ALBERT

Description: The Victoria mode (VM) represents the second dominant mode (EOF2) of North Pacific variability, independent of the Pacific Decadal Oscillation (PDO), and is defined as the EOF2 of SST anomalies (SSTAs) in the North Pacific poleward of 20°N. The present study indicates that the VM is closely linked to the development of El Niño–Southern Oscillation (ENSO). The VM may effectively act as an ocean bridge (or conduit) through which the extratropical atmospheric variability in the North Pacific influences ENSO. The VM can trigger the onset of ENSO via the following two dominant processes: 1) surface air–sea coupling associated with the VM in the subtropical/tropical Pacific and 2) evolution of subsurface ocean temperature anomalies along the equator associated with the VM. These two processes may force sufficient surface warming to occur in the central–eastern equatorial Pacific from spring to summer, which in turn initiates an ENSO event. The VM influence on ENSO relies on a basin-scale air–sea interaction dynamic, as opposed to more local-scale dynamics typically associated with the seasonal footprinting mechanism (SFM) or Pacific meridional mode (PMM). The majority of VM events are followed by ENSO events. These ENSO events triggered by VM include El Niño Modoki (EM) as well as conventional El Niño. There is no evidence that the VM tends to be more conducive to the initialization of EM than conventional El Niño.

Description: This study demonstrates the close connection between the second dominant mode of spring sea surface temperature anomalies (SSTAs) in the North Pacific poleward of 20°N, referred to as the Victoria mode (VM), and the Pacific intertropical convergence zone (ITCZ) precipitation during the following summer. Our analysis shows that strong positive VM cases are followed by positive precipitation anomalies over the central–eastern Pacific ITCZ region, in association with negative precipitation anomalies over the ITCZ regions of the tropical western Pacific and eastern North Pacific. The hypothesized physical mechanism through which the spring VM induces the Pacific ITCZ summer precipitation is similar to but slightly different from the seasonal footprinting mechanism (SFM). During strong positive VM cases, SSTAs in the subtropics associated with the spring VM persist until summer and develop toward the equator, where low-level convergence and divergence caused by SSTA gradients give rise to enhanced precipitation over the central–eastern Pacific ITCZ region, and to reduced precipitation over the ITCZ regions of the tropical western Pacific and eastern North Pacific. The thermodynamic ocean–atmosphere coupling between the ITCZ and SSTAs associated with the VM may play a vital role in the initiation of El Niño–Southern Oscillation (ENSO) events. The VM influence on tropical Pacific summer precipitation can be passed on to the next year through its influence on ENSO. A VM-based linear model is established to predict the tropical Pacific summer precipitation, which yields skillful forecasts for summer precipitation across almost the entire tropical Pacific.

Description: Previous studies have indicated that boreal winter subtropical and extratropical sea surface pressure (SLP) anomalies over both the North and South Pacific are significantly related to the El Niño − Southern Oscillation (ENSO) state in the following boreal winter. Here we use observational data and model simulations to show that the ability of the boreal winter North Pacific SLP anomalies to initiate ENSO events a year later may strongly depend on the state of the simultaneous South Pacific SLP anomalies, and vice versa. When the boreal winter North Pacific SLP anomalies are of the opposite sign to the simultaneous South Pacific anomalies, the correlation of the North or South Pacific anomalies with the following ENSO state becomes much weaker, and the strength of the ENSO events also tends to be weaker. One possible reason for this is that when the boreal winter North and South Pacific SLP anomalies have the opposite sign, the westerly anomalies over the western–central equatorial Pacific during the following boreal summer are greatly reduced by the interference between the antecedent North and South Pacific SLP anomalies, thereby not favoring the development of ENSO events. Further analysis indicates that a combination of North and South Pacific precursor signals may serve to enhance the ENSO prediction skill.

Description: Abstract The North American dipole (NAD) represents a meridional dipole of sea level pressure (SLP) anomalies over the western tropical North Atlantic and northeastern North America. This study demonstrates that the NAD is intimately linked to the development of the Pacific meridional mode (PMM). In addition to the North Pacific Oscillation (NPO), the NAD provides another important remote forcing source to trigger the PMM. The NAD influences the PMM through both direct and indirect pathways. The direct effect is that the winter NAD influences the sea surface temperature (SST) and surface winds over the northeastern subtropical Pacific (NESP) through concurrent anticyclonic flow associated with the NAD, which tends to generate a weak initial warming over the NESP region during late winter and early spring. The indirect effect is that the NAD first induces SST cooling over the northern tropical Atlantic (NTA) during spring, and the NTA SST cooling then generates a low‐level anticyclonic flow anomaly over the NESP, which further strengthens the surface warming over the NESP, thereby causing the development of the PMM in the following months. The NAD can also exert an influence on the El Niño‐Southern Oscillation (ENSO) through its effects on the PMM. In particular, El Niño episodes led by the combined NAD‐PMM events tend to take the form of the Central Pacific El Niño, rather than the canonical eastern Pacific El Niño. We suggest that a better understanding of the NAD‐PMM‐ENSO dynamic link could be useful for the prediction of different types of El Niño event.

Description: The Middle Holocene period witnessed significant environmental and cultural transitions regarding the deceleration of sea‐level rise and the emergence of early agriculture, which profoundly moulded human–environment interactions in coastal regions. The Hemudu Culture (7000–5000 cal. a BP) is primarily distributed in eastern China coastal plains and marks a milestone in the origin of rice agriculture, and thus serves as an ideal case for examining these dramatic transitions. However, most existing studies have only focused on one particular aspect of the interactions amongst cultural evolution, subsistence strategies and sea‐level fluctuations in the Yaojiang Valley, and comprehensive understanding of cultural responses to Middle Holocene sea‐level fluctuations across the Ningshao Plain remains ambiguous. In this multi‐proxy study, pollen, phytolith and diatom remains from a high‐resolution profile of the Xiawangdu site (Hangzhou Bay, eastern China), along with a systematic collection of archaeological sites and radiocarbon dates for the Hemudu and Liangzhu cultures, were employed to assess human adaptation to the coastal environment. Coinciding with the cultural interruption between 6200 and 5600 cal. a BP, induced by Middle Holocene sea‐level transgression, Hemudu Culture diffused from the Yaojiang Valley northward to the Zhoushan Islands and southward to the Ningbo Plain, e.g. the Xiawangdu site. The regional ecological environment has experienced an evolutionary sequence from marine transgression (6200–5600 cal. a BP), to coastal lagoon (5600–5300 cal. a BP), through to freshwater wetland (5300–4300 cal. a BP). Settlements of pile dwellings and terraces were constructed to adapt to the hydrological transition from coastal lagoon to freshwater wetland around 5300 cal. a BP. In addition, evidence of both rice phytoliths and Poaceae pollen suggests that rice had already been domesticated by 5600 cal. a BP. Thus, the dispersal, settlement and subsistence of Hemudu Culture in the coastal lowlands of eastern China were largely dependent upon sea‐level fluctuations.