Modeling Indian Ocean Circulation: Bay of Bengal Fresh Plume and Arabian Sea Mini Warm Pool

P. N. Vinayachandran*¹  and J.  Kurian*^{1, *2}

^*1Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India,

vinay@caos.iisc.ernet.in

^*2 Present affiliation. Department of Atmo spheric and Oceanic Sciences, UCLA, USA

ABSTRACT: The Indian subcontinent divides the north Indian Ocean into two tropical basins, namely the Arabian Sea and the Bay of Bengal.  The Arabian Sea has high salinity whereas the salinity of the Bay of Bengal is much lower due to the contrast in freshwater forcing of the two basins.  The freshwater received by the Bay in large amounts during the summer monsoon through river discharge is flushed out annually by ocean circulation. After the withdrawal of the summer monsoon, the Ganga – Brahmaputra river plume flows fi rst along the Indian coast and then around  Sri Lanka into the Arabian Sea creating a low salinity  pool in the southeastern  Arabian Sea (SEAS). In the same region, during the pre-monsoon months of February – April, a warm pool, known  as the Arabian Sea Mini Warm Pool (ASMWP),  which is distinctly warmer than the rest of the Indian Ocean, takes shape.  In fact, this is the warmest region in the world oceans during this period. Simulation of the river plume and its movement as well as its implications to thermodynamics has been a challenging probl em for models of Indian Ocean. Here we address these issues using an ocean general circulation model – first we show that the model is capable of reproducing fresh plumes in the Bay of Bengal as well as  its movement and then we  use the model to determine the processes that lead to formation of the ASMWP.

Hydrographic observations from the western Bay of Bengal have shown the presence of a fresh plume along the northern part of the Indian coast during summe r monsoon. The Indian Ocean model when forced by realistic winds and climatological rive r discharge reproduces the fresh plume  with reasonable accuracy. The fresh plume does not advect along the Indian coast until the end of  summer monsoon.  The North Bay Monsoon Current, which flows eastward in the northern Bay, separates the low salinity water from the more saline southern parts of the bay and thus plays an important role in the fresh water budget of the Bay of Bengal. The model also reproduces the surge of the fresh-plume along the Indian coast, into the Arabian Sea  during  northeast monsoon.

Mechanisms that lead to the formation of the Arabian Sea Mini Warm Pool are investigated using several numerical experiments. Contrary to the existing theories, we find that salinity effects are not necessary for the formation of the ASMWP. The orographic  effects of the Sahyadris (Western Ghat s) and  resulting reduction in wind speed leads to the formation of the  ASMWP. During November – April, the SEAS behave as a low-wind heat-dominated regime where the evolution of sea surface temperature is solely determined by atmospheric forcing. In such regions the evolution of surface layer temperature is not  dependent on the characteristics of the subsurface ocean such as the barrier layer and temperature inversion.

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Indonesian Rainfall Variability: Impacts of ENSO and Local Air–Sea Interaction

HARRY H. HENDON

NOAA–CIRES Climate Diagnostics Center, Boulder, Colorado

(Manuscript received 9 November 2001, in final form 28 October 2002)

ABSTRACT

     Relationships between Indonesian rainfall and Indo-Pacific sea surface temperatures (SSTs) and circulation anomalies are investigated using observations for 1951–97. Indonesia receives significant rainfall year-round but experiences a wet season that peaks in January and a dry season that peaks in August. Dry season rainfall anomalies are spatially coherent, strongly correlated with SST, and tightly coupled to El Nino–Southern Oscillation (ENSO) variations in the Pacific basin. Drought conditions typically occur during El Nin˜o, when SSTs surrounding Indonesia are cool and the Walker circulation is weakened, resulting in anomalous surface easterlies across Indonesia. The opposite tends to occur during La Nina. Broadscale Indonesian rainfall and SST anomalies tend to not persist from the dry season into the wet season. Rainfall in the heart of the wet season tends to be uncorrelated with SST and spatially incoherent.

     Seasonally varying feedback between Indonesian SST, winds, and rainfall explains the growth, persistence, and coherence of the local anomalies during the dry season and their decay or change in sign once the wet season commences. During the dry season anomalous surface easterlies, remotely driven by warm SSTs in the central Pacific during El Nino, act to increase local wind speed, cooling the ocean surrounding and to the east of Indonesia and thereby increasing the anomalous SST gradient across the Pacific. Hence, local rainfall and the Walker circulation are further reduced. Once the wet season commences and the climatological surface winds across Indonesia shift from southeasterly to northwesterly, the anomalous surface easterlies now act to reduce the wind speed. The initial cold SST anomaly is damped, reducing the negative rainfall anomalies and surface easterlies. The opposite scenario occurs during La Nina.

      Indonesian rainfall variations during the dry season are also coupled to the development of an anomalous zonal SST gradient in the equatorial Indian Ocean. This anomalous gradient is strongly related to ENSO and is most prominent during the dry season. Once the wet season commences, the entire Indian Ocean tends to have the same-signed SST anomaly (positive during El Nin˜o and negative during La Nin˜ a). Development and decay of this anomalous zonal SST gradient in the Indian Ocean is promoted by seasonally varying air–sea interaction in the eastern Indian Ocean in response to ENSO conditions in the Pacific. The eastern Indian Ocean SST changes are driven largely by induced surface heat flux variations (primarily changes in latent heat flux and net shortwave radiation). Biennial variations in the Indonesian region may also be induced by this seasonally varying air–sea interaction associated with ENSO.

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Sea-Breeze Circulation over Jakarta, Indonesia: A Climatology Based on Boundary Layer Radar Observation

TRI W. HADI,* T. HORINOUCHI, T.  TSUDA, H. HASHIGUCHI, AND S. FUKAO

Radio Science Center for Space and Atmosphere, Kyoto University, Gokasho, Uji, Kyoto, Japan

(Manuscript received 7 February 2001, in final form 9 November 2001)

ABSTRACT

Characteristics of sea-breeze circulation over the tropical site of Jakarta, Indonesia, have been documented based on analyses of satellite images and data from long-term L-band boundary layer radar measurements carried out at Serpong (6.48S, 106.78E). Inspection of satellite imagery reveals that a sea-breeze front develops well along the northern coastal plain of West Java and propagates inland until its structure is deformed over complex topography. It is found that the sea-breeze signal detected by the boundary layer radar is most well defined during the dry season months of July–October. In all of these months, radar observations indicate a late afternoon intensification of sea-breeze flow in the 0.5–0.8-km height range between 1700 and 1800 LT, which is not elucidated upon by surface measurements. The effect of weather conditions on the sea-breeze pattern is investigated by using a cloudiness index derived from data of incoming solar radiation. The results show that seabreeze intrusion over the radar site occurs earlier during more cloudy days, whereas the intensity of sea-breeze circulation weakens accordingly. In the rainy season months of January and February, diurnal wind variation is characterized by daytime onshore flow enhancement, which is not likely attributed to sea-breeze circulation.

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The Role of Bay of Bengal Convection in the Onset of the 1998 South China Sea Summer Monsoon

YIMIN LIU

State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG),

Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, and Laboratory for Atmospheric Research,

Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, China

JOHNNY C. L. CHAN

Laboratory for Atmospheric Research, Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, China

JIANGYU MAO AND GUOXIONG WU

State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG),

Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

(Manuscript received 8 January 2002, in final form 1 May 2002)

ABSTRACT

Assimilated analysis fields from the South China Sea Monsoon Experiment and the outgoing longwave radiation data from the National Center for Atmospheric Research (NCAR) have been employed to describe the largescale and synoptic features of the subtropical circulation during the Bay of Bengal (BOB; 68–208N, 808–1008E) and South China Sea (SCS; 78–208N, 1108–1208E) monsoon onsets in 1998. The results show that the Asian monsoon onset during May 1998 exhibited a typical eastward development from the BOB region to the SCS domain. The weakening and retreat of the subtropical anticyclone from the SCS were preceded by the intrusion of westerlies and the development of convective activities over the northern part of the SCS (NSCS; 158–208N, 1108–1208E). As the vertical shear of zonal wind changes in sign, the ridge surface of the subtropical anticyclone tilted northward and the summer pattern was established over the SCS. Based on these observational results, version 4 of the NCAR climate model (CCM3) is used to investigate the physical link between the convection associated with the BOB monsoon vortex and the SCS summer monsoon onset, as well as the mechanism of the evolution of the low-level subtropical anticyclone over the SCS.

Introduction of heating over the BOB results in vigorous convection over the BOB, and the BOB monsoon onset, as well as the development of westerlies and vertical ascent over the NSCS region due to an asymmetric Rossby wave response. Together with the low-level moisture advection, convection is induced over the NSCS. It is the condensation heating over the NSCS that causes the overturning of the meridional gradient of temperature over the SCS. Consequently the subtropical anticyclone in the lower troposphere over the SCS weakened gradually. Eventually as convection develops over the entire SCS domain, the subtropical anticyclone moves out of the region.

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Horizontal Distribution of Salinity and Temperature on Merbok Estuary, Malaysia

Muhammad Syukri

Jurusan Fisika/Geofisika, FMIPA, Universitas Syiah Kuala, Kopelma Darussalam

Banda Aceh 23111,lndonesia. Telp: 0651-7410248,Fax: 0651-7551831

Email: m.syukri@unsyiah.net

Abstrak

 

Telah dilakukan studi mengenai karakteristik dan distribusi salinitas dan temperatur dan water system di estuaria Merbok dan di sekitar perairan pantai. Penelitan ini difokuskan pada proses-proses fisika. Hasil analisis menun-jukkan suatu pola variabilitas khusus mengenai fenomena yang terjadi secara horizontal. Pada kondisi debit air (discharge) yang tinggi, air dengan salinitas (isohalines) rendah akan terdapat di dekat mulut sungai. Pada kondisi debit air rendah, keberadaan airsungai mengisi Iebihjeias di estuaria bagian bawah. Pola yangsamajuga teramati untuk parameter temperatur di daerah yang sama yang disebabkan rendahnya debit sungai. Dengan debit yang lebih tinggi dapat menyebabkan perubahan temperatur air payau dibandingkan debit yang lebih rendah. Hal ini menyebabkan terjadi prases transisi stratifikasi antara debit air yang tinggi dan rendah tersebut. Kata kunci: debit sungai (discharge), salinitas, temperatur, perairan pantai, estuaria.

Abstract

 

The characteristics and distribution of the salinity and temperature and water systems in the Merbok Estuary and nearby coastal waters are examined. This research focuses mainly on physical processes. The results analysis provides an insight the typical scales of variability of the horizontal phenomena. During high discharges, the lower salinity water (isohalines) was more evident near the estuary mouth due to high discharge. During low discharge, the invasion of freshwater in the lower estuary was much less pronounced. Similar temperature trend was observed for the estuary possibly due to low discharge phenomenon. Bigger amount of freshwater can dictate the brackish water temperature compare to smaller amount of freshwater. There was a transition in stratification between high and low river discharge.

 

Key words : river discharge; salinity; temperature; coastal water; estuary

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Coupled Pattern Analysis of Sea Surface Temperature and TOPEX/Poseidon Sea Surface Height

ERIC W. LEULIETTE* AND JOHN M. WAHR
Department of Physics and CIRES, University of Colorado, Boulder, Colorado
(Manuscript received 10 April 1997, in final form 14 May 1998)

ABSTRACT

Though thermal effects dominate steric changes in sea level, the long-period contribution of thermal expansion to sea level is uncertain. Nerem et al. found that a global map of sea surface temperature (SST) trends and a corresponding map of TOPEX/Poseidon-derived sea surface height (SSH) trends were strongly correlated. Thisresult is explored with a coupled pattern analysis (CPA) between five years of global SST and SSH, which allows for matching of modes of common temporal variability. The dominant mode found is an annual cycle that accounts for nearly all (95.3%) of the covariance between the fields and has a strong SST/SSH spatial correlation (0.68). The spatial correlation is strong in both the Atlantic (0.80) and the Pacific (0.70). Good temporal and spatial agreement between the SSH and SST fields for the primary seasonal mode suggests that a robust regression between fields may have some physical significance with respect to thermal expansion and that the regression coefficient might be a proxy for the mixing depth of the mode. The value of the regression coefficient, H, scaled by a thermal expansion coefficient of 2 3 1024 8C21 is 40 m for this mode, and ranges from 33 to 47 m among the basins. The primary mode of a nonseasonal CPA is an interannual mode that captures 38.0% of the covariance and has significant spatial correlations (0.54) between SSH and SST spatial patterns. The spatial pattern and temporal coefficients of this mode are correlated with ENSO events. A robust regression between fields finds that the nonseasonal modes have a regression coefficient 2–4 times that of the seasonal modes, indicative of deeper thermal mixing. The secondary nonseasonal mode captures most of the secular trend in both fields during the period examined. The temporal coefficients of this mode lag those of primary mode. Evidence is presented that this mode is consistent with the behavior expected from secular trends that are dominantly forced by thermal expansion.

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