Capturing the 2018 Monsoon Onset in the Bay of Bengal from in-situ ship and mooring network observations

2020 ◽  
Author(s):  
Amit Tandon ◽  
Emily Shroyer ◽  
Ramasamy Venkatesan ◽  
Andrew Lucas ◽  
J. Thomas Farrar ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Mixed Layer ◽  
Bay Of Bengal ◽  
Asian Summer Monsoon ◽  
Monsoon Onset ◽  
Heat Fluxes ◽  
Surface Mixed Layer ◽  
Seasonal Oscillations

<p class="p1"><span class="s1">Air-Sea interaction in the Bay of Bengal has a strong coupling with the Monsoon rains over the South Asian region. The wet and dry spells, or active-break cycles of the Asian summer monsoon are governed by different modes of intra-seasonal variability with implied northward and westward propagation. Multiple hypotheses exist as to how air-sea interaction and the ocean mixed layer influence the propagation of Monsoon Intra-seasonal Oscillations (MISO), but the multi-scale nature of atmosphere-ocean coupling is not well understood. Multi-country collaborative initiatives MISOBOB (Oceanic Control of Monsoon Intra-seasonal Oscillations in the Tropical Indian Ocean and the Bay of Bengal-USA), RIO-MISO (Role of the Indian Ocean on Monsoon Intra-Seasonal Oscillations-USA), and OMM (Ocean Mixing and Monsoons-India) have led to a combination of ocean observations, atmospheric observations, and associated modeling to study this phenomenon. </span></p> <p class="p2"> </p> <p class="p1"><span class="s1">We present observations analyzed using the OMNI (Ocean Moored Buoy Network for Northern Indian Ocean) buoy network of India and RAMA 15N mooring along with MISOBOB field program in June 2018, which captured the onset of the 2018 Monsoon from a heavily instrumented ship that simultaneously made measurements in the atmospheric and oceanic boundary layers. The shortwave and net heat fluxes show dramatic changes during the active phase with the in-situ net heat flux reversing sign. The Monsoon onset cooled all of the Central and North Bay of Bengal by 1.5 K, leading to large heat losses in the Bay, as the oceanic surface mixed layer deepened from 20m to about 40m. This talk will also explore the role of sub-surface salinity stratification in modulating cooling of the upper ocean at multiple locations across the Bay, providing a basin-wide view. Observations suggest that the air-sea interaction and ocean stratification in the Bay likely has strong feedback on the organized convection in the atmosphere.</span></p>

scholarly journals The Role of Turbulence in Redistributing Upper-Ocean Heat, Freshwater, and Momentum in Response to the MJO in the Equatorial Indian Ocean

2018 ◽  
Vol 48 (1) ◽  
pp. 197-220 ◽  
Author(s):  
Kandaga Pujiana ◽  
James N. Moum ◽  
William D. Smyth
Keyword(s):  
Indian Ocean ◽  
Wind Stress ◽  
Mixed Layer ◽  
Turbulent Mixing ◽  
Active Phase ◽  
Salt Flux ◽  
Surface Mixed Layer ◽  
Turbulent Stress ◽  
The Mean

AbstractThe role of turbulent mixing in regulating the ocean’s response to the Madden–Julian oscillation (MJO) is assessed from measurements of surface forcing, acoustic, and microstructure profiles during October–early December 2011 at 0°, 80.5°E in the Indian Ocean. During the active phase of the MJO, the surface mixed layer was cooled from above by air–sea fluxes and from below by turbulent mixing, in roughly equal proportions. During the suppressed and disturbed phases, the mixed layer temperature increased, primarily because of the vertical divergence between net surface warming and turbulent cooling. Despite heavy precipitation during the active phase, subsurface mixing was sufficient to increase the mixed layer salinity by entraining salty Arabian Sea Water from the pycnocline. The turbulent salt flux across the mixed layer base was, on average, 2 times as large as the surface salt flux. Wind stress accelerated the Yoshida–Wyrtki jet, while the turbulent stress was primarily responsible for decelerating the jet through the active phase, during which the mean turbulent stress was roughly 65% of the mean surface wind stress. These turbulent processes may account for systematic errors in numerical models of MJO evolution.

Genesis of Indian Ocean Mixed Layer Temperature Anomalies: A Heat Budget Analysis

2010 ◽  
Vol 23 (20) ◽  
pp. 5375-5403 ◽  
Author(s):  
Agus Santoso ◽  
Alexander Sen Gupta ◽  
Matthew H. England
Keyword(s):  
Indian Ocean ◽  
Time Scales ◽  
Mixed Layer ◽  
Heat Budget ◽  
Heat Fluxes ◽  
Temperature Anomalies ◽  
Mixed Layer Temperature ◽  
The Indian Ocean ◽  
The Tropics

Abstract The genesis of mixed layer temperature anomalies across the Indian Ocean are analyzed in terms of the underlying heat budget components. Observational data, for which a seasonal budget can be computed, and a climate model output, which provides improved spatial and temporal coverage for longer time scales, are examined. The seasonal climatology of the model heat budget is broadly consistent with the observational reconstruction, thus providing certain confidence in extending the model analysis to interannual time scales. To identify the dominant heat budget components, covariance analysis is applied based on the heat budget equation. In addition, the role of the heat budget terms on the generation and decay of temperature anomalies is revealed via a novel temperature variance budget approach. The seasonal evolution of the mixed layer temperature is found to be largely controlled by air–sea heat fluxes, except in the tropics where advection and entrainment are important. A distinct shift in the importance and role of certain heat budget components is shown to be apparent in moving from seasonal to interannual time scales. On these longer time scales, advection gains importance in generating and sustaining anomalies over extensive regions, including the trade wind and midlatitude wind regimes. On the other hand, air–sea heat fluxes tend to drive the evolution of thermal anomalies over subtropical regions including off northwestern Australia. In the tropics, however, they limit the growth of anomalies. Entrainment plays a role in the generation and maintenance of interannual anomalies over localized regions, particularly off Sumatra and over the Seychelles–Chagos Thermocline Ridge. It is further shown that the spatial distribution of the role and importance of these terms is related to oceanographic features of the Indian Ocean. Mixed layer depth effects and the influence of model biases are discussed.

scholarly journals Role of Diurnal Warm Layers in the Diurnal Cycle of Convection over the Tropical Indian Ocean during MISMO

2010 ◽  
Vol 138 (6) ◽  
pp. 2426-2433 ◽  
Author(s):  
H. Bellenger ◽  
Y. N. Takayabu ◽  
T. Ushiyama ◽  
K. Yoneyama
Keyword(s):  
Indian Ocean ◽  
Diurnal Cycle ◽  
Tropical Indian Ocean ◽  
Early Morning ◽  
Heat Fluxes ◽  
Surface Heat Fluxes ◽  
Boundary Layer Processes ◽  
Early Afternoon

Abstract The role of air–sea interaction in the diurnal variations of convective activity during the suppressed and developing stages of an intraseasonal convective event is analyzed using in situ observations from the Mirai Indian Ocean cruise for the Study of the Madden–Julian oscillation (MJO)-convection Onset (MISMO) experiment. For the whole period, convection shows a clear average diurnal cycle with a primary maximum in the early morning and a secondary one in the afternoon. Episodes of large diurnal sea surface temperature (SST) variations are observed because of diurnal warm layer (DWL) formation. When no DWL is observed, convection exhibits a diurnal cycle characterized by a maximum in the early morning, whereas when DWL forms, convection increases around noon and peaks in the afternoon. Boundary layer processes are found to control the diurnal evolution of convection. In particular, when DWL forms, the change in surface heat fluxes can explain the decrease of convective inhibition and the intensification of the convection during the early afternoon.

scholarly journals Interannual Variability of Sea Surface Temperature off Java and Sumatra in a Global GCM*

2008 ◽  
Vol 21 (11) ◽  
pp. 2451-2465 ◽  
Author(s):  
Yan Du ◽  
Tangdong Qu ◽  
Gary Meyers
Keyword(s):  
Indian Ocean ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Mixed Layer ◽  
Heat Budget ◽  
Sea Surface ◽  
Surface Mixed Layer ◽  
Horizontal Advection ◽  
Cold Anomaly ◽  
Sst Anomalies

Abstract Using results from the Simple Ocean Data Assimilation (SODA), this study assesses the mixed layer heat budget to identify the mechanisms that control the interannual variation of sea surface temperature (SST) off Java and Sumatra. The analysis indicates that during the positive Indian Ocean Dipole (IOD) years, cold SST anomalies are phase locked with the season cycle. They may exceed −3°C near the coast of Sumatra and extend as far westward as 80°E along the equator. The depth of the thermocline has a prominent influence on the generation and maintenance of SST anomalies. In the normal years, cooling by upwelling–entrainment is largely counterbalanced by warming due to horizontal advection. In the cooling episode of IOD events, coastal upwelling–entrainment is enhanced, and as a result of mixed layer shoaling, the barrier layer no longer exists, so that the effect of upwelling–entrainment can easily reach the surface mixed layer. Horizontal advection spreads the cold anomaly to the interior tropical Indian Ocean. Near the coast of Java, the northern branch of an anomalous anticyclonic circulation spreads the cold anomaly to the west near the equator. Both the anomalous advection and the enhanced, wind-driven upwelling generate the cold SST anomaly of the positive IOD. At the end of the cooling episode, the enhanced surface thermal forcing overbalances the cooling effect by upwelling/entrainment, and leads to a warming in SST off Java and Sumatra.

scholarly journals Spring Mixing: Turbulence and Internal Waves during Restratification on the New England Shelf

2005 ◽  
Vol 35 (12) ◽  
pp. 2425-2443 ◽  
Author(s):  
J. A. MacKinnon ◽  
M. C. Gregg
Keyword(s):  
New England ◽  
Internal Waves ◽  
Mixed Layer ◽  
Wave Interaction ◽  
Heat Fluxes ◽  
Surface Mixed Layer ◽  
Turbulent Dissipation ◽  
Surface Heat Fluxes ◽  
Bottom Boundary Layers ◽  
Diapycnal Diffusivity

Abstract Integrated observations are presented of water property evolution and turbulent microstructure during the spring restratification period of April and May 1997 on the New England continental shelf. Turbulence is shown to be related to surface mixed layer entrainment and shear from low-mode near-inertial internal waves. The largest turbulent diapycnal diffusivity and associated buoyancy fluxes were found at the bottom of an actively entraining and highly variable wind-driven surface mixed layer. Away from surface and bottom boundary layers, turbulence was systematically correlated with internal wave shear, though the nature of that relationship underwent a regime shift as the stratification strengthened. During the first week, while stratification was weak, the largest turbulent dissipation away from boundaries was coincident with shear from mode-1 near-inertial waves generated by passing storms. Wave-induced Richardson numbers well below 0.25 and density overturning scales of several meters were observed. Turbulent dissipation rates in the region of peak shear were consistent in magnitude with several dimensional scalings. The associated average diapycnal diffusivity exceeded 10−3 m2 s−1. As stratification tripled, Richardson numbers from low-mode internal waves were no longer critical, though turbulence was still consistently elevated in patches of wave shear. Kinematically, dissipation during this period was consistent with the turbulence parameterization proposed by MacKinnon and Gregg, based on a reinterpretation of wave–wave interaction theory. The observed growth of temperature gradients was, in turn, consistent with a simple one-dimensional model that vertically distributed surface heat fluxes commensurate with calculated turbulent diffusivities.

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.

scholarly journals Mechanisms of Barrier Layer Formation and Erosion from In Situ Observations in the Bay of Bengal

2019 ◽  
Vol 49 (5) ◽  
pp. 1183-1200 ◽  
Author(s):  
Jenson V. George ◽  
P. N. Vinayachandran ◽  
V. Vijith ◽  
V. Thushara ◽  
Anoop A. Nayak ◽  
...  
Keyword(s):  
Time Series ◽  
Barrier Layer ◽  
Bay Of Bengal ◽  
High Salinity ◽  
Layer Structure ◽  
Surface Mixed Layer ◽  
Study Region ◽  
In Situ Observations ◽  
Differential Advection

AbstractDuring the Bay of Bengal (BoB) Boundary Layer Experiment (BoBBLE) in the southern BoB, time series of microstructure measurements were obtained at 8°N, 89°E from 4 to 14 July 2016. These observations captured events of barrier layer (BL) erosion and reformation. Initially, a three-layer structure was observed: a fresh surface mixed layer (ML) of thickness 10–20 m; a BL below of 30–40-m thickness with similar temperature but higher salinity; and a high salinity core layer, associated with the Summer Monsoon Current. Each of these three layers was in relative motion to the others, leading to regions of high shear at the interfaces. However, the destabilizing influence of the shear regions was not enough to overcome the haline stratification, and the three-layer structure was preserved. A salinity budget using in situ observations suggested that during the BL erosion, differential advection brought high salinity surface waters (34.5 psu) with weak stratification to the time series location and replaced the three-layer structure with a deep ML (~60 m). The resulting weakened stratification at the time series location then allowed atmospheric wind forcing to penetrate deeper. The turbulent kinetic energy dissipation rate and eddy diffusivity showed elevated values above 10−7 W kg−1 and 10−4 m2 s−1, respectively, in the upper 60 m. Later, the surface salinity decreased again (33.8 psu) through differential horizontal advection, stratification became stronger and elevated mixing rates were confined to the upper 20 m, and the BL reformed. A 1D model experiment suggested that in the study region, differential advection of temperature–salinity characteristics is essential for the maintenance of BL and to the extent to which mixing penetrates the water column.

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