Structure of the Indian Ocean Meridional Overturning Circulation and its relationship with the zonal wind stress

2013 ◽  
Vol 57 (2) ◽  
pp. 351-358
Author(s):  
YaoKun Li ◽  
JiPing Chao
Keyword(s):  
Indian Ocean ◽  
Wind Stress ◽  
Zonal Wind ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Zonal Wind Stress ◽  
Meridional Overturning ◽  
The Indian Ocean

Impact of the Equatorial Wind Stress on the Indian Ocean Shallow Meridional Overturning Circulation During the IOD Mature Phase

2020 ◽  
Author(s):  
Linfang Zhang ◽  
Yaokun Li ◽  
Jianping Li
Keyword(s):  
Indian Ocean ◽  
Wind Stress ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Overturning Circulation ◽  
Mature Phase ◽  
Meridional Overturning ◽  
The Indian Ocean ◽  
Upper Level ◽  
The Impact

<p>            This paper investigates the impact of the equatorial wind stress on the Indian Ocean Shallow Meridional Overturning Circulation (SMOC) during the India Ocean Dipole (IOD) mature phase. The results show that the equatorial zonal wind stress directly drives the meridional motion of seawater at the upper level. In normal years, the wind stress in the Indian Ocean is easterly between 30°S-0°and the westerly wind is between 0°and 30°N, which contributes to a southward Ekman transport at the upper level to form the climatological SMOC. During the years of positive IOD events, abnormal easterly wind near the equator, accompanying with the cold sea surface temperature anomaly (SSTA) along the coast of Sumatra and Java and the warm SSTA along the coast of East Africa, brings southward Ekman transport south of the equator while northward Ekman transport north of the equator. This leads the seawaters moving away from the equator and hence upwelling near the equator as a consequence, to form a pair of small circulation cell symmetric about the equator.</p>

scholarly journals Reconstructing the Meridional Overturning Circulation from Boundary Densities and the Zonal Wind Stress

2007 ◽  
Vol 37 (3) ◽  
pp. 743-763 ◽  
Author(s):  
J. Hirschi ◽  
J. Marotzke
Keyword(s):  
Time Scales ◽  
Wind Stress ◽  
Zonal Wind ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Temperature Conductivity ◽  
Boundary Density ◽  
Zonal Wind Stress ◽  
Meridional Overturning ◽  
Temporal And Spatial

Abstract Numerical models are used to test whether the meridional overturning circulation (MOC) can be reconstructed from boundary densities and the wind stress. In idealized model setups without topography the strength as well as the temporal and spatial variability of the MOC cell can largely be reproduced from boundary densities and the zonal wind stress. With added slopes along the meridional boundaries, most of the depth-averaged flow is missed and neither strength nor spatial structure of the MOC is well reproduced. However, the temporal evolution of both MOC and its estimate are similar. In an eddy-permitting model with realistic bottom topography the contribution of the depth-averaged meridional flow to the MOC is captured at some places while it is missed at others. Nevertheless, boundary densities and the zonal wind stress allow the leading modes of the temporal and spatial MOC variability to be reproduced. On seasonal time scales most of the MOC variability is due to the wind stress but changes in the boundary density affect the MOC as well. On interannual time scales the MOC variability largely reflects changes in the boundary density. Generally, the MOC reconstructions are accurate when bottom velocities are small, an assumption made in the reconstruction approach. The results are relevant for estimates of both the modern and the past MOC. In the real ocean, boundary densities can be obtained from measurements of temperature, conductivity, and pressure in the water column, whereas past seawater densities have left their imprint in sea sediments.

scholarly journals Impact of Equatorial Wind Stress on Ekman Transport During the Mature Phase of the Indian Ocean Dipole

2021 ◽  
Author(s):  
Linfang Zhang ◽  
Yaokun Li ◽  
Jianping Li
Keyword(s):  
Indian Ocean ◽  
Wind Stress ◽  
Zonal Wind ◽  
Indian Ocean Dipole ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Zonal Wind Stress ◽  
Mature Phase ◽  
The Indian Ocean ◽  
The Impact

Abstract This paper investigates the impact of equatorial wind stress on the equatorial Ekman transport during the Indian Ocean dipole (IOD) mature phase. The results show that the equatorial zonal wind stress directly drives the meridional motion of seawater at the upper levels. In normal years, the zonal wind stress south of the equator is easterly and that north of the equator is westerly, which contributes to southward Ekman transport at the upper levels to form the climatological Indian Ocean shallow meridional overturning circulation. During the years of positive IOD events, abnormal easterly winds near the equator bring southward Ekman transport south of the equator while they bring northward Ekman transport north of the equator. This causes the seawater to move away from the equator and hence induces upwelling near the equator, which forms a pair of small circulation cells that are symmetric about the equator at the upper levels (approximately 100 m deep). The abnormal circulation cell south (north) of the equator strengthens (weakens) the southward (southward) motion south (north) of the equator. During years with negative IOD events, the opposite occurs. In addition, during the mature period of IOD, the remote sea surface temperature anomaly (SSTA) such as El Niño–Southern Oscillation (ENSO) may exert some influence on equatorial wind stress and Ekman transport anomaly but the influence is weak.

scholarly journals Using the Helmholtz Decomposition to Define the Indian Ocean Meridional Overturning Streamfunction

2020 ◽  
Vol 50 (3) ◽  
pp. 679-694
Author(s):  
Lei Han ◽  
Rui Xin Huang
Keyword(s):  
Indian Ocean ◽  
Zonal Flow ◽  
Circulation Pattern ◽  
Ocean Basin ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Open Boundaries ◽  
Meridional Overturning ◽  
The Indian Ocean ◽  
The Antarctic

AbstractThe zonally integrated flow in a basin can be separated into the divergent/nondivergent parts, and a uniquely defined meridional overturning circulation (MOC) can be calculated. For a basin with significant volume exchange at zonal open boundaries, this method is competent in removing the components associated with the nonzero source terms due to zonal transports at open boundaries. This method was applied to the zonally integrated flow in the Indian Ocean basin extended all the way to the Antarctic by virtue of the ECCO dataset. The contributions due to two major zonal flow systems at open boundaries, the Indonesian Throughflow (ITF) and the Antarctic Circumpolar Current (ACC), were well separated from the rotational flow component, and a nondivergent overturning circulation pattern was identified. Comparisons with previous studies on the MOC of the Indian Ocean in different seasons showed overall consistency but with refinements in details to the south of the entry of the ITF, reflecting the influence of ITF on the MOC pattern in the domain. Other options of decomposition are also examined.

scholarly journals Deep Meridional Overturning Circulation in the Indian Ocean and Its Relation to Indian Ocean Dipole

2014 ◽  
Vol 27 (12) ◽  
pp. 4508-4520 ◽  
Author(s):  
Weiqiang Wang ◽  
Xiuhua Zhu ◽  
Chunzai Wang ◽  
Armin Köhl
Keyword(s):  
Indian Ocean ◽  
Indian Ocean Dipole ◽  
Deep Ocean ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Indian Ocean Dipole Mode ◽  
Residual Term ◽  
Temperature Anomalies ◽  
Meridional Overturning ◽  
The Indian Ocean

Abstract This paper uses the 42-yr German Estimating the Circulation and Climate of the Ocean (GECCO) synthesis data to analyze and examine the relationship of the Indian Ocean deep meridional overturning circulation (DMOC) with the Indian Ocean dipole mode (IOD). Contributions of various dynamical processes are assessed by decomposing the DMOC into the Ekman and geostrophic transport, the external mode, and a residual term. The first three terms successfully describe the DMOC with a marginal residual term. The following conclusions are obtained. First, the seasonal cycle of the DMOC is mainly determined by the Ekman component. The exception is during the transitional seasons (March–April and September–October) in the northern Indian Ocean Basin, where the geostrophic component dominates. Second, at the beginning phase of the IOD (May–June), the Ekman component dominates the DMOC structure; at and after the peak phase of the IOD (September–December), the DMOC structure is primarily determined by the geostrophic component in correspondence with the well-developed sea surface temperature anomalies, while the wind (and thus the Ekman component) plays a secondary role south of 10°S and contributes negatively within the zonal band of 10° on both sides of the equator. Therefore, there exists a surface to deep-ocean connection through which IOD-related surface wind and ocean temperature anomalies are transferred down to the deep ocean. Westward-propagating signals are observed even in the deep ocean, suggesting possible roles of Rossby waves in transferring the surface signal to the deep ocean.

Reduced Stability of the Atlantic Meridional Overturning Circulation due to Wind Stress Feedback during Glacial Times

2008 ◽  
Vol 21 (23) ◽  
pp. 6260-6282 ◽  
Author(s):  
Olivier Arzel ◽  
Matthew H. England ◽  
Willem P. Sijp
Keyword(s):  
Sea Ice ◽  
Wind Stress ◽  
Surface Wind ◽  
Meridional Overturning Circulation ◽  
Nonlinear Dependence ◽  
Upper Ocean ◽  
Overturning Circulation ◽  
Climate Conditions ◽  
Meridional Overturning ◽  
The Impact

Abstract A previous study by Mikolajewicz suggested that the wind stress feedback stabilizes the Atlantic thermohaline circulation. This result was obtained under modern climate conditions, for which the presence of the massive continental ice sheets characteristic of glacial times is missing. Here a coupled ocean–atmosphere–sea ice model of intermediate complexity, set up in an idealized spherical sector geometry of the Atlantic basin, is used to show that, under glacial climate conditions, wind stress feedback actually reduces the stability of the meridional overturning circulation (MOC). The analysis reveals that the influence of the wind stress feedback on the glacial MOC response to an external source of freshwater applied at high northern latitudes is controlled by the following two distinct processes: 1) the interactions between the wind field and the sea ice export in the Northern Hemisphere (NH), and 2) the northward Ekman transport in the tropics and upward Ekman pumping in the core of the NH subpolar gyre. The former dominates the response of the coupled system; it delays the recovery of the MOC, and in some cases even stabilizes collapsed MOC states achieved during the hosing period. The latter plays a minor role and mitigates the impact of the former process by reducing the upper-ocean freshening in deep-water formation regions. Hence, the wind stress feedback delays the recovery of the glacial MOC, which is the opposite of what occurs under modern climate conditions. Close to the critical transition threshold beyond which the circulation collapses, the glacial MOC appears to be very sensitive to changes in surface wind stress forcing and exhibits, in the aftermath of the freshwater pulse, a nonlinear dependence upon the wind stress feedback magnitude: a complete and irreversible MOC shutdown occurs only for intermediate wind stress feedback magnitudes. This behavior results from the competitive effects of processes 1 and 2 on the midlatitude upper-ocean salinity during the shutdown phase of the MOC. The mechanisms presented here may be relevant to the large meltwater pulses that punctuated the last glacial period.

scholarly journals The sloshing and diapycnal meridional overturning circulations in the Indian Ocean

2020 ◽  
Author(s):  
Lei Han
Keyword(s):  
Indian Ocean ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Western Coast ◽  
Ekman Pumping ◽  
Meridional Heat Transport ◽  
Vertical Movements ◽  
Sloshing Mode ◽  
Meridional Overturning ◽  
The Indian Ocean

AbstractThe meridional overturning circulation (MOC) seasonality in the Indian Ocean is investigated with the ocean state estimate product, ECCO v4r3. The vertical movements of water parcels are predominantly due to the heaving of the isopycnals all over the basin except off the western coast. Aided by the linear propagation equation of long baroclinic Rossby waves, the driving factor determining the strength of the seasonal MOC in the Indian Ocean is identified as the zonally-integrated Ekman pumping anomaly, rather than the Ekman transport concluded in earlier studies. A new concept of sloshing MOC is proposed, and its difference with the classic Eulerian MOC leads to the so-called diapycnal MOC. The striking resemblance of the Eulerian and sloshing MOCs implies the seasonal variation of the Eulerian MOC in the Indian Ocean is a sloshing mode. The shallow overturning cells manifest themselves in the diapycnal MOC as the most remarkable structure. New perspectives on the upwelling branch of the shallow overturn in the Indian Ocean are offered based on diapycnal vertical velocity. The discrepancy among the observation-based estimates on the bottom inflow across 32°S of the basin is interpreted with the seasonal sloshing mode. Consequently, the “missing mixing” in the deep Indian Ocean is attributed to the overestimated diapycnal volume fluxes. Decomposition of meridional heat transport (MHT) into sloshing and diapycnal components clearly shows the dominant mechanism of MHT in the Indian Ocean in various seasons.

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