scholarly journals Distinct Mechanisms of Decadal Subsurface Heat Content Variations in the Eastern and Western Indian Ocean Modulated by Tropical Pacific SST

2018 ◽  
Vol 31 (19) ◽  
pp. 7751-7769 ◽  
Author(s):  
Xiaolin Jin ◽  
Young-Oh Kwon ◽  
Caroline C. Ummenhofer ◽  
Hyodae Seo ◽  
Yu Kosaka ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Rossby Waves ◽  
Surface Wind ◽  
Heat Content ◽  
Tropical Indian Ocean ◽  
Western Indian Ocean ◽  
Tropical Pacific ◽  
Ekman Pumping ◽  
The Pacific ◽  
Decadal Variations

Decadal variability of the subsurface ocean heat content (OHC) in the Indian Ocean is investigated using a coupled climate model experiment, in which observed eastern tropical Pacific sea surface temperature (EPSST) anomalies are specified. This study intends to understand the contributions of external forcing relative to those of internal variability associated with EPSST, as well as the mechanisms by which the Pacific impacts Indian Ocean OHC. Internally generated variations associated with EPSST dominate decadal variations in the subsurface Indian Ocean. Consistent with ocean reanalyses, the coupled model reproduces a pronounced east–west dipole structure in the southern tropical Indian Ocean and discontinuities in westward-propagating signals in the central Indian Ocean around 100°E. This implies distinct mechanisms by which the Pacific impacts the eastern and western Indian Ocean on decadal time scales. Decadal variations of OHC in the eastern Indian Ocean are attributed to 1) western Pacific surface wind anomalies, which trigger oceanic Rossby waves propagating westward through the Indonesian Seas and influence Indonesian Throughflow transport, and 2) zonal wind anomalies over the central tropical Indian Ocean, which trigger eastward-propagating Kelvin waves. Decadal variations of OHC in the western Indian Ocean are linked to conditions in the Pacific via changes in the atmospheric Walker cell, which trigger anomalous wind stress curl and Ekman pumping in the central tropical Indian Ocean. Westward-propagating oceanic Rossby waves extend the influence of this anomalous Ekman pumping to the western Indian Ocean.

scholarly journals Influences of Pacific Climate Variability on Decadal Subsurface Ocean Heat Content Variations in the Indian Ocean

2018 ◽  
Vol 31 (10) ◽  
pp. 4157-4174 ◽  
Author(s):  
Xiaolin Jin ◽  
Young-Oh Kwon ◽  
Caroline C. Ummenhofer ◽  
Hyodae Seo ◽  
Franziska U. Schwarzkopf ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Boundary Conditions ◽  
Heat Content ◽  
Western Indian Ocean ◽  
Ocean Heat Content ◽  
Ekman Pumping ◽  
Subsurface Ocean ◽  
The Pacific ◽  
The Indian Ocean

Abstract Decadal variabilities in Indian Ocean subsurface ocean heat content (OHC; 50–300 m) since the 1950s are examined using ocean reanalyses. This study elaborates on how Pacific variability modulates the Indian Ocean on decadal time scales through both oceanic and atmospheric pathways. High correlations between OHC and thermocline depth variations across the entire Indian Ocean Basin suggest that OHC variability is primarily driven by thermocline fluctuations. The spatial pattern of the leading mode of decadal Indian Ocean OHC variability closely matches the regression pattern of OHC on the interdecadal Pacific oscillation (IPO), emphasizing the role of the Pacific Ocean in determining Indian Ocean OHC decadal variability. Further analyses identify different mechanisms by which the Pacific influences the eastern and western Indian Ocean. IPO-related anomalies from the Pacific propagate mainly through oceanic pathways in the Maritime Continent to impact the eastern Indian Ocean. By contrast, in the western Indian Ocean, the IPO induces wind-driven Ekman pumping in the central Indian Ocean via the atmospheric bridge, which in turn modifies conditions in the southwestern Indian Ocean via westward-propagating Rossby waves. To confirm this, a linear Rossby wave model is forced with wind stresses and eastern boundary conditions based on reanalyses. This linear model skillfully reproduces observed sea surface height anomalies and highlights both the oceanic connection in the eastern Indian Ocean and the role of wind-driven Ekman pumping in the west. These findings are also reproduced by OGCM hindcast experiments forced by interannual atmospheric boundary conditions applied only over the Pacific and Indian Oceans, respectively.

scholarly journals Indian Ocean impact on ENSO evolution 2014–2016 in a set of seasonal forecasting experiments

2021 ◽  
Author(s):  
Michael Mayer ◽  
Magdalena Alonso Balmaseda
Keyword(s):  
Pacific Ocean ◽  
Indian Ocean ◽  
El Niño ◽  
Initial Conditions ◽  
El Nino ◽  
Southern Oscillation ◽  
Heat Content ◽  
Tropical Pacific ◽  
Decadal Variations ◽  
The Indian Ocean

AbstractThis study investigates the influence of the anomalously warm Indian Ocean state on the unprecedentedly weak Indonesian Throughflow (ITF) and the unexpected evolution of El Niño-Southern Oscillation (ENSO) during 2014–2016. It uses 25-month-long coupled twin forecast experiments with modified Indian Ocean initial conditions sampling observed decadal variations. An unperturbed experiment initialized in Feb 2014 forecasts moderately warm ENSO conditions in year 1 and year 2 and an anomalously weak ITF throughout, which acts to keep tropical Pacific ocean heat content (OHC) anomalously high. Changing only the Indian Ocean to cooler 1997 conditions substantially alters the 2-year forecast of Tropical Pacific conditions. Differences include (i) increased probability of strong El Niño in 2014 and La Niña in 2015, (ii) significantly increased ITF transports and (iii), as a consequence, stronger Pacific ocean heat divergence and thus a reduction of Pacific OHC over the two years. The Indian Ocean’s impact in year 1 is via the atmospheric bridge arising from altered Indian Ocean Dipole conditions. Effects of altered ITF and associated ocean heat divergence (oceanic tunnel) become apparent by year 2, including modified ENSO probabilities and Tropical Pacific OHC. A mirrored twin experiment starting from unperturbed 1997 conditions and several sensitivity experiments corroborate these findings. This work demonstrates the importance of the Indian Ocean’s decadal variations on ENSO and highlights the previously underappreciated role of the oceanic tunnel. Results also indicate that, given the physical links between year-to-year ENSO variations, 2-year-long forecasts can provide additional guidance for interpretation of forecasted year-1 ENSO probabilities.

scholarly journals Decadal Variability of the Indian and Pacific Walker Cells since the 1960s: Do They Covary on Decadal Time Scales?

2017 ◽  
Vol 30 (21) ◽  
pp. 8447-8468 ◽  
Author(s):  
Weiqing Han ◽  
Gerald A. Meehl ◽  
Aixue Hu ◽  
Jian Zheng ◽  
Jessica Kenigson ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Decadal Variability ◽  
Tropical Indian Ocean ◽  
Warm Pool ◽  
Past Century ◽  
The Pacific ◽  
Decadal Variations ◽  
Long Term Trends ◽  
The Indian Ocean ◽  
The 1960S

Previous studies have investigated the centennial and multidecadal trends of the Pacific and Indian Ocean Walker cells (WCs) during the past century, but have obtained no consensus owing to data uncertainties and weak signals of the long-term trends. This paper focuses on decadal variability (periods of one to few decades) by first documenting the variability of the WCs and warm-pool convection, and their covariability since the 1960s, using in situ and satellite observations and reanalysis products. The causes for the variability and covariability are then explored using a Bayesian dynamic linear model, which can extract nonstationary effects of climate modes. The warm-pool convection exhibits apparent decadal variability, generally covarying with the Indian and Pacific Ocean WCs during winter (November–April) with enhanced convection corresponding to intensified WCs, and the Indian–Pacific WCs covary. During summer (May–October), the warm-pool convection still highly covaries with the Pacific WC but does not covary with the Indian Ocean WC, and the Indian–Pacific WCs are uncorrelated. The wintertime coherent variability results from the vital influence of ENSO decadal variation, which reduces warm-pool convection and weakens the WCs during El Niño–like conditions. During summer, while ENSO decadal variability still dominates the Pacific WC, decadal variations of ENSO, the Indian Ocean dipole, Indian summer monsoon convection, and tropical Indian Ocean SST have comparable effects on the Indian Ocean WC overall, with monsoon convection having the largest effect since the 1990s. The complex causes for the Indian Ocean WC during summer result in its poor covariability with the Pacific WC and warm-pool convection.

scholarly journals Interannual to Decadal Variability of Tropical Indian Ocean Sea Surface Temperature: Pacific Influence versus Local Internal Variability

2020 ◽  
pp. 1-50
Author(s):  
Lei Zhang ◽  
Gang Wang ◽  
Matthew Newman ◽  
Weiqing Han
Keyword(s):  
Indian Ocean ◽  
Indian Ocean Dipole ◽  
Tropical Indian Ocean ◽  
Tropical Pacific ◽  
Sea Surface ◽  
External Influence ◽  
Sst Variability ◽  
The Pacific ◽  
The Indian Ocean ◽  
The Tropical Pacific

AbstractThe Indian Ocean has received increasing attention for its large impacts on regional and global climate. However, sea surface temperature (SST) variability arising from Indian Ocean internal processes has not been well understood particularly on decadal and longer timescales, and the external influence from the Tropical Pacific has not been quantified. This paper analyzes the interannual-to-decadal SST variability in the Tropical Indian Ocean in observations and explores the external influence from the Pacific versus internal processes within the Indian Ocean using a Linear Inverse Model (LIM). Coupling between Indian Ocean and tropical Pacific SST anomalies (SSTAs) is assessed both within the LIM dynamical operator and the unpredictable stochastic noise that forces the system. Results show that the observed Indian Ocean Basin (IOB)-wide SSTA pattern is largely a response to the Pacific ENSO forcing, although it in turn has a damping effect on ENSO especially on annual and decadal timescales. On the other hand, the Indian Ocean Dipole (IOD) is an Indian Ocean internal mode that can actively affect ENSO; ENSO also has a returning effect on the IOD, which is rather weak on decadal timescale. The third mode is partly associated with the Subtropical Indian Ocean Dipole (SIOD), and it is primarily generated by Indian Ocean internal processes, although a small component of it is coupled with ENSO. Overall, the amplitude of Indian Ocean internally generated SST variability is comparable to that forced by ENSO, and the Indian Ocean tends to actively influence the tropical Pacific. These results suggest that the Indian-Pacific Ocean interaction is a two-way process.

Observations on Seychelles-Chagos Thermocline Ridge (SCTR) upwelling during April-May 2019 in the western tropical Indian Ocean

2021 ◽  
Author(s):  
Suyun Noh ◽  
SungHyun Nam
Keyword(s):  
Indian Ocean ◽  
Tropical Indian Ocean ◽  
Meridional Overturning Circulation ◽  
Time Varying ◽  
Ekman Pumping ◽  
Wind Stress Curl ◽  
The Pacific ◽  
Equatorial Upwelling ◽  
Meridional Overturning ◽  
Upwelling Intensity

<p>The Seychelles-Chagos Thermocline Ridge (SCTR) in the western tropical Indian Ocean is known as a region of off-equatorial upwelling contrasting to equatorial upwelling in the Pacific and Atlantic where the most wide open-ocean upwelling occurs corresponding to ascending branch of one of the meridional overturning cells in the Indian Ocean, yet detailed stratification, upwelling intensity, and dynamics of SCTR upwelling variability are still poorly understood. Here, we present observational results on the SCTR upwelling based on ship-based data collected during April-May 2019 as a part of the Korea-US inDian Ocean Scientific Research Program (KUDOS). The upwelling structure is confirmed from 20 ℃ and 10 ℃ isotherms (D20 and D10) shoaling up in the center of SCTR, from 200 m to 100 m (D20) and from 600 m to 400 m (D10), respectively. Horizonal divergence at the upper 250 m within an 1° by 1° area in the SCTR center (8 °S, 61 °E) estimated from currents measurements along the boundaries (1.0 x 10<sup>-3</sup> Sv) supports a mean upwelling intensity of 7.0 x 10<sup>-3</sup> m day<sup>-1</sup> (1.0 x 10<sup>-3</sup> Sv divided by the area). The upwelling intensity generally decreases with depth but shows multiple peaks within the upper water column, yielding the maximum peak (5.0 x 10<sup>-2</sup> m day<sup>-1</sup>) at 60 m and the minimum peak (1.4 x 10<sup>-2</sup> m day<sup>-1</sup>) at 230 m, with negative peaks (downwelling) at depths around 100 and 210 m. Our results on the observed structure and intensity of SCTR upwelling are discussed in comparison to time-varying local wind stress curl-driven Ekman pumping, D20-based Seychelles Upwelling Index (SUI), and Indian Dipole Mode Index (DMI). Detailed observations on the structure and intensity of SCTR upwelling presented here have important implications on time-varying SCTR upwelling (e.g., weakened upwelling peaked in fall 2019) and climate via meridional overturning circulation in the upper Indian Ocean.</p>

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>

scholarly journals Diagnosing Heat and Vorticity Budgets of Annual Coupled Rossby Waves

2005 ◽  
Vol 35 (7) ◽  
pp. 1173-1189 ◽  
Author(s):  
Warren B. White ◽  
Jeffrey L. Annis
Keyword(s):  
Indian Ocean ◽  
Latent Heat ◽  
Rossby Waves ◽  
Surface Wind ◽  
Phase Speed ◽  
Western Indian Ocean ◽  
Heat Fluxes ◽  
Eastern Indian Ocean ◽  
Wind Stress Curl ◽  
Upper Level

Abstract Annual coupled Rossby waves are generated at the west coast of Australia and propagate westward across the eastern Indian Ocean from 10° to 30°S in covarying sea level height (SLH), sea surface temperature (SST), and meridional surface wind (MSW) residuals, generally traveling slower than uncoupled Rossby waves while increasing amplitude. The waves decouple in the western Indian Ocean as SST and SLH residuals become decorrelated, with wave amplitudes decreasing and westward phase speeds increasing. Here, the ocean and atmosphere thermal and vorticity budgets of the coupled Rossby waves in the eastern Indian Ocean along 20°S are diagnosed. In the upper ocean, these diagnostics find the residual SST tendency driven by the residual meridional geostrophic advection of mean temperature with warm SST residuals dissipated by upward latent heat flux to the atmosphere. In the troposphere, these upward latent heat fluxes drive mid-to-upper-level residual diabatic heating via excess condensation, balanced there by upward residual vertical thermal advection. The resulting upward residual vertical velocity drives residual upper-level divergence and lower-level convergence, the latter balanced in the troposphere vorticity budget by the residual meridional advection of planetary vorticity. This yields poleward MSW residuals collocated with warm SST residuals, as observed. The SLH tendency is modified by a positive feedback from wind stress curl residuals, the latter acting to increase the amplitude and decrease the westward phase speed of the wave. These diagnostics allow a more exact analytical model for coupled Rossby waves to be constructed, yielding wave characteristics as observed.

scholarly journals Idealized Modeling of the Atmospheric Boundary Layer Response to SST Forcing in the Western Indian Ocean

2019 ◽  
Vol 76 (7) ◽  
pp. 2023-2042 ◽  
Author(s):  
Adam V. Rydbeck ◽  
Tommy G. Jensen ◽  
Matthew R. Igel
Keyword(s):  
Boundary Layer ◽  
Indian Ocean ◽  
Atmospheric Boundary Layer ◽  
Rossby Waves ◽  
Surface Wind ◽  
Western Indian Ocean ◽  
Overturning Circulation ◽  
Low Level ◽  
Sst Gradient ◽  
Equatorial Rossby Waves

Abstract The atmospheric response to sea surface temperature (SST) variations forced by oceanic downwelling equatorial Rossby waves is investigated using an idealized convection-resolving model. Downwelling equatorial Rossby waves sharpen SST gradients in the western Indian Ocean. Changes in SST cause the atmosphere to hydrostatically adjust, subsequently modulating the low-level wind field. In an idealized cloud model, surface wind speeds, surface moisture fluxes, and low-level precipitable water maximize near regions of strongest SST gradients, not necessarily in regions of warmest SST. Simulations utilizing the steepened SST gradient representative of periods with oceanic downwelling equatorial Rossby waves show enhanced patterns of surface convergence and precipitation that are linked to a strengthened zonally overturning circulation. During these conditions, convection is highly organized, clustering near the maximum SST gradient and ascending branch of the SST-induced overturning circulation. When the SST gradient is reduced, as occurs during periods of weak or absent oceanic equatorial Rossby waves, convection is much less organized and total rainfall is decreased. This demonstrates the previously observed upscale organization of convection and rainfall associated with oceanic downwelling equatorial Rossby waves in the western Indian Ocean. These results suggest that the enhancement of surface fluxes that results from a steepening of the SST gradient is the leading mechanism by which oceanic equatorial Rossby waves prime the atmospheric boundary layer for rapid convective development.

A new mode of decadal variability in the Tropical Indian Ocean subsurface temperature and its association with heat redistribution

2021 ◽  
Author(s):  
Sandeep Mohapatra ◽  
Chellappan Gnanaseelan
Keyword(s):  
Indian Ocean ◽  
Wind Stress ◽  
Decadal Variability ◽  
Surface Wind ◽  
Heat Content ◽  
Tropical Indian Ocean ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Subsurface Temperature ◽  
Wind Stress Curl

<p>Similar to the Pacific and Atlantic, Tropical Indian Ocean (TIO) has its own internal climate mode of variabilities such as Indian Ocean Dipole (IOD) and subsurface mode (SSM). A typical interannual SSM is characterized by the meridional gradient in opposing subsurface temperature anomalies in the eastern equatorial IO and in the southwestern IO. Here in the present study, we have explored the structure and the underlying dynamics for the SSM in decadal time scale which has not been reported before. By analyzing different reanalysis products we observe that decadal SSM is characterized by a pure north-south pattern with the northern mode covering the entire equatorial belt which is different from interannual SSM. A north-south SSM is the leading mode of decadal variability in the thermocline and subsurface temperature over the TIO. Our preliminary analysis suggests that the decadal variability in the surface winds along the equatorial IO and the associated wind stress curl are found to be the primary forcing mechanisms for the decadal evolution of the north-south mode. Positive wind stress curl anomalies south of 8<sup>o</sup>S intensify the downwelling Rossby waves in the south during the positive phase of the decadal SSM. On the other hand, the northern cooling is driven mostly by the equatorial upwelling Kelvin waves and the Ekman divergence. Further, the phase transition in the SSM is primarily determined by the strength of the surface wind and the associated Ekman transport. The equatorial easterlies (westerlies) diverge (converge) the meridional Ekman transport, transporting heat towards the off-equatorial (equatorial) region during the positive (negative) phase. Consistently with SSM, upper 500m oceanic heat content reveals a conventional north-south dipole highlighting the importance of SSM on the TIO heat redistribution. This is further supported by the modulation of meridional overturning circulation and the meridional heat balance across the southern Indian Ocean (SIO). Overall the present study explores the underlying mechanism responsible for decadal SSM and its association with the heat distribution across the SIO.</p>

scholarly journals Indian Ocean Warming

2020 ◽  
pp. 191-206 ◽  
Author(s):  
M. K. Roxy ◽  
C. Gnanaseelan ◽  
Anant Parekh ◽  
Jasti S. Chowdary ◽  
Shikha Singh ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Climate Models ◽  
Ghg Emissions ◽  
Heat Content ◽  
Tropical Indian Ocean ◽  
Western Indian Ocean ◽  
Ocean Warming ◽  
Marine Phytoplankton ◽  
Indian Ocean Warming ◽  
The Future

Abstract Sea surface temperature (SST) and upper ocean heat content (OHC, upper 700 m) in the tropical Indian Ocean underwent rapid warming during 1950–2015, with the SSTs showing an average warming of about 1 °C. The SST and OHC trends are very likely to continue in the future, under different emission scenarios. Climate models project a rise in tropical Indian Ocean SST by 1.2–1.6 °C and 1.6–2.7 °C in the near (2040–2069) and far (2070–2099) future across greenhouse gas (GHG) emissions scenarios RCP4.5 and RCP8.5, relative to the reference period of 1976–2005. Indian Ocean warming has very likely resulted in decreasing trend in oxygen (O2) concentrations in the tropical Indian Ocean, and declining trends in pH and marine phytoplankton over the western Indian Ocean. The observed trends in O2, pH and marine phytoplankton are projected to increase in the future with continued GHG emissions.

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