Enhanced tropospheric biennial oscillation of the East Asian summer monsoon since the late-1970s

The tropospheric biennial oscillation (TBO) of East Asian summer monsoon (EASM) has major impacts on East Asian climate. Here it is shown that, since the late-1970s, the TBO signal of EASM has strengthened significantly. The EASM TBO in wind anomalies undergoes a transition from a cyclone over the western North Pacific (WNPC) in preceding summer to an anticyclone over the western North Pacific (WNPAC) in following summer, with the anomalies strengthening remarkably after the late-1970s. Correspondingly, the biennial component of precipitation anomalies in eastern China show different distributions. Both observational and numerical simulation analyses demonstrate that these changes are caused by the westward shift of El Niño warming and enhanced Indo-Pacific and Atlantic-Pacific coupling. The positive sea surface temperature (SST) anomalies associated with the TBO of EASM shift toward the central Pacific after the late-1970s, which favor the strengthening of the WNPC and cause a weakened EASM. In following summer, both the north Indian Ocean and tropical north Atlantic SST warming are closely coupled with El Niño since the late-1970s, which favor the strengthening of WNPAC and cause an intensified EASM. Together, these changes provide more favorable background state for the transition of circulation anomalies over the western North Pacific, giving rise to enhanced biennial variability in EASM in the late-1970s.

Global Patterns of Carbon Dioxide Variability from Satellite Observations

Advanced satellite technology has been providing unique observations of global carbon dioxide (CO2) concentrations. These observations have revealed important CO2variability at different timescales and over regional and planetary scales. Satellite CO2retrievals have revealed that stratospheric sudden warming and the Madden-Julian Oscillation can modulate atmospheric CO2concentrations in the mid-troposphere. Atmospheric CO2also demonstrates variability at interannual timescales. In the tropical region, the El Niño–Southern Oscillation and the Tropospheric Biennial Oscillation can change atmospheric CO2concentrations. At high latitudes, mid-tropospheric CO2concentrations can be influenced by the Northern Hemispheric annular mode. In addition to modulations by the large-scale circulations, sporadic events such as wildfires, volcanic eruptions, and droughts, which change CO2surface emissions, can cause atmospheric CO2concentrations to increase significantly. The natural variability of CO2summarized in this review can help us better understand its sources and sinks and its redistribution by atmospheric motion. ▪ Global satellite CO2data offer a unique opportunity to explore CO2variability in different regions. ▪ Atmospheric CO2concentration demonstrates variations at intraseasonal, seasonal, and interannual timescales. ▪ Both large-scale circulations and variations of surface emissions can modulate CO2concentrations in the atmosphere.

A Recent Shift in the Monsoon Centers Associated with the Tropospheric Biennial Oscillation

The tropospheric biennial oscillation (TBO) is conventionally considered to involve transitions between the Indian and Australian summer monsoons and the interactions between these two monsoons and the underlying Indo-Pacific Oceans. Here it is shown that, since the early 1990s, the TBO has evolved to mainly involve the transitions between the western North Pacific (WNP) and Australian monsoons. In this framework, the WNP monsoon replaces the Indian monsoon as the active Northern Hemisphere TBO monsoon center during recent decades. This change is found to be caused by stronger Pacific–Atlantic coupling and an increased influence of the tropical Atlantic Ocean on the Indian and WNP monsoons. The increased Atlantic Ocean influence damps the Pacific Ocean influence on the Indian summer monsoon (leading to a decrease in its variability) but amplifies the Pacific Ocean influence on the WNP summer monsoon (leading to an increase in its variability). These results suggest that the Pacific–Atlantic interactions have become more important to the TBO dynamics during recent decades.

Statistical Prediction of Winter Haze Days in the North China Plain Using the Generalized Additive Model

Winter (December–February) haze days in the North China Plain (WHDNCP) have recently dramatically increased. In addition to human activities, climate change and variability also contributed to the severe situation and supported the possibility of seasonal predictions. In this study, using the generalized additive model (GAM), the sea surface temperature around the Alaska Gulf and sea ice area of the Beaufort Sea were selected as the predictors to establish a statistical prediction model (SPM). The difference between the current and previous year of WHDNCP (WDY) was predicted first and was then added to the observation of the previous year to obtain the final predicted WHDNCP. For WDY prediction, the root-mean-square error of the SPM using GAM was 3.01 days. In addition to the annual variation, the tropospheric biennial oscillation features and the dramatically increasing trend after 2010 were both captured successfully. Furthermore, for the final predicted WHDNCP anomalies, the long-term trend and turning points were simulated well, and the percentage of the same mathematical sign was 91.7%. Independent prediction tests were performed for 2014 and 2015, and the forecast bias was 0.86 and 0.19 days, respectively. To assess the predictive ability, recycling independent tests (including real-time hindcasts for the period 2005–15) were also applied, and the percentage of the same sign was 100%.

Indo-Pacific Variability on Seasonal to Multidecadal Time Scales. Part I: Intrinsic SST Modes in Models and Observations

The variability of Indo-Pacific SST on seasonal to multidecadal time scales is investigated using a recently introduced technique called nonlinear Laplacian spectral analysis (NLSA). Through this technique, drawbacks associated with ad hoc prefiltering of the input data are avoided, enabling recovery of low-frequency and intermittent modes not previously accessible via classical approaches. Here, a multiscale hierarchy of spatiotemporal modes is identified for Indo-Pacific SST in millennial control runs of CCSM4 and GFDL CM3 and in HadISST data. On interannual time scales, a mode with spatiotemporal patterns corresponding to the fundamental component of ENSO emerges, along with ENSO-modulated annual modes consistent with combination mode theory. The ENSO combination modes also feature prominent activity in the Indian Ocean, explaining a significant fraction of the SST variance in regions associated with the Indian Ocean dipole and suggesting a deterministic relationship between these patterns. A pattern representing the tropospheric biennial oscillation also emerges along with its associated annual cycle combination modes. On multidecadal time scales, the dominant NLSA mode in the model data is predominantly active in the western tropical Pacific; this pattern is referred to here as the west Pacific multidecadal mode (WPMM). The interdecadal Pacific oscillation also emerges as a distinct NLSA mode, though with smaller explained variance than the WPMM. Analogous modes on interannual and decadal time scales are also identified in HadISST data for the industrial era, as well as in model data of comparable time span, though decadal modes are either absent or of degraded quality in these datasets.