Changes In Thermocline Depth Research Articles

Spatiotemporal Variations of Ocean Upwelling and Downwelling Induced by Wind Wakes of Offshore Wind Farms

Offshore wind farms (OWFs) generate large-scale wind wakes, which might lead to upwelling/downwelling. Understanding the vertical marine response to the wake effects is crucial for assessing the ecological impacts of OWFs and optimizing their co-deployments with mariculture. In this study, we employ a high-resolution ocean model to investigate the spatiotemporal variations of upwelling and downwelling induced by the wind wakes of OWFs through idealized numerical experiments. We have two main findings. First, the wind-wake-induced upwelling and downwelling are not balanced in the north–south direction, resulting in a net effect of thermocline rising. Second, the thermocline depth changes caused by wind wakes develop nonlinearly over time. Specifically, when the elevated thermocline approaches the sea surface, the upwelling slows down significantly. The spatially asymmetric pattern of the upwelling is attributed to horizontal Ekman transport, while its temporal nonlinear evolution is caused by stratification changes. By utilizing the simulated change law of thermocline depth, we calculate the ocean response of OWF wakes in China’s adjacent waters. The results suggest that baroclinic theory overestimates the ocean response in the Bohai Sea, the Yellow Sea, and the nearshore waters of the East China Sea. However, in the open seas and the South China Shelf, the upwelling/downwelling is expected to be close to the theoretical calculations. This study provides a foundation for conducting regional simulations with high resolutions in areas where OWFs will be constructed.

Mesophotic coral bleaching associated with changes in thermocline depth

As global temperatures continue to rise, shallow coral reef bleaching has become more intense and widespread. Mesophotic coral ecosystems reside in deeper (30–150 m), cooler water and were thought to offer a refuge to shallow-water reefs. Studies now show that mesophotic coral ecosystems instead have limited connectivity with shallow corals but host diverse endemic communities. Given their extensive distribution and high biodiversity, understanding their susceptibility to warming oceans is imperative. In this multidisciplinary study of an atoll in the Chagos Archipelago in the central Indian Ocean, we show evidence of coral bleaching at 90 m, despite the absence of shallow-water bleaching. We also show that the bleaching was associated with sustained thermocline deepening driven by the Indian Ocean Dipole, which might be further enhanced by internal waves whose influence varied at a sub-atoll scale. Our results demonstrate the potential vulnerability of mesophotic coral ecosystems to thermal stress and highlight the need for oceanographic knowledge to predict bleaching susceptibility and heterogeneity.

Analysis of models describing thermocline depth-ocean temperature and dissolved oxygen concentration in the ocean-plankton community

In the present work, the two mathematical models describing components of the climate are examined using the numerical method. The first model depicts the relation between the change in thermocline depth and ocean temperature. The second model is simplified by eliminating the natural depletion of the Phytoplankton population. The stability and convergences analysis is conducted for the modified system. The numerical simulation is conducted with the help of the most proven numerical method, called the Adams Predictor-Corrector method. The effect of the oxygen production rate by Phytoplankton (τ) is illustrated with 2D plots. The graphs of the solutions of the models demonstrate the influence of different parameter values on the solutions. The present investigation helps to work for more complex models, and the model can be examined with novel operators.

Persistent Discrepancies between Observed and Modeled Trends in the Tropical Pacific Ocean

Abstract The trends over recent decades in tropical Pacific sea surface and upper ocean temperature are examined in observations-based products, an ocean reanalysis and the latest models from the Coupled Model Intercomparison Project phase six and the Multimodel Large Ensembles Archive. Comparison is made using three metrics of sea surface temperature (SST) trend—the east–west and north–south SST gradients and a pattern correlation for the equatorial region—as well as change in thermocline depth. It is shown that the latest generation of models persist in not reproducing the observations-based SST trends as a response to radiative forcing and that the latter are at the far edge or beyond the range of modeled internal variability. The observed combination of thermocline shoaling and lack of warming in the equatorial cold tongue upwelling region is similarly at the extreme limit of modeled behavior. The persistence over the last century and a half of the observed trend toward an enhanced east–west SST gradient and, in four of five observed gridded datasets, to an enhanced equatorial north–south SST gradient, is also at the limit of model behavior. It is concluded that it is extremely unlikely that the observed trends are consistent with modeled internal variability. Instead, the results support the argument that the observed trends are a response to radiative forcing in which an enhanced east–west SST gradient and thermocline shoaling are key and that the latest generation of climate models continue to be unable to simulate this aspect of climate change.

Fokker\u2013Planck dynamics of the El Ni\xf1o-Southern Oscillation

The asymmetric nature of the El Niño-Southern Oscillation (ENSO) is explored by using a probabilistic model (PROM) for ENSO. Based on a Fokker–Planck Equation (FPE), PROM describes the dynamics of a nonlinear stochastic ENSO recharge oscillator model for eastern equatorial Pacific temperature anomalies and equatorial Pacific basin-averaged thermocline depth changes. Eigen analyses of PROM provide new insights into the stationary and oscillatory solutions of the stochastic dynamical system. The first probabilistic eigenmode represents a stationary mode, which exhibits the asymmetric features of ENSO, in case deterministic nonlinearities or multiplicative noises are included. The second mode is linked to the oscillatory nature of ENSO and represents a cyclic asymmetric probability distribution, which emerges from the key dynamical processes. Other eigenmodes are associated with the temporal evolution of higher order statistical moments of the ENSO system. The model solutions demonstrate that the deterministic nonlinearity plays a stronger role in establishing the observed asymmetry of ENSO as compared to the multiplicative stochastic part.

Thermocline state change in the eastern equatorial Pacific during the late Pliocene/early Pleistocene intensification of Northern Hemisphere glaciation

Abstract. The late Pliocene/early Pleistocene intensification of Northern Hemisphere glaciation (iNHG) ∼2.5 million years ago (marine isotope stages, MIS, 100–96) stands out as an important tipping point in Earth's climate history, which strongly influenced oceanographic and climatic patterns including trade wind and upwelling strength in the eastern equatorial Pacific (EEP). The thermocline depth in the EEP, in turn, plays a pivotal role in the Earth's climate system: small changes in its depth associated with short-term climate phenomena such as the El Niño–Southern Oscillation can affect surface-water properties and the ocean–atmosphere exchange. However, thermocline dynamics in the EEP during the iNHG still remain unclear. While numerous studies have suggested a link between a thermocline shoaling in the EEP and Northern Hemisphere ice growth, other studies have indicated a stable thermocline depth during the iNHG; consequently, a causal relationship between thermocline dynamics and ice-sheet growth has been excluded. In light of these contradictory views, we have generated geochemical (planktic foraminiferal δ18O, δ13C and Mg ∕ Ca), sedimentological (sand accumulation rates) and faunal (abundance data of thermocline-dwelling foraminifera) records for Ocean Drilling Program Site 849 located in the central region of the EEP. Our records span the interval from ∼2.75 to 2.4 Ma (MIS G7–95), which is critical for understanding thermocline dynamics during the final phase of the iNHG. Our new records document a thermocline shoaling from ∼2.64 to 2.55 Ma (MIS G2–101) and a relatively shallow thermocline from ∼2.55 Ma onwards (MIS 101–95). This indicates a state change in thermocline depth at Site 849 shortly before the final phase of the iNHG. Ultimately, our data support the hypothesis that (sub-)tropical thermocline shoaling may have contributed to the development of large Northern Hemisphere ice sheets.

Reverberation of Wideband Signals in Shallow Water when Using Sound Focusing

Within the framework of the normal mode approximation, expressions are obtained for calculating bottom reverberation signals recorded by a horizontal array in an inhomogeneous shallow-water waveguide in a wide frequency band. These expressions can be used to simulate bottom-scattered signals both for a monostatic and bistatic geometry, as well as in the case when sound focusing is applied. The constructed model is used to numerically study the structure of bottom reverberation in a waveguide with different parameters and characteristics of the receiver and source systems. The considered bottom inhomogeneities are the slope of the bottom, change in thermocline depth, and wind waves. The study demonstrates the promise of using sound focusing as time reversal using a single receiver–transmitter element to enhance the reverberation signal arriving from a given bottom area.

Effects of Climate Change on Lake Thermal Structure and Biotic Response in Northern Wilderness Lakes

Climate disrupts aquatic ecosystems directly through changes in temperature, wind, and precipitation, and indirectly through watershed effects. Climate-induced changes in northern lakes include longer ice-free season, stronger stratification, browning, shifts in algae, and more cyanobacterial blooms. We compared retrospective temperature-depth relationships modeled using MINLAKE2012 with biogeochemical changes recorded in sediment cores. Four lakes in Voyageurs National Park (VOYA) and four lakes in Isle Royale National Park (ISRO) were studied. Meteorological data from International Falls and Duluth, Minnesota, were used for VOYA and ISRO, respectively. Model output was processed to analyze epilimnetic and hypolimnetic water temperatures and thermal gradients between two periods (1962–1986, 1987–2011). Common trends were increased summer epilimnion temperatures and, for deep lakes, increased frequency and duration of thermoclines. Changes in diatom communities differed between shallow and deep lakes and the parks. Based on changes in benthic and tychoplanktonic communities, shallow lake diatoms respond to temperature, mixing events, pH, and habitat. Changes in deep lakes are evident in the deep chlorophyll layer community of Cyclotella and Discostella species, mirroring modeled changes in thermocline depth and stability, and in Asterionella and Fragilaria species, reflecting the indirect effects of in-lake and watershed nutrient cycling and spring mixing.

Ventilation time scales of the North Atlantic subtropical cell revealed by coral radiocarbon from the Cape Verde Islands

AbstractWe present coral‐ and sclerosponge‐based reconstructions of the 14C content in North Atlantic dissolved inorganic carbon (DIC) during the last ~100 years from the subtropical cells (STCs). These waters are sensitive to the dynamics of the shallow overturning meridional circulation that transports heat and water masses from the subtropics to the tropics. We use these records to investigate the circulation patterns of the off‐equatorial upwelling regions of the STCs, which are not well understood. Coral and sclerosponge skeletons provide long time series of ocean DIC 14C content, a tracer of oceanic circulation, effectively extending the observational record back in time. Sclerosponge data from the Bahamas were used to extend the existing subtropical 14C time series to the 21st century. Coral 14C data from the Cape Verde Islands (1890–2002) captured the 14C signature of water brought to the surface in the off‐equatorial regions of the STC present near the West African coast. We observe a unique postbomb trend at Cape Verde that is similar to the upwelling regions in the Pacific, and we interpret this trend as the result of the slow penetration of bomb 14C into the interior ocean as part of the STC circulation. Using a multibox mixing model we constrain the time history of bomb 14C in the eastern tropical Atlantic, and we estimate a 20 year time scale for ventilation of the thermocline in this area of the ocean. The similarity between the Atlantic and Pacific 14C‐based records of upwelling suggests that both are caused by bomb 14C penetration rather than more complex explanations that invoke changes in thermocline depth (e.g., related to El Niño–Southern Oscillation variability) or changes in the strength of the subtropical cells. Our results offer constraints for models of tropical ocean circulation and anthropogenic CO2 uptake that attempt to reproduce the characteristics of the shallow wind‐driven circulation in the Atlantic.

Spatial trend patterns in the Pacific Ocean sea level during the altimetry era: the contribution of thermocline depth change and internal climate variability

This study investigates the spatial trend patterns and variability of observed sea level and upper ocean thermal structure in the Pacific Ocean during the altimetry era (1993–2012), and the role of thermocline depth changes. The observed sea level trend pattern in this region results from the superposition of two main signals: (1) a strong broad-scale V-shaped positive trend anomaly extending to mid-latitudes in the central Pacific and (2) another very strong positive trend anomaly located in the western tropical Pacific within about 120° E–160° E and 20° S–20° N latitude. In this study, we focus on the tropical Pacific (20° N–20° S) where the strongest trends in sea level are observed. By making use of in situ observational data, we study the impact of thermocline depth changes on steric sea level between the surface and 700 m and its relation with the altimetry-based observed sea level changes. This is done by calculating the time-varying thermocline depth (using the 20 °C isotherm depth as a proxy) and estimating the sea level trend patterns of the thermocline-attributed individual steric components. We show that it is essentially the vertical movement of the thermocline that governs most of the observed sea level changes and trends in the tropical Pacific. Furthermore, we also show that in the equatorial band, the changes in the upper ocean thermal structure are in direct response to the zonal wind stress. Away from the equatorial band (say, within 5°–15° latitude), the changes in the upper ocean thermal structure are consistent with the wind stress-generated Rossby waves. We also estimate the contribution of the Interdecadal Pacific Oscillation (IPO) on the vertical thermal structure of the tropical Pacific Ocean. Removing the IPO contribution to the upper layer steric sea level provides a non-negligible residual pattern, suggesting that IPO-related internal ocean variability alone cannot account for the observed trend patterns in the Pacific sea level. It is likely that the residual signal may also reflect non-linear interactions between different natural modes like El Nino Southern Oscillation (ENSO), IPO, etc.

Revisiting the Wintertime Intraseasonal SST Variability in the Tropical South Indian Ocean: Impact of the Ocean Interannual Variation*

Abstract Intraseasonal sea surface temperature (SST) variability over the Seychelles–Chagos thermocline ridge (SCTR; 12°–4°S, 55°–85°E) induced by boreal wintertime Madden–Julian oscillations (MJOs) is investigated with a series of OGCM experiments forced by the best available atmospheric data. The impact of the ocean interannual variation (OIV), for example, the thermocline depth changes in the SCTR, is assessed. The results show that surface shortwave radiation (SWR), wind speed–controlled turbulent heat fluxes, and wind stress–driven ocean processes are all important in causing the MJO-related intraseasonal SST variability. The effect of the OIV is significant in the eastern part of the SCTR (70°–85°E), where the intraseasonal SSTs are strengthened by about 20% during the 2001–11 period. In the western part (55°–70°E), such effect is relatively small and not significant. The relative importance of the three dominant forcing factors is adjusted by the OIV, with increased (decreased) contribution from wind stress (wind speed and SWR). The OIV also tends to intensify the year-to-year variability of the intraseasonal SST amplitude. In general, a stronger (weaker) SCTR favors larger (smaller) SST responses to the MJO forcing. Because of the nonlinearity of the upper-ocean thermal stratification, especially the mixed layer depth (MLD), the OIV imposes an asymmetric impact on the intraseasonal SSTs between the strong and weak SCTR conditions. In the eastern SCTR, both the heat flux forcing and entrainment are greatly amplified under the strong SCTR condition, but only slightly suppressed under the weak SCTR condition, leading to an overall strengthening effect by the OIV.

Can tidal perturbations associated with sea level variations in the western Pacific Ocean be used to understand future effects of tidal evolution?

This study examines connections between mean sea level (MSL) variability and diurnal and semidiurnal tidal constituent variations at 17 open-ocean and 9 continental shelf tide gauges in the western tropical Pacific Ocean, a region showing anomalous rise in MSL over the last 20 years and strong interannual variability. Detrended MSL fluctuations are correlated with detrended tidal amplitude and phase fluctuations, defined as tidal anomaly trends (TATs), to quantify the response of tidal properties to MSL variation. About 20 significant amplitude and phase TATs are found for each of the two strongest tidal constituents, K1 (diurnal) and M2 (semidiurnal). Lesser constituents (O1 and S2) show trends at nearly half of all gauges. Fluctuations in MSL shift amplitudes and phases; both positive and negative responses occur. Changing overtides suggest that TATs are influenced by changing shallow water friction over the equatorial Western Pacific and the eastern coast of Australia (especially near the Great Barrier Reef). There is a strong connection between semidiurnal TATs at stations around the Solomon Islands and changes in thermocline depth, overtide generation, and the El Nino Southern Oscillation (ENSO). TATs for O1, K1, and M2 are related to each other in a manner that suggests transfer of energy from M2 to the two diurnals via resonant triad interactions; these cause major tidal variability on sub-decadal time scales, especially for M2. The response of tides to MSL variability is not only spatially complex, it is frequency dependent; therefore, short-term responses may not predict long-term behavior.

Southwest Pacific subtropics responded to last deglacial warming with changes in shallow water sources

This study examined sources of mixed layer and shallow subsurface waters in the subtropical Bay of Plenty, New Zealand, across the last deglaciation (~30–5 ka). δ18O and δ13C from planktonic foraminifera Globgerinoides bulloides and Globorotalia inflata in four sediment cores were used to reconstruct surface mixed layer thickness, δ18O of seawater (δ18OSW) and differentiate between high- and low-latitude water provenance. During the last glaciation, depleted planktonic δ18OSW and enriched δ13C (−0.4–0.1‰) indicate surface waters had Southern Ocean sources. A rapid δ13C depletion of ~1‰ in G. bulloides between 20 and 19 ka indicates an early, permanent shift in source to a more distal tropical component, likely with an equatorial Pacific contribution that persisted into the Holocene. At 18 ka, a smaller but similar shift in G. inflata δ13C depletion of ~0.3‰ suggests that deeper subsurface waters had a delayed reaction to changing conditions during the deglaciation. This contrasts with the isotopic records from nearby Hawke Bay, to the east of the North Island of New Zealand, which exhibited several changes in thermocline depth indicating switches between distal subtropical and proximal subantarctic influences during the early deglaciation ending only after the Antarctic Cold Reversal. Our results identify the midlatitude subtropics, such as the area around the North Island of New Zealand, as a key region to decipher high- versus low-latitude influences in Southern Hemisphere shallow water masses.

Relationships between lake transparency, thermocline depth, and sediment oxygen demand in Arctic lakes

The burial of organic matter within lake sediments can be a significant component of landscape carbon cycling. Whether organic matter deposited in lake sediments is sequestered or mineralized depends on factors limiting the decomposition rate of organic matter, such as temperature and the availability of oxygen. In stratified lakes, the distribution of temperature and oxygen is determined by the depth of the thermocline, and therefore sediment organic matter burial should be sensitive to changes in thermocline depth. Using a survey of more than 30 lakes over 3 years in the Alaskan Arctic, we found that thermocline depth during the summer was positively correlated with water transparency. Furthermore, using sediment incubations from 3 lakes, we found that variation in sediment oxygen demand is primarily affected by variation in temperature and the availability of oxygen with limited effect of the source of the sediments. Because variation in temperature and oxygen concentration in stratified lakes is mainly determined by the depth of thermocline, these results indicate that changes in transparency can have indirect effects on the rate of organic matter mineralization in lakes. A reduction in thermocline depth that results from decreased lake transparency may decrease the breakdown of sediment organic matter and increase the storage of organic carbon in lake sediments.

Physical‐biological synchrony in the global ocean associated with recent variability in the central and western equatorial Pacific

Synchrony in the second modes of an empirical orthogonal function (EOF) analysis of global physical and biological properties is described for the 1993–2010 time period. High correlations are found between the El Niño Modoki index and principal component time series of sea surface temperature, sea surface salinity, photosynthetically active radiation, precipitation, surface currents, surface chlorophyll concentration, and equatorial temperature profiles. Spatial patterns indicate that the second mode is also associated with the North Pacific Gyre Oscillation (NPGO). Biological changes during traditional El Niños (first EOF mode) have been explained on the basis of strong and coherent variations in thermocline depth, wind‐driven upwelling and light, but changes associated with the second mode are more subtle and complex. Equatorial temperature profiles indicate that the warming is confined to the mixed layer and that changes in thermocline depth are small. The biological changes associated with the second mode may be driven by a combination of weak perturbations to vertical nutrient supply and the strength of subtropical gyres. In the western tropical Pacific, the site of some of the strongest perturbations, the biological changes can be associated with the occurrence and thickness of barrier layers and to island effects downstream of the Kiribati Islands. Globally integrated impacts of the second mode are much weaker than those associated with the traditional ENSO. During the positive phase, chlorophyll is strongly enhanced in the tropics and weakly enhanced at the global scale in sharp contrast to traditional El Niño effects. The analysis improves our understanding of global‐scale physical and biological coupling associated with the so‐called El Niño Modoki and the NPGO.

Climate-driven limnological changes determine ecological thresholds in an alpine lake

AB Aquatic Biology Contact the journal Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AB 18:47-58 (2013) - DOI: https://doi.org/10.3354/ab00487 Climate-driven limnological changes determine ecological thresholds in an alpine lake Tomi P. Luoto1,2,*, Liisa Nevalainen1,3 1Research Institute for Limnology, University of Innsbruck, 5310 Mondsee, Austria 2Department of Geosciences and Geography, University of Helsinki, 00014 University of Helsinki, Finland 3Department of Environmental Sciences, University of Helsinki, 15140 Lahti, Finland *Email: tomi.luoto@helsinki.fi ABSTRACT: The consequences of climate warming may be dramatic in lakes because changes in surface heating can affect physical and biological processes. We examined sedimentary remains of Cladocera and Chironomidae from a climatically sensitive lake in the Austrian Alps to test the hypothesis that changes in surface water temperature cause direct and indirect shifts in species composition. Contrary to the expectation that oxygen availability in lakes generally decreases under climate warming, our records showed that the assemblages experienced a succession towards a state with increased oxygen availability. Among the benthic fauna, an ecological threshold was crossed around the year 1850, when taxa tolerant of low oxygen concentrations disappeared and oxyphilous taxa began to dominate. In the zooplankton community, the keystone grazer Daphnia was replaced by Bosmina. These ecological changes, especially during the past 20 yr, were most likely caused by improved oxygen conditions following the decrease in depth of the thermocline. As the faunal succession was strongly correlated with alpine air temperature trends, the change in thermocline depth has likely been driven by the increasing air temperatures, which have caused warming of the epilimnion and consequently, in combination with wind-induced mixing, deepening of the thermocline. These recent changes occur alongside further increasing alpine temperatures and may be reflected in further deepening of the thermocline. We conclude that, in addition to direct climate influences, changes in summer stratification and mixing depth may cause significant changes in aquatic community compositions through changes in oxygen availability under the present climate warming conditions. KEY WORDS: Austrian Alps · Chironomidae · Cladocera · Climate change · Little Ice Age · Mixing depth Full text in pdf format PreviousNextCite this article as: Luoto TP, Nevalainen L (2013) Climate-driven limnological changes determine ecological thresholds in an alpine lake. Aquat Biol 18:47-58. https://doi.org/10.3354/ab00487 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AB Vol. 18, No. 1. Online publication date: January 23, 2013 Print ISSN: 1864-7782; Online ISSN: 1864-7790 Copyright © 2013 Inter-Research.

Dual overflows into the deep Sulu Sea

[1] The Sulu Sea, isolated from the neighboring ocean below 570 m, is nearly isothermal below 1250 m but with a marked salinity increase with depth. The source of the deep Sulu Sea water has been attributed to South China Sea water overflowing the 570 m topographic sill of Panay Strait. However, the Panay overflow (estimated as 0.32 × 106 m3/sec) is an unlikely source for the saltier water Sulu Sea deep water. We propose that deep Sulu Sea ventilation is derived from the south, from the Sulawesi Sea through Sibutu Passage. Sulawesi Sea water between 245 to 527 m, is mixed and heaved over the Sibutu Passage 234 m sill by the energetic tidal environment. Oxygen concentrations within the deep Sulu Sea suggest that the Sulawesi overflow is 0.15 × 106 m3/sec, with a residence time of Sulu Sea deep water of 60 years. The deep tropical Sulu Sea has the unique distinction of being ventilated from two separate sources, whose ratio may fluctuate across a range of temporal scales, associated with regional thermocline depth changes.

Composition and daytime vertical distribution of the ichthyoplankton assemblage in the Central Cantabrian Sea shelf, during summer: An Eulerian study

During summer, wind driven coastal upwelling dominates in the Central Cantabrian Sea (southern Bay of Biscay). Nevertheless, atmospheric forcing is highly variable and wind pulses may cause noticeable and fast hydrographic responses in the shelf region. In this paper, the composition and vertical distribution of the summer ichthyoplankton assemblage during the daytime at a fixed station, located on the Central Cantabrian Sea shelf, are documented. Also, the impact of a short-time scale hydrographic event on the abundance and structure of the larval fish assemblage is examined. Significant small-scale temporal hydrographic variability was observed. Currents showed changes in speed and direction and significant changes in thermocline depth were also observed. A total of 34 taxa of fish larvae were identified. Engraulis encrasicolus eggs and larvae of the shelf-dwelling species Trachurus trachurus, Capros aper and E. encrasicolus dominated the ichthyoplankton assemblage. The distribution of E. encrasicolus eggs and fish larvae was vertically structured. E. encrasicolus egg concentration increased exponentially towards the surface. Fish larvae showed a subsurface peak of concentration and their vertical distribution was not conditioned by thermocline depths. The short term hydrographic event did not affect the vertical distribution of fish larvae but it accounted for significant temporal changes in larval fish assemblage structure and abundance. Results suggest that temperature and light intensity are important factors in the vertical distribution of fish larvae. They also indicate that the temporal monitoring of the larval fish assemblage in this region requires multiple sampling sites.

Intraseasonal variability in the Makassar Strait thermocline

Intraseasonal variability [ISV] in the Makassar Strait thermocline is examined through the analysis of along-channel flow, regional sea level anomaly and wind fields from January 2004 through November 2006. The dominant variability of 45–90 day in the Makassar Strait along-channel flow is horizontally and vertically coherent and exhibits vertical energy propagation. The majority of the Makassar ISV is uncoupled to the energy exerted by the local atmospheric ISV: instead the Makassar ISV is due to the combination of a remotely forced baroclinic wave radiating from Lombok Strait and deep reaching ISV originating in the Sulawesi Sea. Thermocline depth changes associated with ENSO influence the ISV characteristics in the Makassar Strait lower thermocline, with intensified ISV during El Nino when the thermocline shallows and weakened ISV during La Nina.

Temporal evolution of the equatorial thermocline associated with the 1991–2006 ENSO

An oceanic reanalysis data set for the period 1991–2006 has been used to investigate the temporal evolution of the thermocline depth during El Niño–Southern Oscillation (ENSO) cycles. We have examined the dynamical behavior of the thermocline depth in the central eastern equatorial basin by carrying out a detailed investigation into the water mass budget in a selected region lying east of the equatorial Pacific. The result shows that the time change in thermocline depth (i.e., the speed of the local vertical motion) caused by the meridional movement of warm water is consistent with the recharge‐oscillator theory. On the other hand, the time change associated with Bjerknes feedback via zonal mass movement seems to be linked to the development of the El Niño as often suggested by previous observational studies. The speed of the local vertical motion estimated from the meridional mass transport is negatively correlated with that estimated from the zonal mass transport and lags in time by three months. This relationship stems from the fact that rapid adjustments to changes in the surface zonal pressure gradient are observed in the equatorial basin, whereas delayed adjustments through the propagation of baroclinic Rossby waves take place in the central eastern equatorial basin. This time‐lagged relationship introduces a phase shift in the temporal evolution of the thermocline depth. The implications of these results for a better understanding and more accurate prediction of the ENSO time evolution are discussed.