Western Tropical Indian Ocean Research Articles

Seasonal forecasting of the Ethiopian summer rains

AbstractWe present a new seasonal forecasting model for the June–September rains in Ethiopia. It has previously been found that the total June–September rainfall over the whole country is difficult to predict using statistical methods. A detailed study of all available data shows the rainfall seasonality varies greatly from one region to another, which would explain why the total June–September rainfall over all regions is a difficult property to forecast. In addition, the correlation between rainfall and the southern oscillation index varies spatially, with a strong teleconnection present only in some regions. This study accounts for the spatial variability in rainfall by grouping the rain gauge stations into four geographical clusters based on seasonality and cross‐correlation of rainfall anomalies. Linear regression equations are then developed separately for each cluster. The variables we use for the regressions are sea‐surface temperature anomalies in the preceding March, April and May of the tropical western Indian Ocean, the tropical eastern Indian Ocean, and Niño3.4. Formal skill testing of the equations shows that the new forecasting scheme is more effective in central western Ethiopia than either climatology or persistence—the methods currently used by the Ethiopian National Meteorological Services Agency. Copyright © 2004 Royal Meteorological Society

Role of winds in estimation of ocean heat storage anomaly using satellite data

The heat storage anomaly of the upper ocean can be estimated using altimeter data, based on the thermal dynamics equation. In this study, we analyzed both nonthermal and thermal steric height measured by altimetry, and removed nonthermal steric height from altimeter measurements to calculate the heat storage anomaly from thermal steric height alone in the global oceans, but with a focus on the Pacific Ocean. After examining nonthermal steric height (salinity, waves, and wind), we believe that wind stress is a major component to the change in sea surface height. Blended TOPEX/Poseidon and ERS‐1/2 altimeter 1° × 1° sea surface height anomaly from January 1993 to October 2000 were used to calculation of the heat storage anomaly. In addition, we used expendable bathythermograph (XBT) data to the global oceans to determine a reference heat storage, from which the altimeter estimation of the heat storage was compared. The wind stress curls, from scatterometer data obtained from January 1993 to October 2000, were used for analyzing the wind effects on sea surface height variation. The correlation coefficients between heat storage from altimeter data (HAlt′) and that from the XBT (HXBT′) are larger than 0.6 in the global oceans, except in the regions of 20°N in the eastern Pacific, the eastern tropical Pacific, the tropical Atlantic, and the western tropical Indian Ocean. The root mean square (RMS) of the differences (HAlt′ − HXBT′) has large values in the Gulf Stream and the Kuroshio Extension regions as well as the eastern and western tropical Pacific, and eastern Indian Ocean. The wind‐induced sea surface height variation exhibits strong annual and interannual cycles, so the empirical orthogonal function (EOF) method was employed to extract the cycles. The annual cycle has two modes, whereas the interannual cycle is characterized by El Niño events. Wind stress can affect the estimation of the heat storage through mechanisms derived from quasi‐geostrophic and barotropic theory. We calculated the correlation coefficient between the differences (HAlt′ − HXBT′) and wind stress curl. The result shows that El Niño Southern Oscillation (ENSO) related wind stress curl anomalies are responsible for estimating discrepancy in the regions west of South and North America and tropical oceans. The high correlation suggests that in these regions the barotropic and linear theory is the dynamic mechanism underlying the wind effects on heat storage estimation. Furthermore, we focus on baroclinic signal in sea level resulting from Ekman upwelling in the tropical oceans. The effect of wind‐forcing has the most important role in sea level change during the 1997–1998 El Niño event. For instance, the heat storage anomaly (Q′) due to the strong coastal upwelling off the west coast of Peru closely relates to sea surface temperature (SST), wind‐forcing, remnant sea level height anomaly (Δη′) after removing the contributions of the heat, salt, and waves to the sea level height, and Ekman depth (DEK). However, the Q′ in the west coast of America and western Pacific warm pool areas did not closely relate to SST, wind‐forcing, Δη′, and DEK.

Feeding Partitioning among Tuna Taken in Surface and Mid-water Layers: The Case of Yellowfin ( Thunnus albacares ) and Bigeye ( T. obesus ) in the Western Tropical Indian Ocean

2 , R. Sabatie 3 Abstract—The trophic relations of two apex predators, yellowfin and bigeye tuna (Thunnus albacares and T. obesus), and their prey were investigated in the western tropical Indian Ocean. The contents of 173 non-empty stomachs were analysed from specimens caught with longlines and purse seine during scientific and fishing cruises. Diet data were processed by occurrence, by number, and by wet weight and a comparison of diets between surface and deep swimmers made. Crustaceans were the almost exclusive food source of surface-swimming bigeye tuna, with the stomatopod (Natosquilla investigatoris) being the sole prey item recorded in this category. The diet of deep-swimming yellowfin tuna was balanced between epipelagic fish, crustaceans and cephalopods. Bigeye tuna fed predominantly on cephalopods and mesopelagic fish (Scopelarchidae and Paralepididae), for which this predator appeared to be the most active chaser. The diet of the two predators reflects their ability to catch the prey, and their vertical distribution.

Pacific Interdecadal Climate Variability: Linkages between the Tropics and the North Pacific during Boreal Winter since 1900

This study examines the tropical linkages to interdecadal climate fluctuations over the North Pacific during boreal winter through a comprehensive and physically based analysis of a wide variety of observational datasets spanning the twentieth century. Simple difference maps between epochs of high sea level pressure over the North Pacific (1900‐24 and 1947‐76) and epochs of low pressure (1925‐46 and 1977‐97) are presented for numerous climate variables throughout the tropical Indo-Pacific region, including rainfall, cloudiness, sea surface temperature (SST), and sea level pressure. The results support the notion that the Tropics play a key role in North Pacific interdecadal climate variability. In particular, SST anomalies in the tropical Indian Ocean and southeast Pacific Ocean, rainfall and cloudiness anomalies in the vicinity of the South Pacific convergence zone, stratus clouds in the eastern tropical Pacific, and sea level pressure differences between the tropical southeast Pacific and Indian Oceans all exhibit prominent interdecadal fluctuations that are coherent with those in sea level pressure over the North Pacific. The spatial patterns of the interdecadal tropical climate anomalies are compared with those associated with ENSO, a predominantly interannual phenomenon; in general, the two are similar with some differences in relative spatial emphasis. Finally, a published 194-yr coral record in the western tropical Indian Ocean is shown to compare favorably with the twentieth-century instrumental records, indicating the potential for extending knowledge of tropical interdecadal climate variability to earlier time periods.

Variability of Mixed-Layer Heights over the Indian Ocean and Central Arabian Sea during INDOEX, IFP-99

The upper air data collected from the balloon-borne GLASS Sondes launched from the Oceanic Research Vessel (ORV) Sagar Kanya during the Intensive Field Phase of the Indian Ocean experiment (INDOEX, IFP-99;SK-141 Cruise) are utilized forstudying the variability in the mixed-layer heights observed over the western tropical Indian Ocean and central Arabian Sea. During the entire cruise, typical daytime convective mixed-layer heights (roughly corresponding to 1400 LT) obtained from θV and q profiles, were observed to be in the range 200–900 m. Shallowmixed -layer heights are observed, in general, over the Inter-Tropical Convergence Zone (ITCZ). Over the central Arabian Sea, vertical profiles of θV and q demonstrate a double mixed-layer structure of the marine atmospheric boundary layer (MABL), which gradually disappears close to the Indian coastline.

Chlorophyll modulation of mixed layer thermodynamics in a mixed-layer isopycnal General Circulation Model — An example from Arabian Sea and equatorial Pacific

Western tropical Indian Ocean, Arabian Sea, and the equatorial Pacific are known as regions of intense bio-chemical-physical interactions: the Arabian Sea has the largest phytoplankton bloom with seasonal signal, while the equatorial Pacific bloom is perennial with quasi-permanent upwelling. Here, we studied three dimensional ocean thermodynamics comparing recent ocean observation with ocean general circulation model (OPYC) experiment combined with remotely sensed chlorophyll pigment concentrations from the Coastal Zone Color Scanner (CZCS). Using solar radiation parameterization representing observations that a higher abundance of chlorophyll increases absorption of solar irradiance and heating rate in the upper ocean, we showed that the mixed layer thickness decreases more than they would be under clear water conditions. These changes in the model mixed layer were consistent with Joint Global Ocean Flux Study (JGOFS) observations during the 1994-1995 Arabian Sea experiment and epi-fluorescence microscopy (EFM) on samples collected during Equatorial Pacific Ocean Climate Study (EPOCS) in November, 1988. In the Arabian Sea, as the chlorophyll concentrations peak in October (3 mg/m3) after the summer plankton bloom induced by coastal upwelling, the chlorophyll induced biological heating enhanced the sea surface temperature (SST) by as much as 0.6‡C and sub-layer temperature decreases and sub-layer thickness increases. In the equatorial Pacific, modest concentrations of chlorophyll less than 0.3 mg/m3 is enough to introduce a meridional differential heating, which results in reducing the equatorial mixed layer thickness to more than 20 m. The anomalous meridional tilting of the mixed layer bottom enhances off equatorial westward geostrophic currents. Consequently, the equatorial undercurrent transports more water from west to east. We proposed that these numerical model experiments with use of satellite andin situ ocean observations are consistent under three dimensional ocean circulation theory combined with solar radiation transfer process.

Stolidobranch ascidians from the tropical western Indian Ocean

Coastal ascidians collected over two centuries from Suez to Mozambique have been successively deposited in the MNHN and are now described and figured. Some of them were already known from the Indian Ocean, others are common to the Pacific, and some others are new species. Even though the present taxonomic work notably increases our knowledge of the tropical eastern African coast, it comprises so many miscellaneous collections from such distant points that it can only begin to suggest the diversity of ascidians there.

Interannual variability of the sea-surface temperature in the Indian Ocean in response to the air–sea turbulent heat exchange

Abstract We investigate the interannual variability of the sea-surface temperature (SST) regulated by the thermal and dynamical forcings at the sea surface associated with seasonal changes of the monsoon circulation in the Indian Ocean. Analysis is done with satellite observations from July 1987 to December 1998. Interannual variations of the air–sea thermal exchange are examined along a path that defines the main monsoon circulation. Taking into account the advection by meridional atmospheric circulation, we show that the anomalous moisture contents are transferred meridionally from south of the equator to the north during the southwest monsoon. It is also shown that the interannual signals from north and south of the equator can be reduced in the equatorial region as a result of convergence of the wind during the northeast monsoon. Due to the synchronous changes of wind and SST anomalies, which have opposite effects on the turbulent heat flux, the air–sea thermal coupling is minimized in the western tropical Indian Ocean on an interannual time scale. In the equatorial region, the resultant air–sea thermal exchange at low latitude almost depends on the atmospheric condition in the mid latitude instead of the thermal condition at the sea surface. In addition, it is thought that the interannual variation of the SST at the equatorial zone is mainly controlled by the wind-induced heat transport in the western and the central areas. In contrast, the SST anomalies in the eastern half of the basin are usually controlled by the surface thermal forcing except for the marked ENSO years. It is suggested that the east–west contrast of heating anomalies due to the wind driven heat transport may affect the oceanic heat balance in the eastern tropical Indian Ocean. The SST anomaly in the southern mid latitude is thermally controlled by the air–sea heat exchange. Our results suggest that the SST anomalies in the mid and low latitudes are linked and modulated through the meridional transport of atmospheric anomalies, although they are determined by the distinct processes.

Interannual upper ocean variability in the tropical Indian Ocean

Empirical orthogonal function (EOF) analysis is used to characterize interannual upper ocean temperature variability since mid‐1980s along two repeated XBT sections across the Indian Ocean. The mode 1 EOF loadings have deepened thermocline in the western tropical Indian Ocean and a shoal thermocline in the east/southeast Indian Ocean during the 1994 and 1997/98 Indian Ocean dipole (IOD) events. Reversed anomalies occur during 1988/89, 1996, and 1998/99. The mode 2 EOF loadings only have significant magnitudes related to the IOD, with upwelling (downwelling) along the Sumatra‐Java coast and downwelling (upwelling) in the southern subtropics leading (lagging) the IOD peaks. Lagged correlations between sea level anomaly and westward wind stress anomaly along the equatorial Indian Ocean (an IOD index) support the EOF analysis: the two EOF modes are linked by propagation and reflection of subtropical Rossby waves and equatorial Kelvin waves, which also contribute to reverse the IOD through a delayed action.

Teleconnections between global sea-surface temperatures and the interannual variability of observed and model simulated rainfall over southern Africa

Sea-surface temperatures (SSTs) for the thirty-year period 1961–1990 are prescribed as boundary forcing for a series of atmospheric general circulation model (AGCM) experiments. The results of five separate multidecadal AGCM runs, each forced with the same SSTs, but different initial conditions, have been used to form an ensemble. The interannual variability of AGCM simulated rainfall over South Africa and Namibia (southern Africa) are compared with observations. Empirical orthogonal function (EOF) analyses revealed a strong correlation between the dominant observed austral summer season (October–March) and mid-summer season (December and January) rainfall patterns over southern Africa and the corresponding model results. It is shown that the AGCM simulated austral summer season rainfall variability in the dominant amplitude time series (first principle component or PC1 time series) compare well with SST fluctuations over the equatorial Pacific and tropical western Indian Oceans. Although the associated dominant observed rainfall time series accounts for a larger fraction of the total variability (relative to the AGCM), it also compares well with equatorial Pacific and tropical western Indian Ocean SST fluctuations. To a certain degree these results accentuates the model's ability to capture the major summer seasonal rainfall variability as a response of global SST forcing, which might make the AGCM suitable for future use in seasonal rainfall forecasting research. Similar associations could not be found for the austral winter season (May–August).

The Indian Ocean SST dipole simulated in a coupled general circulation model

We are successful in simulating the recently discovered ocean‐atmosphere coupled phenomenon called the Indian Ocean Dipole for the first time, using a coupled general circulation model without flux correction. During the analyzed 50 years of model integration, the anomalous climate events have appeared 8 times over the Indian Ocean (IO). They are characterized by the cooling of the sea surface temperature (SST) in the southeastern tropical IO and the warming of the SST in the western tropical IO, associated with the anomalous easterly winds along the equator. The spatial pattern of the anomalous SST shows an east‐west dipole mode (DM) structure that is similar to the recent reports. The simulated DM events are independent of the El Niño simulated in the same model. The heat budget analysis shows that the tropical air‐sea interaction, which is strongly influenced by ocean dynamics, is crucial in generating the model DM events.

On the semi-diagnostic computation of climatological circulation in the western tropical Indian Ocean

The seasonal mean climatological circulation in the Indian Ocean north of 20°S and west of 80°E during the summer and winter has been investigated using a 3-dimensional, fully non-linear, semi-diagnostic circulation model. The model equations include the basic ocean hydrothermodynamic equations of momentum, hydrostatics, continuity, sea surface topography and temperature and salt transport equations. Model is driven with the seasonal mean data on wind stress at the ocean surface and thermohaline forcing at different levels. The circulation in the upper levels of the ocean at 20, 50, 150, 300, 500 and 1000 m depths during the two contrasting seasons has been obtained using the model, and the role of steady, local forcing of wind and internal density field on the dynamical balance of circulation in the western tropical Indian Ocean is explained. The climatological temperature and salinity data used to drive the model is found to be hydrodynamically adjusted with surface wind, flow field and bottom relief during the adaptation stages. Semi-diagnostic technique is found to be very effective for the smoothening of climatic temperature and salinity data and also to obtain the 3-dimensional steady state circulation, which would serve as initial condition in simulation models of circulation.

Simulation of Interannual SST Variability in the Tropical Indian Ocean

A 2.5-layer thermodynamic ocean model is used to investigate interannual variability in sea surface temperature (SST) of the tropical Indian Ocean. Simulated SST agrees well with the data. Model and observed SSTs exhibit large seasonal and interannual variability in the western and southeastern tropical Indian Ocean. Three processes, namely, latent heat flux, radiative flux, and entrainment, play major roles in the evolution of model SST anomalies. Interannual heat flux is found to have greater influence on the SST anomalies in most parts of the model domain. On the other hand, influence of interannual wind is only pronounced near the coasts in the Arabian Sea during the Asian summer monsoon season and a region in the central part of the southern tropical Indian Ocean (STIO) during boreal winter. Besides the El Niño–Southern Oscillation related basinwide warming, empirical orthogonal function analysis shows a dipole structure in both model and observed SST anomalies in the STIO. The eastern pole is anomalously cold with a peak occurring 4–8 months prior to the negative peak of the Southern Oscillation. A similar dipole structure in the latent heat flux anomalies explains the dipole in the SST anomalies.

Anomalies in the tropics associated with the heavy rainfall in east asia during the summer of 1998

The summer of 1998 was characterised by a severe flood in East Asia. The possible linkages were examined among the anomalies in the tropics that may be associated with the severe flood. The anomalies of 1998 are obtained by removing the climatology, which is the average of the ECMWF (European Centre for Medium Range Weather Forecasts) Re—Analysis (ERA) data over 15 years from 1979 to 1993, from the corresponding fields of 1998, which are obtained from the ECMWF operational analyses. In comparison to the results of Nitta (1990), it was found that there are considerable similarities in the atmospheric circulation anomalies between the summers of 1998 and 1988, in the tropics as well as in middle—high latitudes. It was shown that the atmospheric convection is slightly suppressed over the tropical western Pacific. In general, the suppressed convection corresponds to a negative anomaly of SST in the warm pool region. In the summers of 1998 and 1988, however, there are positive anomalies of SSTs in the tropical western Pacific, corresponding to the suppressed convection over there. This slightly suppressed convection may not provide a viable forcing mechanism for the severe flood in East Asia. It was postulated that the zonal wind anomalies in the tropics, in addition to the atmospheric convection over the tropical western Pacific, influence the position and intensity of the North Pacific subtropical high. In both summers of 1998 and 1988, while the stronger convection occurs over the warmer tropical Indian Ocean, the suppressed convection corresponds to the positive anomalies of SSTs in the tropical western Pacific. A possible explanation was given for the broken relationship between SSTs and OLR, (Outgoing Longwave Radiation) by analyzing the large—scale atmospheric circulation anomalies in the tropics. The heat fluxes at the surface in the warm pool of the tropical western Pacific and tropical Indian Ocean were also examined by using the ERA–15 data. To avoid the inconsistency between the ERA–15 and the operational analyses, the anomalies of the heat fluxes at the surface in the warm pool region in the summer of 1988, instead of the summer of 1998, were examined. The anomalous latent heat flux and the net solar radiation flux are the main reason for the positive anomalies of SSTs in the tropical Indian Ocean and in the tropical western Pacific, respectively. The suppressed convection over the tropical western Pacific allows more solar radiation fluxes downward at the surface, which would increase the SSTs.

A multi-level adaptation model of circulation for the western Indian Ocean

A three-dimensional, fully non-linear semi-diagnostic (adaptation) model is described. This model is used to compute the climatological mean circulation and to understand the role of local, steady forcing of the wind and thermohaline forcing on the observed circulation in the western tropical Indian Ocean. The model consists of equations of motion and continuity, sea surface topography, equations of state and temperature, and salinity diffusion equations. While the sea surface topography equation is solved by a successive overrelaxation technique, the other model equations are solved by a leap-frog numerical scheme. Two versions of the model, having 18 and 33 levels in the vertical direction, were prepared to study climatological mean circulation in the western tropical Indian Ocean. The first numerical experiment is carried out with the 18-level adaptation model to study the sensitivity of the solution to different values of eddy coefficients. The main scientific rationale behind these numerical experiments was to obtain the most appropriate values of the eddy coefficients for the realistic computation of climatological circulation in the western tropical Indian Ocean. Three numerical experiments were conducted for the month of February to understand the sensitivity of the model solution to different eddy coefficients. The model reproduced the circulation features during February, even with low values of horizontal and vertical eddy coefficients. In the second experiment, the adaptation model, with 33 levels in the vertical direction, is applied to study the seasonal mean climatological circulation at selected depths during Spring in the western tropical Indian Ocean. Adapted (steady state) results of currents, sea surface topography, temperature and salinity anomaly fields are presented. Reasonable agreement is obtained between the model results on currents and the observational data. The computed anomaly fields for temperature and salinity at selected depths during Spring show that the observed temperature and salinity data were adapted with surface wind, flow field and bottom relief of the ocean and that the observed data were found to be fully smoothed during the adaptation stage. Copyright © 1999 John Wiley & Sons, Ltd.

Change in the association between Indian Ocean sea-surface temperatures and summer rainfall over South Africa and Namibia

The El Niño–Southern Oscillation (ENSO) signal in Indian Ocean sea-surface temperatures has weakened since the late-1970s, resulting in important changes in the association between tropical western Indian Ocean sea-surface temperatures and December–February rainfall over South Africa and Namibia. Prior to the late-1970s, warm events in the tropical Indian Ocean occurred frequently in association with ENSO warm events, and both were typically associated with anomalously dry conditions over much of South Africa and Namibia. Over the most recent two decades, however, sea-surface temperature variability in the tropical western Indian Ocean has become significantly less dependent upon ENSO. At the same time, warm (cold) events in the tropical western Indian Ocean have become associated with wet (dry) conditions over the north-eastern half of South Africa and northern Namibia since the late-1970s. These changes in rainfall–sea-surface temperature associations can be successfully simulated using a general circulation model. Such changes in the climate system have important implications for the predictability of southern African rainfall, and highlight the need for an operational forecast system to predict Indian Ocean sea-surface temperatures. Copyright © 1999 Royal Meteorological Society

Intraseasonal Surface Fluxes in the Tropical Western Pacific and Indian Oceans from NCEP Reanalyses

Abstract Reliability of the surface fluxes from National Centers for Environmental Prediction (NCEP) reanalyses is assessed across the warm pool of the western Pacific and Indian Oceans. Emphasis is given to the spatial distribution and coherence of the fluxes on intraseasonal (25–100 day) periods, as intraseasonal variability predominates the subseasonal variability across the warm pool. Comparison is made with surface fluxes estimated from data collected at a mooring during the Coupled Ocean–Atmosphere Response Experiment and with independent gridded estimates based on operational wind and surface pressure analyses and satellite observations of rainfall, shortwave radiation, and outgoing longwave radiation. In general, fluxes that depend primarily on surface wind variations (e.g., stress and latent heat flux) agree more favorably than fluxes that are largely dependent on fluctuations of convection (e.g., surface shortwave radiation and freshwater or precipitation). In particular, the intraseasonal varia...

Mixed Layer Modeling of Intraseasonal Variability in the Tropical Western Pacific and Indian Oceans

Sea surface temperature (SST) variations associated with the atmospheric intraseasonal oscillation in the tropical Indian and western Pacific Oceans, are examined using a one-dimensional mixed layer model. Surface fluxes associated with 10 well-defined intraseasonal events from the period 1986‐93 are used to force the model. Surface winds from the European Centre for Medium-Range Weather Forecasts daily analyses and SST from the mixed layer model are used to compute latent and sensible heat fluxes and wind stress with the TOGA COARE bulk flux algorithm. Surface freshwater flux is estimated from the Microwave Sounding Unit precipitation data. Net shortwave radiation is estimated, via regression analysis, from outgoing longwave radiation. An idealized diurnal cycle of shortwave radiation is also imposed. The intraseasonal SST variation from the model, when forced by the surface fluxes estimated from gridded analyses, agrees well with the SST observed at a mooring during the COARE. The model was then integrated for the 10 well-defined intraseasonal events at grid points from 758 to 1758 Ea t 58S, which spans the warm pool of the equatorial Indian and western Pacific Oceans. The one-dimensional model is able to simulate the amplitude of the observed intraseasonal SST variation throughout this domain. Variations of shortwave radiation and latent heat flux are equally important for driving the SST variations in the western Pacific, while latent heat flux variations are less important in the Indian Ocean. The phasing of the intraseasonal variation of precipitation relative to wind stress results in little impact of the freshwater flux variation on the intraseasonally varying mixed layer. The diurnal cycle of shortwave radiation is found to significantly increase the intraseasonal amplitude of SST over that produced by daily mean insolation.

CETACEANS OF THE WESTERN TROPICAL INDIAN OCEAN: DISTRIBUTION, RELATIVE ABUNDANCE, AND COMPARISONS WITH CETACEAN COMMUNITIES OF TWO OTHER TROPICAL ECOSYSTEMS

AbstractWe conducted a cetacean survey in the pelagic western tropical Indian Ocean (WTIO) aboard an 85‐m research vessel from March to July 1995, covering 9,784 linear km. Using 25 × binoculars and line‐transect methods, we recorded 589 sightings of 21 species. Stenella longirostris was the most abundant cetacean, in terms of number of individuals sighted, by an order of magnitude above any other species, while Physeter macrocephalus was the most frequently sighted, in terms of number of schools. Twelve species were widespread, seven were rare, and two were localized; our sightings include new distributional records for 12 species. Significant observations included the following: (1) Delphinus cf. tropicalis was abundant off the coast of Oman (16 sightings) and readily distinguishable in the field from D. delphis and D. capensis, (2) Balaenoptera musculus was fairly common and localized in the area of the Maldives (17 sightings), and (3) three sightings were made of an unidentified bottlenose whale tentatively referred to as Indopacetus (i. e., Mesoplodon) pacificus. We recorded 26 mixed‐species cetacean schools, 43 schools with which seabirds associated, and 17 schools associated with tuna. Notable among these were mixed aggregations of Stenella attenuata, S. longirostris, yellowfin tuna, and seabirds.The cetacean community of the WTIO was similar to that of the eastern tropical Pacific (ETP) and the Gulf of Mexico (GM) in several respects. First, differences in abundance rank of individual species were small, with the result that common species were common and rare species were rare, regardless of ocean. Second, these differences in abundance were due primarily to differences in encounter rate, which varied with ocean by as much as 3,000%, and less so to school size, which generally varied less than 100%. Third, regardless of ocean, three species comprised the majority of cetaceans in the community, Stenella attenuata, S. longirostris, and S. coeruleoalba, representing 62%‐82% of all individuals for all species. However, the rank order of abundance for these three species differed with ocean. Most notably, S. attenuata was abundant in the ETP and GM (abundance rank = 2 and 1, respectively) but much less common in the WTIO (abundance rank = 6). Although habitat preferences for S. attenuata appear to overlap considerably with those of S. longirostris in the ETP, our results suggest there may actually be significant differences between these two species. Detailed analysis of oceanographic correlates of distribution will be necessary in order to understand fully the habitat requirements of these pelagic dolphins, often the most conspicuous elements of tropical cetacean communities around the world.

Intraseasonal Variability of Surface Fluxes and Sea Surface Temperature in the Tropical Western Pacific and Indian Oceans

Composites of sea surface temperature (SST), surface heat, momentum, and freshwater flux anomalies associated with intraseasonal oscillations of convection are developed for the warm pool of the western Pacific and Indian Oceans during 1986–93. The composites are based on empirical orthogonal function analysis of intraseasonally filtered outgoing longwave radiation (OLR), which efficiently extracts the Madden–Julian oscillation (MJO) in convection. Surface fluxes are estimated using gridded analyses from the European Centre for Medium-Range Weather Forecasts, weekly SST, OLR, microwave sounding unit precipitation, and the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) bulk flux algorithm. At intraseasonal timescales, these surface flux estimates agree reasonably well with estimates based on mooring observations collected during TOGA COARE. The amplitude of the composite SST variation produced by the MJO is about 0.25°C in the western Pacific, 0.35°C in the Indonesian region, and 0.15°C in the Indian Ocean. The intraseasonal anomalies of SST and net surface heat flux propagate eastward at about 4 m s−1 along with the convective anomaly. The amplitude of the net surface heat flux variation is 50–70 W m−2 in the western Pacific, with anomalous insolation and latent heat flux making similar contributions. Across the Indian Ocean, the net surface heat flux anomaly is weaker (30–40 W m−2), and anomalous insolation appears to make a greater contribution than anomalous latent heat flux. Across the entire warm pool, the net surface heat flux leads the SST variation by about one-quarter cycle, which is consistent with the notion that surface heat flux variations are driving the SST variations at these intraseasonal timescales. The intraseasonal SST variation, however, is estimated to significantly reduce the amplitude of the latent and sensible heat fluxes produced by the MJO.