Impact of North America snow cover on tropical cyclogenesis over the western North Pacific

This study reveals a connection of summer–fall (JJASO) tropical cyclone (TC) genesis over the western North Pacific (WNP) to preceding boreal spring (MAM) North America snow cover (NASC). Sea surface temperature (SST) anomalies in the tropical central Pacific and subtropical eastern Pacific play a crucial role in relaying influence of the MAM NASC on the following JJASO WNP TC genesis frequency. The increased NASC leads to a decrease in upward sensible heat flux and the atmospheric cooling over the North America. The atmospheric cooling enhances the meridional thermal contrast and geopotential height gradient, which is favorable for the occurrence of lower-level westerly wind anomalies and positive precipitation anomalies over the tropical eastern Pacific. The lower-level northeasterly wind anomalies over the subtropical northeastern Pacific as a Gill-type atmospheric response to positive precipitation anomalies induce ocean surface cooling via the enhanced wind speed. A positive feedback between the northeasterly wind anomalies and negative SST anomalies leads to a westward extension of the easterly flows to the western Pacific. The easterly wind anomalies along with the negative specific humidity anomalies and negative lower-level vorticity anomalies, and enhanced vertical wind shear suppress the TC genesis over the WNP during JJASO.

Variability of the Red River Plume in the Gulf of Tonkin as Revealed by Numerical Modeling and Clustering Analysis

We study the daily to interannual variability of the Red River plume in the Gulf of Tonkin from numerical simulations at high resolution over 6 years (2011–2016). Compared with observational data, the model results show good performance. To identify the plume, passive tracers are used in order to (1) help distinguish the freshwater coming from different continental sources, including the Red River branches, and (2) avoid the low salinity effect due to precipitation. We first consider the buoyant plume formed by the Red River waters and three other nearby rivers along the Vietnamese coast. We show that the temporal evolution of the surface coverage of the plume is correlated with the runoff (within a lag), but that the runoff only cannot explain the variability of the river plume; other processes, such as winds and tides, are involved. Using a K-means unsupervised machine learning algorithm, the main patterns of the plume and their evolution in time are analyzed and linked to different environmental conditions. In winter, the plume is narrow and sticks along the coast most of the time due to the downcoast current and northeasterly wind. In early summer, the southwesterly monsoon wind makes the plume flow offshore. The plume reaches its highest coverage in September after the peak of runoff. Vertically, the plume thickness also shows seasonal variations. In winter, the plume is narrow and mixed over the whole water depth, while in summer, the plume can be detached both from the bottom and the coast. The plume can deepen offshore in summer, due to strong wind (in May, June) or specifically to a recurrent eddy occurring near 19°N (in August). This first analysis of the variability of the Red River plume can be used to provide a general picture of the transport of materials from the river to the ocean, for example in case of anthropogenic chemical substances leaked to the river. For this purpose, we provide maps of the receiving basins for the different river systems in the Gulf of Tonkin.

The internal seiche field in the changing South Aral Sea (2006–2013)

Internal standing waves (seiches) in the South Aral Sea are studied for the first time. The study, based on numerical simulations and field data, focuses on two different campaigns: the first in autumn 2006, when the stratification was weak and there was a mild prevailing northeasterly wind, and the second in autumn 2013, when the stratification was strong and there was a mild easterly wind. Between these two campaigns, the sea surface level decreased by 3.2 m. The periods of the fundamental modes were identified as 36 and 14 h, respectively. In both years, either second or third vertical modes were found. In general, the vertical modes in 2013 were higher because of the broad and strong pycnocline. For both years, it was found that the deep quasi-homogeneous mixed upper layer could sustain internal waves under mild wind conditions. The observed first and second vertical modes in 2006 are the first and second horizontal modes and the second and third vertical modes in 2013 are the second and third horizontal modes. The results suggest that, due to sea level variations, the neck connecting the Chernyshev Bay to the main body of the lake can become a critical location for the development of a nodal line for all principal oscillation modes. Rotation effects on waves were not analyzed in this study.

The internal seiche field in the changing South Aral Sea (2006–2013)

Internal standing waves (seiches) in the South Aral Sea are studied for the first time. The study, based on numerical simulations and field data and, focuses on two different campaigns: the first in autumn 2006, when the stratification was weak, and the second in autumn 2013, when it was strong. During this time, the sea surface level decreased 3.2 m but differences in stratification depend only on the time when the campaigns were carried out because the maximum density gradient decreases even though the sea continues to become saltier. In 2006 there was a mild prevailing northeasterly wind and in 2013 a mild easterly wind. The fundamental modes were identified as 36 h and 14 h, respectively. For both years we focused on the sub-inertial modes which were found to be either second or third vertical modes. In general, the vertical modes in 2013 are higher because of the strong stratification. For both years, it was found that the quasi homogeneous mixed deep upper layer could sustain internal waves under mild wind conditions. Regarding the horizontal structure, in 2006 they are first and second modes and in 2013 second and third horizontal modes. The results suggest that, due to sea level variations, the neck connecting the Chernyshev Bay to the main body of the lake can become critical for the development a nodal line in that neck.

Seasonal Exchanges of the Kuroshio and Shelf Waters and Their Impacts on the Shelf Currents of the East China Sea

Previous in situ observations and modeling studies have indicated that, through mass and momentum exchanges across the shelf edge, the Kuroshio can significantly influence the shelf currents of the East China Sea (ECS). Here, instead of localized observations, this study uses 25 yr of drifter data, supported by satellite and other data to identify seasonal cross-shelf exchanges along the entire shelf edge. The authors show that Kuroshio meanders onshore from fall to winter and offshore from spring to summer, with the largest amplitude northeast of Taiwan. The influence is limited to the shelf edge when the Kuroshio meanders offshore in spring and summer. By contrast, strong onshelf intrusions and cross-shelf exchanges occur when the Kuroshio meanders onshore in fall and winter. Drifters intrude onshelf northeast of Taiwan and spread as far north as 30°N against the strong northeasterly wind. The forcing on the shelf is identified as a northward downsloping of the sea level that is steepest north of Taiwan at 25°–28°N, but which is 3 times weaker farther north. The vorticity budget computed from a numerical model indicates that intrusion during fall and winter is primarily a result of balance between onshelf advection of ambient potential vorticity and vorticity production by the along-isobath pressure gradient acting on the changing mass of water column across the continental slope.

Radar observations of the seasonal migration of brown planthopper (Nilaparvata lugens Stål) in Southern China

The summer and autumn migrations of the brown planthopper (Nilaparvata lugens) were observed in Southern China with a millimetric scanning entomological radar and a searchlight trap supplemented with capture in field cages, field surveys, and dissections of females. Nilaparvata lugens took off at dusk and dawn in summer, but in autumn there was sometimes only a dusk take-off. The variation of the area density of the radar targets indicated that flight durations were about 9–10 h. In summer, planthopper-size targets generally flew below 1800 m above ground level (AGL), although some insects reached 2000 m AGL; in autumn, they flew lower, generally below 1100 m although some insects reached 1700 m AGL. Multiple layer concentrations were seen every night in both summer and autumn. The depths of these layers in autumn were less than in summer. Nilaparvata lugens flew in strong winds; wind shear may be the main factor causing them to accumulate and form dense layers at certain heights. Nilaparvata lugens emigrating in summer from the vicinity of the radar site in the Northeastern Guangxi Zhuang Autonomous Region, and carried by the prevailing southwesterly wind, would have travelled northeastwards and reached Northern Hunan Province. In autumn, with the prevailing northeasterly wind, emigrants would have reached overwintering areas (south of 21°N).

Qualitative Discussion of Influences of some Factors on the Occurrence of Wind-Driven Coastal Upwelling Associated with “Aoshio” on the Northeast Shore of Tokyo Bay Using the Developed Analytical Model

An analytical model based on some solutions in the context of a two-layered fluid was developed to estimate the occurrence of northeasterly wind-driven coastal upwelling associated with “Aoshio” on the northeast shore of Tokyo Bay, and its validity was verified by comparing with observation data [1]. In this study, influences of all of the factors incorporated into this analytical model (including densities and thicknesses of the upper and lower layers, the parameter expressing the influences of interfacial friction and bottom friction) on the model are analyzed. The analytical model is found to express the competition between the wind-shear effect and the stratification effect: when the former dominates over the latter, Aoshio will occur on the northeast shore of the bay. The parameter that can be used to characterize the stratification effect can be simply expressed in terms of the product of density contrast and the square of thickness of the upper layer. Using different values of this parameter corresponding to different months in the model can simply estimate in which months it is easy for Aoshio phenomenon to happen on the northeast shore of the bay, and the result is roughly consistent with an observation phenomenon that Aoshio was frequently observed on the northeast shore of the bay in September and May and relatively less observed in June and July during 1978-2010.

Cross Flows in the Taiwan Strait in Winter*

In winter, a branch of the China Coastal Current can turn in the Taiwan Strait to join the poleward-flowing Taiwan Coastal Current. The associated cross-strait flows have been inferred from hydrographic and satellite data, from observed abundances off northwestern Taiwan of cold-water copepod species Calanus sinicus and, in late March of 2012, also from debris found along the northwestern shore of Taiwan of a ship that broke two weeks earlier off the coast of China. The dynamics related to such cross flows have not been previously explained and are the focus of this study using analytical and numerical models. It is shown that the strait’s currents can be classified into three regimes depending on the strength of the winter monsoon: equatorward (poleward) for northeasterly winds stronger (weaker) than an upper (lower) bound and cross-strait flows for relaxing northeasterly winds between the two bounds. These regimes are related to the formation of the stationary Rossby wave over the Changyun Ridge off midwestern Taiwan. In the weak (strong) northeasterly wind regime, a weak (no) wave is produced. In the relaxing wind regime, cross-strait currents are triggered by an imbalance between the pressure gradient and wind and are amplified by the finite-amplitude meander downstream of the ridge where a strong cyclone develops.

Mechanism for initiation of the offshore phytoplankton bloom in the Taiwan Strait during winter: a physical–biological coupled modeling study

In situ observations showed phytoplankton blooms appear during winter in the Taiwan Strait (TWS), but the mechanism for bloom initiation was unclear. With the use of a coupled physical–biological numerical model, we find the winter bloom is triggered by the relaxation of the northeasterly monsoon. Thus, the aim of this study is to investigate the mechanism for bloom formation using the model. The model results show the weakening of the northeasterly wind generates a current that carries the fresh eutrophic Min-Zhe coastal water (MZCW) off the west coast of the TWS; then a stable stratification is formed in the upper ocean of the western strait, which significantly limits the turbulence. Via diagnostic analysis of the model output, we illustrate that the reduced turbulence allows the phytoplankton to accumulate within the upper layer of the western strait, which leads to an increase in chlorophyll. The analysis is further verified by the critical turbulence theory about the bloom. In addition to reduced turbulence, the lag between zooplankton and phytoplankton responses to the offshore extension of the MZCW is responsible for the formation of the bloom at the front. Therefore, we propose the observed offshore bloom in winter in the TWS is induced by the stable water stratification and the biological processes during the relaxation of the northeasterly wind.

Wind Regimes Associated with a Mountain Gap at the Northeastern Adriatic Coast

Winds through the Vratnik Pass, a mountain gap in the Dinaric Alps, Croatia, are polarized along the gap axis that extends in the northeast–southwest direction. Although stronger northeasterly wind at the Vratnik Pass is frequently related to the Adriatic bora wind, especially at the downstream town of Senj, there are many cases in which the wind at Senj is directionally decoupled from the wind at the Vratnik Pass. A cluster analysis reveals that this decoupling is sometimes related to lower wind speeds or a shallow southeasterly sirocco wind along the Adriatic, but in many cases the bora blows over a wider region, while only Senj has a different wind direction. Several mechanisms can be responsible for the latter phenomenon, including the formation of a lee wave rotor. A numerical model simulation corroborates the decoupling caused by a rotor for a single case. The prevalence of northeasterly winds at the Vratnik Pass during southeasterly sirocco episodes is another result that challenges the current understanding. It is shown that, at least in one of these episodes, this phenomenon is related to a secondary mesoscale low pressure center in the eastern lee of the Apennines that forms as a subsystem of a broader Genoa cyclone. Less frequent southwesterly winds through the gap are predominantly related to the thermal sea breeze and anabatic circulations that are sometimes superimposed on the geostrophic wind.