[1] Summer thermal structure and winds over Asia show a larger land-ocean thermal gradient in the upper than in the lower troposphere, implying a bigger role of the upper troposphere in driving the Asian summer monsoon circulation. Using data from atmospheric re-analyses and model simulations, we show that the land-ocean thermal contrast in the mid-upper (200-500 hPa) troposphere (TCupper) contributes about three times as much as the thermal contrast in the mid-lower (500-850 hPa) troposphere (TClower) in determining both the strength and variations of Asian summer monsoon circulations. Tropical sea surface temperature anomalies associated with the annual cycle, El Niño-Southern Oscillation, decadal changes, and global warming all are accompanied with much larger variations and changes in TCupper than in TClower, partly due to enhanced latent heating aloft from convection. The variations and changes in TCupper and TClower are highly correlated with the strength of the South Asian Summer Monsoon (SASM) and the East Asian Summer Monsoon (EASM) in their respective sectors during the past 50-60 years. In particular, the weakening of the EASM since the 1950s is caused by the weakening mainly in TCupper and secondarily in TClower induced mainly by recent tropical surface warming, although spurious cooling over East Asia seen in reanalysis data may have enhanced this weakening. However, the strength of the SASM and EASM monsoons follows TCupper but decouples with TClower in the global warming case in the 21 st century. The results suggest that the TCupper plays a dominant role and provides an efficient mechanism through which tropical oceans can influence extratropical monsoons.

A distinct class of El Niño events with extreme magnitude (termed "super El Niño" events in this study) is identified after removing decadal variation. These events occurred in 1972/1973, 1982/1983, and 1997/1998. They are distinguished not only by their size but also by associated features such as a Southern Hemispheric transverse circulation that is not similarly robust in other El Niño events. This transverse circulation is characterized by a low-level equatorward flow, which spins off from a high sea-level-pressure anomaly around Australia and then merges into the deep convection anomalies over the central Pacific, and by an upper-level southward divergent flow, which branches off from the convection center and connects to the subsidence of the Australia high. It is suggested that this transverse cell, peaking in boreal summer, serves as an effective booster during the developing stage of a super El Niño by intensifying tropical Pacific low-level westerly winds.

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The Pacific-North American (PNA) teleconnection pattern is one of the prominent atmospheric circulation modes in the extra-tropical Northern Hemisphere, and its seasonal-to-interannual predictability is suggested to originate from the El Niño–Southern Oscillation (ENSO). Intriguingly, the PNA exhibits variance at near-annual frequencies, which is related to a rapid phase reversal of the PNA during ENSO years, whereas the ENSO sea surface temperature (SST) anomalies in the tropical Pacific are evolving much slower in time. This distinct seasonal feature of the PNA can be explained by an amplitude modulation of the interannual ENSO signal by the annual cycle (i.e., the ENSO Combination Mode). The ENSO-related seasonal phase transition of the PNA is reproduced well in an atmospheric general circulation model when both the background SST annual cycle and ENSO SST anomalies are prescribed. In contrast, this characteristic seasonal evolution of the PNA is absent when the tropical Pacific background SST annual cycle is not considered in the modeling experiments. The background SST annual cycle in the tropical Pacific modulates the ENSO-associated tropical Pacific convection response, leading to a rapid enhancement of convection anomalies in winter. The enhanced convection results in a fast establishment of the large-scale PNA teleconnection during ENSO years. The dynamics of this ENSO-annual cycle interaction fills an important gap in our understanding of the seasonally modulated PNA teleconnection pattern during ENSO years.