Eastern Equatorial Indian Ocean Research Articles

Evolution of Sea Surface Salinity Anomalies in the Southwestern Tropical Indian Ocean During 2010–2011 Influenced by a Negative IOD Event

AbstractUsing Argo observations and Estimating the Circulation and Climate of the Ocean data sets, this study investigates the sea surface salinity (SSS) variability and its relationship with ocean dynamics in the southwestern tropical Indian Ocean (SWTIO) associated with the 2010 negative Indian Ocean dipole (IOD) event. The results show that the ocean circulation in the tropical southern Indian Ocean (IO) significantly contributes to the SSS anomalies during the evolution of the negative IOD event. The anomalous eastward current advected the high‐salinity water into the eastern equatorial IO during July–October 2010. Then the positive SSS anomalies expanded southward to 6–8°S due to the southeastward mean seasonal currents in the eastern equatorial IO during November–December 2010. A salinity budget analysis has been used to identify key processes in contributing to the SSS variations in the SWTIO. The decreased precipitation in the western and central equatorial IO leads to high SSS anomalies from January to April 2011. Then the anomalous southwestward currents transport the higher SSS water from the eastern equatorial IO to the SWTIO until May–June 2011. At the same time, the upwelling Rossby waves shoal the thermocline and mixed layer depth, bringing high‐salinity subsurface water to the surface layer and cooling the sea surface temperature, which further depresses local precipitation to provide a positive feedback for the SSS increase.

A PMIP3 narrative of modulation of ENSO teleconnections to the Indian summer monsoon by background changes in the Last Millennium

Using nine model simulations from the PMIP3, we study simulated mean Indian summer (June–September) climate and its variability during the Last Millennium (LM; CE0850-1849) with emphasis on the Medieval Warm Period (MWP; CE1000-1199) and Little Ice Age (LIA; CE1550-1749), after validation of the simulated ‘current day (CE1850-2005)’ climate and trends. We find that the simulated above (below) mean-LM summer temperatures during the MWP (LIA) are associated with relatively higher (lower) moisture, and relatively higher (lower) number of concurrent El Ninos (La Ninas). Importantly, the models simulate higher (lower) Indian summer monsoon rainfall (ISMR) during the MWP (LIA) compared to the LM-mean, notwithstanding a strong simulated negative correlation between NINO3.4 index and the area-averaged ISMR. Interestingly, the percentage of the simulated strong El Ninos (La Ninas) associated with negative (positive) ISMR anomalies is higher (lower) in the LIA (MWP). This nonlinearity is explained by the simulated background climate changes, as follows. Distribution of simulated anomalous 850 hPa boreal summer velocity potential during MWP in models indicates, relative to the mean LM conditions, a zone of anomalous convergence in the central tropical Pacific flanked by two zones of divergence, i.e. a westward shift in the Walker circulation. The anomalous divergence centre in the west during the MWP also extends into the equatorial eastern Indian Ocean, triggering in an anomalous convergence zone over India and relatively higher moisture transport therein and therefore excess rainfall during the MWP as compared to the LM-mean, and hence an apparent weakening in the El Nino impact.

Intraseasonal Oscillation of Tropospheric Ozone over the Indian Summer Monsoon Region

Boreal summer intraseasonal oscillation (BSISO) of lower tropospheric ozone is observed in the Indian summer monsoon (ISM) region on the basis of ERA-Interim reanalysis data and ozonesonde data from the World Ozone and Ultraviolet Radiation Data Centre. The 30–60-day intraseasonal variation of lower-tropospheric ozone shows a northwest–southeast pattern with northeastward propagation in the ISM region. The most significant ozone variations are observed in the Maritime Continent and western North Pacific. In the tropics, ozone anomalies extend from the surface to 300 hPa; however, in extratropical areas, it is mainly observed under 500 hPa. Precipitation caused by BSISO plays a dominant role in modulating the BSISO of lower-tropospheric ozone in the tropics, causing negative/positive ozone anomalies in phases 1–3/5–6. As the BSISO propagates northeastward to the western North Pacific, horizontal transport becomes relatively more important, increasing/reducing tropospheric ozone via anticyclonic/cyclonic anomalies over the western North Pacific in phases 3–4/7–8. As two extreme conditions of the ISM, most of its active/break events occur in BSISO phases 4–7/1–8 when suppressed/enhanced convection appears over the equatorial eastern Indian Ocean and enhanced/suppressed convection appears over India, the Bay of Bengal, and the South China Sea. As a result, the BSISO of tropospheric ozone shows significant positive/negative anomalies over the Maritime Continent, as well as negative/positive anomalies over India, the Bay of Bengal, and the South China Sea in active/break spells of the ISM. This BSISO of tropospheric ozone is more remarkable in break spells than in active spells of the ISM, due to the stronger amplitude of BSISO in the former.

Wyrtki Jets: Role of intraseasonal forcing

Direct current measurements observed from the acoustic Doppler current profilers in the equatorial Indian Ocean (EIO) and solutions from an ocean general circulation model are investigated to understand the dynamics of the Wyrtki jet. These jets are usually described as semiannual direct wind forced zonal currents along the central and eastern EIO. We show that both, spring and fall, Wyrtki jets show predominant semiannual spectral peaks, but significant intraseasonal energy is evident during spring in the central and eastern EIO. We find that for the semiannual band, there is a strong spectral coherence between the overlying winds and the currents in the central EIO, but no coherency is observed in the eastern part of the EIO. Moreover, for the intraseasonal band, strong coherency between the winds and currents is evident. During spring, intraseasonal currents induced by the Madden–Julian oscillation (MJO) superimpose constructively with semiannual currents and thus intensify the strength of the spring Wyrtki jet. Also, the atmospheric intraseasonal variability accounts for the interannual variabilities observed in spring Wyrtki jets.

Dynamics of 2015 positive Indian Ocean Dipole

Evolution of typical positive Indian Ocean Dipole (pIOD) event was dominated by a significant sea-surface temperature (SST) cooling in the south-eastern tropical Indian Ocean. Interestingly, during the evolution of 2015 pIOD event, the SST in the south-eastern tropical Indian Ocean did not reveal significant cooling, instead anomalous strong SST warming took place in the western tropical Indian Ocean off the East African coast. This anomalous SST warming was associated with a weakening of the Asian summer monsoon. Furthermore, analysis on the mixed layer heat budget demonstrated that the evolution of the 2015 pIOD event could be attributed mainly to the air-sea heat flux. By decomposing the air-sea heat flux, it is found that reduced latent heat loss plays an important role on the SST warming in the western pole and keeping SST warm in the eastern pole. We note that a residual term also may play a role during the initial development of the event. In contrast to the SST pattern, the subsurface temperature revealed a clear positive dipole pattern. Shallow (deep) 20°C isothermal layer in the eastern (western) equatorial Indian Ocean was observed during boreal summer. This robust subsurface dipole pattern indicated that the subsurface ocean response was largely wind driven through the equatorial wave dynamics as previously suggested.

Intraseasonal Variability of the Equatorial Undercurrent in the Indian Ocean

AbstractBy analyzing in situ observations and conducting a series of ocean general circulation model experiments, this study investigates the physical processes controlling intraseasonal variability (ISV) of the Equatorial Undercurrent (EUC) of the Indian Ocean. ISV of the EUC leads to time-varying water exchanges between the western and eastern equatorial Indian Ocean. For the 2001–14 period, standard deviations of the EUC transport variability are 1.92 and 1.77 Sv (1 Sv ≡ 106 m3 s−1) in the eastern and western basins, respectively. The ISV of the EUC is predominantly caused by the wind forcing effect of atmospheric intraseasonal oscillations (ISOs) but through dramatically different ocean dynamical processes in the eastern and western basins. The stronger ISV in the eastern basin is dominated by the reflected Rossby waves associated with intraseasonal equatorial zonal wind forcing. It takes 20–30 days to set up an intraseasonal EUC anomaly through the Kelvin and Rossby waves associated with the first and second baroclinic modes. In the western basin, the peak intraseasonal EUC anomaly is generated by the zonal pressure gradient force, which is set up by radiating equatorial Kelvin and Rossby waves induced by the equatorial wind stress. Directly forced and reflected Rossby waves from the eastern basin propagate westward, contributing to intraseasonal zonal current near the surface but having weak impact on the peak ISV of the EUC.

Weak El Niño and Winter Climate in the Mid- to High Latitudes of Eurasia

AbstractAccording to the sea surface temperature anomaly (SSTA) intensity in the Niño-3.4 region and the east–west gradient across the Pacific, three types of El Niño are identified in this work. An event with larger than average intensity is defined as a strong El Niño, all others are considered to be weak events. Almost all strong El Niños are concurrent with a large gradient, which is featured by negative SSTAs in the western Pacific and positive SSTAs in the equatorial eastern Pacific (EP) and Indian Ocean (IO). According to the east–west gradient, the weak events can be subdivided into gradient-weak (GW) El Niño and equatorial-weak (EW) El Niño. The GW El Niño characterizes a great east–west gradient without a significant IO SSTA. In contrast, the EW event features a positive SSTA over the tropical IO and EP. The impact of GW El Niño on the North Atlantic–Eurasia continent (NA–Eurasia) displays a negative North Atlantic Oscillation (NAO)-like atmospheric anomaly, resulting in a drier and cooler-than-normal winter over Eurasia. Observational and numerical evidence indicate that the prolonged subtropical jet from the North Pacific to NA acts as a waveguide that captures the planetary Rossby waves generated by the GW El Niño. This waveguide favors the propagation of the perturbations into the downstream regions, which would affect the NA–Eurasian climate. However, the EW El Niño is accompanied by a relatively weak subtropical jet that cannot impact the NA–Eurasian climate significantly. For the strong El Niño, the absence of the NAO signal can be attributed to the counteracting of the teleconnections triggered by the Pacific and the tropical IO.

Interdecadal change in the South China Sea summer monsoon withdrawal around the mid-2000s

In this study, a significant interdecadal change in the South China Sea (SCS) summer monsoon (SCSSM) withdrawal, which occurred around the mid-2000s, is revealed using NCEP-DOE reanalysis, CMAP rainfall, and OLR data. The withdrawal of the SCSSM occurred much later (about 2 weeks) after the mid-2000s. The westerlies and rainfall are stronger around the SCS during the period after the mid-2000s compared to those that occurred before the mid-2000s, which is consistent with the delayed SCSSM withdrawal. The robust and significant increases in rainfall and convection around the SCS and Philippine Sea are dynamically associated with the appearance of an anomalous low-level cyclone around the northern SCS, and the anomalous westerlies at approximately 10°N extend eastward from the Indo-China Peninsula to the western North Pacific (WNP). Anomalous mid-level ascending motion and upper-level divergence were also observed around the Philippine Sea, together with anomalous descending motion and upper-level convergence over the equatorial eastern Indian Ocean. Correspondingly, an anomalous zonal circulation formed between the WNP (upward motion) and eastern Indian Ocean (downward motion). Further analysis indicates that the increasing number and frequent visits to the SCS by the tropical cyclones and enhanced quasi-biweekly oscillation activities may both contribute to the delayed SCSSM withdrawal around the mid-2000s.

Changes in the temperature distribution in the top 150 m of the equatorial Indian Ocean in association with El Niño and monsoon drought

The vertical temperature profiles of the equatorial Indian Ocean between the latitudes 5°S and 5°N in years of El Niño and monsoon drought have been studied using quality controlled ocean temperature (EN4) data set of the UK Met Office Hadley Centre and Simple Ocean Data Assimilation (SODA) reanalysis data. The temperature anomalies in the equatorial Indian Ocean were studied in three composites: (a) years of El Niño with no drought monsoon (composite A), (b) years of El Niño associated with drought monsoon (composite B) and (c) years of monsoon drought without El Niño (composite C). In composite A, a prominent subsurface dipole (SSD) in temperature is seen between depths of 60 and 120 m during the months September–November. It is not present in composite B. SSD is associated with surface wind anomalies, easterly in the equatorial central Indian Ocean and southeasterly in the equatorial eastern Indian Ocean. Drought monsoons have strong westerly surface wind anomalies in the equatorial central Indian Ocean, which reduces the strength of the equatorial easterly wind anomaly in composite B. In composite C, a negative SSD developed due to the strong anomalous westerly wind over the equatorial Indian Ocean associated with monsoon drought and the absence of El Niño induced equatorial easterly winds.

Climatological characteristics of the synoptic changes accompanying South China Sea summer monsoon withdrawal

This manuscript analyses the climatological characteristics (especially the synoptic phenomena and changes) of the South China Sea (SCS) summer monsoon withdrawal (SCSSMW) based on monsoon retreat dates from the National Climate Center of China Meteorological Administration. The SCSSMW is mainly defined as the westerly to easterly shift in the zonal winds due to the westwards intrusion of the western North Pacific (WNP) subtropical high in the northern SCS region. At the low level (850 hPa), the weakening and retreat of the intertropical convergence zone (ITCZ) and rain belt in the SCS–WNP are pertinent, and the southwesterly winds over the north Indian Ocean and the SCS also weaken and retreat. The anomalous anticyclone centred over the northern SCS resembles a Rossby wave response to the reduced precipitation over the SCS and the Philippine Sea. Changes in the upper‐level (200 hPa) circulations include the deceleration of the tropical easterly stream (from the Indo‐China Peninsula to the Arabian Sea) and northerly cross‐equatorial flow (around the equatorial eastern Indian Ocean and Maritime Continent). At the mid‐level (500 hPa), the ascending centre moves equatorwards, shifting from the northern to the southern SCS. There appears an anomalous vertical circulation cell (descending/ascending in the northern/southern SCS), which links the anomalous horizontal circulation through the Sverdrup balance. In addition, the upper‐level divergent centre is shifted southeastwards, as is the generating centre of tropical cyclones. Moreover, an anomalous convergent centre over Taiwan Island is prominent in the upper level. The time series of several atmospheric variables (e.g., zonal wind, precipitation, convection) also exhibited abrupt changes during the SCSSMW. The reasons for not simultaneous retreat of monsoonal westerlies and rainy season in the SCS are also explored.

Distribution of living coccolithophores in eastern Indian Ocean during spring intermonsoon

We studied the biodiversity of autotrophic calcareous coccolithophore assemblages at 30 locations in the Eastern Equatorial Indian Ocean (EEIO) (80°–94°E, 6°N–5°S) and evaluated the importance of regional hydrology. We documented 26 species based on the identification of coccospheres and coccoliths, respectively. The coccolithophore community was dominated by Gephyrocapsa oceanica, Emiliania huxleyi, Florisphaera profunda, Umbilicosphaera sibogae, and Helicosphaera carteri. The abundance of coccoliths and coccospheres ranged from 0.2 × 103 to 160 × 103 coccoliths l−1 and 0.2 × 103 to 68 × 103 cells l−1, averaged 23 × 103 coccoliths l−1 and 9.4 × 103 cells l−1, respectively. Biogenic PIC, POC, and rain ratio mean values were 0.50 μgC l−1, 1.047 μgC l−1, and 0.10 respectively. High abundances of both coccoliths and coccospheres in the surface ocean layer occurred on the north of the equator. Vertically, the great majority of coccoliths and coccospheres were concentrated in water taken from depths of <75 m. The ratios between the number of coccospheres and free coccoliths indicated that coccoliths experience different levels of dissolution when transported to deep water. Abundant coccolithophores mainly occurred at the west of 90°E, which is in accordance with the presence of Wyrtki jets. Patterns of coccolithosphores and of coccoliths have been reflected in hydrological processes.

Ocean Surface Layer Response to Convectively Coupled Kelvin Waves in the Eastern Equatorial Indian Ocean

AbstractMoored measurements between June 2011 and April 2012 at 0°, 90°E in the Indian Ocean are analyzed to assess the surface layer response to the passage of 15 convectively coupled Kelvin waves. During the suppressed phase of the Kelvin waves, intense downward net surface heat flux, weak winds, and a strong diurnal cycle were evident. The net surface heat flux was 3 times larger than turbulent cooling at the base of the surface layer, accounting for an increase of surface layer temperature by +0.2 °C on average. In contrast, strong winds, heavy rainfall, diminished diurnal variations, and air‐sea heat loss marked the active phase, during which surface layer cooling from above by air‐sea fluxes and from below by subsurface turbulent mixing led to a decrease in surface layer temperature by −0.3 °C. Strong salinity stratification formed a thick barrier layer during the active phase, inhibiting the downward diffusion of heat below the surface layer.

Origin of intraseasonal variability in the eastern equatorial Indian Ocean: intrinsic variability and local and remote wind stress forcings

The eastern equatorial Indian Ocean (EEIO) experiences considerable intraseasonal variability (ISV) that arises from intrinsic variability in the ocean and/or is induced by atmospheric forcings. This study investigates the relative contributions to ISV from intrinsic variability and local and remote wind forcings using the Regional Ocean Modeling System. First, to evaluate the contributions of intrinsic variability and forced responses, we conducted three ensemble experiments with different initial conditions. The ensemble mean represents forced ISV whereas differences from the ensemble mean indicate intrinsic ISV. In the central EEIO, the forced ISV of temperature and zonal velocity is larger than the intrinsic ISV above 500 m of depth. This is strongly related to equatorial wave dynamics: the spatial and temporal variability of the forced ISV of temperature (zonal velocity) shows characteristics of equatorial Kelvin waves (Rossby waves). Second, to understand the roles of local and remote atmospheric forcings, especially those of wind stress, we conducted four regional forcing experiments in which intraseasonal wind forcings were retained in selected regions only and suppressed elsewhere. At the surface, local forcing dominates the ISV in the equatorial Indian Ocean. By contrast, in the subsurface, remote forcing also plays an important role in ISV. The deep penetration of oceanic ISV in the EEIO is caused by wind forcing in the western (west of 80°E) and middle-eastern (80°E‒100°E) regions. The influence of surface forcing penetrates eastward–downward for temperature and westward–downward for zonal velocity, consistent with Kelvin and Rossby waves, respectively.

Modeling of sea surface temperature using linear models with autocorrelation Indian Ocean

Sea surface temperature (SST) is one of several indicators of climate system of the Earth. We used model SST to observe SST dataset from buoy arrays in the eastern equatorial Indian Ocean. Relationship between SST and other climate parameters can be represented in linearity approach. This approach shows that temporal variability of the SST as a dominant effect. Linear model fitting (LMF) has been examined with four treatments, with and without: covariate transformation, interaction, centering, and addition time covariate in the model. The LMF chosen as basic construction in the model with covariate interaction combination and transformation, which increases magnitude of multiple-R2 (56.62%) and adjusted-R2 (56.13%), i.e. 0.31% and 0.43% respectively. This shows that time covariates have a strong significance effect in the model, compared to continuous covariates. However, the model has autocorrelation, which has large Akaike Information Criterion (AIC) value then this deletion of effects can be done through the autoregressive moving average. Moreover we obtained that LMF which suitable to SST is model with AIC value 403.2987 by using three climate features include two time covariates. Furthermore, we observed that using GAM model fitting showed an increase in explained deviance to 65.90%, a significant decrease in AIC from 678.24 to 634.99 and significant increase in adjusted-R2 from 51.20% to 64.40% by using sixteen climate features include two times covariates without interaction and transformation.

Characteristics of northward propagating intraseasonal oscillation in the Indian summer monsoon

Indian summer monsoon exhibits strong northward propagation in intraseasonal timescale [intraseasonal oscillations (ISO)] from the equatorial Indian Ocean (EIO) to the foothills of the Himalayas. Initiation of this northward march is often marked by a strong convective activity over the EIO, which could be associated with Madden–Julian Oscillation. Satellite derived rainfall and reanalysis products are used to unravel different characteristics of ISO in terms of strength, extent and speed at different times. Eastern EIO convective events are grouped into two categories based on the intensities of the simultaneous rainfall anomalies over central India (CI). Northward propagation from EIO is observed to be much stronger and slower when CI experiences strong dry anomaly at the time of initiation of northward march of convection near the equator. Essentially, a first northward propagating dry lobe seems to impact the behavior of the next arriving wet lobe. Distinctive features in Rossby waves emanated from the eastern EIO and different surface and atmospheric conditions over the region in the two different cases are observed. Strong dry conditions over CI favors strong easterly vertical wind-shear, which eventually helps destabilizing the atmosphere. Upper tropospheric meridional temperature gradient over the region modulates the variation in vertical wind-shear via thermal wind balance. These conditions are favorable for generation of strong boundary layer convergence to the north of the convective band, producing strong northward propagation. These results indicate the dual role of convection near the equator and dryness over CI in modulating the northward march of rainfall over India.

Impact of riverine freshwater forcing on the sea surface salinity simulations in the Indian Ocean

Abstract A regional ocean circulation model with eddy-permitting resolution (1/8°) is used to examine the impact of continental (riverine) freshwater discharge on the sea surface salinity (SSS) simulations over the Indian Ocean (IO). The daily climatological atmospheric forcing and monthly river discharges are used to force the ocean model. All the rivers draining into the IO from the Indian subcontinent, Africa, Australia, and Indonesia are incorporated in the model. The validation of numerical simulations shows the model's capability in simulating the surface circulation and space and time variability of SSS over the IO. Two numerical experiments are carried out with different freshwater forcing to identify the impact of river discharges on the salinity variability. The first one, i.e. No-River Experiment (NR-Expt), treats the evaporation (E) and precipitation (P) as the surface freshwater flux (E-P). The riverine freshwater discharge (R) is added to the surface freshwater flux in the second experiment, i.e. River Experiment (R-Expt). The model simulated SSS in the R-Expt shows a better agreement with the observed SSS from Aquarius satellite and buoys, as compared to the NR-Expt. The mixed layer salt budget analysis highlights the dominant contributions from E-P and horizontal advection terms in controlling the mixed layer salinity (MLS) over most of the IO domain. Seasonal variability in the salinity and salt budget terms over the ten hydrologically different regions in the northern and southern tropical IO are analyzed. The northern BoB (N-BoB) found to have the largest impact of river discharges on the SSS and MLS simulations during the southwest monsoon (June–September) season. Freshening up to −4psu with salinity tendency up to −0.05 psu/day is simulated in the N-BoB when river discharges are incorporated in the model. The effect of river discharges on the MLS in the southeastern AS appears in the late winter and pre-monsoon seasons. The eastern equatorial IO (EEIO) acts as a source of low-saline waters in the IO. As compared to the northern IO, the southern tropical IO regions exhibit smaller seasonal variability in the MLS and salinity budget terms. The effect of river discharges on the salinity budget terms is noticed with various temporal scales in the hydrologically different regions over the IO. The sector-wise inter-comparison of the salt budget terms derived from the NR-Expt and R-Expt provides insight into the relative impact of rivers in regulating the salinity variability in different parts of the northern and southern tropical IO.

Factors controlling the interannual variation of 30–60-day boreal summer intraseasonal oscillation over the Asian summer monsoon region

The 30–60-day boreal summer intraseasonal oscillation (BSISO) is a dominant variability of the Asian summer monsoon (ASM), with its intensity being quantified by intraseasonal standard deviations based on OLR data. The spatial and interannual variations of the BSISO intensity are identified via empirical orthogonal function (EOF) analysis for the period 1981–2014. The first EOF mode (EOF1) shows a spatially coherent enhancement or suppression of BSISO activity over the entire ASM region, and the interannual variability of this mode is related to the sea surface temperature anomaly (SSTA) contrast between the central–eastern North Pacific (CNP) and tropical Indian Ocean. In contrast, the second mode (EOF2) exhibits a seesaw pattern between the southeastern equatorial Indian Ocean (EIO) and equatorial western Pacific (EWP), with the interannual fluctuation linked with developing ENSO events. During strong years of EOF1 mode, the enhanced low-level westerlies induced by the summer-mean SSTA contrast between the warmer CNP and cooler tropical Indian Ocean tend to form a wetter moisture background over the eastern EIO, which interacts with intraseasonal low-level convergent flows, leading to stronger equatorial eastward propagation. The intensified easterly shear favors stronger northward propagation over the South Asian and Eastern Asian/Western North Pacific sectors, respectively. Opposite situation is for weak years. For interannual variations of EOF2 mode, the seesaw patterns with enhanced BSISO activity over the southeastern EIO while weakened activity over the EWP mostly occur in the La Nina developing summers, but inverse patterns appear in the El Nino developing summers.

Picophytoplankton size and biomass around equatorial eastern Indian Ocean

The cellular size and biomass of picophytoplankton were studied by flow cytometer during spring monsoon (March–May of 2015) in equatorial eastern Indian Ocean. We established an empirical relationship between forward scatter and cellular size to address the size and biomass of picophytoplankton. Results indicated that mean cell diameter of Prochlorococcus (0.60 μm) was the smallest, and then followed by Synechococcus (0.98 μm) and picoeukaryotic phytoplankton (1.05 μm). Thereafter, the biomass converted by abundance reached 0.64 μg·C·L−1 for Prochlorococcus, 0.34 μg·C·L−1 for Synechococcus, and 0.20 μg·C·L−1 for picoeukaryotic phytoplankton. Additionally, the distinct biomass contribution of picophytoplankton appeared to be affected by abundance, but not changes in cellular size. Vertically, the cellular sizes of picophytoplankton were remarkably small in upper waters, which was predominantly controlled by the nutrient availability. In contrast, they were larger in deeper waters, which was primarily attributed to the combined effects of low temperature and reduced light availability. Spatially, under the influence of high nutrient concentration induced by the different circulations and coastal upwelling, slightly high carbon biomass of picophytoplankton was observed around the coastal zones of Sri Lanka island and Sumatra, as well as the southern Bay of Bengal.

Unusual Premonsoon Eddy and Kelvin Wave Activities in the Bay of Bengal During Indian Summer Monsoon Deficit in June 2009 and 2012

An investigation of the eddy and coastal Kelvin wave activities in the Bay of Bengal (BoB) is carried out during premonsoon season in two years of Indian summer monsoon deficit in June (2009 and 2012), occurred in the recent warming hiatus period. Using altimeter observations, our study reveals that over the northern BoB cyclonic eddy kinetic energy is reduced by 35% and 50% from the climatology during premonsoon seasons in 2009 and 2012, respectively, while the cyclonic eddy area is reduced by 18% and 24%, respectively. A concurrent reduction is observed in the first upwelling Kelvin wave (uKW) activities in the eastern equatorial Indian Ocean as well as in the coastal BoB for these years. The reduction in the generation of the first uKW in the eastern equatorial Indian Ocean is attributed to the westerly wind anomalies in January–March of these years. Additionally, meridional wind stress anomalies during March–April in these years are found to be southerly, causing anomalous coastal downwelling in the eastern rim of BoB. This coastal downwelling blocks the propagation of the first uKW. The decrease in the first uKW activities in the coastal waveguide of the BoB reduces the radiation of upwelling Rossby waves, thereby decreasing the cyclonic eddy activities in the northern BoB. The results from this letter could be helpful for further understanding of upper ocean mixing processes in the BoB during monsoon deficit years.

Influence of western boundary reflection on seasonal circulation in the equatorial Indian Ocean

The influence of western boundary reflection (WBR) of Rossby waves on the seasonal circulation in the equatorial Indian Ocean is investigated using an ocean model LICOM , forced by daily winds and surface fluxes of the National Centers for Environmental Prediction (NCEP) reanalysis I during 2000–2010. LICOM has reproduced seasonal variations of Ocean Surface Current Analysis Real-time (OSCAR) zonal currents and satellite altimeter sea level well. Comparisons of the experimental results with and without WBR suggest that the WBR enhances the zonal currents and sea level variations in the central and eastern equatorial Indian Ocean. Long equatorial wave dynamics analyses suggest that the difference of the second baroclinic mode waves with and without WBR are evidently larger than those of the first baroclinic mode, suggesting the importance of the second baroclinic mode waves in controlling the sea level and zonal current seasonal variations. Analyses also suggest that reflected Kelvin waves from the first and second baroclinic modes have a much larger contribution to the semiannual oscillations of the sea level than Rossby waves. The first meridional Rossby wave of the first and second baroclinic modes have larger contribution to zonal currents than the Kelvin waves from the WBR. These results suggest the important role of the WBR in controlling the seasonal circulation in the equatorial Indian Ocean.