Cold-core Eddy Research Articles

Research on Cold Core Eddy Change and Phytoplankton Bloom Induced by Typhoons: Case Studies in the South China Sea

The effects of 8 typhoons which passed by coldcore eddy (CCE) areas in the South China Sea (SCS) from 1997 to 2009 were observed and evaluated. The changes in the preexisting CCE acted upon by typhoons were described by eddy kinetic energy (EKE) and eddy available gravitational potential energy (EAGPE). The mechanical energy of CCE was estimated from a two-layer reduced gravity model. Comparing with the scenario that typhoon passes by the region without CCEs, the preexisting CCE area plays an important role in the increase of chlorophyll-a (chl-a) concentration in the CCEs impacted by the typhoons. The preexisting chl-a in CCE is about 25%~45% (8%~25%) of postexisting chl-a in CCE for higher (slower) transit speed typhoons. If the EAGPE of CCE increases greatly after typhoon passing by with slow transit speed, so does the chl-a in the CCE area. The EKE (EAGPE) changes of the preexisting CCE are in the order of O(1014~1015 J). EKE and EAGPE of CCE are dominantly enhanced by typhoon with slow transit speed (<3 m/s) and the posttyphoon EAGPE is always larger than posttyphoon EKE for 8 cases. The maximum EAGPE change of the preexisting CCE reaches5.11×1015 J, which was induced by typhoon Hagibis.

A radiolarian-based palaeoclimate history of Core Y9 (Northeast of Campbell Plateau, New Zealand) for the last 160 kyr

Abstract Sea-surface temperatures (SSTs) based on radiolarian assemblage changes are estimated for the last 160 kyr, from a sediment core (Y9) recovered from Pukaki Saddle, northeast of Campbell Plateau. Site Y9 lies beneath Subantarctic Surface Water (SAW) immediately to the north of the Subantarctic Front (SAF), which in this region is bathymetrically constrained by the edges of Campbell Plateau and defines the northern boundary of the Antarctic Circumpolar Current (ACC). Radiolarian assemblages are characterised by an exceptionally high abundance of the Antarctic to subantarctic species Antarctissa spp. (up to 68%), especially during glacial intervals. SST estimates are derived using Factor Analysis and the Modern Analog Technique. Both methods capture the glacial-interglacial (G-I) pattern. The SST reconstructions show the changing relative influence of distinct water masses during the past G-I cycle, with major temperature variations of the order of 7–9 °C at glacial Terminations. Glacials (marine isotope stages (MIS) 6 and 2) are associated with particularly cool SSTs that are indicative of a more vigorous SAF/ACC and an enhancement of the inflow through Pukaki Saddle and/or frequent development of cold-core eddies at the SAF. By contrast, the influence of warmer waters and relaxation of the ACC during interglacials can be inferred from temperatures slightly warmer (e.g., mid-Holocene) and/or comparable to present day (e.g., MIS 5e). During these intervals, relatively warmer temperatures most likely indicate a higher warm-core eddy activity due to a strengthened Subtropical Front and/or a weakened inflow of cool water through Pukaki Saddle and/or an increased stratification in the Campbell Plateau region. Furthermore, the SST record is characterised by an abrupt warming at ca. 10 kyr (i.e., Termination I), the occurrence of a reversal at Termination I, and a warming event at the end of MIS 4, coinciding with the A4 event in the Byrd ice core. These characteristics, together with the pronounced G-I cycle shown by the SST estimates, suggest that Site Y9 is influenced by major oceanographic changes in the SW Pacific and responds to thermal changes at high southern latitudes.

Stable composition of the nano- and picoplankton community during the ocean iron fertilization experiment LOHAFEX.

The iron fertilization experiment LOHAFEX was conducted in a cold-core eddy in the Southern Atlantic Ocean during austral summer. Within a few days after fertilization, a phytoplankton bloom developed dominated by nano- and picoplankton groups. Unlike previously reported for other iron fertilization experiments, a diatom bloom was prevented by iron and silicate co-limitation. We used 18S rRNA gene tag pyrosequencing to investigate the diversity of these morphologically similar cell types within the nano- and picoplankton and microscopically enumerated dominant clades after catalyzed reported deposition fluorescence in situ hybridization (CARD-FISH) with specific oligonucleotide probes. In addition to Phaeocystis, members of Syndiniales group II, clade 10–11, and the Micromonas clades ABC and E made up a major fraction of the tag sequences of the nano- and picoplankton community within the fertilized patch. However, the same clades were also dominant before the bloom and outside the fertilized patch. Furthermore, only little changes in diversity could be observed over the course of the experiment. These results were corroborated by CARD-FISH analysis which confirmed the presence of a stable nano- and picoplankton community dominated by Phaeocystis and Micromonas during the entire course of the experiment. Interestingly, although Syndiniales dominated the tag sequences, they could hardly be detected by CARD-FISH, possibly due to the intracellular parasitic life style of this clade. The remarkable stability of the nano- and picoplankton community points to a tight coupling of the different trophic levels within the microbial food web during LOHAFEX.

Slow translation speed causes rapid collapse of northeast Pacific Hurricane Kenneth over cold core eddy

AbstractCategory 4 Hurricane Kenneth (HK) experienced unpredicted rapid weakening when it stalled over a cold core eddy (CCE) on 19–20 September 2005, 2800 km SE of Hawaii. Maximum sea surface temperature (SST) cooling of 8–9°C and a minimum aerially averaged SST of 18.3°C (over 8750 km2) characterized its cool wake. A 3‐D mixed‐layer model enabled estimation of enthalpy fluxes (latent and sensible heat), as well as the relative importance of slow translation speed (Uh) compared with the preexisting CCE. As Uh dropped below 1.5 m s−1, enthalpy fluxes became negative, cutting off direct ocean energy flux to HK. Although HK's weakening was attributed to wind shear, our results indicate that slow Uh and consequent intense SST cooling were the main causes. The tropical cyclone‐intensified CCE experienced rapid growth in magnitude (−6 to −40 cm), increased diameter (60 to 350 km), elevated chlorophyll a for 4 months, and 12 month longevity.

Colored dissolved organic matter signature and phytoplankton response in a coastal ecosystem during mesoscale cyclonic (cold core) eddy

Chromophoric dissolved organic matter (CDOM) and hydrochemical parameters were measured in the nearshore region of the western Bay of Bengal with and without significant terrestrial influence. A meso-scale cyclonic eddy that occupied the northern part of the study area set up a nutrient enriched distinct ecosystem in April (premonsoon) attended with increased levels of DOM fluorescence, particularly the protein tyrosine like fluorescence (B). A new (minor) fluorescence component, attributed to land source was revealed which contained two fluorophores, the red-shifted tryptophan-like (TU) hypothesized as the “unfolded protein” and the petroleum hydrocarbon-like (P). During the eddy, pennate diatom population increased, bringing the centric:pennate diatom ratio to half of what it was during the remaining period (monsoon season). The nutrients distribution suggested that when pennates are favored (premonsoon), orthophosphate and silicate are the limiting nutrients and that when centric diatoms are favored (monsoon season), the limitation is by nitrate.

Mesozooplankton community structure in the upper 1,000 m along the western Bay of Bengal during the 2002 fall intermonsoon

Stratification, no upwelling, sediment load, and heavy cloud cover are known to limit primary production in the western Bay of Bengal. Studies on primary consumers in this area are few. Recent studies in the Bay have shown the role of cold-core eddies in enhancing the biological production. This study was carried out to provide a detailed account of variation in mesozooplankton biomass, abundance, and copepod assemblages between cold-core eddy and non-eddy regions in the western Bay. In this study carried out in the western Bay during fall 2002 intermonsoon, we observed a very high zooplankton biovolume of 2.2 ml/m3 in the mixed layer at station WB3 located within a shallow cold-core eddy. Zooplankton from 29 groups were observed during this study. Calanoid and poecilostomatoid copepods substantially contributed to the total zooplankton abundance and carbon biomass. Below the 200 m depth, there were fewer groups but higher proportion of copepods. Copepodites of calanoids were more abundant between 500 and 1,000 m at the WB3 eddy station. Invertebrate eggs made up a staggering 65% of the total collection in the 200- to 300-m stratum at WB1, a location in the other cold-core eddy. Copepod species diversity (3.39 to 4.77) and richness (2.32 to 4.84) were lower at WB3. Among 147 copepod species in 69 genera found, Oncaea venusta (17% of the total copepod abundance), Paracalanus indicus (5.4%), Lucicutia flavicornis (5.1%), and Pleuromamma indica (4.5%) were the four most dominant ones. High copepod diversity throughout upper 1,000 m of the western Bay is attributed to the moderate oligotrophy. We reported 93 copepod species for the first time from this region, from which 7 are first records for the Indian Ocean. Cold-core eddies seem to play a pivotal role in sustaining zooplankton in nutrient-limiting regions such as the western Bay of Bengal.

BIOGEOCHEMICAL SIGNIFICANCE OF EDDIES OF THE EASTERN ARABIAN SEA

Within the Arabian Sea, the western part is consid ered to be highly productive when compared to the northern, central and eastern parts of the b asin. Despite the contrasting chlorophyll and prima ry productivity patterns, sediment trap data show that annual fluxes of organic carbon reach comparable rates in most regions of this highly productive bas in. The traditional mechanisms of nutrient supply t o the upper ocean waters of the Arabian Sea cannot only a ccount for this. Episodic injections of nutrients associated with cold-core eddies stimulate rapid bi ological activity and increase biological productio n in the open ocean waters of the eastern Arabian Sea du ring the demise of the southwest monsoon. In addition to wind-driven coastal upwelling during th e monsoons, annually, an eddy-mediated enhanced biological production, at regional to basin-wide sc ales is proposed. The enhanced production caused by a high nutrient supply mechanism such as a meso-scale eddy leads directly to organic carbon fluxes to th e deep Arabian Sea. The elevated export flux occurred in association with the rapid response of grazers was imported to the deeper layers which leads to a spec tacular enhancement of biogenic particle sedimentation.

Cold core eddies and fronts of the Antarctic Circumpolar Current south of New Zealand from in situ and satellite data

The meridional heat flux required to balance the heat lost by ocean to atmosphere at high latitudes must be accomplished by some mechanism other than mean advection and the heat flux by eddies crossing the Antarctic Circumpolar Current (ACC) may be a candidate. In this study, the positions of the main ACC fronts are determined based on 23 expendable bathythermographs (XBT) transects collected from 1994 to 2010 and are compared with those detected through satellite altimetry. Then, cold core anomalies in XBT sections are identified and altimetry is used to follow the spatial‐temporal evolution of these cold, low sea level anomalies. Mean values of main parameters, such as speed (0.35 km/h), lifetime (79 weeks), and diameter (105 km), are estimated. Moreover, estimations of rotational speed (0.9–76.8 cm/s), ocean surface layer heat content along temperature sections and eddy available heat anomaly (mean value −9.74 × 109 Jm−2) give a wider description of the detected eddies. In our study area, the spawning of eddies is found to occur downstream of the Southeast Indian Ridge and in correspondence of the polar front (PF) with regard to the ACC frontal structure. The contribution of eddies to the global heat budget is not only linked to their ability to cross the ACC fronts but also to the capacity of keeping partially unaltered the properties of water inside them. Analysis of the relation between the translation and rotational speeds shows that a typical eddy may effectively be a significant part (0.8%) of the net meridional heat transport across the PF with a mean heat content/anomaly of −7.65 × 1019 J.

Effects of the Cold Core Eddy on Tropical Cyclone Intensity and Structure under Idealized Air–Sea Interaction Conditions

Abstract The impacts of ocean feedback on tropical cyclones (TCs) are investigated using a coupled atmosphere–ocean model under idealized TC and cold core eddy (CCE) conditions. Results reveal negative impacts of the ocean coupling on TC development. The cold wake induced by a TC not only weakens the TC intensity but also limits the expansion of the storm circulation. The presence of CCE has boosted the TC-induced sea surface temperature cooling, which conversely inhibits the TC development. The TC appears to be weakened as it encounters the CCE edge. The intensity reduction attains a maximum shortly after the TC passes over the CCE center, and simultaneously the CCE-induced asymmetry of the storm structure is most significant as well. The TC undergoes a period of recovery after departure from the CCE, lasting about 36–48 h. During this time the residual asymmetry caused by the CCE is smoothed gradually by storm axisymmetrization. The CCE has induced smaller TC size throughout the simulation even after the TC intensity has completely recovered, an indication of longer recovery time for the TC size. Notably cooler and moister eye air in the lower troposphere, just under the warm-core height, is found in the experiment with CCE. The water vapor mixing ratio budget analysis indicates that it is primarily attributed to changes in vertical advection that occurred in the eye, that is, the undermined eye subsidence associated with the suppressed eyewall convection. The horizontal patterns of vertical motion in the boundary layer are also distinctly changed by the CCE.

Three-dimensional structure of a swarm of the salpThalia democraticawithin a cold-core eddy off southeast Australia

[1] Swarms of the salp Thalia democratica periodically occur off southeast Australia following the austral spring bloom of phytoplankton. In October 2008 a filament of upwelled water was advected south by the adjacent East Australian Current and formed a 30 km diameter cold-core eddy (CCE). The three-dimensional structure of a subsurface swarm of T. democratica within the eddy was examined using both oblique and vertical hauls and an optical plankton counter (OPC) deployed on a towed body. The CCE displayed distinct uplift of the nutricline and elevated fluorescence. Net samples show the zooplankton community was dominated by T. democratica, comprising 73%–88% of zooplankton abundance. The size distribution of T. democratica measured from net samples was 0.5–5 mm and was used to interpret the OPC transects, which showed the swarm formed a 15 km diameter disc located 20–40 m deep in the center of the eddy. The maximum salp abundance was in the pycnocline and coincided with the subsurface fluorescence maximum. The mean abundance of T. democratica size particles within the disc was 5003 individuals m−3 (ind. m−3), contrasted with only 604 ind. m−3 at the outer edge of the eddy. The vertically concentrated and horizontally constrained disc-shaped salp swarm occurred at the interface of salp-bearing inner shelf water and nutrient-rich upwelled water in a CCE. The physical processes that formed the CCE on the inshore edge of the western boundary current led to the largest density of salps recorded.

English

Eddy pumping is a process by which mesoscale eddies induce isopycnal displacements that lift nutrient-replete waters into the euphotic zone, driving a set of biogeochemical processes in the ocean. To investigate the potential effect of physical processes on the microbial community functions, the microbial community metaproteomic profiles were determined in a cold-core cyclonic eddy in the South China Sea (SCS). The significant differences of metaproteomic patterns accurately predicted their distinct functional responses to environmental changes caused by the mesoscale cyclonic eddy perturbations. From the protein gel profiles, 11 proteins were successfully identified by MALDI TOF MS/MS, including nitrogen regulatory proteins, ribosomal proteins and substrate transport proteins, etc, which implied the metabolic responses of the microbial populations to the nutrients change within the cyclonic eddies. These results highlight the potential of metaproteomics for studying complex microbial consortium in the ocean and contributed to our understanding of the biological consequences of a mesoscale cyclonic eddy in the SCS.

Impact of an unusually large warm-core eddy on distributions of nutrients and phytoplankton in the southwestern Canada Basin during late summer/early fall 2010

[1] Recent freshening of the Arctic Ocean due to melting of sea ice and enhanced Ekman pumping has deepened the nutricline over the Canada Basin and reduced nutrient concentrations in the euphotic zone. Cold-core eddies frequently transport nutrient-rich shelf water to the Canada Basin, but the eddies are much deeper than the euphotic zone. Because warm-core eddies appear near the surface or at a depth range shallower than that of the cold-core eddies, they may play a crucial role in determining nutrient distributions in the euphotic zone and hence may affect primary production. During late summer/early fall 2010, we conducted detailed surveys of a warm-core eddy, which was unusually large (∼100 km in diameter). We suggest that this warm-core eddy which contained high-ammonium shelf water could supply ammonium to the euphotic zone in the southwestern Canada Basin and may sustain ∼30% higher biomass of picophytoplankton (<2 μm) than that in the surrounding water in the basin. The role of warm-core eddies in supplying nutrients to the euphotic zone and controlling phytoplankton distributions seems to be more important than previously because the recent deepening of the nutricline in the Canada Basin has decreased the nutrient supply to the euphotic zone.

Ensemble prediction study of the East Australian Current

We examine the role of dynamical instabilities and flow dependent, growing errors in an ensemble prediction study of the East Australian Current. In the region of the Tasman front where the East Australian Current separates from the coast, the circulation is characterised by meandering and significant mesoscale eddy variability, and is dominated by nonlinear dynamics, thereby representing a severe challenge for operational forecasting. We explore the mechanisms for and structures of forecast errors over a month using the Australian operational ocean forecast system. To examine the role of dynamical instabilities, forecast ensemble perturbations are generated as bred vectors allowing the identification of those perturbations to a given initial state that grow most rapidly. We consider a period spanning the Austral autumn/summer corresponding to the seasons of maximum eddy variability. We find that the bred vectors span a low dimensional subspace whose structures correspond to regions of large forecast error. Further, these structures typically occur in regions of instability and in particular in the vicinity of the Tasman front and the East Australian Current extension. Our results suggest that even a very few bred vectors will be useful in adaptive sampling and targeted observations. References G. R. Brassington, P. R. Oke, and T. Pugh, BLUElink Operational ocean prediction in Australia, vol. 5, ch. 11, pp. 87--95, World Scientific Publishing, 2006. M. Corazza, E. Kalnay, D. J. Patil, S.-C Yang, R. Morss, M. Cai, I. Szunyogh, B. R. Hunt, and J. A. Yorke, Use of the breeding technique to estimate the structure of the analysis errors of the day, Nonlin. Proc. in Geophys. 10 (2003), 233--243. http://hal.archives-ouvertes.fr/hal-00302201/ S. M. Griffies, M. J. Harrison, R. C. Pacanowski, and A. Rosati, A technical guide to {MOM4} {GFDL} ocean technical group report no 5, 2003. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.143.8321&rep=rep1&type=pdf H. E. Hurlburt, G. B. Brassington, Y. Drillet, M. Kamachi, M. Benkiran, R. Bourdalle-Badie, E. P. Chassignet, G. A. Jacobbs, O. Le Galloudec, J.-M. Lellouche, E. J. Metzger, P. R. Oke, T. F. Pugh, A. Schiller, O. M. Smedstad, B. Tranchant, H. Tsujino, N. Usui and A. J. Walcraft, High-resolution global and basin-scale ocean analyses and forecasts., Oceanography 22 (2009), no. 3, 80--97. http://handle.dtic.mil/100.2/ADA508067 M. M. Mata, M. Tomczak, S. Wijffels, and J. A. Church, East Australian Current volume transports at 30S: Estimates from the World Ocean Circulation Experiment hydrographic sections PR11/P6 and the PCM3 current meter array., J. Geophys. Res. 105 (2000), 28, 509--28, 526. doi:10.1029/1999JC000121 M. M. Mata, S. E. Wijffels, J. A. Church, and M. Tomczak, Eddy shedding and energy conversion in the East Australian Current, J. Geophys. Res. 111 (2006), C09034. doi:10.1029/2006JC003592 T. J. O'Kane and J. S. Frederiksen, Statistical dynamical subgrid-scale parameterizations for geophysical flows, Physica Scripta T132 (2008), 014033. doi:10.1088/0031-8949/2008/T132/014033 T. J. O'Kane, P. R. Oke and P. A. Sandery, Ocean Modelling, to appear (2011). doi:10.1016/j.ocemod.2011.04.003 P. R. Oke, M. Balmaseda, M. Benkiran, J. A. Cummings, E. Dombrowsky, Y. Fujii, S. Guinehut, G. Larnicol, P.-Y. Le Traon, and M. J. Martin, Observing System Evaluations using GODAE systems, Oceanography (2009), 144--153. http://handle.dtic.mil/100.2/ADA511325. P. R. Oke and D. A. Griffin, The cold-core eddy and strong upwelling off the coast of New South Wales in early 2007, Deep Sea Research 58 (2011), 574--591. doi:10.1016/j.dsr2.2010.06.006 D. Pazo, M. A. Rodr\OT1\i guez, and J. M. Lopez, Spatio-temporal evolution of perturbations in ensembles initialized by bred, Lyapunov and singular vectors, Tellus 62A (2010), 10--23. doi:10.1111/j.1600-0870.2009.00419.x P. A. Sandery, G. B. Brassington, and J. Freeman, Adaptive nonlinear dynamical initialization., J. Geophys. Res. 116 (2011), C01021. doi:10.1029/2010JC006260 A. Schiller, P. R. Oke, G. B. Brassington, M. Entel, R. Fiedler, D. A. Griffin, and J.V. Mansbridge, Eddy-resolving ocean circulation in the Asian-Australian region inferred from an ocean reanalysis effort., Progress in Oceanography 76 (2008), 334--365. doi:10.1016/j.pocean.2008.01.003 Z. Toth and E. Kalnay, Ensemble forecasting at {NMC}, Bull. Amer. Meteor. Soc. 74 (1993), 2317--2330. doi:10.1175/1520-0477(1993)074&lt;2317:EFANTG>2.0.CO;2

Structure and Causes of the Pulsation Mode in the Antarctic Circumpolar Current South of Australia

Abstract The subsurface variability of potential temperature and salinity south of Australia along 130°E is studied over a 25-yr period (1980–2004). The study is done with fields provided by a global eddy-permitting model of the DRAKKAR project forced by atmospheric reanalysis. The analysis performed by C. Sun and D. R. Watts with in situ hydrographic data is repeated. Sun and Watts have investigated the EOF modes in streamfunction space along the World Ocean Circulation Experiment (WOCE) SR3 section. In particular, they found that an EOF mode, which they called the “pulsation mode,” strongly dominates subsurface thermohaline variations. Here, it is found that, in the model, an EOF mode with spatial structure similar to the Sun and Watts pulsation mode dominates subsurface thermohaline variations in streamfunction space. The mode displays a maximum of variability at the Subantarctic Front (SAF) between Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). The associated time series exhibits an intermittent interseasonal frequency (3–6 months), especially during three periods (1983–84, 1990, and 1994–96). Some energy is also found with a 4-yr period. Further analyses reveal that the pulsation mode can also be observed in physical space. The pulsation mode is found to be related to movements of the SAF constrained by the bathymetry of the Southeast Indian Ridge. The pulsation mode displays many similarities with cold-core eddy events rather than being related to variations of the westerly wind stress, as previously proposed. The impact of those events on SAMW properties remains unclear.

The biological oceanography of the East Australian Current and surrounding waters in relation to tuna and billfish catches off eastern Australia

The surface and sub-surface biological oceanography of tuna fishing grounds within the East Australian Current (EAC) was compared in 2004 with two other fishing areas further offshore. Our aim was to determine whether the biological oceanography of the region could explain the distribution and intensity of pelagic fishery catches inside and outside the EAC at that time. The EAC fishing area was noticeably warmer, less saline and lower in nutrients than waters in the other fishing areas. The EAC waters were dominated by large diatoms, the biomass of which was significantly higher than in the seamount and offshore areas, apparently the result of a cold core eddy beneath the EAC surface filament. Over the seamount and offshore more typical Tasman Sea waters prevailed, although the presence of a relatively deeper oxygen minimum layer over the seamount suggested topographically induced mixing in the area. Notably, sub-surface zooplankton and micronekton biomass was significantly higher around the seamount than in the two other areas. The offshore region was characterised by frontal activity associated with the Tasman front. Micronekton net biomass was generally highest in surface waters in this region. Examination of tuna catch records at that time showed yellowfin tuna ( Thunnus albacares) dominated the catches of the EAC, whereas swordfish ( Xiphias gladius) and bigeye tuna ( Thunnus obesus) were the main species caught offshore. We suggest the yellowfin tuna concentrate in waters that are not only warmer but where prey species are concentrated near the surface. Offshore, deeper living species such as swordfish and bigeye tuna ( T. obesus) can take advantage of prey species that are distributed deeper in the water column and along the flanks of the many seamounts in the region, or that are concentrated at fronts associated with the Tasman Front. Although only a snapshot of the region, relatively consistent catch data over time suggests the underlying biological oceanography may persist over longer time periods, particularly during the Austral spring.

Temporal variations of the net Kuroshio transport and its relation to atmospheric variations

Temporal variations of the net Kuroshio transport are investigated using long-term hydrographic data from repeat section of the 137°E meridian from the south of Japan (34°N) to New Guinea (1°S) conducted by the Japan Meteorological Agency. In this study, boundaries of the Kuroshio and the Kuroshio Counter Current (KCC) are defined based on the sea surface dynamic height distribution. Westward flows associated with the KCC and cold-core eddy north of the Kuroshio are removed from the eastward flow associated with the Kuroshio in order to estimate the net Kuroshio transport strictly. The net Kuroshio transport reveals low-frequency variations: significant signals on a decadal (about 10-year) timescale. The variations of net Kuroshio transport are predominantly caused by changes in the magnitude of oceanic current speed fields associated with a vertical movement of the main pycnocline depth around the southern boundary of the Kuroshio: deepening of the main pycnocline around the southern boundary of the Kuroshio forms a sharp northern upward-tilting slope of the isopycnal surfaces at the Kuroshio region, and eventually the net Kuroshio transport increases together with the Kuroshio eastward transport. By using a wind-driven hindcast model, it is found that the main pycnocline depth variation results from the first-mode baroclinic Rossby waves attributable to two types of Aleutian Low (AL) changes: a change in the magnitude of AL and meridional movement of AL.

The cold-core eddy and strong upwelling off the coast of New South Wales in early 2007

During the Austral summer of 2006–07 a series of extreme oceanic events occurred in the Tasman Sea. Following a series of strong wind-driven upwelling events, an intense cold-core eddy developed off Sydney, Australia. A data-assimilating, eddy-resolving ocean model is used to create a three-dimensional time-varying reanalysis of these events. The reanalysis indicates that the cold anomalies associated with the upwellings were in excess of −5 °C near the coast, where sea level decreased by as much as 0.2 m. The reanalysed three-dimensional structure of the cold-core eddy shows the eddy "leaning" to the west–north–west, in towards the continental shelf. The diameter of the eddy is about 100 km and the sea-level anomaly at the eddy centre peaks at around −1 m, with an associated sub-surface temperature anomaly in excess of −8 °C at 200 m depth, corresponding to an upward isotherm excursion of 600 m. The circulation around the cyclonic eddy is ageostrophic, with upwelling in the southern sector of the eddy (where flow is onshore and climbing the continental slope) and downwelling in the northern sector (where flow is descending off the slope). Three-dimensional trajectories of water parcels around the eddy involve 50-100-m vertical excursions. Based on the reanalysed circulation and composite satellite images of Chlorophyll-a, we hypothesise that the circulation around the eddy led to significant nutrient enrichment in the euphotic zone around the perimeter of the eddy.

Characterisation of water masses and phytoplankton nutrient limitation in the East Australian Current separation zone during spring 2008

This study focuses on the comparison of oceanic and coastal cold-core eddies with inner-shelf and East Australian Current (EAC) waters at the time of the spring bloom (October 2008). The surface water was biologically characterised by the phytoplankton biomass, composition, photo-physiology, carbon fixation and by nutrient-enrichment experiments. Marked differences in phytoplankton biomass and composition were observed. Contrasted biomarker composition suggests that biomarkers could be used to track water masses in this area. Divinyl chlorophyll a, a biomarker for tropical Prochlorophytes, was found only in the EAC. Zeaxanthin a biomarker for Cyanophytes, was found only within the oceanic eddy and in the EAC, whereas chlorophyll b (Chlorophytes) was only present in the coastal eddy and at the front between the inner-shelf and EAC waters. This study showed that cold-core eddies can affect phytoplankton, biomass, biodiversity and productivity. Inside the oceanic eddy, greater phytoplankton biomass and a more complex phytoplankton community were observed relative to adjacent water masses (including the EAC). In fact, phytoplankton communities inside the oceanic eddy more closely resembled the community observed in the inner-shelf waters. At a light level close to half-saturation, phytoplankton carbon fixation (gC d −1) in the oceanic eddy was 13-times greater than at the frontal zone between the eddy and the EAC and 3-times greater than in the inner-shelf water. Nutrient-enrichment experiments demonstrated that nitrogen was the major macronutrient limiting phytoplankton growth in water masses associated with the oceanic eddy. Although the effective quantum yield values demonstrate healthy phytoplankton communities, the phytoplankton community bloomed and shifted in response to nitrogen enrichments inside the oceanic eddy and in the frontal zone between this eddy and the EAC. An effect of Si enrichment was only observed at the frontal zone between the eddy and the EAC. No response to nutrient enrichment was observed in the inner-shelf water where ambient NO x , Si and PO 4 concentrations were up to 14, 4 and 3-times greater than in the EAC and oceanic eddy. Although results from the nutrient-enrichment experiments suggest that nutrients can affect biomass and the composition of the phytoplankton community, the comparison of all sites sampled showed no direct relationship between phytoplankton biomass, nutrients and the depth of the mixed layer. This is probably due to the different timeframe between the rapidly changing physical and chemical oceanography in the separation zone of the EAC.

Mesoscale distribution of larval Euphausia similis in various water masses of the East Australian Current

Larval Euphausia similis were collected off temperate eastern Australia in spring 2004 and 2006 to evaluate the relationships between larval populations, mesoscale oceanographic variability, and the wider planktonic community. Larval E. similis were present in greater numbers in the East Australian Current (EAC) relative to productive coastal waters. Larval E. similis density was homogenous across the EAC—Tasman Sea frontal region, but larvae were smaller in the Tasman Sea. Larval E. similis density was not enhanced within a cold core eddy relative to the surrounding EAC. We observed a negative correlation between larval E. similis density and larval fish density, and a weak positive correlation with fluorescence. Evaluation of a significant fish density×fluorescence interaction term showed that the effect of fish density was reduced at high fluorescence values. Analysis of normalized biomass size spectrum (NBSS) provided evidence for potential competitive exclusion of copepods by krill. Data presented in this study suggest a predatory influence on surface E. similis populations by mesopelagic larval fish. The degree of predation appears to be dependent on food availability, potentially mediated by changes in the physiological condition of krill.

Upper-Ocean Processes under the Stratus Cloud Deck in the Southeast Pacific Ocean

Abstract The annual mean heat budget of the upper ocean beneath the stratocumulus/stratus cloud deck in the southeast Pacific is estimated using Simple Ocean Data Assimilation (SODA) and an eddy-resolving Hybrid Coordinate Ocean Model (HYCOM). Both are compared with estimates based on Woods Hole Oceanographic Institution (WHOI) Improved Meteorological (IMET) buoy observations at 20°S, 85°W. Net surface heat fluxes are positive (warming) over most of the area under the stratus cloud deck. Upper-ocean processes responsible for balancing the surface heat flux are examined by estimating each term in the heat equation. In contrast to surface heat fluxes, geostrophic transport in the upper 50 m causes net cooling in most of the stratus cloud deck region. Ekman transport provides net warming north of the IMET site and net cooling south of the IMET site. Although the eddy heat flux divergence term can be comparable to other terms at a particular location, such as the IMET mooring site, it is negligible for the entire stratus region when area averaged because it is not spatially coherent in the open ocean. Although cold-core eddies are often generated near the coast in the eddy-resolving model, they do not significantly impact the heat budget in the open ocean in the southeast Pacific.