Western Arabian Sea Research Articles

Study of Impact of Climatic Variability on the Sea Surface Temperature and Chlorophyll-a Concentration Using Statistical Analysis on Satellite Derived Data for the Arabian Sea

The impact of long term variation in sea surface temperature (SST) on marine biomass using long-term satellite remote sensing data (1985 - 2006) for SST and (1998 – 2010) for chlorophyll-a was studies. This paper presents the result of the analysis conducted on t wenty one year’s data on high-quality satellite derived Sea Surface Temperature, and twelve years SeaWiFS chlorophyll-a data from Arabian Sea. The results are extracted after processing the data using statistical methods for computation of inter-annual seasonal mean and standard deviation. The mean winter data show that region consisting on Gulf of Oman is colder (< 21 o C) than the rest of Arabian Sea. However, annual mean of winter show the intermittent rise and fall in SST in response to El-Nino occurred in 1986, 1988, 1991–1995, 1997-1998, 2002, 2006 and La-Nino events which occurred in 1985, 1986, 1989, 1996, 1999 and 2000 years. The results of SST means for summer and Post-Summer seasons differ from the phenomenon observed in winter and Pre-Summer. During summer, SST of major part of the western Arabian Sea lower to 15 o C under the influence of up-welled water off Somalia and Oman coast. The results of chlorophyll-a concentration depict the increase of concentration with the lowering of SST in the area of northern Arabian Sea. Similar results are obtained for standard deviation computations of both parameters of sea surface during winter monsoon season. Whereas, the results of Standard Deviation for summer monsoon show the minimum values (<1) in the northern and central Arabian Sea and maximum in the alignment of zone under Ekman process. The comparison with temperature confirms the dependency of productivity of the sea surface.

Seasonality of biological feedbacks on sea surface temperature variations in the Arabian Sea: The role of mixing and upwelling

The effects of biological heating on upper-ocean temperature and circulation in the Arabian Sea are investigated using an ocean general circulation model. We find that the change of sea surface temperature (SST) is not only dependent on the variation of chlorophyll concentration, but also the dynamic processes, e.g., mixing and upwelling. Biological heating can warm the SST in the north Arabian Sea during spring and the central Arabian Sea during autumn when the mixed layer depth is shallow. However, the situation is quite different during winter and summer. Although the chlorophyll concentration is high in the north Arabian Sea during winter and in the western Arabian Sea during summer, the SSTs become significantly cool instead of warm. The heat budget analyses indicate that the cold SSTs result from both the strong convective mixing during the winter and the strong upwelling during the summer, which bring the cold water below the mixed layer to the surface.

Species diversity variations in Neogene deep-sea benthic foraminifera at ODP Hole 730A, western Arabian Sea

Deep-sea benthic foraminifera are an important and widely used marine proxy to understand paleoceanographic and paleoclimatic changes on regional and global scales, owing to their sensitivity to oceanic and climatic turnovers. Some species of benthic foraminifera are sensitive to changes in water mass properties whereas others are sensitive to organic fluxes and deep-sea oxygenation. Benthic faunal diversity has been found closely linked to food web, bottom water oxygen levels, and substrate and water mass stability. The present study is aimed at analyzing species diversity trends in benthic foraminifera and their linkages with Indian monsoon variability during the Neogene. Species diversity of benthic foraminifera is examined in terms of number of species (S), information function (H), equitability (E) and Sanders’ rarefied values, which were combined with relative abundances of high and low productivity benthic foraminifera at Ocean Drilling Program Hole 730A, Oman margin, western Arabian Sea. The Oman margin offers the best opportunity to understand monsoon-driven changes in benthic diversity since summer monsoon winds have greater impact on the study area. The species diversity was higher during the early Miocene Climatic Optimum (∼17.2–16.4 Ma) followed by a decrease during 16.4–13 Ma coinciding with a major increase in Antarctic ice volume and increased formation of Antarctic Bottom Water. All the diversity parameters show an increase during 13–11.6 Ma, a gradual decrease during 11.6–9 Ma and then an increase with a maximum at 7 Ma. Thereafter the values show little change until 1.2 Ma when all the parameters abruptly decrease. The benthic foraminiferal populations and diversity at Hole 730A were mainly driven by the Indian monsoon, and polar waters might have played a minor or no role since early Neogene period as the Arabian Sea is an enclosed basin.

Simulations of the Asian monsoon using a regionally coupled-global model

This study examines the simulation of June–September mean of the Asian monsoon in three different numerical experiments with a global climate model in which the atmosphere and ocean are coupled (coupled general circulation model, CGCM), regionally coupled (“Pacemaker”), and with specified sea surface temperature as in the Atmospheric Model Inter-comparison Project (AMIP) applied to a global atmospheric general circulation model (AGCM). The main features of the climatology for wind at 850 hPa, rainfall and zonal wind shear (U200–U850 hPa) over the South China Sea, subtropical western Pacific and Arabian Sea regions are remarkably well simulated in the Pacemaker experiment, compared with observations, whereas the CGCM and AGCM experiments either underestimate or overestimate magnitude of anomalies. The observed relationships between NINO3.4 (an index of El Nino and the Southern Oscillation, ENSO) and the Indian summer monsoon indices are remarkably better captured in the Pacemaker experiment than in the CGCM or AGCM/AMIP experiments. The pattern correlations between the first empirical orthogonal function (EOF) of the model simulations and that of the observed precipitation is higher in the Pacemaker experiment than in the CGCM or AGCM experiments over a large region (40°E–80°W, 20°S–30°N). The temporal correlation between the first principal component of the model simulation and the observations is also higher for the Pacemaker experiment than for the AGCM experiment. The northward/eastward propagation features and the spectral peaks (30–60 days) of rainfall are significantly more realistically captured by Pacemaker in comparison to CGCM and AGCM/AMIP. Based on the correlation coefficient between seasonal and pentad EOF1 and the related composite analysis, we have found that the Pacemaker is able to reproduce the relationship between the intraseasonal and interannual variability of the South Asian monsoon. The observed improvement in simulation in the Pacemaker experiment, as compared to the CGCM experiment, illustrates that a simple system having only local air–sea feedback is capable of greater realism than sophisticated coupled models.

Influence of the Reflected Rossby Waves on the Western Arabian Sea Upwelling Region

Abstract The sea surface temperature (SST) in the western Arabian Sea upwelling region is known to influence the amount of precipitation associated with the Indian summer monsoon. Thus, understanding what determines the SST in this region is an important issue. Using outputs from an ocean general circulation model with and without strong damping in the eastern equatorial Indian Ocean, this study examines how the reflection of semiannual Kelvin waves at the eastern boundary of the Indian Ocean may influence the western Arabian Sea upwelling region. The downwelling Kelvin waves generated in boreal spring are reflected at the eastern boundary and reach the western equatorial Indian Ocean as reflected Rossby waves about 6 months later. The resulting westward current along the equator in the western equatorial Indian Ocean transports warmer water to the western Arabian Sea upwelling region. Thus, the SST in this region becomes colder especially in boreal fall without the reflected Rossby waves. These results are further supported by the analysis of the mixed layer temperature balance. Surprisingly, vertical processes do not contribute to the SST difference, even though the thermocline becomes shallower without the downwelling Rossby waves. This is because the mixed layer is shoaling rapidly from September to November, and there is basically no entrainment of water from below. In contrast, the reflected Rossby waves do not have large impacts on the SST in other seasons mainly because the zonal SST gradient is not as strong and/or the amplitude of Rossby waves is weaker.

Boron/calcium ratios in Globigerinoides ruber from the Arabian Sea: Implications for controls on boron incorporation

Abstract Culture and downcore studies have been used to argue that B/Ca ratios in planktic foraminifera are correlated with seawater pH and may record seawater borate/bicarbonate ratios, although other factors may also control B/Ca. Specimens of Globigerinoides ruber from the Ocean Drilling Program (ODP) Site 723A (western Arabian Sea) and Core AAS9/21 (eastern Arabian Sea) from the last 22 kyr were analysed for B/Ca, δ 18 O and Mg/Ca. B/Ca was compared to reconstructed salinity and sea surface temperature (SST) records, to gain insight into the processes controlling the incorporation of boron. In both cores, B/Ca and Mg/Ca are positively correlated which suggests that temperature governs the incorporation of boron in G. ruber to some extent. Higher B/Ca ratios at the site affected by upwelling (ODP 723A) than in the non-upwelling region (Core AAS9/21), and a positive correlation between B/Ca and shell size in G. ruber are observed. We observe no significant correlation between B/Ca in G. ruber and salinity, pH or dissolved inorganic carbon (DIC) in either the western or eastern Arabian Sea which throws considerable doubt on the utility of B/Ca ratios as a pH or pCO 2 proxy.

Tropical Indian Ocean Surface and Subsurface Temperature Fluctuations in a Climate Change Scenario

Tropical Indian Ocean temperature plays a vital on Indian monsoon system and also cyclone genesis. The feature and evolution mechanisms of the tropical Indian Ocean temperature between pre warming and warming period and also during Indian Ocean Dipole (IOD) events that co-occurred with El Nino are studied using Simple Ocean Data Assimilation (SODA) data set. The southeastern Arabian Sea, Western Arabian Sea and Seychelles Chaogos Thermocline Ridge regions subsurface temperature anomalies are cooled and the southern gyre, south Bay of Bengal, off java regions show warming trend from surface to subsurface. During the positive IOD with co-occurred El Nino years in a climate change period (1970-2008) surface warm temperatures are extended from the Sumatra region to off the African coast in pre-monsoon season. But the subsurface temperature shows different pattern, the warming is more in the central Arabian Sea and south Bay of Bengal (BoB) area.

New biological data on the rare, threatened shark Carcharhinus leiodon (Carcharhinidae) from the Persian Gulf and Arabian Sea

New biological data relevant to the conservation of the rare and threatened shark Carcharhinus leiodon are presented, based on specimens sampled in fish markets in Kuwait, the UAE and Yemen. The maximum size of this species is extended to 1648 mm total length (TL); females are mature by at least 1312 mm TL and demonstrate placental viviparity with litters of 4–6 embryos. In the north-western Persian Gulf there is evidence that parturition occurs in spring when embryos are ~350–515 mm TL, with at least some neonate individuals probably remaining in the area through the summer. Further records of C. leiodon from the western Arabian Sea indicate that adults are present in this region throughout the year. Landings of C. leiodon apparently caught in the eastern Persian Gulf may extend the highly fragmented known distribution of this species. Contrary to an earlier study, the first detailed examination of dissected adult C. leiodon jaws revealed that fine serrations are present on upper teeth, and characters are provided to separate the dentition and jaws of C. leiodon from congeners. The stomach of an adult C. leiodon contained bentho-demersal fish, and an individual with fin abnormalities is noted.

Primary Growth Increments in Otoliths of Six Tropical Myctophid Species

AbstractThe otoliths of six species of myctophids belonging to the genera Benthosema, Diaphus, Myctophum, Lampanyctus, and Symbolophorus were collected in the western Arabian Sea in 1979 and revealed primary growth increments; the readability of the otoliths, however, varied considerably among the six species. The hypothesis of a daily deposition rate of the increments could not be verified, but strong circumstantial evidence for this was put forward. Under this hypothesis, power curves were fitted to the length at age data, yielding length = 0.59 Age0.77 and length = 0.69 Age0.79 for B. pterotum in the Gulf of Oman and the Gulf of Aden respectively, and length = 2.65 Age0.53 for B. fibulatum. A prominent check in the otoliths is suggested to be caused by metamorphosis.

Performance Evaluation of Spread Spectrum Communication System in some Key Locations of North Western Arabian Sea

Multi-path interference due to boundary reflection and variation of sound speed profile in underwater acoustic channel pose a major barrier to dependable underwater acoustic communications. Based on actual data of northwestern Arabian Sea, multipath impulse response profiles of the area were obtained previously. These channel models are used in spread spectrum communication simulation described in this paper. The transmitted signals, which consist of frames of multiple spreading sequences, are modulated using coherent phase shift keying. The use of multiple pseudo noise sequences (PN) in a single frame allows one to achieve robust communication with a relatively simple receiver structure as compared to traditional single spreading sequence approach. This long code multiple sequence approach also helps in realizing low probability of intercept(LPI) communication. On the receiver side, a rake type receiver collects the energy present in multiple propagation paths after synchronization and Doppler compensation. Channel estimation necessary for the correct working of rake receiver is done together with a Doppler tracking that allows an adaptation to the instantaneous shift. The performance of underwater acoustic communication scheme has been evaluated by simulation. Bit error rate (BER) has been used as a performance metric and effects of different channels, types of noises, data rates, chip rates etc. have been evaluated. The results indicate that such communication systems overcome multipath channel distortion and can perform at low signal to noise ratio. This work can be used as a lead for the practical design of underwater acoustic DSSS communication/telemetry system for this area.

A comparison of Globigerinoides ruber calcification between upwelling and non-upwelling regions in the Arabian Sea

Shell weights of planktonic foraminifera species Globigerinoides ruber in the size range of 300–355 $\upmu$ m were measured from sediment traps in the western and eastern Arabian Sea which represent upwelling and non-upwelling conditions respectively. In the Western Arabian Sea Trap (WAST), G. ruber flux ranged from 33.3 to 437.3 #/m2/day and shell weights ranged from 6.7 to 14.2 $\upmu$ g. Whereas, in the Eastern Arabian Sea Trap (EAST), flux ranged from 0.7 to 164.6 #/m2/day and shell weights ranged from 10.4 to 14.8 $\upmu$ g. Shell weights of G. ruber versus flux showed significant correlation at both the sites which reveals that shell calcification mainly depends on optimal growth conditions. Though the WAST and EAST location have distinct difference in pCO2 and sea surface temperature (SST), the shell weights of G. ruber are similar in these two regions which suggest that surface water pCO2 and SST do not show dominant influence on shell calcification on a seasonal timescale.

Dynamics of the Indian-Ocean oxygen minimum zones

In the Indian Ocean, mid-depth oxygen minimum zones (OMZs) occur in the Arabian Sea and the Bay of Bengal. The lower part of the Arabian-Sea OMZ (ASOMZ; below 400m) intensifies northward across the basin; in contrast, its upper part (above 400m) is located in the central/eastern basin, well east of the most productive regions along the western boundary. The Bay-of-Bengal OMZ (BBOMZ), although strong, is weaker than the ASOMZ. To investigate the processes that maintain the Indian-Ocean OMZs, we obtain a suite of solutions to a coupled biological/physical model. Its physical component is a variable-density, 612-layer model, in which each layer corresponds to a distinct dynamical regime or water-mass type. Its biological component has six compartments: nutrients, phytoplankton, zooplankton, two size classes of detritus, and oxygen. Because the model grid is non-eddy resolving (0.5°), the biological model also includes a parameterization of enhanced mixing based on the eddy kinetic energy derived from satellite observations. To explore further the impact of local processes on OMZs, we also obtain analytic solutions to a one-dimensional, simplified version of the biological model.Our control run is able to simulate basic features of the oxygen, nutrient, and phytoplankton fields throughout the Indian Ocean. The model OMZs result from a balance, or lack thereof, between a sink of oxygen by remineralization and subsurface oxygen sources due primarily to northward spreading of oxygenated water from the Southern Hemisphere, with a contribution from Persian-Gulf water in the northern Arabian Sea. The northward intensification of the lower ASOMZ results mostly from horizontal mixing since advection is weak in its depth range. The eastward shift of the upper ASOMZ is due primarily to enhanced advection and vertical eddy mixing in the western Arabian Sea, which spread oxygenated waters both horizontally and vertically. Advection carries small detritus from the western boundary into the central/eastern Arabian Sea, where it provides an additional source of remineralization that drives the ASOMZ to suboxic levels. The model BBOMZ is weaker than the ASOMZ because the Bay lacks a remote source of detritus from the western boundary. Although detritus has a prominent annual cycle, the model OMZs do not because there is not enough time for significant remineralization to occur.

Long-term Changes in Sea Surface Temperature at Selected Locations in the Sea of Oman and the Arabian Sea off Oman

Long-term changes in the sea surface temperature (SST) at two locations off Oman were investigated using Hadley Center SST for the period 1961~2009. A mean annual increase in SST by 0.32℃ was noticed in the Sea of Oman while an increase of 0.53℃ was noticed in the western Arabian Sea. The shift in SST is higher off Muscat than off Masirah during the study period. The bi-modal variability of the SST in the study region is successfully simulated by a cyclic model developed utilizing SST data for 1961~2009 period. An increase in summer warming and decrease in winter cooling are evident in the annual SST distribution at both the locations. The decadal variability off Masirah in the western Arabian Sea showed that the standard deviation of SST switched its character post-1990. The SST variance in Sea of Oman showed a decadal-scale change but in western Arabian Sea, it was nearly unchanged until 1990 and rapidly declined post-1990 period. The large shifts in SST apparently caused higher variability in the sea surface height (SSH) anomalies post-1990 period.

Radiocarbon Calibration/Comparison Records Based on Marine Sediments from the Pakistan and Iberian Margins

We present a new record of radiocarbon ages measured by accelerator mass spectrometry (AMS) on a deep-sea core collected off the Pakistan Margin. The 14C ages measured on the planktonic foraminifera Globigerinoides ruber from core MD04-2876 define a high and stable sedimentation rate on the order of 50 cm/kyr over the last 50 kyr. The site is distant from the main upwelling zone of the western Arabian Sea where 14C reservoir age is large and may be variable. Many independent proxies based on elemental analyses, mineralogy, biomarkers, isotopic proxies, and foraminiferal abundances show abrupt changes correlative with Dansgaard-Oeschger and Heinrich events. It is now common knowledge that these climatic events also affected the Arabian Sea during the last glacial period through changes in the Indian monsoon and in ventilation at intermediate depths. The stratigraphic agreement between all proxies, from fine- to coarse-size fractions, indicates that the foraminiferal 14C ages are representative of the different sediment fractions.To build a calendar age scale for core MD04-2876, we matched its climate record to the oxygen isotopic (δ18O) profile of Hulu Cave stalagmites that have been accurately dated by U-Th (Wang et al. 2001; Southon et al. 2012; Edwards et al., submitted). Both archives exhibit very similar signatures, even for century-long events linked to monsoonal variations. For comparison, we have also updated our previous work on core MD95-2042 from the Iberian Margin (Bard et al. 2004a,b,c), whose climate record has likewise been tuned to the high-resolution δ18O Hulu Cave profile. Sophisticated and novel statistical techniques were used to interpolate ages and calculate uncertainties between chronological tie-points (Heaton et al. 2013, this issue). The data from the Pakistan and Iberian margins compare well even if they come from distant sites characterized by different oceanic conditions. Collectively, the data also compare well with the IntCal09 curve, except for specific intervals around 16 cal kyr BP and from 28 to 31 cal kyr BP. During these intervals, the data indicate that 14C is somewhat older than indicated by the IntCal09 curve. Agreement between the data from both oceanic sites suggests that the discrepancy is not due to local changes of sea-surface 14C reservoir ages, but rather that the IntCal09 curve needed to be updated in these intervals as has been done in the framework of IntCal13 (Reimer et al. 2013a, this issue).

Is the trend in chlorophyll‐a in the Arabian Sea decreasing?

Recent studies of satellite‐derived Chlorophyll concentrations (Chl‐a) in the western Arabian Sea (AS) have suggested an increasing temporal trend, but the length of the records used have typically been too short to resolve longer‐term trends, if any. Our analysis of a long term satellite ocean color data shows a change of trend in the summer chlorophyll for the western AS before and after 2003; Chl‐aconcentration was indeed increasing till 2003, but appears to be declining since then, indicating a secular multi‐year trend in Chl‐avariability. However, this trend is not uniform over the entire region. Analysis of wind, sea surface temperature (SST), Sea Level Anomaly (SLA) and thermocline depth, suggests that the declining summer monsoon chlorophyll‐a(Chl‐a) concentration may be due to increasing SLA in this region. The earlier observed biological changes in the western AS could be an artifact of the change in local winds and ocean dynamics, which may be a part of the natural long‐term variability.

Controlling factors of the oxygen balance in the Arabian Sea's OMZ

Abstract. The expansion of OMZs (oxygen minimum zones) due to climate change and their possible evolution and impacts on the ecosystems and the atmosphere are still debated, mostly because of the unability of global climate models to adequatly reproduce the processes governing OMZs. In this study, we examine the factors controlling the oxygen budget, i.e. the equilibrium between oxygen sources and sinks in the northern Arabian Sea OMZ using an eddy-resolving biophysical model. Our model confirms that the biological consumption of oxygen is most intense below the region of highest productivity in the western Arabian Sea. The oxygen drawdown in this region is counterbalanced by the large supply of oxygenated waters originated from the south and advected horizontally by the western boundary current. Although the biological sink and the dynamical sources of oxygen compensate on annual average, we find that the seasonality of the dynamical transport of oxygen is 3 to 5 times larger than the seasonality of the biological sink. In agreement with previous findings, the resulting seasonality of oxygen concentration in the OMZ is relatively weak, with a variability of the order of 15% of the annual mean oxygen concentration in the oxycline and 5% elsewhere. This seasonality primarily arises from the vertical displacement of the OMZ forced by the monsoonal reversal of Ekman pumping across the basin. In coastal areas, the oxygen concentration is also modulated seasonally by lateral advection. Along the western coast of the Arabian Sea, the Somali Current transports oxygen-rich waters originated from the south during summer and oxygen-poor waters from the northeast during winter. Along the eastern coast of the Arabian Sea, we find that the main contributor to lateral advection in the OMZ is the Indian coastal undercurrent that advects southern oxygenated waters during summer and northern low-oxygen waters during winter. In this region, our model indicates that oxygen concentrations are modulated seasonally by coastal Kelvin waves and westward-propagating Rossby waves. Whereas on seasonal time scales the sources and sinks of oxygen are dominated by the mean vertical and lateral advection (Ekman pumping and monsoonal currents), on annual time scales we find that the biological sink is counterbalanced by the supply of oxygen sustained by mesoscale structures (eddies and filaments). Eddy-driven advection hence promotes the vertical supply of oxygen along the western coast of the Arabian Sea and the lateral transport of ventilated waters offshore the coast of Oman and southwest India.

Spatial-temporal variation and mechanism associated with mini-cold pool off the southern tip of India during summer and winter monsoon season

Sea surface temperature (SST) from different sources suggests that the occurrence of a mini-cold pool (MCP) off the southern tip of India (STI) is a persistent phenomenon which occurs during both the summer and the winter monsoon seasons. However, the associated mechanism is different in both scenarios and, hence, numerical experiments are conducted to study and ascertain the mechanism. The dynamics that govern the occurrence of MCP during the summer season is mainly due to upwelling, caused by the divergence in the near-surface circulation off STI, advection of the cold upwelled water from the western Arabian Sea and the southwest coast of India. In contrast, during the winter monsoon, the model studies suggest that circulations driven by positive Ekman dynamics and outgoing heat flux are mainly responsible for the formation of MCP off STI during December–February. The cold water intrusion in both seasons occurs in accordance with the monsoon and coastal currents, which underlines the importance of advection. The position and extent of cooling differs during both seasons because wind stress varies significantly.

Sea Surface Height Variability in the Tropical Indian Ocean: Steric Contribution

Variability of Sea level and its steric contribution in the Tropical Indian Ocean (TIO) was studied based on 15 years (1993–2007) satellite altimeter observations of sea surface height (SSH) anomaly and steric height (STH) anomaly computed using temperature and salinity fields obtained from Simple Ocean Data Assimilation (SODA) product. Complex Empirical Orthogonal Function (CEOF) analysis was carried out to decompose variability of SSH and STH into various modes to examine the coherency between them. It is revealed that both the parameters exhibit variability in all the time scales. First three major modes of CEOF corresponds to 90% and 84% of the total variability of SSH and STH respectively. There exists strong coherence between the respective CEOF modes of SSH and STH. The first mode of CEOF contributes around ~50% of the total signal corresponds to the annual cycle exhibit large variability in the western Arabian Sea along the Somali and Arabia Coast, latitudinal strip between 2 and 10°N extending from Somali-coast to the west coast of India, coastal oceans around India, and the south eastern TIO. The second CEOF with 25% of total signal contains mixed signature of intra-seasonal and inter-annual periodicities. This exhibit large amplitude in the central south TIO, western and eastern parts of Equatorial Indian Ocean (EIO). Computed long term linear growth rate of sea level anomaly suggests that increase of sea level varies from small (1–3 mm yr−1) in the north TIO to large (8 mm yr−1) in the south TIO. Further analysis suggests that SSH trend in the south TIO was mostly governed by steric contribution while the variability of SSH trend in the north TIO could be explained partially by the variability in STH.

Recent developments in tropical cyclone analysis using observations and high resolution models

Affecting scores of people with seemingly insurmountable and unmanageable costs, tropical cyclones (TCs) are one of the most devastating extreme weather events. In this special issue, we highlight and summarize some of the recent studies that make use of detailed observations and high-resolution numerical models to analyze tropical cyclones from their genesis to post-landfall impacts. Focused around TC activity in the Atlantic, North Indian, and Western Pacific basins, the different papers in this issue contribute to an increased understanding of TCs and to our scientific and operational ability to predict their nature and characteristics. Opening the issue is a comprehensive review from the India Meteorological Department operational forecasters Mohapatra, Bandyopadhyay, and Tyagi on TC climatology over the North Indian Ocean (NIO). The best track parameters related to the position and intensity of a TC were analyzed from the pre-1877 era to 2010. This paper provides an authoritative summary on the errors and uncertainty associated with TC tracks and intensity and offers insight into the methodology, trends, and future consideration for improved TC analysis over the NIO. Mohanty et al. extend this perspective and review storm characteristics over the Bay of Bengal and the Arabian Sea under two epochs—one until 1950 and another from 1950 onward. Their analysis of the two epochs, corresponding to a so-called past cooling period and current warming period, showed that in recent decades, there is a statistically significant increase in the number of severe cyclones, particularly over the Bay of Bengal in the post-monsoon season. The Mohanty et al. study also finds an overall increase in severe cyclones in the western Arabian Sea and southern Bay of Bengal. From the modeling perspective, Osuri et al. reviewed the performance of the Advanced Research Weather Research and Forecasting (WRF-ARW) model over NIO. They

Inter-annual Variability of Chlorophyll-a in the Arabian Sea and its Gulfs

The analysis of chlorophyll-a for a 7 year period from the ocean color images of SeaWiFS in the Arabian Sea and its marginal seas showed considerable inter-annual and geographical variability. The variability in chlorophyll-a was essentially centered on the Southwest and the Northeast monsoon seasons. The mean seasonal chlorophyll-a of the Northeast Monsoon in the western Arabian Sea showed an increasing trend from 1998 to 2004. The period of elevated chlorophyll-a levels in the Gulf of Mannar is apparently extended due to advection of chlorophyll-a rich water from the Bay of Bengal along the east coast of India. The mean distribution of chlorophyll-a from the SeaWiFS images shows a coastally trapped chlorophyll-a rich front from the east coast of India that enriches the Gulf of Mannar and continues northward along the west coast of India. As this front moves northward along the coast, the southern coastal waters turn oligotrophic. The chlorophyll-a concentration in the region is also influenced by the large-scale climatic events such as El Nino/La Nina during 1998-1999. The chlorophyll-a data from selected locations were subjected to spectral analysis to extract periodic signals. The annual signal was found to be dominant in the areas affected by only one of the two monsoons such as the Gulf of Mannar. The semiannual signal was predominant in areas affected by both the monsoons such as western and northern Arabian Sea. We built an empirical model incorporating the annual and semiannual modes of chlorophyll-a from the ocean color data. The ratio of the standard deviations of SeaWiFS-observed to the modeled chlorophyll-a, taken as a measure of model suitability, varied between 0.71 and 0.86. Thus the chlorophyll-a variability in this region can reasonably be predicted using the empirical model developed in this study.