Effects Of Wind Events Research Articles

Surface Heat Fluxes over the Northern Arabian Gulf and the Northern Red Sea: Evaluation of ECMWF-ERA5 and NASA-MERRA2 Reanalyses

The air–sea heat fluxes in marginal seas and under extreme weather conditions constitute an essential source for energy transport and mixing dynamics. To reproduce these effects in numerical models, we need a better understanding of these fluxes. In response to this demand, we undertook a study to examine the surface heat fluxes in the Arabian Gulf (2013 to 2014) and Red Sea (2008 to 2010)—the two salty Indian Ocean marginal seas. We use high-quality buoy observations from offshore meteorological stations and data from two reanalysis products, the Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA2) from the National Aeronautics and Space Administration (NASA) and ERA5, the fifth generation of the European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalyses of global climate. Comparison of the reanalyses with the in situ-derived fluxes shows that both products underestimate the net heat fluxes in the Gulf and the Red Sea, with biases up to −45 W/m 2 in MERRA2. The reanalyses reproduce relatively well the seasonal variability in the two regions and the effects of wind events on air–sea fluxes. The results suggest that when forcing numerical models, ERA5 might provide a preferable dataset of surface heat fluxes for the Arabian Gulf while for the Red Sea the MERRA2 seems preferable.

Inflow of Warm Water to the Inner Hornsund Fjord, Svalbard: Exchange Mechanisms and Influence on Local Sea Ice Cover and Glacier Front Melting

AbstractObservational data covering currents and temperature through two contrasting autumn to spring periods (2010/2011 and 2013/2014) at the constricted entrance to the inner basin of the Hornsund fjord, Svalbard, are investigated. This fjord is presently undergoing significant changes, manifested by reduction of the seasonal sea ice cover and retreating tidewater glacier fronts. The data set presented here shows how local wind, tides, and longer period current fluctuations allow warm water of Atlantic origin to reach the innermost basin of the fjord. While the forcing factors of the exchange mechanisms are not seen to vary much between the two studied seasons, the effect of wind events on exchange is assumed to be modified by local stratification and sea ice concentration. The resulting heat transport depends on availability of heat in the central part of the fjord. The observations indicate that available heat in autumn and winter 2013/2014 effectively hindered formation of a stable sea ice cover. While the present data set does not enable direct quantification of the role of oceanic heat in tidewater glacier melting, we find that the net retreat of glacier fronts was 10 times larger in the warm (ocean) period 2013/2014 compared to the colder 2010/2011.

Accelerated succession following an intense wind storm in an oak-dominated forest

Catastrophic wind events often contribute to stand dynamics of central hardwood forests, yet opportunities to study them are limited. As luck would have it, we sampled 54 long-term monitoring plots (0.04ha) on the Shawnee National Forest in southern Illinois prior to an intense 2009 windstorm that swept across the central United States. The sampled plots, which were resampled immediately after the storm, were situated across a gradient of topographic positions. Overstory trees (⩾6.6cmdbh), saplings (⩾1m tall and <6.6cmdbh), and seedlings (<1m tall) were tallied and recorded by species within each plot and used to document post-storm changes in community composition and density. Prior to the storm, species abundance differed among the three strata, and within each strata tree communities were most closely related to slope position and elevation. This relationship did not change after the storm, however there was a shift in species composition of the overstory and seedling strata. Due in part to their overwhelming pre-windstorm abundance, oaks suffered greater losses compared to non-oak species in the overstory strata, accounting for 62% and 53% of all reductions in basal area and stem density, respectively. The lack of understory oak and release of more shade-tolerant species in the understory by the wind storm could potentially accelerate succession. Although wind has likely always played a role in the disturbance regime of central hardwood forests, the effect of wind events has changed with increased fire suppression and threatens the perpetuity of oak-dominated forests in this region.

Short‐term variability in physical forcing in temperate reservoirs: effects on phytoplankton dynamics and sedimentary fluxes

Summary1. The effects of wind events on phytoplankton dynamics were investigated in two temperate reservoirs.2. Meteorological forcing, change in physical and chemical structure of the water column and biological responses of phytoplankton communities were followed for 3 weeks in three seasons.3. Depending on the season, the phytoplankton response differed in response to nutrient and light conditions, and to the intensity of stratification and mixing.4. We demonstrated that, on a time scale of a few days, wind events can modify phytoplankton dynamics, in terms of size structure and exported biomass. An increase of mixing favoured the largest size class and disadvantaged the smallest size class, while an increase in stratification had the opposite effects. The short‐term change in size structure was reflected in the sedimentary fluxes but with a time lag.

Plankton dynamics controlled by hydrodynamic processes near a submarine canyon off NW corsican coast: A numerical modelling study

A three-dimensional (3D) non-linear high-resolution hydrodynamic model coupled to a coastal plankton ecosystem model is used to estimate the impact of hydrodynamic processes on the evolution of the spring phytoplankton bloom in the vicinity of a submarine canyon. Model results for the plankton distribution showed a clear 3D character around and in the canyon, with large horizontal and vertical gradients, induced by the hydrodynamic constraints. Phytoplankton concentrations were significantly larger all along the slope domain with maximum values obtained over the canyon. Upwelling of deep water rich in nitrate takes place both upstream (with respect to the current direction normal to the central axis of the canyon) and downstream of the canyon enhancing primary production. As phytoplankton-rich water enters into the western part of the canyon it is downwelled and trapped by the cyclonic circulation leading to accumulation of phytoplankton biomass there. The effect of wind events was to induce an upward nitrate flux into the upper layer through vertical turbulent diffusion, allowing the start of a short-live phytoplankton bloom. Maximum surface nitrate concentrations were found along the slope and particularly upstream and downstream of the canyon just after the wind stopped. Enhanced turbulent diffusion combined with upwelling motion in these areas resulted in larger upward nitrate transports, further enhancing primary production.

Effect of wind events on phytoplankton blooms in the Pearl River estuary during summer

The Pearl River estuary fuels phytoplankton productivity in the estuarine coastal waters in the oligotrophic South China Sea. Cruises were conducted to investigate the spatial distribution of primary productivity, and two size fractionations (<5 and >5μm) of chlorophyll in the Pearl River estuary and across the coastal plume south of Hong Kong during the season of high river discharge (July, 1999 and 2000). During 18–19 July 1999, phytoplankton biomass and productivity were low in the freshwater dominated estuary (<20mg chl am−2 and <100mgCm−2d−1, respectively), and were maximal at the edge of the estuarine coastal plume south of Hong Kong, reaching 70mg chl am−2 and 4.2gCm−2d−1, respectively. Chl a was dominated by the >5μm size fractionation (>50%). The Pearl River estuary displayed a typical spatial variability of phytoplankton biomass: low biomass and productivity due to rapid dilution and light limitation in turbid estuarine waters, a regional maximum of biomass and productivity at intermediate salinities in the coastal plume and low biomass due to nutrient limitation in oceanic waters. In summer when south-southwest monsoonal winds blow, part of the coastal plume moves eastwards. A regional maximum of phytoplankton usually develops in the coastal plume south of Hong Kong. However, a change in wind direction to the east or northeast can cause the following effects: pushing the coastal plume westward to the estuary, holding (retaining) the estuarine plume in the estuary (due to the Coriolis effect) and at the same time, mixing the water column vertically. Such a strong east or northeast wind event was observed during July 22–26 1999 and caused an increase in salinity at the surface in the estuary compared to before the wind event. This wind event increased the residence time of the estuarine plume inside the estuary, and as a result, a phytoplankton bloom (52–151mg chlam−2) developed during July 25–26 inside the estuary. This similar easterly wind effect was observed during July 2000. After a strong easterly wind event (speed 10ms−1) on July 11, the coastal plume was absent in waters south of Hong Kong on July 13, but it returned on July 19 after winds blew from the south and southwest direction. The results suggest that wind events not only changed the spatial movement of the coastal plume, but also the spatial distribution of phytoplankton biomass and productivity in the Pearl River estuary and adjacent coastal waters.

Zooplankton Community During the Transition from Spring Bloom to Oligotrophy in the Open NW Mediterranean and Effects of Wind Events. 2. Vertical Distributions and Migrations

Vertical distribution of zooplankton was investigated over a 3-week period at a fixed station in the open NW Mediterranean during the DYNAPROC cruise (May 1995). The observational period coincided with the transition from spring bloom to oligotrophy in this area. Vertical distribution and migration in the 0‐1000 m water column are described in detail for the main species of copepods (>500 µm). Ontogenetic migration is reported for Neocalanus gracilis, Euchaeta acuta and Pleuromamma abdominalis. This study also documented the downwards seasonal migration of Calanus helgolandicus. Short-term changes in the vertical distribution of the small-sized organisms (>50 µm) in the upper 100 m are discussed in relation to the two major wind events that occurred during the cruise. The naupliar stages of copepods and euphausiids appeared unable to counter the upwards vertical advection and the wind-induced mixing, while no marked changes were observed for Oithona spp. and Microsetella spp.

Zooplankton Community During the Transition from Spring Bloom to Oligotrophy in the Open NW Mediterranean and Effects of Wind Events. 1. Abundance and Specific Composition

Short-term changes in the zooplankton community were investigated at a time-series station in the open NW Mediterranean during the DYNAPROC cruise programme (May 1995). Based on the samples obtained with three devices (12 l bottle, 200 μm and 500 μm mesh nets), the abundance and specific composition are preseuled for mesozooplankton (copepods mainly), macroplankton and micronekton. Over the 3 week sampling period, overall density of the copepod community decreased and changes in the specific composition occurred. The decline was also observed in nauplii and was more marked in small-sized copepod species than in larger ones; one species, Centropages typicus, even disappeared. An increase in the proportion of carnivorous organisms appeared in both copepods (Euchaeta acuta, Heterorhabdus spp.) and euphausiids (Nematoscelis megalops, Stylocheiron longicorne). These changes are discussed in relation to environmental features and in the context of seasonal variability, the observational period coinciding with the transition from spring bloom to oligotrophy in this area. Effects of a major wind event that appeared to have induced long-lasting changes are also explored.

One-month study in the open NW Mediterranean Sea (DYNAPROC experiment, May 1995): overview of the hydrobiogeochemical structures and effects of wind events

Time-series observations were made over a one-month period (May 1995, DYNAPROC cruise) in the open northwestern Mediterranean at a fixed station where horizontal advection remained weak throughout the observational period. Changes in the dynamics of the pelagic ecosystem and influence of wind events were examined at time scales of a few hours to a few days. This paper gives a summary of the strategies and multidisciplinary observation methods of the cruise program. It describes in detail the physical and meteorological background and provides an overview of the chemical and biological features encountered over the course of the study. Effects of two different wind events were observed. The first wind event was associated with a low pressure system and an important wind stress curl. During this event, an upward advective shift of deeper layers, related to Ekman pumping, was superimposed upon the wind-induced mixing of the superficial layers. Despite the resulting enhanced availability of nitrate in the euphotic layer, phytoplankton biomass decreased drastically; the processes (vertical advection, primary production, grazing pressure, etc.) controlling this decrease are examined. In contrast, the second wind event, slightly lower and of longer duration, resulted in increased downward mixing and a higher phytoplankton biomass. However, it is shown that this transient increase would not have occurred without the previous influence of the first wind event. This study also allowed documentation of the transition from a mesotrophic to an oligotrophic system over a short time scale.