scholarly journals Spiciness Anomalies in the Upper South Indian Ocean

2018 ◽  
Vol 48 (9) ◽  
pp. 2081-2101 ◽  
Author(s):  
Motoki Nagura ◽  
Shinya Kouketsu
Keyword(s):  
Indian Ocean ◽  
Southern Oscillation ◽  
Mixed Layer Depth ◽  
Surface Heat ◽  
Surface Fluxes ◽  
Sea Surface Salinity ◽  
South Indian Ocean ◽  
South Indian ◽  
Generation Region ◽  
Upper South

AbstractThis study investigates an isopycnal temperature/salinity T/S, or spiciness, anomaly in the upper south Indian Ocean for the period from 2004 to 2015 using observations and reanalyses. Spiciness anomalies at about 15°S on 24–26σθ are focused on, whose standard deviation is about 0.1 psu in salinity and 0.25°C in temperature, and they have a contribution to isobaric temperature variability comparable to thermocline heave. A plausible generation region of these anomalies is the southeastern Indian Ocean, where the 25σθ surface outcrops in southern winter, and the anticyclonic subtropical gyre advects subducted water equatorward. Unlike the Pacific and Atlantic, spiciness anomalies in the upper south Indian Ocean are not T/S changes in mode water, and meridional variations in SST and sea surface salinity in their generation region are not density compensating. It is possible that this peculiarity is owing to freshwater originating from the Indonesian Seas. The production of spiciness anomalies is estimated from surface heat and freshwater fluxes and the surface T/S relationship in the outcrop region, based on several assumptions including the dominance of surface fluxes in the surface T/S budget and effective mixed layer depth proposed by Deser et al. The result agrees well with isopycnal salinity anomalies at the outcrop line, which indicates that spiciness anomalies are generated by local surface fluxes. It is suggested that the Ningaloo Niño and El Niño–Southern Oscillation lead to interannual variability in surface heat flux in the southeastern Indian Ocean and contribute to the generation of spiciness anomalies.

Spiciness Anomalies of Subantarctic Mode Water in the South Indian Ocean

2021 ◽  
Vol 34 (10) ◽  
pp. 3927-3953
Author(s):  
Motoki Nagura
Keyword(s):  
Indian Ocean ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Surface Mixed Layer ◽  
The South ◽  
South Indian Ocean ◽  
Mode Water ◽  
Horizontal Advection ◽  
South Indian ◽  
Subantarctic Mode Water

AbstractThis study investigates spreading and generation of spiciness anomalies of the Subantarctic Mode Water (SAMW) located on 26.6 to 26.8 σθ in the south Indian Ocean, using in situ hydrographic observations, satellite measurements, reanalysis datasets, and numerical model output. The amplitude of spiciness anomalies is about 0.03 psu or 0.13°C and tends to be large along the streamline of the subtropical gyre, whose upstream end is the outcrop region south of Australia. The speed of spreading is comparable to that of the mean current, and it takes about a decade for a spiciness anomaly in the outcrop region to spread into the interior up to Madagascar. In the outcrop region, interannual variability in mixed layer temperature and salinity tends to be density compensating, which indicates that Eulerian temperature or salinity changes account for the generation of isopycnal spiciness anomalies. It is known that wintertime temperature and salinity in the surface mixed layer determine the temperature and salinity relationship of a subducted water mass. Considering this, the mixed layer heat budget in the outcrop region is estimated based on the concept of effective mixed layer depth, the result of which shows the primary contribution from horizontal advection. The contributions from Ekman and geostrophic currents are comparable. Ekman flow advection is caused by zonal wind stress anomalies and the resulting meridional Ekman current anomalies, as is pointed out by a previous study. Geostrophic velocity is decomposed into large-scale and mesoscale variability, both of which significantly contribute to horizontal advection.

Indian Ocean Subtropical Underwater and the Interannual Variability in its Annual Subduction Rate Associated with the Southern Annular Mode

2022 ◽  
Keyword(s):  
Indian Ocean ◽  
Interannual Variability ◽  
Mixed Layer Depth ◽  
Potential Temperature ◽  
Southern Annular Mode ◽  
Sea Surface Salinity ◽  
Annular Mode ◽  
South Indian ◽  
Subtropical Underwater ◽  
Subduction Rate

Abstract In this study, the Indian Ocean subtropical underwater (IOSTUW) was investigated as a subsurface salinity maximum using Argo floats (2000–2020) for the first time. It has mean salinity, potential temperature and potential density values of 35.54 ± 0.29 psu, 17.91 ± 1.66 °C, and 25.56 ± 0.35 kg m−3, respectively, and mainly extends between 10°S and 30°S along the isopycnal surface in the subtropical south Indian Ocean. The annual subduction rate of the IOSTUW during the period of 2004-2019 was investigated based on a gridded Argo dataset. The results revealed a mean value of 4.39 Sv (1 Sv=106 m3s−1) with an interannual variability that is closely related to the Southern Annular Mode (SAM). The variation in the annual subduction rate of the IOSTUW is dominated by the lateral induction term, which largely depends on the winter mixed layer depth (MLD) in the sea surface salinity (SSS) maximum region. The anomalies of winter MLD is primarily determined by SAM-related air-sea heat flux and zonal wind anomalies through modulation of the buoyancy. As a result, the annual subduction rate of the IOSTUW generally increased when the SAM index showed negative anomalies and decreased when the SAM index showed positive anomalies. Exceptional cases occurred when the wind anomaly within the SSS maximum region was weak or was dominated by its meridional component.

Changes in the relationship between spring precipitation in southern China and tropical Pacific-South Indian Ocean SST

2021 ◽  
pp. 1-37
Author(s):  
XiaoJing Jia ◽  
Chao Zhang ◽  
Renguang Wu ◽  
QiFeng Qian
Keyword(s):  
Indian Ocean ◽  
Southern Oscillation ◽  
Southern China ◽  
Tropical Pacific ◽  
Western Pacific ◽  
The South ◽  
South Indian Ocean ◽  
South Indian ◽  
The Western Pacific ◽  
Sst Anomalies

AbstractThe present study explores the changed relationship between the interannual variations in spring (April-May) precipitation over southern China (SPSC) and sea surface temperature (SST) anomalies in the tropical Pacific and South Indian Ocean during the 1960-2017 period. Observational analysis shows that the relation between SPSC and the El Niño-Southern Oscillation (ENSO) was significant before the mid-1980s (P1) and after the early 2000s (P3) but insignificant from the mid-1980s to the early 2000s (P2). In P2, positive anomalous SPSC was significantly correlated with negative anomalous SST in the South Indian Ocean. During this period, an anomalous anticyclone and intensified southwesterly winds tended to appear over tropical India accompanied by a negative anomalous South Indian Ocean SST, which caused anomalous low-level convergence over the western Pacific. As a result, the western Pacific subtropical high (WPSH) tended to weaken and retreat eastward. This resulted in anomalous moisture convergence in southern China, favoring enhanced SPSC. Further analysis shows that the negative South Indian Ocean SST anomalies tended to induce anomalous cross-equatorial vertical circulation where the South Indian Ocean and southern China are controlled by descent and ascent air flow. The ascent motion may also contribute to positive anomalous SPSC. The observed contribution of the South Indian Ocean SST anomalies to the SPSC variation is confirmed by numerical experiments using an atmospheric model. The intensified variance of SST in the South Indian Ocean and the eastward shift of the ENSO-related circulation anomalies over the western tropical Pacific may partly account for the changes in the SST-SPSC relationship.

scholarly journals Diagnosing Kenya Rainfall in Boreal Autumn: Further Exploration

2012 ◽  
Vol 25 (12) ◽  
pp. 4323-4329 ◽  
Author(s):  
Charles Mutai ◽  
Dierk Polzin ◽  
Stefan Hastenrath
Keyword(s):  
Indian Ocean ◽  
Southern Oscillation ◽  
South Indian Ocean ◽  
The West ◽  
Trade Winds ◽  
South Indian ◽  
Indian Ocean Trade ◽  
Circulation Cell ◽  
The Indian Ocean ◽  
High Phase

Abstract A powerful zonal vertical circulation cell along the Indian Ocean equator controls the boreal autumn rains in Kenya, with a tight negative correlation between surface westerlies (UEQ) and rainfall. UEQ is favored by a steep eastward pressure gradient (PWE) and slow winds in the downstream portion of the South Indian Ocean trade winds (SIW). The high phase of the Southern Oscillation (SO) favors weak SIW, lower pressure in the east, and thus steeper PWE, but that is also affected by pressure in the west. In 1958–97 circulation causalities were most distinct in the regime of abundant rain and slow UEQ, with the SO in the low phase. In the regime of deficient rain and fast UEQ, relationships were less distinct, although the SO was in the high phase almost throughout. In the decade 2001–10, UEQ had a weaker (stronger) correlation with PWE (SIW), the SO was more active and contributing in the proper sense to pressure in the east, but PWE was mainly controlled by pressure in the west. UEQ strongly controlled the rainfall in 1958–97 and 2001–10.

scholarly journals Variability in the Mozambique Channel Trough and Impacts on Southeast African Rainfall

2020 ◽  
Vol 33 (2) ◽  
pp. 749-765 ◽  
Author(s):  
Rondrotiana Barimalala ◽  
Ross C. Blamey ◽  
Fabien Desbiolles ◽  
Chris J. C. Reason
Keyword(s):  
Indian Ocean ◽  
Austral Summer ◽  
Western Indian Ocean ◽  
Strong Relationship ◽  
Ocean Basin ◽  
Moist Convection ◽  
South Indian Ocean ◽  
Mozambique Channel ◽  
Mascarene High ◽  
South Indian

AbstractThe Mozambique Channel trough (MCT) is a cyclonic region prominent in austral summer in the central and southern Mozambique Channel. It first becomes evident in December with a peak in strength in February when the Mozambique Channel is warmest and the Mascarene high (MH) is located farthest southeast in the Indian Ocean basin. The strength and the timing of the mean MCT are linked to that of the cross-equatorial northeasterly monsoon in the tropical western Indian Ocean, which curves as northwesterlies toward northern Madagascar. The interannual variability in the MCT is associated with moist convection over the Mozambique Channel and is modulated by the location of the warm sea surface temperatures in the south Indian Ocean. Variability of the MCT shows a strong relationship with the equatorial westerlies north of Madagascar and the latitudinal extension of the MH. Summers with strong MCT activity are characterized by a prominent cyclonic circulation over the Mozambique Channel, extending to the midlatitudes. These are favorable for the development of tropical–extratropical cloud bands over the southwestern Indian Ocean and trigger an increase in rainfall over the ocean but a decrease over the southern African mainland. Most years with a weak MCT are associated with strong positive south Indian Ocean subtropical dipole events, during which the subcontinent tends to receive more rainfall whereas Madagascar and northern Mozambique are anomalously dry.

scholarly journals Structure and origin of the subtropical South Indian Ocean Countercurrent

2006 ◽  
Vol 33 (24) ◽  
Author(s):  
Gerold Siedler ◽  
Mathieu Rouault ◽  
Johann R. E. Lutjeharms
Keyword(s):  
Indian Ocean ◽  
South Indian Ocean ◽  
South Indian
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