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A new data set of seasonal stable water isotopes (δD and δ18O) and temperature‐salinity profiles was applied to improve our understanding of water mass distributions and their impact on the environment of the Beibu Gulf (BG). Our study revealed that the coastal current (CC), West‐Guangdong coastal current (WGCC), and South China Sea water (SCSW) were the three dominant water masses in the BG, and their influence was exhibited in seasonal variations. The CC was the dominant contributor to the BG water during summer (43%) and fall (45%), while it changed to the intrusion of SCSW with higher salinity in winter (57%). The contribution of WGCC to the BG was relatively stable during the three seasons (24%–31%). In addition, the nutrients in the BG were greatly affected by different water mixing occurring in the gulf. The nutrients mainly originated from the CC in summer (52%–68%) and fall (32%–69%), while the dominant source shifted to the WGCC in winter (36%–69%). Moreover, the contribution of SCSW to the nutrients loading (15%–49%) in the BG was relatively high due to its high contribution (57%) to the BG water during winter. These indicated that the BG has a stable input of external nutrients from different water masses to sustain primary production in the BG. Our study uses dual water isotopes to quantify the seasonal intrusion of water masses and their impact on nutrients, providing a new method to study the impact of the distribution of water masses on nutrients in the gulf.

The causes of ENSO diversity, although being of great interest in recent research, do not have a consistent explanation. This study provides a possible mechanism focused on the background change of the tropical Pacific as a response to global warming. The second empirical orthogonal function mode of the sea surface temperature anomalies (SSTA) in the tropical Pacific, namely the cold tongue mode (CTM), represents the background change of the tropical Pacific under global warming. Using composite analysis with surface observations and subsurface ocean assimilation data sets, we find ENSO spatial structure diversity is closely associated with the CTM. A positive CTM tends to cool the SST in the eastern equatorial Pacific and warm the SST outside, as well as widen (narrow) zonal and meridional scales for El Niño (La Niña), and vice versa. Particularly in the positive CTM phase, the air‐sea action center of El Niño moves west, resembling the spatial pattern of CP‐El Niño. This westward shift of center is related to the weakened Bjerknes feedback (BF) intensity by the CTM. By suppressing the SSTA growth of El Niño in the eastern equatorial Pacific, the CTM contributes to more frequent occurrence of CP‐El Niño under global warming.

The East Asian summer monsoon affects precipitation and hence vegetation in the densely populated Northwest Pacific region, yet a long-standing controversy exists concerning the spatial and temporal dynamics of the Holocene Optimum (HO) in the East Asian Monsoon Region. Here we use a detailed 14,000-year record reconstructing vegetation variations from a strategically selected crater lake from Northeast China, as well as a compilation of previous paleoclimatic studies, to show that the HO began around 6,000 Cal a BP in Northeast China, significantly later than generally recognized. By comparing our paleoenvironmental data with Holocene vegetation records from other regions of East Asia, we identified a marked northward shift for the onset of the HO from ~10,260 Cal a BP in South China to ~6,000 Cal a BP in Northeast China. The gradual northward transgression of the vegetation change could be caused by both the temperature and precipitation changes in different regions. Finally, we fitted a regression model of the start of the HO period versus latitude, which allowed us to make predictions for the beginning of the HO based at different geographical locations. This study reveals a strong relationship between latitude and the initiation of the HO, and provides a window towards better understanding the forcing of vegetation changes in the East Asian monsoon region.

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