scholarly journals Biophysical modelling of coastal upwelling variability and circulation along the Tanzanian and Kenyan coasts

2021 ◽  
pp. 43-61
Author(s):  
Issufo Halo ◽  
Philip Sagero ◽  
Majuto Manyilizu ◽  
Shigalla B. Mahongo
Keyword(s):  
Chlorophyll A ◽  
Ocean Circulation ◽  
Coastal Upwelling ◽  
Monsoon Season ◽  
Western Indian Ocean ◽  
Water Volume ◽  
Coastal Current ◽  
Spatial And Temporal Variability ◽  
Northeast Monsoon ◽  
Monsoon Period

Ocean circulation, upwelling phenomena and chlorophyll-a concentrations were investigated within the framework of numerical model simulations with 1/12° nested horizontal grid-size, in the tropical western Indian Ocean, along the coasts of Tanzania and Kenya. Ekman driven upwelling exhibited high levels of spatial and temporal variability in the region, characterized by a more vigorous occurrence/intensification during the Northeast than the Southwest Monsoon season. A similar trend was observed for chlorophyll-a distribution, but with an additional strong contribution during the inter-monsoon period from March to April. Trend analysis of a SST-derived coastal upwelling index (CUI) computed over the Pemba Channel and offshore of the East African Coastal Current (EACC), for 24 years (1990 - 2013), revealed a general linear relation of the form CUI(yr) = 2.4x10-7yr – 285, with a steady small annual increase of the upwelling phenomena by 0.0024/year ≃ 4% during the whole period of the simulation, which could be attributed to documented increasing trends of wind intensity and water volume transport in the region. The CUI exhibited the two most dominant peaks of variabilities on the range of annual and semi-annual timescales. The wind-stress southward component and the easting/westing veering of the northward EACC at 6°S revealed that these parameters were moderate and significantly correlated with the CUI (r = - 0.53 and 0.52, p<0.05) respectively, further suggesting its intensification during the Northeast Monsoon season.

scholarly journals Relationship between Lightning Activity over Peninsular India and Sea Surface Temperature

2010 ◽  
Vol 49 (4) ◽  
pp. 828-835 ◽  
Author(s):  
M. I. R. Tinmaker ◽  
Kaushar Ali ◽  
G. Beig
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Monsoon Season ◽  
Lightning Activity ◽  
Sea Surface ◽  
Lightning Flash ◽  
Northeast Monsoon ◽  
Monsoon Period ◽  
Peninsular India ◽  
Seasonal Time Scale

Abstract This paper presents a study of spatiotemporal variation of lightning activity over Peninsular India (8°–22°N, 72°–88°E) by using monthly satellite-based lightning flash grid (1° × 1°) data for a period of 10 yr (1998–2007). The data are examined in terms of spatial, annual, and seasonal distribution of the lightning activity. It is found that lightning activity is higher over south Peninsular India and eastern India. On a seasonal time scale, the lightning activity shows two maxima—first in the month of May and then in the month of September. The lightning activity in the monsoon period is noticed to be considerable because of the occurrence of the low-level jet and increase in the monsoon break period. During the postmonsoon, the activity is mainly due to the presence of the convective nature of the disturbed weather during the northeast monsoon season over most parts of the east coast of south Peninsular India. The relationship between lightning activity over Peninsular India and sea surface temperature in the bordering seas (Arabian Sea and Bay of Bengal) is also examined. The results disclose a significant link between them.

scholarly journals Coastal upwelling and seasonal variation in phytoplankton biomass in the Pemba Channel

2021 ◽  
pp. 19-32
Author(s):  
Margareth S. Kyewalyanga ◽  
Nyamisi Peter ◽  
Masumbuko Semba ◽  
Shigalla B. Mahongo
Keyword(s):  
Phytoplankton Biomass ◽  
Coastal Upwelling ◽  
Monsoon Season ◽  
Northeast Monsoon ◽  
Chl A ◽  
Physico Chemical ◽  
Northern Tanzania ◽  
High Nutrient ◽  
Chemical Factors ◽  
Physical And Chemical

This study was conducted in the Pemba Channel off Tanga Region in northern Tanzania to investigate physical and chemical factors that drive changes in phytoplankton biomass. Three transects off Mwaboza, Vyeru and Sahare were selected. For each transect, ten stations were sampled. Phytoplankton biomass was determined as chlorophyll-a (Chl-a) concentration. Similarly, physico-chemical variables (temperature, salinity, dissolved oxygen, pH and nutrients) were determined. It was observed that the Chl-a concentration was significantly higher during the northeast monsoon (median 1.44 mg m-3) as compared to the southeast monsoon (median 1.19 mg m-3; W = 2216, p = 0.029). The higher productivity during the northeast monsoon is attributed to the presence of high-nutrient water caused by coastal upwelling. It is concluded that indication of upwelling, observed through relatively low temperatures during the northeast monsoon season, could be responsible for bringing nutrient-rich waters to the surface, which in turn stimulated the increase in Chl-a concentration.

scholarly journals Seasonal coastal upwelling in the Bali Strait: a model study

2021 ◽  
Vol 944 (1) ◽  
pp. 012055
Author(s):  
A Suprianto ◽  
A S Atmadipoera ◽  
J Lumban-Gaol
Keyword(s):  
Chlorophyll A ◽  
Coastal Upwelling ◽  
Fish Distribution ◽  
Sea Surface ◽  
Monsoon Period ◽  
Easterly Wind ◽  
Chl A ◽  
Model Study ◽  
Velocity Surface ◽  
Upwelling Event

Abstract Bali Strait is part of fisheries management zone (WPP 573), where abundant fishery potential, of lemuru fish commodity. Here, physical oceanographic setting such as upwelling event plays an important role on maintaining high primary productivity and lemuru fish distribution. This study aims to describe physical process and dynamics of seasonal coastal upwelling using time-series datasets (2008 and 2014) of temperature, salinity, current velocity, surface chlorophyll-a (chl-a) from INDESO model and satellite imagery. The results showed that upwelling in the Bali Strait only during the southeast monsoon period when the south-easterly wind force surface Ekman drift of about 5.5 × 10−3 Sv flowing south-eastward (toward offshore). Upwelling event is characterized by minimum parameter of sea surface temperature (24.93 °C), and sea level anomaly (0.75 m), but maximum of surface chlorophyll-a (1.33 mg/m3). Furthermore, isotherm of 26 °C and Isohaline 33.7 psu are outcropped at sea surface in the center of upwelling zone. In contrast, during the nortwest monsoon period these isolines remain at deeper layer of about 80-90 m depth. Mean temperature-based upwelling index during peak of upwelling in August (1.19±0.19 °C). Upwelling impact on high abundance of lemuru fish (Sardinella sp.) production two month later after peak of chl-a.

scholarly journals Primary productivity and its variability in the equatorial South China Sea during the northeast monsoon

2013 ◽  
Vol 13 (8) ◽  
pp. 21573-21608 ◽  
Author(s):  
S. H. Ooi ◽  
A. A. Samah ◽  
P. Braesicke
Keyword(s):  
South China Sea ◽  
Chlorophyll A ◽  
South China ◽  
Primary Productivity ◽  
Monsoon Season ◽  
Winter Monsoon ◽  
Phytoplankton Growth ◽  
Northeast Monsoon ◽  
Cold Surge ◽  
China Sea

Abstract. Near coastal areas of the equatorial South China Sea (SCS) are one of the world's regions with highest primary productivity (phytoplankton growth). Concentrations of phytoplankton in the SCS depend significantly on atmospheric forcings and the oceanic state, in particular during the northeast (winter) monsoon season from November to March. Aided by new ocean-observing satellite data, we present a climatological overview of recent surface atmospheric and oceanic features in the equatorial SCS during the northeast monsoon to identify the dominant air-sea processes influencing and modulating the primary productivity of the region. Measured chlorophyll a concentrations are used as a proxy for phytoplankton amounts and the spatial and temporal variations are characterized according to meteorological conditions. Converging northeasterly surface winds support high chlorophyll a concentrations along East Malaysia's coastline in conjunction with a continual nutrient supply from the bottom of the continental shelf by vertical mixing. The mixing can be enhanced due to increased turbulence by wind-generated high waves when they approach shallow water from the deep basin during strong cold surges and monsoon disturbances. Intraseasonal variability during the winter monsoon is characterized by a coastal increase of chlorophyll a starting in November and peaking in January. A general decrease is observed in March. Interannual variability of chlorophyll a concentrations is influenced by ENSO (due to the known modulation of cold surge occurrences), with decreases during El Niño and increases during La Niña in early winter along the shore of East Malaysia. As an example, we discuss an enhanced phytoplankton growth event that occurred due to a typical cold surge-induced Borneo vortex event in January 2010.

Spatial and temporal variability and size fractionation of chlorophyll a in the tropical and subtropical Pacific Ocean

2012 ◽  
Vol 31 (3) ◽  
pp. 120-131 ◽  
Author(s):  
Dongsheng Zhang ◽  
Chunsheng Wang ◽  
Zhensheng Liu ◽  
Xuewei Xu ◽  
Xiaogu Wang ◽  
...  
Keyword(s):  
Pacific Ocean ◽  
Chlorophyll A ◽  
Temporal Variability ◽  
Spatial And Temporal Variability ◽  
Size Fractionation

scholarly journals Variations in O<sub>3</sub>, CO, and CH<sub>4</sub> over the Bay of Bengal during the summer monsoon season: Ship-borne measurements and model simulations

2016 ◽  
Author(s):  
Imran A. Girach ◽  
Narendra Ojha ◽  
Prabha R. Nair ◽  
Andrea Pozzer ◽  
Yogesh K. Tiwari ◽  
...  
Keyword(s):  
Summer Monsoon ◽  
Bay Of Bengal ◽  
Monsoon Season ◽  
Surface Ozone ◽  
Ozone Production ◽  
Spatial And Temporal Variability ◽  
Summer Monsoon Season ◽  
Rainfall Events ◽  
Model Simulations ◽  
Mixing Ratios

Abstract. We present ship-borne measurements of surface ozone, carbon monoxide and methane over the Bay of Bengal (BoB), the first time such measurements have been taken during the summer monsoon season, as a part of the Continental Tropical Convergence Zone (CTCZ) experiment during 2009. O3, CO, and CH4 mixing ratios exhibited significant spatial and temporal variability in the ranges of 8–54 nmol mol−1, 50–200 nmol mol−1, and 1.57–2.15 µmol mol−1, with means of 29.7 ± 6.8 nmol mol−1, 96 ± 25 nmol mol−1, and 1.83 ± 0.14 µmol mol−1, respectively. The average mixing ratios of trace gases over northern BoB (O3: 30 ± 7 nmol mol−1, CO: 95 ± 25 nmol mol−1, CH4: 1.86 ± 0.12 µmol mol−1), in airmasses from northern or central India, did not differ much from those over central BoB (O3: 27 ± 5 nmol mol−1, CO: 101 ± 27 nmol mol−1, CH4: 1.72 ± 0.14 µmol mol−1), in airmasses from southern India. Spatial variability is observed to be most significant for CH4. The ship-based observations, in conjunction with backward air trajectories and ground-based measurements over the Indian region, are analyzed to estimate a net ozone production of 1.5–4 nmol mol−1 day−1 in the outflow. Ozone mixing ratios over the BoB showed large reductions (by ~ 20 nmol mol−1) during four rainfall events. Temporal changes in the meteorological parameters, in conjunction with ozone vertical profiles, indicate that these low ozone events are associated with downdrafts of free-tropospheric ozone-poor airmasses. While the observed variations in O3 and CO are successfully reproduced using the Weather Research and Forecasting model with Chemistry (WRF-Chem), this model overestimates mean concentrations by about 20 %, generally overestimating O3 mixing ratios during the rainfall events. Analysis of the chemical tendencies from model simulations for a low-O3 event on August 10, 2009, captured successfully by the model, shows the key role of horizontal advection in rapidly transporting ozone-rich airmasses across the BoB. Our study fills a gap in the availability of trace gas measurements over the BoB, and when combined with data from previous campaigns, reveals large seasonal amplitude (~ 39 and ~ 207 nmol mol−1 for O3 and CO, respectively) over the northern BoB.

scholarly journals Tidally induced lateral dispersion of the Storfjorden overflow plume

Ocean Science ◽  
2013 ◽  
Vol 9 (5) ◽  
pp. 885-899 ◽  
Author(s):  
F. Wobus ◽  
G. I. Shapiro ◽  
J. M. Huthnance ◽  
M. A. M. Maqueda ◽  
Y. Aksenov
Keyword(s):  
Ocean Circulation ◽  
Circulation Model ◽  
Bottom Layer ◽  
Horizontal Resolution ◽  
Open Boundary ◽  
Coastal Current ◽  
Fram Strait ◽  
Open Boundary Conditions ◽  
Shear Dispersion ◽  
Set Up

Abstract. We investigate the flow of brine-enriched shelf water from Storfjorden (Svalbard) into Fram Strait and onto the western Svalbard Shelf using a regional set-up of NEMO-SHELF, a 3-D numerical ocean circulation model. The model is set up with realistic bathymetry, atmospheric forcing, open boundary conditions and tides. The model has 3 km horizontal resolution and 50 vertical levels in the sh-coordinate system which is specially designed to resolve bottom boundary layer processes. In a series of modelling experiments we focus on the influence of tides on the propagation of the dense water plume by comparing results from tidal and non-tidal model runs. Comparisons of non-tidal to tidal simulations reveal a hotspot of tidally induced horizontal diffusion leading to the lateral dispersion of the plume at the southernmost headland of Spitsbergen which is in close proximity to the plume path. As a result the lighter fractions in the diluted upper layer of the plume are drawn into the shallow coastal current that carries Storfjorden water onto the western Svalbard Shelf, while the dense bottom layer continues to sink down the slope. This bifurcation of the plume into a diluted shelf branch and a dense downslope branch is enhanced by tidally induced shear dispersion at the headland. Tidal effects at the headland are shown to cause a net reduction in the downslope flux of Storfjorden water into the deep Fram Strait. This finding contrasts previous results from observations of a dense plume on a different shelf without abrupt topography.

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