Interannual monsoon wind variability over the South tropical Indian ocean drives East African small pelagic fisheries

2020 ◽  
Author(s):  
Fatma Jebri ◽  
Zoe Jacobs ◽  
Dionysios Raitsos ◽  
Meric Srokosz ◽  
Stuart Painter ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Phytoplankton Biomass ◽  
Critical Role ◽  
Tropical Indian Ocean ◽  
Nutrient Concentrations ◽  
Northeast Monsoon ◽  
The South ◽  
East African ◽  
Monsoon Wind ◽  
Pelagic Fisheries

<p>Small pelagic fisheries play a critical role in food security and economic stability for East African coastal communities ― a region of least developed countries. Using satellite and field observations together with modelling, we show the links between the small pelagic fisheries along the East African coast and the changes in Western Indian Ocean currents due to the interannual variability of the monsoonal wind field. The annual variations in phytoplankton biomass and fisheries yield are strongly associated. During the Northeast monsoon, the enhanced phytoplankton biomass is triggered by local wind-driven upwelling. During the Southeast monsoon, however, the enhanced phytoplankton biomass is due to two current induced mechanisms: coastal “dynamic uplift” upwelling; and westward advection of waters with higher nutrient concentrations. This biological response to the Southeast monsoon is greater than that to the Northeast monsoon. Interannually, an extreme increase (decrease) in chlorophyll concentrations is induced by strengthened (weakened) surface currents, which occur during anomalously “strong” (“weak”) Southeast monsoon years. For years where the effects of El Niño / La Niña are weak, the Southeast monsoon wind strength over the south tropical Indian Ocean is the main driver of year-to-year variability. Such changes have important implications for the predictability of fisheries yield, its response to climate change, policy and resource management.  </p>

An Enhanced Influence of Tropical Indian Ocean on the South Asia High after the Late 1970s

2012 ◽  
Vol 25 (20) ◽  
pp. 6930-6941 ◽  
Author(s):  
Xia Qu ◽  
Gang Huang
Keyword(s):  
Indian Ocean ◽  
South Asia ◽  
Tropical Indian Ocean ◽  
Tropospheric Temperature ◽  
Decadal Change ◽  
The South ◽  
South Asia High ◽  
The Impact ◽  
Change In Mean ◽  
Sst Anomalies

Abstract The tropical Indian Ocean (TIO)’s influence on the South Asia high (SAH)’s intensity experiences a decadal change in the late 1970s; after (before) the decadal shift, the influence is significant (insignificant). The present study investigates the role of tropospheric temperature in relaying the impact of sea surface temperature (SST) to the SAH and the change in the TIO’s influence. During the two epochs, the local tropospheric temperature responses to the TIO warming are distinct—more significant during the second epoch. It is inferred that this change may be responsible for the strengthening of the TIO’s influence on the SAH. Encouragingly, the ensemble simulations accurately capture the time of the decadal change, indicating that the enhanced influence is attributed to the SST forcing. There are two possible reasons for the change in the TIO–SAH relationship. The first reason is the change in the locations of the SST anomalies in the TIO. During the second epoch, positive SST anomalies lie in the Indian Ocean warm pool. Through the background vigorous convection and moist adjustment, the SST anomalies affect largely the tropospheric temperature and thus the SAH. The second reason is the decadal change in mean SST and the SST variability. During the recent decades, both the background SST and the variability of the TIO SST increase, which enhance the influence of the SST anomalies on the atmosphere. The influence of the remote oceanic forcing on the enhanced TIO–SAH relationship and its comparison with the contribution of the TIO SST are also discussed.

Variability of boreal spring Hadley circulation over the Asian monsoon domain and its relationship with tropical SST

2020 ◽  
Author(s):  
Yaqi Wang ◽  
Juan Feng ◽  
Jianping Li ◽  
Ran An ◽  
Lanning Wang
Keyword(s):  
Indian Ocean ◽  
Asian Monsoon ◽  
Southern Oscillation ◽  
Critical Role ◽  
Hadley Circulation ◽  
Tropical Indian Ocean ◽  
Dominant Mode ◽  
Main Body ◽  
Boreal Spring ◽  
Sst Anomalies

<p>The variability of boreal spring Hadley circulation (HC) over the Asian monsoon domain over the last four decades is explored. The climatological distribution of the regional HC is symmetric of the equator, with the ascending branch around the equator and sinking branch around the subtropics in each hemisphere. The first dominant mode (EOF1) of the regional HC is equatorial asymmetric, with the main body in the Southern Hemisphere (SH) and the ascending branch to the north of the equator. This mode is mainly characterized by interannual variation and is related to El Niño-Southern Oscillation (ENSO). Significant negative sea surface temperature (SST) anomalies are observed over the tropical Indian Ocean (TIO) along with the development of La Niña events; however, the magnitude of SST anomalies in the southern Indian Ocean is greater than that in the northern counterpart, contributing to EOF1 formation. The spatial distribution of the second dominant mode (EOF2) is with the main body lying in the Northern Hemisphere (NH) and the ascending branch located to the south of the equator. The temporal variation of this mode is connected to the warming of the TIO. The warming rate of the southern TIO SST is faster than that in the northern counterpart, resulting in the southward migration of the rising branch. The above result indicates the critical role of the meridional distribution of SST on the variability of the regional HC.</p>

scholarly journals The sensitivity of the Seychelles–Chagos thermocline ridge to large-scale wind anomalies

2009 ◽  
Vol 66 (7) ◽  
pp. 1455-1466 ◽  
Author(s):  
Juliet C. Hermes ◽  
Chris J. C. Reason
Keyword(s):  
Indian Ocean ◽  
Large Scale ◽  
Regional Climate ◽  
Austral Summer ◽  
Tropical Indian Ocean ◽  
Ocean Model ◽  
The South ◽  
South Indian Ocean ◽  
Southwest Indian Ocean ◽  
South Indian

Abstract Hermes, J. C., and Reason, C. J. C. 2009. The sensitivity of the Seychelles–Chagos thermocline ridge to large-scale wind anomalies. – ICES Journal of Marine Science, 66: 1455–1466. The Seychelles–Chagos thermocline ridge (SCTR) in the southwest tropical Indian Ocean is important for regional climate, the Madden–Julian Oscillation, as well as upper-ocean nutrients and related phytoplankton and zooplankton densities. Subsurface variability in this region has been proved to influence the overlying sea surface temperatures, which in turn can influence eastern African rainfall. There is evidence that austral summers with a deeper (shallower) SCTR tend to have more (less) tropical cyclone (TC) days in the Southwest Indian Ocean. The importance of this relationship was underlined during the 2006/2007 austral summer, when areas of Madagascar and central Mozambique experienced devastating floods, because of ten named tropical storms, including several intense TCs, effecting on these areas. At the same time, the SCTR during this season was anomalously deep, partly because of a downwelling Rossby wave that propagated across the South Indian Ocean during the previous austral winter/spring. In this paper, a regional ocean model is used to investigate the effect of remote forcing on this region and to study the sensitivity of the SCTR to changes in the large-scale winds over the South Indian Ocean, with a particular focus on the events of the 2006/2007 austral summer.

scholarly journals Satellite and Argo Observed Surface Salinity Variations in the Tropical Indian Ocean and Their Association with the Indian Ocean Dipole Mode

2015 ◽  
Vol 28 (2) ◽  
pp. 695-713 ◽  
Author(s):  
Yan Du ◽  
Yuhong Zhang
Keyword(s):  
Indian Ocean ◽  
Ocean Circulation ◽  
Indian Ocean Dipole ◽  
Coastal Upwelling ◽  
Tropical Indian Ocean ◽  
Sea Surface Salinity ◽  
The South ◽  
Surface Salinity ◽  
Indian Ocean Dipole Mode ◽  
The Indian Ocean

Abstract This study investigates sea surface salinity (SSS) variations in the tropical Indian Ocean (IO) using the Aquarius/Satelite de Aplicaciones Cientificas-D (SAC-D) and the Soil Moisture and Ocean Salinity (SMOS) satellite data and the Argo observations during July 2010–July 2014. Compared to the Argo observations, the satellite datasets generally provide SSS maps with higher space–time resolution, particularly in the regions where Argo floats are sparse. Both Aquarius and SMOS well captured the SSS variations associated with the Indian Ocean dipole (IOD) mode. Significant SSS changes occurred in the central equatorial IO, along the Java–Sumatra coast, and south of the equatorial IO, due to ocean circulation variations. During the negative IOD events in 2010, 2013, and 2014, westerly wind anomalies strengthened along the equator, weakening coastal upwelling off Java and Sumatra and decreasing SSS. South of the equatorial IO, an anomalous cyclonic gyre changed the tropical circulation, which favored the eastward high-salinity tongue along the equator and the westward low-saline tongue in the south. An upwelling Rossby wave favored the increase of SSS farther to the south. During the positive IOD events in 2011 and 2012, the above-mentioned processes reversed, although the decrease of SSS was weaker in magnitude.

scholarly journals Seasonal Characteristics of Circulation in the Southeastern Tropical Indian Ocean*

2005 ◽  
Vol 35 (2) ◽  
pp. 255-267 ◽  
Author(s):  
Tangdong Qu ◽  
Gary Meyers
Keyword(s):  
Indian Ocean ◽  
Annual Variation ◽  
Maximum Amplitude ◽  
Tropical Indian Ocean ◽  
Surface Flow ◽  
Sea Surface ◽  
The South ◽  
Near Surface ◽  
Seasonal Characteristics ◽  
Southeastern Tropical Indian Ocean

Abstract The circulation in the southeastern tropical Indian Ocean is studied using historical temperature and salinity data. A southward shift of the subtropical gyre at increasing depth dominates the structure of the annual mean circulation. Near the southern Indonesian coast the westward South Equatorial Current (SEC) is at the sea surface and strongest near 10°–11°S, reflecting strong influence of the Indonesian Throughflow (ITF). In latitudes 13°–25°S the SEC is a subsurface flow and its velocity core deepens toward the south, falling below 500 m at 25°S. The eastern gyral current (EGC) is a surface flow overlying the SEC, associated with the meridional gradients of near-surface temperature and salinity. The ITF supplies water to the SEC mainly in the upper 400 m, and below that depth the flow is reversed along the coast of Sumatra and Java. Monsoon winds strongly force the annual variation in circulation. Dynamic height at the sea surface has a maximum amplitude at 10°–13°S, and the maximum at deeper levels is located farther south. Annual variation is also strong in the coastal waveguides, but is mainly confined to the near-surface layer. Although the South Java Current at the sea surface is not well resolved in the present dataset, semiannual variation is markedly evident at depth and tends to extend much deeper than the annual variation along the coast of Sumatra and Java.

scholarly journals Evolution of two troughs in the tropical Indian Ocean and their characteristic features

MAUSAM ◽  
2021 ◽  
Vol 43 (4) ◽  
pp. 395-398
Author(s):  
M.S. SINGH ◽  
B. Lakshmanaswamy
Keyword(s):  
Indian Ocean ◽  
Southern Hemisphere ◽  
Northern Hemisphere ◽  
South Asian ◽  
Tropical Indian Ocean ◽  
The Other ◽  
The South ◽  
The Indian Ocean ◽  
Characteristic Features

Evolution and characteristic features of double trough systems in the tropical Indian Ocean have been studied with the help of Climatological Atlas (Part I andIl) ~f the Tropical Indian Oc.ean (Hastenrath and Lamb 1979). It is confirmed that there are two troughs (Northern Hemisphere EquatorIal Trough and Southern Hemisphere Equatorial Trough) in this region (including south Asian landmass) all the year round, one in northern hemisphere and the other in southern. Both are migratory in nature and, perhaps, thermal in origin.  In the convergent zones of the two troughs, there is extensive cloudiness. The migration of these trough systems during their respective summer seasons appear to be related to the extensive heating of the south Asian/ African land masses surrounding the Indian Ocean in north and west.  

scholarly journals Nutrient enrichment induced by tropical cyclone Seroja in the southeastern tropical Indian Ocean

2021 ◽  
Vol 925 (1) ◽  
pp. 012021
Author(s):  
D W Purnaningtyas ◽  
F Khadami ◽  
Avrionesti
Keyword(s):  
Tropical Cyclone ◽  
Indian Ocean ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Nutrient Enrichment ◽  
Tropical Indian Ocean ◽  
Sea Surface ◽  
Nutrient Concentrations ◽  
Biochemical Alterations ◽  
Southeastern Tropical Indian Ocean

Abstract Tropical cyclone (TC) passage triggers a complex response from the adjacent ocean, including vertical mixing, leading to biochemical alterations and affecting the surrounding ecosystem’s dynamics. In previous studies, increased nutrient concentrations and primary production were observed along the cyclone track after the storm. TC Seroja was awakened near the equator in the southeastern tropical Indian Ocean, making it interesting to investigate how the ambient ecosystem responds. Hence, we analyzed the sea surface temperature and nutrient changes during the Seroja event using multi-satellite remote sensing and numerical model data in the south of Indonesia and East Timor along the Seroja track between April 2 and 10, 2021. Immediately after the TC Seroja passed, the sea surface temperature cooled to 3 °C around the TC lane. At the same time, the spatial distribution patterns showed the upsurge of some nutrients in response to the passage of TC Seroja; the surface nitrate swells up to 1.5 mmol/m3, while phosphate increased up to 0.2 mmol/m3, and the dissolved silicate concentration enhanced up to 1.0 mmol/m3. The responses recover within 2-7 days. These results indicate that tropical cyclones contribute to nutrient enrichment in oligotrophic areas outside of their usual annual upwelling time, thereby further supporting ecosystem sustainability.

Dynamics of 2015 positive Indian Ocean Dipole

2019 ◽  
Vol 69 (1) ◽  
pp. 75
Author(s):  
Putri Adia Utari ◽  
Mokhamad Yusup Nur Khakim ◽  
Dedi Setiabudidaya ◽  
Iskhaq Iskandar
Keyword(s):  
Heat Flux ◽  
Indian Ocean ◽  
Indian Ocean Dipole ◽  
Tropical Indian Ocean ◽  
Boreal Summer ◽  
The South ◽  
South Eastern ◽  
Positive Indian Ocean Dipole ◽  
Sst Warming ◽  
Dipole Pattern

Evolution of typical positive Indian Ocean Dipole (pIOD) event was dominated by a significant sea-surface temperature (SST) cooling in the south-eastern tropical Indian Ocean. Interestingly, during the evolution of 2015 pIOD event, the SST in the south-eastern tropical Indian Ocean did not reveal significant cooling, instead anomalous strong SST warming took place in the western tropical Indian Ocean off the East African coast. This anomalous SST warming was associated with a weakening of the Asian summer monsoon. Furthermore, analysis on the mixed layer heat budget demonstrated that the evolution of the 2015 pIOD event could be attributed mainly to the air-sea heat flux. By decomposing the air-sea heat flux, it is found that reduced latent heat loss plays an important role on the SST warming in the western pole and keeping SST warm in the eastern pole. We note that a residual term also may play a role during the initial development of the event. In contrast to the SST pattern, the subsurface temperature revealed a clear positive dipole pattern. Shallow (deep) 20°C isothermal layer in the eastern (western) equatorial Indian Ocean was observed during boreal summer. This robust subsurface dipole pattern indicated that the subsurface ocean response was largely wind driven through the equatorial wave dynamics as previously suggested.

Role of friction and orography in the Asian-African monsoonal system

2020 ◽  
Author(s):  
Giovanni Dalu ◽  
Marco Gaetani ◽  
Cyrille Flamant ◽  
Marina Baldi
Keyword(s):  
Indian Ocean ◽  
West African Monsoon ◽  
Tropical Indian Ocean ◽  
Western Indian Ocean ◽  
Ekman Pumping ◽  
The South ◽  
The West ◽  
Impermeable Barrier ◽  
The Indian Ocean ◽  
Marine Air

<p>The West African monsoon (WAM) originates in the Gulf of Guinea when the intertropical convergence zone (ITCZ) makes its landfall; whilst, the south Asian monsoon (SAM) originates in the Indian ocean when the ITCZ crosses the equator. The monsoonal dynamics are here studied after landfall using Gill’s tropospheric model with an implanted Ekman frictional layer (EFL). Ekman pumping increases low level convergence, making the lower monsoonal cyclone deeper and more compact that the upper anticyclone, by transferring tropospheric vorticity into the EFL. In the upper troposphere, air particles spiral-out anticyclonically away from the monsoons, subsiding over the Tropical Atlantic, the Tropical Indian ocean, or transiting into the southern hemisphere across the equator. Whilst marine air particles spiral-in cyclonically towards the WAM or the SAM, the latter appears to be a preferred ending destination in the absence of orography. The Himalayas introduced as a barrier to the monsoonal winds, strengthen the tropospheric winds by tightening the isobars. The Somali mountains (SMs), introduced as a barrier to the Ekman winds, separates the WAM and the SAM catch basins; thus, the Atlantic air particles converge towards the WAM and the Indian ocean particles converge towards the SAM. The Indian Ghats (IGs), introduced as a semi-impermeable barrier to the Ekman winds, deflect the marine air particles originated in the western Indian ocean towards the south-eastern flank of the SAM. In short, an upper single anticyclone encircles both monsoons; the Himalayas strengthen the upper-level winds by increasing the pressure gradients; the SMs split the EFL cyclone, keeping the marine air particles to the west of SMs in the WAM basin and the particles to the east of SMs in the SAM basin; the IGs guides transmit the air particles, deflecting them towards Bangladesh.</p>

Sign in / Sign up

Export Citation Format

Share Document