Local Circulation Features Research Articles

Phytoplankton Biomass and the Hydrodynamic Regime in NEOM, Red Sea

NEOM (short for Neo-Mustaqbal) is a $500 billion coastal city megaproject, currently under construction in the northwestern part of the Red Sea, off the coast of Tabuk province in Saudi Arabia, and its success will rely on the preservation of biodiverse marine ecosystems. Monitoring the variability of ecological indicators, such as phytoplankton, in relation to regional environmental conditions, is the foundation for such a goal. We provide a detailed description of the phytoplankton seasonal cycle of surface waters surrounding NEOM using satellite-derived chlorophyll-a (Chl-a) observations, based on a regionally-tuned product of the European Space Agency’s Ocean Colour Climate Change Initiative, at 1 km resolution, from 1997 to 2018. The analysis is also supported with in situ cruise datasets and outputs of a state-of-the-art high-resolution hydrodynamic model. The open waters of NEOM follow the oligotrophic character of the Northern Red Sea (NRS), with a peak during late winter and a minimum during late summer. Coral reef-bound regions, such as Sindala and Sharma, are characterised by higher Chl-a concentrations that peak during late summer. Most of the open waters around NEOM are influenced by the general cyclonic circulation of the NRS and local circulation features, while shallow reef-bound regions are more isolated. Our analysis provides the first description of the phytoplankton seasonality and the oceanographic conditions in NEOM, which may support the development of a regional marine conservation strategy.

Local Circulation Features in the Eastern Amazon: High-resolution Simulation

A high-resolution simulation of a real case is carried out in Eastern Amazon (Northeastern coast of State of Pará) with the Weather Research and Forecasting (WRF) model in order to analyze/detect local circulation features in the region. To this end, we simulated a case with low interference of synoptic weather systems and precipitation aiming to eliminate the effects of convective systems in local circulations. The performance of the model for the day studied was evaluated and the existence of circulations found in the simulation was confirmed through observational data. The model performance evaluation showed that the model was able to capture most of the patterns for the studied area. The simulation captured the presence of three main circulations: the Sea Breeze (SB), on the coast; the Land Breeze (LB), also on the coast; and the Bay Breeze (BB), which forms on both banks of Marajó Bay. In addition to these three major circulations, the high-resolution simulations also revealed the presence of a river breeze (RB) from Guamá River and a fan-shaped circulation on the entrance of Guajará Bay. The observational data confirmed the presence of Guamá River breeze from the Southeast, which is more active from July to September.

Systematic Errors in South Asian Monsoon Precipitation: Process-Based Diagnostics and Sensitivity to Entrainment in NCAR Models

AbstractIn simulations of the boreal summer Asian monsoon, generations of climate models show a persistent climatological wet bias over the tropical western Indian Ocean and a dry bias over South Asia. Here, focusing on the monsoon developing stages (May–June), process-based diagnostics are first applied to a suite of NCAR models and reanalysis products. Two primary factors are identified for the initiation and maintenance of the wet bias over the northwestern Indian Ocean (NWIO; 5°–15°N, 52°–67°E): (i) excessive tropospheric moisture and (ii) restrained horizontal advection of the 1000–800-hPa levels cold–dry air couplet that originates offshore of Somalia. Second, guided by the diagnostics, we hypothesized that insufficient dilution of convective updrafts is one possible candidate for model bias and performed a series of enhanced entrainment sensitivity experiments with NCAR CAM4. Over the NWIO, the results suggest that globally increasing the maximum entrainment rateεmaxleads to a drier free troposphere, arrests the vertical extension of clouds, and weakens moisture–convection and cloud–radiation feedbacks; each factor contributes to a reduced wet bias. Moreover, a higherεmaxleads to a reduced dry bias over South Asia through changes in the local circulation features. In CAM4, improved precipitation climatology due to increasedεmaxsuggests that insufficient dilution is one factor, but not the only one, that contributes to systematic errors. Rather, realistic representation of boundary layer processes in climate models arising out of local ocean–atmosphere interaction processes off Somalia’s coast deserves attention in reducing the NWIO wet bias.

The large-scale circulation during air quality hazards in Bergen, Norway

In this study, we assessed the large-scale circulation anomalies leading to wintertime air pollution episodes in Bergen, Norway. Bergen is an example of a city, where a strong interplay between large-scale and local circulation features is relevant. Certain large-scale circulation regimes that are different from the usually assumed large-scale stagnation, lead to local air pollution episodes. We assessed these circulation regimes and their predictability. For this, we modified and applied a previously developed atmospheric circulation proxy for the identification of air pollution episodes. Use of this proxy on data from a high-resolution atmospheric general circulation model showed a good reproduction of the total number of potentially polluted days per month and their inter-monthly variability. We also found a link between the persistence of the flow above the Bergen valley and the occurrence and severity of the local air pollution episodes. Analysis of the large-scale circulation over the North Atlantic-European region, with respect to air pollution in Bergen, revealed that the persistence in the meteorological conditions connected to the air pollution episodes is not necessarily caused by large-scale anomalies of the atmospheric circulation over the Norwegian west coast. It is rather connected to anomalies further upstream as far away as Greenland.

Analysis of the internal boundary layer formation on tropical coastal regions using SODAR data in Rio de Janeiro (Brazil)

This paper investigates local circulation features in the industrialised coastal region of Santa Cruz, Rio de Janeiro, Brazil. A better understanding of local circulation affects directly the choice of suitable dispersion models and/or development of new ones to plan mitigation strategies to reduce air pollution in the area. The topographic characteristics and the variety of atmospheric phenomena acting over the area suggest the formation of internal boundary layers (IBLs) during cold front and bay breeze flows with direction upstream of the major industries. Vertical profiles of main meteorological variables are evaluated together with surface weather stations and satellite data to derive a detailed physics-based description of the various stages of bay breeze in terms of the main forces, duration, and atmospheric stabilities. It is found that the development phase of the sea breeze coincides with the largest bay/land differences and, consequently, it is the most pronounced condition to observe the IBL formation.

Analysis of the internal boundary layer formation on tropical coastal regions using SODAR data in Rio de Janeiro (Brazil)

This paper investigates local circulation features in the industrialised coastal region of Santa Cruz, Rio de Janeiro, Brazil. A better understanding of local circulation affects directly the choice of suitable dispersion models and/or development of new ones to plan mitigation strategies to reduce air pollution in the area. The topographic characteristics and the variety of atmospheric phenomena acting over the area suggest the formation of internal boundary layers (IBLs) during cold front and bay breeze flows with direction upstream of the major industries. Vertical profiles of main meteorological variables are evaluated together with surface weather stations and satellite data to derive a detailed physics-based description of the various stages of bay breeze in terms of the main forces, duration, and atmospheric stabilities. It is found that the development phase of the sea breeze coincides with the largest bay/land differences and, consequently, it is the most pronounced condition to observe the IBL formation.

Combination Mode Dynamics of the Anomalous Northwest Pacific Anticyclone*

Abstract Nonlinear interactions between ENSO and the western Pacific warm pool annual cycle generate an atmospheric combination mode (C-mode) of wind variability. The authors demonstrate that C-mode dynamics are responsible for the development of an anomalous low-level northwest Pacific anticyclone (NWP-AC) during El Niño events. The NWP-AC is embedded in a large-scale meridionally antisymmetric Indo-Pacific atmospheric circulation response and has been shown to exhibit large impacts on precipitation in Asia. In contrast to previous studies, the authors find the role of air–sea coupling in the Indian Ocean and northwestern Pacific only of secondary importance for the NWP-AC genesis. Moreover, the NWP-AC is clearly marked in the frequency domain with near-annual combination tones, which have been overlooked in previous Indo-Pacific climate studies. Furthermore, the authors hypothesize a positive feedback loop involving the anomalous low-level NWP-AC through El Niño and C-mode interactions: the development of the NWP-AC as a result of the C-mode acts to rapidly terminate El Niño events. The subsequent phase shift from retreating El Niño conditions toward a developing La Niña phase terminates the low-level cyclonic circulation response in the central Pacific and thus indirectly enhances the NWP-AC and allows it to persist until boreal summer. Anomalous local circulation features in the Indo-Pacific (e.g., the NWP-AC) can be considered a superposition of the quasi-symmetric linear ENSO response and the meridionally antisymmetric annual cycle modulated ENSO response (C-mode). The authors emphasize that it is not adequate to assess ENSO impacts by considering only interannual time scales. C-mode dynamics are an essential (extended) part of ENSO and result in a wide range of deterministic high-frequency variability.

Improving the seasonal forecast for summertime South China rainfall using statistical downscaling

AbstractThe performance of various seasonal forecast systems in predicting the station‐scale summer rainfall in South China (SC) was assessed and was compared with that based on a statistical downscaling scheme. Hindcast experiments from 11 dynamical models covering the period of 1983–2003 were taken from the Asia‐Pacific Economic Cooperation Climate Center multimodel ensemble. Based on observations, singular value decomposition analysis (SVDA) showed that SC precipitation is strongly related to the broad‐scale sea level pressure (SLP) variation over Southeast Asia, western north Pacific, and part of the Indian Ocean. Analogous covariability was also found between model hindcasts and the observed station precipitation. Based on these results from SVDA, a statistical downscaling scheme for predicting SC station rainfall with model SLP as predictor was constructed. In general, the statistical scheme is superior to the original model prediction in two geographical regions, namely, western SC (near Guangxi) and eastern coastal SC (eastern Guangdong to part of Fujian). Further analysis indicated that dynamical models are able to reproduce the large‐scale circulation patterns associated with the recurrent modes of SC rainfall, but not the local circulation features. This probably leads to erroneous rainfall predictions in some locations. On the other hand, the statistical scheme was able to map the broad‐scale SLP patterns onto the station‐scale rainfall anomalies, thereby correcting some of the model biases. Overall, our results demonstrate how SC summer rainfall predictions can be improved by tapping the source of predictability related to large‐scale circulation signals from dynamical models.

Oceanography of Mississippi Delta Sedimentary Environments

Oceanography of Mississippi Delta Sedimentary Environments