A numerical investigation of eddy-induced chlorophyll bloom in the southeastern tropical Indian Ocean during Indian Ocean Dipole—2006

Abstract:

An eddy-resolving coupled physical–biological model is used to study the effect of cyclonic eddy in enhancing offshore chlorophyll-a (Chl-a) bloom in the southeastern tropical Indian Ocean during boreal summer–fall 2006. The results demonstrate that the offshore Chl-a blooms are markedly coincident with the high eddy kinetic energy. Moreover, the vertical variations in Chl-a, nitrate, temperature, and mixed-layer depth (MLD) strongly imply that the cyclonic eddies induce surface Chl-a bloom through the injection of nutrient-rich water into the upper layer. Interestingly, we found that the surface bloom only occurs when the deep Chl-a maximum is located within the MLD. On the other hand, the response of subsurface Chl-a to the eddy pumping is remarkable, although it is hardly observable at the surface.

Keywords: Chlorophyll-a - Upwelling - Coupled physical–biological model - Indian Ocean Dipole - Southeastern tropical Indian Ocean.

Published as:

Iskandar, I.,  H. Sasaki, Y. Sasai, Y. Masumoto and K. Mizuno, Numerical investigation of eddy-induced chlorophyll bloom in the southeastern tropical Indian Ocean during 2006, Ocean Dynamics,  doi:10.1007/s10236-010-0290-6.

Seasonal and interannual patterns of sea surface temperature in Banda Sea as revealed by self-organizing map

Abstract:

Seasonal and interannual variations of sea surface temperature (SST) in the Banda Sea are studied for the period of January 1985 through December 2007. A neural network pattern recognition approach based on self-organizing map (SOM) has been applied to monthly SST from the Advanced Very High Resolution Radiometer (AVHRR) Oceans Pathfinder. The principal conclusions of this paper are outlined as follows. There are three different patterns associated with the variations in the monsoonal winds: the southeast and northwest monsoon patterns, and the monsoon-break patterns. The southeast monsoon pattern is characterized by low SST due to the prevailing southeasterly winds that drive Ekman upwelling. The northwest monsoon pattern, on the other hand, is one of high SST distributed uniformly in space. The monsoon-break pattern is a transitional pattern between the northwest and southeast monsoon patterns, which is characterized by moderate SST patterns. On interannual time-scale, the SST variations are significantly influenced by the El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) phenomena. Low SST is observed during El Niño and/or positive IOD events, while high SST appears during La Niña event. Low SST in the Banda Sea during positive IOD event is induced by upwelling Kelvin waves generated in the equatorial Indian Ocean which propagate along the southern coast of Sumatra and Java before entering the Banda Sea through the Lombok and Ombai Straits as well as through the Timor Passage. On the other hand, during El Niño (La Niña) events, upwelling (downwelling) Rossby waves associated with off-equatorial divergence (convergence) in response to the equatorial westerly (easterly) winds in the Pacific, partly scattered into the Indonesian archipelago which in turn induce cool (warm) SST in the Banda Sea.

Published as:

Iskandar, I. (2010), Seasonal and interannual patterns of sea surface temperature in Banda Sea as revealed by self-organizing map, Continental Shelf Research, Volume 30, Issue 9, 31 May 2010, Pages 1136-1148

Subsurface equatorial zonal current in the eastern Indian Ocean

Abstract:

Variations of subsurface zonal current in the eastern equatorial Indian Ocean are investigated by examining 6-year data (December 2000–November 2006) from acoustic Doppler current profiler (ADCP) mooring at 0°S, 90°E. The analysis indicates the presence of an eastward equatorial subsurface current between 90 and 170 m depths during both boreal winter and summer. During boreal winter, the generation of eastward pressure gradient, which drives an eastward flow in the thermocline, is caused primarily by upwelling equatorial Kelvin waves excited by prevailing easterly winds. On the other hand, the downwelling Rossby waves generated by the reflection of the spring downwelling Kelvin waves in the eastern boundary, as well as the upwelling equatorial Kelvin waves triggered by easterlies, create an oceanic state that favors the generation of the eastward pressure gradient during boreal summer. The subsurface current reveals a distinct seasonal asymmetry. The maximum eastward speed of 63 cm s⁻¹ is observed in April, and secondary maximum of 49 cm s⁻¹ is seen in October. The zonal transport per unit width within depth of the subsurface current exhibits similar variations: reaching maximum eastward transport of 35 m² s⁻¹ in April and secondary maximum of 29 m² s⁻¹ in October. Moreover, the subsurface current during boreal summer undergoes significant interannual variations; it was absent in 2003, but it was anomalously strong during 2006.

Published as:

Iskandar, I., Y. Masumoto, and K. Mizuno (2009), Subsurface equatorial zonal current in the eastern Indian Ocean, J. Geophys. Res., 114, C06005, doi:10.1029/2008JC005188.

Chlorophyll-a bloom along the southern coasts of Java and Sumatra during 2006

Abstract:

Nine years of chlorophyll-a concentration data provided by the Sea-viewing Wide-Field of view Sensor (SeaWiFS) revealed an unusual bloom along the southern coastal area of Java and Sumatra during 2006. The bloom was generated by anomalous strong southeasterly winds along the coasts of Java and Sumatra associated with the Indian Ocean Dipole (IOD) event. The bloom evolution started in July 2006 and intensified during August 2006. Peak positive anomalies exceeding 4.0 mg m^-3 were evident in September-November coinciding with the peak phase of the IOD. The blooms, thereafter, diminished rapidly in December 2006. In addition, there was an offshore intensification of chlorophyll-a distribution off eastern Java during November initiated by upwelling-favourable winds along the coast. Concurrent altimeter data show that the offshore intensification was co-located with the cyclonic eddies that further enhanced the intensification by increasing the concentration of nutrients in the euphotic zone.

Published as:

I. Iskandar; S. A. Rao; T. Tozuka (2009), Chlorophyll-a bloom along the southern coasts of Java and Sumatra during 2006, International Journal of Remote Sensing, 1366-5901, Volume 30, Issue 3, 2009, Pages 663 – 671

Impact of Indian Ocean Dipole on intraseasonal zonal currents at 90°E on the equator as revealed by self-organizing map

Abstract:

A neural network pattern recognition approach called self-organizing map (SOM) has been used to examine the impact of the Indian Ocean Dipole (IOD) on intraseasonal zonal currents in the eastern equatorial Indian Ocean. This study shows that during negative IOD events the intraseasonal zonal currents are mostly dominated by the first two modes. On the other hand, contributions from the higher modes to the intraseasonal zonal current significantly increase during positive IOD events. This is attributed to the change in the background stratification associated with the IOD events; the sharp pycnocline in the eastern basin during the positive IOD events causes the wind forcing to project more onto the higher modes.

Published as:

Iskandar, I., T. Tozuka, Y. Masumoto, and T. Yamagata (2008), Impact of Indian Ocean Dipole on intraseasonal zonal currents at 90°E on the equator as revealed by self-organizing map, Geophys. Res. Lett., 35, L14S03, doi:10.1029/2008GL033468.

Intraseasonal variations of surface and subsurface currents off Java as simulated in a high-resolution ocean general circulation model

Abstract:

A high-resolution ocean general circulation model (OGCM) is used to explore dynamics of intraseasonal variability in surface and subsurface currents off Java. The results indicate that the surface current, the so-called South Java Coastal Current (SJCC), is dominated by variations with a period of 90 days. In the subsurface current, which is referred to as the South Java Coastal Undercurrent (SJCU), 60-day variations are the most prominent feature. A normal mode analysis demonstrates that the first baroclinic mode is the leading mode, which accounts for 70% of the total variance, whereas the second baroclinic mode explains 24% of the total variance. The 90-day variations in the SJCC captured mostly by the first baroclinic mode are found to be primarily driven by winds. Those are associated with propagation of the first baroclinic Kelvin waves generated in the central equatorial Indian Ocean. On the other hand, the 60-day variations in the SJCU enhanced by wind forcing over the eastern equatorial Indian Ocean off Sumatra are mostly captured by the second baroclinic mode.

Published as:

Iskandar, I., T. Tozuka, H. Sasaki, Y. Masumoto, and T. Yamagata (2006), Intraseasonal variations of surface and subsurface currents off Java as simulated in a high-resolution ocean general circulation model, J. Geophys. Res., 111, C12015, doi:10.1029/2006JC003486.

Intraseasonal Kelvin waves along the southern coast of Sumatra and Java

Abstract:

The intraseasonal sea level variations along the southern coast of Sumatra and Java are investigated using in situ data and the TOPEX/Poseidon satellite altimetry data. The analysis shows that there are two intraseasonal variations of distinct timescale: 20–40 days during boreal summer (JJA) and 60–90 days during boreal winter (DJF). During boreal summer the shorter time variations of the sea level along the coast of Sumatra and Java are traced back to the eastern equatorial Indian Ocean (EIO); this indicates the importance of the remotely forced equatorial Kelvin waves. During boreal winter, on the other hand, both the remote winds over the EIO and the local alongshore winds are important in explaining the longer time variations of the sea level along the coast. Further analysis indicates that these intraseasonal variations are associated with the coastal Kelvin waves with phase speed ranging from 1.5 to 2.86 m/s. A simple analytical model forced by daily wind stress confirms the above intraseasonal variations along the southern coast of Sumatra and Java.

Published as:

Iskandar, I., W. Mardiansyah, Y. Masumoto, and T. Yamagata (2005), Intraseasonal Kelvin waves along the southern coast of Sumatra and Java, J. Geophys. Res., 110, C04013, doi:10.1029/2004JC002508.