Department Surface Waters - Research and Management

Periodic winds acting on a stratified waterbody can amplify normal modes of motion and enhance the basin-scale circulation via resonance. Here, we use idealized large-eddy simulations to investigate the flow features and quantify the horizontal transport in periodically wind-forced stratified basins. Motivated by observations in lakes, we focus on systems in which daily winds either resonate with the second vertical basin-scale internal mode, V2H1 (case 1), or the first vertical basin-scale internal mode, V1H1 (case 2). In particular, we analyze the case when strong nonlinearities affect the evolution of the V2H1 mode (case 3). To achieve these three resonance scenarios, we hold the basin morphology and the periodic forcing invariant, but change the background stratification. Our results show that a quasilinear V2H1 modal response has more active mass transport in the boundary regions than does the quasilinear V1H1 case. This difference arises from the lack of midlayer horizontal transport in the V1H1 mode, whereas the midlayer current in the V2H1 mode intensifies the transport along the slopes in both directions by splitting the flow into two branches, one running upslope and one running downslope. Nonlinear dynamics further amplify the along-slope transport in case 3, in which a second mode, an undular borelike wave, emerges from the periodic forcing. This study shows that the horizontal transport under wind-induced resonance is sensitive to the amplified mode of motion in the stratified basin and that nonlinear flow dynamics can considerably enhance mass transport in sloping regions.

Ecological responses to past climate change as determined from palaeorecords offer insights into responses that may accompany future climate change. In arid and semi‐arid lands, the interactions between regional vegetation and climate change are not yet well understood, partly due to a lack of suitable palaeovegetation proxies that can provide accurate and continuous tracers for past vegetation dynamics. To gain a better understanding of long‐term vegetation dynamics, this study employs a multiproxy approach applied to sand‐palaeosol sediments of northeastern China's Songnen grasslands. Phytolith analyses and data on the stable carbon isotope composition (δ¹³C) of organic matter are used to reconstruct palaeovegetation composition, namely, the changing abundance of C₃ and C₄ species, whereas a geochemical weathering index (Fed/Fet ratios) tracks past East Asian summer monsoon (EASM) intensity. The phytolith assemblages and indices and δ¹³C of the soil indicate that C₄ species' abundance has been increasing in the Songnen grasslands since the mid‐Holocene, although C₃ vegetation is still dominant. Statistically significant negative correlations between the δ¹³C data and Fet/Fed ratios suggest that continuous weakening of the EASM since the mid‐Holocene may be responsible for the ¹³C‐enrichment of the sediments in the Songnen grasslands. Field vegetation surveys, modern topsoil phytoliths and δ¹³C calibration data indicate that the expansion of C₄ species since the mid‐Holocene is mainly due to their ability to cope with aridity when growing season temperature is not undergoing a significant decrease. Future precipitation decreases in arid and semi‐arid lands should make C₄ species more competitive in the grasslands of northeastern China.