a marked shift since 1979. Diagnostic analysis indicates that active convection over Taiwan has occurred later in the season, from late May to early June, with a tendency of increasingly intense rainfall. This timing shift of convection results from a southward migration of Meiyu rainband, driven by an upper-level cyclonic anomaly over eastern China and a lower-level anticyclonic anomaly in the subtropical Western Pacific. Together, these two circulation patterns enhance both the moisture transport and baroclinic forcing. The role of Western Pacific warming and anthropogenic greenhouse gases in these changes is suggested.

Severe flooding occurred in Thailand during the 2011 summer season, which resulted in more than 800 deaths and affected 13.6 million people. The unprecedented nature of this flood in the Chao Phraya River basin (CPRB) was examined and compared with historical flood years. Climate diagnostics were conducted to understand the meteorological conditions and climate forcing that led to the magnitude and duration of this flood. Neither the monsoon rainfall nor the tropical cyclone frequency anomalies alone was sufficient to cause the 2011 flooding event. Instead, a series of abnormal conditions collectively contributed to the intensity of the 2011 flood: anomalously high rainfall in the premonsoon season, especially during March; record-high soil moisture content throughout the year; elevated sea level height in the Gulf of Thailand, which constrained drainage; and other water management factors. In the context of climate change, the substantially increased premonsoon rainfall in CPRB after 1980 and the continual sea level rise in the river outlet have both played a role. The rainfall increase is associated with a strengthening of the premonsoon northeasterly winds that come from East Asia. Attribution analysis using phase 5 of the Coupled Model Intercomparison Project historical experiments pointed to anthropogenic greenhouse gases as the main external climate forcing leading to the rainfall increase. Together, these findings suggest increasing odds for potential flooding of similar intensity to that of the 2011 flood.

In January 2016, a robust reversal of the Arctic Oscillation took place associated with a rapid tropospheric warming in the Arctic region; this was followed by the occurrence of a classic sudden stratospheric warming in March. The succession of these two distinct Arctic warming events provides a stimulating opportunity to examine their characteristics in terms of similarities and differences. Historical cases of these two types of Arctic warming were identified and validated based upon tropical linkages with the Madden‐Julian Oscillation and El Niño as documented in previous studies. The analysis indicates a recent and seemingly accelerated increase in the tropospheric warming type versus a flat trend in stratospheric warming type. The shorter duration and more rapid transition of tropospheric warming events may connect to the documented increase in midlatitude weather extremes, more so than the route of stratospheric warming type. Forced simulations with an atmospheric general circulation model suggest that the reduced Arctic sea ice contributes to the observed increase in the tropospheric warming events and associated remarkable strengthening of the cold Siberian high manifest in 2016.

In the long term, precipitation in the Central U.S. decreases by 25% during the seasonal transition from June to July. This precipitation decrease has intensified since 1979 and such intensification could have enhanced spring drought occurrences in the Central U.S., in which conditions quickly evolve from being abnormally dry to exceptionally dry. Various atmospheric and land reanalysis data sets were analyzed to examine the trend in the June-July seasonal transition. The intensified deficit in precipitation is accompanied by increased downward shortwave radiation flux, tropospheric subsidence, enhanced evaporative fraction, and elevated planetary boundary layer height, all of which can lead to surface drying. The change in tropospheric circulation was characterized by an anomalous ridge over the western U.S. and a trough on either side-a pattern known to suppress rainfall in the Central U.S. This trending pattern shows similarity with the progression of the 2012 record drought.

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