Shelf Transport Pathways Adjacent to the East Australian Current Reveal Sources of Productivity for Coastal Reefs

The region where the East Australian Current (EAC) separates from the coast is dynamic and the shelf circulation is impacted by the interplay of the western boundary current and its eddy field with the coastal ocean. This interaction can drive upwelling, retention or export. Hence understanding the connection between offshore waters and the inner shelf is needed as it influences the productivity potential of valuable coastal rocky reefs. Near urban centres, artificial reefs enhance fishing opportunities in coastal waters, however these reefs are located without consideration of the productivity potential of adjacent waters. Here we identify three dominant modes of mesoscale circulation in the EAC separation region (~31.5−34.5°S); the ‘EAC mode’ which dominates the flow in the poleward direction, and two eddy modes, the ‘EAC eddy mode’ and the ‘Eddy dipole mode’, which are determined by the configuration of a cyclonic and anticyclonic eddy and the relationship with the separated EAC jet. We use a Lagrangian approach to reveal the transport pathways across the shelf to understand the impact of the mesoscale circulation modes and to explore the productivity potential of the coastal waters. We investigate the origin (position and depth) of the water that arrives at the inner-mid shelf over a 21-day period (the plankton productivity timescale). We show that the proportion of water that is upwelled from below the euphotic zone varies spatially, and with each mesoscale circulation mode. Additionally, shelf transport timescales and pathways are also impacted by the mesoscale circulation. The highest proportion of upwelling (70%) occurs upstream of 32.5°S, associated with the EAC jet separation, with vertical displacements of 70–120 m. From 33 to 33.5°S, water comes from offshore above the euphotic layer, and shelf transport timescales are longest. The region of highest retention over the inner shelf is immediately downstream of the EAC separation region. The position of the EAC jet and the location of the cyclonic eddy determines the variability in shelf-ocean interactions and the productivity of shelf waters. These results are useful for understanding productivity of temperate rocky reefs in general and specifically for fisheries enhancements along an increasingly urbanised coast.

JOANNE: Joint dropsonde Observations of the Atmosphere in tropical North atlaNtic meso-scale Environments

As part of the EUREC4A field campaign which took place over the tropical North Atlantic during January–February 2020, 1215 dropsondes from the HALO and WP-3D aircraft were deployed through 26 flights to characterize the thermodynamic and dynamic environment of clouds in the trade-wind regions. We present JOANNE (Joint dropsonde Observations of the Atmosphere in tropical North atlaNtic meso-scale Environments), the dataset that contains these dropsonde measurements and the products derived from them. Along with the raw measurement profiles and basic post-processing of pressure, temperature, relative humidity and horizontal winds, the dataset also includes a homogenized and gridded dataset with 10 m vertical spacing. The gridded data are used as a basis for deriving diagnostics of the area-averaged mesoscale circulation properties such as divergence, vorticity, vertical velocity and gradient terms, making use of sondes dropped at regular intervals along a circular flight path. A total of 85 such circles, ∼ 222 km in diameter, were flown during EUREC4A. We describe the sampling strategy for dropsonde measurements during EUREC4A, the quality control for the data, the methods of estimation of additional products from the measurements and the different post-processed levels of the dataset. The dataset is publicly available (https://doi.org/10.25326/246, George et al., 2021b) as is the software used to create it (https://doi.org/10.5281/zenodo.4746312, George, 2021).

Large-scale versus regional drivers of climate change in the Lake Victoria basin

<p>Severe thunderstorms pose a constant threat to more than 30 million people living along the shores of Lake Victoria (East Africa). Thousands of fishermen lose their lives on the lake every year, and capsizing accidents with passenger ferries and transport boats are frequently reported. Moreover, hazardous thunderstorms affect people living inland, continuously facing flood risks.</p><p>In this data scarce region, atmospheric models are particularly useful tools to better understand the region’s complex climate, especially when simulated at convection-permitting resolution. For example, such models already demonstrated the importance of the lake in determining the diurnal precipitation cycle, and highlighted the role that mountain blocking of easterly trade winds plays in explaining the regional rainfall pattern.</p><p>Such models also allow us to generate high-resolution future projections for this region. In this study, a surrogate global warming approach has been applied. In a first simulation, the ensemble mean of the recent global climate projections from the CMIP6 data set was used to perturb the lateral boundary conditions from the ERA 5 reanalysis. In this ensemble mean, variations in (large scale) atmospheric dynamics are negligible and the climate change signal is mainly determined by the increased water vapour related to the warming and the response of the mesoscale circulation to differential lake/land heating. Specifically, while increased water vapour tends to increase total precipitation, weakened mesoscale circulation makes the over-lake rainfall to reduce instead. In a second simulation, a CMIP6 member with larger large-scale dynamical changes in the region was chosen to perturb the ERA5 lateral boundary data, thereby changing both the thermodynamics and the dynamical fields. Combining both simulations enables us to study the effects of changed large-scale dynamics and its interaction with the mountain peaks on mean and extreme precipitation in the region, thereby gaining insight in expected future changes of the region’s hazardous thunderstorms.</p>

Observational Evidence that Radiative Heating Modifies the Life Cycle of Tropical Anvil Clouds

A variety of satellite and ground-based observations are used to study how diurnal variations of cloud radiative heating affect the life cycle of anvil clouds over the tropical western Pacific Ocean. High clouds thicker than 2 km experience longwave heating at cloud base, longwave cooling at cloud top, and shortwave heating at cloud top. The shortwave and longwave effects have similar magnitudes during midday, but only the longwave effect is present at night, so high clouds experience a substantial diurnal cycle of radiative heating. Furthermore, anvil clouds are more persistent or laterally expansive during daytime. This cannot be explained by variations of convective intensity or geographic patterns of convection, suggesting that shortwave heating causes anvil clouds to persist longer or spread over a larger area. It is then investigated if shortwave heating modifies anvil development by altering turbulence in the cloud. According to one theory, radiative heating drives turbulent overturning within anvil clouds that can be sufficiently vigorous to cause ice nucleation in the updrafts, thereby extending the cloud lifetime. High-frequency air motion and ice-crystal number concentration are shown to be inversely related near cloud top, however. This suggests that turbulence depletes or disperses ice crystals at a faster rate than it nucleates them, so another mechanism must cause the diurnal variation of anvil clouds. It is hypothesized that radiative heating affects anvil development primarily by inducing a mesoscale circulation that offsets gravitational settling of cloud particles.

High-Resolution COSMO-CLM Modeling and an Assessment of Mesoscale Features Caused by Coastal Parameters at Near-Shore Arctic Zones (Kara Sea)

Coastal Arctic regions are characterized by severe mesoscale weather events that include extreme wind speeds, and the rugged shore conditions, islands, and mountain ranges contribute to mesoscale event formation. High-resolution atmospheric modeling is a suitable tool to reproduce and estimate some of these events, and so the regional non-hydrostatic climate atmospheric model COSMO-CLM (Consortium for Small-scale Modeling developed within the framework of the international science group CLM-Community) was used to reproduce mesoscale circulation in the Arctic coast zone under various surface conditions. Mid-term experiments were run over the Arctic domain, especially over the Kara Sea region, using the downscaling approach, with ≈12 km and ≈3 km horizontal grid sizes. The best model configuration was determined using standard verification methods; however, the model run verification process raised questions over its quality and aptness based on the high level of small-scale coastline diversity and associated relief properties. Modeling case studies for high wind speeds were used to study hydrodynamic mesoscale circulation reproduction, and we found that although the model could not describe the associated wind dynamic features at all scales using ≈3 km resolution, it could simulate different scales of island wind shadow effects, tip jets, downslope winds, vortex chains, and so on, quite realistically. This initial success indicated that further research could reveal more about the detailed properties of mesoscale circulations and extreme winds by applying finer resolution modeling.

Effects of the Sea Breeze Circulation on Soil Temperature Over Kuwait Using in Situ Observations and the ECMWF Model

Background: The mesoscale circulation over Kuwait is an important influence on changes in surface temperatures and soil temperatures. Introduction: This paper presents two common summertime atmospheric features over Kuwait linking wind circulation to soil temperatures. Methods: In this study, we use the European Centre for Medium-range Weather Forecasts ECMWF reanalysis ERA-Interim dataset to investigate effects of the synoptic scale and mesoscale circulations. Results: The results show that a large-scale pressure gradient in summer typically leads to northerly winds over Kuwait, while a weak synoptic-scale pressure gradient leads to light easterly humid winds from the Persian Gulf, consistent with a mesoscale circulation. Conclusions: The results demonstrate the significance of wind circulations in driving the Soil Temperature (SOILT). Using the Era-Interim/Land reanalysis dataset for August 2015 over Kuwait, the average SOILT on days of sea breeze is higher than the average SOILT on days dominated by a synoptic-scale pressure gradient.