scholarly journals On the semi-diagnostic computation of climatological circulation in the western tropical Indian Ocean

MAUSAM ◽  
2021 ◽  
Vol 51 (4) ◽  
pp. 329-348
Author(s):  
C. SHAJI ◽  
A. D. RAO ◽  
S. K. DUBE ◽  
N. BAHULAYAN
Keyword(s):  
Indian Ocean ◽  
Surface Wind ◽  
Diagnostic Technique ◽  
Circulation Model ◽  
Tropical Indian Ocean ◽  
Salt Transport ◽  
3 Dimensional ◽  
Sea Surface Topography ◽  
Salinity Data ◽  
Climatological Circulation

The seasonal mean climatological circulation in the Indian Ocean north of 20°S and west of 80°E during the summer and winter has been investigated using a 3-dimensional, fully non-linear, semi-diagnostic circulation model. The model equations include the basic ocean hydrothermodynamic  equations of momentum, hydrostatics, continuity, sea surface topography and temperature and salt transport equations. Model is driven with the seasonal mean data on wind stress at the ocean surface and thermohaline forcing at different levels. The circulation in the upper levels of the ocean at 20, 50, 150, 300, 500 and 1000 m depths during the two contrasting seasons has been obtained using the model, and the role of steady, local forcing of wind and internal density field on the dynamical balance of circulation in the western tropical Indian Ocean is explained. The climatological temperature and salinity data used to drive the model is found to be hydrodynamically adjusted with surface wind, flow field and bottom relief during the adaptation stages. Semi-diagnostic technique is found to be very effective for the smoothening of climatic temperature and salinity data and also to obtain the 3-dimensional steady state circulation, which would serve as initial condition in simulation models of circulation.

scholarly journals Contrasting Eastern-Pacific and Central-Pacific Types of ENSO

2009 ◽  
Vol 22 (3) ◽  
pp. 615-632 ◽  
Author(s):  
Hsun-Ying Kao ◽  
Jin-Yi Yu
Keyword(s):  
Indian Ocean ◽  
Southern Oscillation ◽  
Surface Wind ◽  
Tropical Indian Ocean ◽  
Eastern Pacific ◽  
Structure Evolution ◽  
Interdecadal Change ◽  
Central Pacific ◽  
Enso Events ◽  
Dominant Period

Abstract Surface observations and subsurface ocean assimilation datasets are examined to contrast two distinct types of El Niño–Southern Oscillation (ENSO) in the tropical Pacific: an eastern-Pacific (EP) type and a central-Pacific (CP) type. An analysis method combining empirical orthogonal function (EOF) analysis and linear regression is used to separate these two types. Correlation and composite analyses based on the principal components of the EOF were performed to examine the structure, evolution, and teleconnection of these two ENSO types. The EP type of ENSO is found to have its SST anomaly center located in the eastern equatorial Pacific attached to the coast of South America. This type of ENSO is associated with basinwide thermocline and surface wind variations and shows a strong teleconnection with the tropical Indian Ocean. In contrast, the CP type of ENSO has most of its surface wind, SST, and subsurface anomalies confined in the central Pacific and tends to onset, develop, and decay in situ. This type of ENSO appears less related to the thermocline variations and may be influenced more by atmospheric forcing. It has a stronger teleconnection with the southern Indian Ocean. Phase-reversal signatures can be identified in the anomaly evolutions of the EP-ENSO but not for the CP-ENSO. This implies that the CP-ENSO may occur more as events or epochs than as a cycle. The EP-ENSO has experienced a stronger interdecadal change with the dominant period of its SST anomalies shifted from 2 to 4 yr near 1976/77, while the dominant period for the CP-ENSO stayed near the 2-yr band. The different onset times of these two types of ENSO imply that the difference between the EP and CP types of ENSO could be caused by the timing of the mechanisms that trigger the ENSO events.

scholarly journals Contributions of Indian Ocean and Monsoon Biases to the Excessive Biennial ENSO in CCSM3

2009 ◽  
Vol 22 (7) ◽  
pp. 1850-1858 ◽  
Author(s):  
Jin-Yi Yu ◽  
Fengpeng Sun ◽  
Hsun-Ying Kao
Keyword(s):  
Indian Ocean ◽  
Southern Oscillation ◽  
Surface Wind ◽  
Tropical Indian Ocean ◽  
Global Ocean ◽  
Climate System Model ◽  
Wind Pattern ◽  
Enso Variability ◽  
Monsoon Variability ◽  
Ocean Atmosphere

Abstract The Community Climate System Model, version 3 (CCSM3), is known to produce many aspects of El Niño–Southern Oscillation (ENSO) realistically, but the simulated ENSO exhibits an overly strong biennial periodicity. Hypotheses on the cause of this excessive biennial tendency have thus far focused primarily on the model’s biases within the tropical Pacific. This study conducts CCSM3 experiments to show that the model’s biases in simulating the Indian Ocean mean sea surface temperatures (SSTs) and the Indian and Australian monsoon variability also contribute to the biennial ENSO tendency. Two CCSM3 simulations are contrasted: a control run that includes global ocean–atmosphere coupling and an experiment in which the air–sea coupling in the tropical Indian Ocean is turned off by replacing simulated SSTs with an observed monthly climatology. The decoupling experiment removes CCSM3’s warm bias in the tropical Indian Ocean and reduces the biennial variability in Indian and Australian monsoons by about 40% and 60%, respectively. The excessive biennial ENSO is found to reduce dramatically by about 75% in the decoupled experiment. It is shown that the biennial monsoon variability in CCSM3 excites an anomalous surface wind pattern in the western Pacific that projects well into the wind pattern associated with the onset phase of the simulated biennial ENSO. Therefore, the biennial monsoon variability is very effective in exciting biennial ENSO variability in CCSM3. The warm SST bias in the tropical Indian Ocean also increases ENSO variability by inducing stronger mean surface easterlies along the equatorial Pacific, which strengthen the Pacific ocean–atmosphere coupling and enhance the ENSO intensity.

A Model-Based Assessment and Design of a Tropical Indian Ocean Mooring Array

2007 ◽  
Vol 20 (13) ◽  
pp. 3269-3283 ◽  
Author(s):  
Peter R. Oke ◽  
Andreas Schiller
Keyword(s):  
Indian Ocean ◽  
Interannual Variability ◽  
Time Scales ◽  
Ocean Circulation ◽  
Mixed Layer Depth ◽  
Intraseasonal Variability ◽  
Circulation Model ◽  
Tropical Indian Ocean ◽  
Empirical Orthogonal Functions ◽  
Analysis System

Abstract A series of observing system simulation experiments (OSSEs) are performed for the tropical Indian Ocean (±15° from the equator) using a simple analysis system. The analysis system projects an array of observations onto the dominant empirical orthogonal functions (EOFs) derived from an intermediate-resolution (2° × 0.5°) ocean circulation model. This system produces maps of the depth of the 20°C isotherm (D20), representing interannual variability, and the high-pass-filtered mixed layer depth (MLD), representing intraseasonal variability. The OSSEs are designed to assess the suitability of the proposed Indian Ocean surface mooring array for resolving intraseasonal to interannual variability. While the proposed array does a reasonable job of resolving the interannual time scales, it may not adequately resolve the intraseasonal time scales. A procedure is developed to rank the importance of observation locations by determining the observation array that best projects onto the EOFs used in the analysis system. OSSEs using an optimal array clearly outperform the OSSEs using the proposed array. The configuration of the optimal array is sensitive to the number of EOFs considered. The optimal array is also different for D20 and MLD, and depends on whether fixed observations are included that represent an idealized Argo array. Therefore, a relative frequency map of observation locations identified in 24 different OSSEs is compiled and a single, albeit less optimal, array that is referred to as a consolidated array is objectively determined. The consolidated array reflects the general features of the individual optimal arrays derived from all OSSEs. It is found that, in general, observations south of 8°S and off of the Indonesian coast are most important for resolving the interannual variability, while observations a few degrees south of the equator, and west of 75°E, and a few degrees north of the equator, and east of 75°E, are important for resolving the intraseasonal variability. In a series of OSSEs, the consolidated array is shown to outperform the proposed array for all configurations of the analysis system for both D20 and MLD.

scholarly journals Predictions of Indian Ocean SST Indices with a Simple Statistical Model: A Null Hypothesis

2009 ◽  
Vol 22 (18) ◽  
pp. 4930-4938 ◽  
Author(s):  
Dietmar Dommenget ◽  
Malte Jansen
Keyword(s):  
Indian Ocean ◽  
Statistical Model ◽  
General Circulation Model ◽  
Null Hypothesis ◽  
General Circulation ◽  
Circulation Model ◽  
Tropical Indian Ocean ◽  
Model Studies ◽  
Sst Variability ◽  
The Indian Ocean

Abstract Several recent general circulation model studies discuss the predictability of the Indian Ocean dipole (IOD) mode, suggesting that it is predictable because of coupled ocean–atmosphere interactions in the Indian Ocean. However, it is not clear from these studies how much of the predictability is due to the response to El Niño. It is shown in this note that a simple statistical model that treats the Indian Ocean as a red noise process forced by tropical Pacific SST shows forecast skills comparable to those of recent general circulation model studies. The results also indicate that some of the eastern tropical Indian Ocean SST predictability in recent studies may indeed be beyond the skill of the simple model proposed in this note, indicating that dynamics in the Indian Ocean may have caused this improved predictability in this region. The model further indicates that the IOD index may be the least predictable index of Indian Ocean SST variability. The model is proposed as a null hypothesis for Indian Ocean SST predictions.

scholarly journals Extreme IOD induced tropical Indian Ocean warming in 2020

2021 ◽  
Vol 8 (1) ◽  
Author(s):  
Ying Zhang ◽  
Yan Du
Keyword(s):  
Indian Ocean ◽  
Surface Wind ◽  
Tropical Indian Ocean ◽  
First Record ◽  
Early Summer ◽  
Wind Anomaly ◽  
Surface Warming ◽  
Positive Indian Ocean Dipole ◽  
Indian Ocean Warming ◽  
The 1960S

AbstractThe tropical Indian Ocean (TIO) basin-wide warming occurred in 2020, following an extreme positive Indian Ocean Dipole (IOD) event instead of an El Niño event, which is the first record since the 1960s. The extreme 2019 IOD induced the oceanic downwelling Rossby waves and thermocline warming in the southwest TIO, leading to sea surface warming via thermocline-SST feedback during late 2019 to early 2020. The southwest TIO warming triggered equatorially antisymmetric SST, precipitation, and surface wind patterns from spring to early summer. Subsequently, the cross-equatorial “C-shaped” wind anomaly, with northeasterly–northwesterly wind anomaly north–south of the equator, led to basin-wide warming through wind-evaporation-SST feedback in summer. This study reveals the important role of air–sea coupling processes associated with the independent and extreme IOD in the TIO basin-warming mode, which allows us to rethink the dynamic connections between the Indo-Pacific climate modes.

scholarly journals Simulated Sea Surface Salinity Variability in the Tropical Indian Ocean

2010 ◽  
Vol 23 (24) ◽  
pp. 6542-6554 ◽  
Author(s):  
Rashmi Sharma ◽  
Neeraj Agarwal ◽  
Imran M. Momin ◽  
Sujit Basu ◽  
Vijay K. Agarwal
Keyword(s):  
Indian Ocean ◽  
Time Scales ◽  
General Circulation ◽  
Circulation Model ◽  
Equatorial Indian Ocean ◽  
Tropical Indian Ocean ◽  
Sea Surface ◽  
Sea Surface Salinity ◽  
Surface Salinity ◽  
Salinity Variability

Abstract A long-period (15 yr) simulation of sea surface salinity (SSS) obtained from a hindcast run of an ocean general circulation model (OGCM) forced by the NCEP–NCAR daily reanalysis product is analyzed in the tropical Indian Ocean (TIO). The objective of the study is twofold: assess the capability of the model to provide realistic simulations of SSS and characterize the SSS variability in view of upcoming satellite salinity missions. Model fields are evaluated in terms of mean, standard deviation, and characteristic temporal scales of SSS variability. Results show that the standard deviations range from 0.2 to 1.5 psu, with larger values in regions with strong seasonal transitions of surface currents (south of India) and along the coast in the Bay of Bengal (strong Kelvin-wave-induced currents). Comparison of simulated SSS with collocated SSS measurements from the National Oceanographic Data Center and Argo floats resulted in a high correlation of 0.85 and a root-mean-square error (RMSE) of 0.4 psu. The correlations are quite high (>0.75) up to a depth of 300 m. Daily simulations of SSS compare well with a Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA) buoy in the eastern equatorial Indian Ocean (1.5°S, 90°E) with an RMSE of 0.3 psu and a correlation better than 0.6. Model SSS compares well with observations at all time scales (intraseasonal, seasonal, and interannual). The decorrelation scales computed from model and buoy SSS suggest that the proposed 10-day sampling of future salinity sensors would be able to resolve much of the salinity variability at time scales longer than intraseasonal. This inference is significant in view of satellite salinity sensors, such as Soil Moisture and Ocean Salinity (SMOS) and Aquarius.

Deep Signatures of Southern Tropical Indian Ocean Annual Rossby Waves*

2011 ◽  
Vol 41 (10) ◽  
pp. 1958-1964 ◽  
Author(s):  
Gregory C. Johnson
Keyword(s):  
Indian Ocean ◽  
Annual Cycle ◽  
Surface Wind ◽  
Phase Speed ◽  
Tropical Indian Ocean ◽  
Energy Propagation ◽  
Deep Basin ◽  
Zonal Velocity ◽  
Vertical Displacements ◽  
Phase Propagation

Abstract The southern tropical Indian Ocean contains a striking forced annual Rossby wave studied previously using satellite altimeter sea surface height data, surface wind fields, expendable bathythermograph ocean temperature data, and models. Here, the deep reach of this wave and its velocity are analyzed using density–depth profiles and 1000-dbar horizontal drift data from Argo. Significant annual cycles in isopycnal vertical displacements and zonal velocity persist to the deepest pressures to which Argo data can be mapped reliably in the region, 1600–1900 dbar. Phase propagation of the annual cycle of the directly measured zonal velocities at 1000 dbar suggests a zonal wavelength of about 6000 km—about the length of the deep basin in which the wave is found—and a westward phase speed of ~0.2 m s−1. Apparent upward phase propagation in isopycnal vertical displacements suggests energy propagation downward into the abyss. This pattern is clearer when accounting for both the potential and kinetic energy of the wave. The largest zonal current associated with this wave has a middepth maximum that decays rapidly up through the pycnocline and less rapidly with increasing depth, suggesting a first-vertical-mode structure. The anomalous zonal volume transport of this annually reversing current is ~27 × 106 m3 s−1 across 80°E in mid-November. The peak zonal velocity of 0.06 m s−1 implies a maximum zonal excursion of about 600 km associated with the wave over an annual cycle.

scholarly journals A Short Surface Pathway of the Subsurface Indonesian Throughflow Water from the Java Coast Associated with Upwelling, Ekman Transport, and Subduction

2010 ◽  
Vol 2010 ◽  
pp. 1-15 ◽  
Author(s):  
Vinu Valsala ◽  
Shamil Maksyutov
Keyword(s):  
Indian Ocean ◽  
General Circulation ◽  
Circulation Model ◽  
Tropical Indian Ocean ◽  
Ocean General Circulation Model ◽  
Reanalysis Data ◽  
Ekman Transport ◽  
Indonesian Throughflow ◽  
The South ◽  
Wind Stress Curl

A surface pathway of the subsurface Indonesian Throughflow (ITF) in the southeastern Indian Ocean is proposed using a combined analysis of Lagrangian particles and passive tracers derived from two independent tools: an Ocean General Circulation Model (OGCM) and Simple Ocean Data Assimilation (SODA.2.0.2) reanalysis data. This newly suggested pathway follows the processes in succession as upwelling in the south Java coast, offshore Ekman drift and subduction into the thermocline centered on 20∘S. The upwelling of subsurface ITF along the south Java coast is found to occur from August to October. Upon surfacing, the ITF advects southwestward being trapped in the surface Ekman layer for an approximate period of 260 days and reaches the southeastern tropical Indian Ocean subduction zone centered on 20∘S which is demarcated by the Zero Wind Stress Curl (ZWSC) and subducts there. The particle trajectory revealed that during the subduction within the ZWSC region, the surface eastward flow above 120 m depth carries the particle about 10∘ to the east and westward flow below this depth carries the particle to the western Indian Ocean along the thermocline. These pathways are confirmed by a series of tracer experiments using SODA reanalysis data. The effects of vertical mixing and entrainment on the surfacing of the ITF at south Java coast were identified.

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