Interannual Variability Of Circulation Research Articles

Impacts of Interannual Ocean Circulation Variability on Japanese Eel Larval Migration in the Western North Pacific Ocean.

The Japanese eel larvae hatch near the West Mariana Ridge seamount chain and travel through the North Equatorial Current (NEC), the Kuroshio, and the Subtropical Countercurrent (STCC) region during their shoreward migration toward East Asia. The interannual variability of circulation over the subtropical and tropical regions of the western North Pacific Ocean is affected by the Philippines–Taiwan Oscillation (PTO). This study examines the effect of the PTO on the Japanese eel larval migration routes using a three-dimensional (3D) particle tracking method, including vertical and horizontal swimming behavior. The 3D circulation and hydrography used for particle tracking are from the ocean circulation reanalysis produced by the Japan Coastal Ocean Predictability Experiment 2 (JCOPE2). Our results demonstrate that bifurcation of the NEC and the strength and spatial variation of the Kuroshio affect the distribution and migration of eel larvae. During the positive phase of PTO, more virtual eels (“v-eels”) can enter the Kuroshio to reach the south coast of Japan and more v-eels reach the South China Sea through the Luzon Strait; the stronger and more offshore swing of the Kuroshio in the East China Sea leads to fewer eels entering the East China Sea and the onshore movement of the Kuroshio to the south of Japan brings the eels closer to the Japanese coast. Significant differences in eel migration routes and distributions regulated by ocean circulation in different PTO phases can also affect the otolith increment. The estimated otolith increment suggests that eel age tends to be underestimated after six months of simulation due to the cooler lower layer temperature. Underestimation is more significant in the positive PTO years due to the wide distribution in higher latitudes than in the negative PTO years.

Connectivity in the Intra-American Seas and implications for potential larval transport

A major challenge in marine ecology is to describe patterns of larval dispersal and population connectivity, as well as their underlying processes. We re-assessed broad-scale population connectivity with a focus on the 18 coral reef hot spots in the Intra-American Seas described in Roberts (Science 278:1454–1457, 1997), by including seasonal and inter-annual variability in potential larval dispersal. While overall dispersal patterns were in agreement with previous findings, further statistical analyses show that dispersal patterns driven by mean circulation initially described by Roberts (Science 278:1454–1457, 1997) can significantly underestimate particle connectivity envelopes. The results from this study indicate that seasonal and inter-annual variability in circulation are crucial in modulating both dispersal distance and directional anisotropy of virtual larvae over most coral reef sites and that certain larval hotspots are likely more strongly connected than originally thought. Improved larval dispersal transport envelopes can enhance the accuracy of probability estimates which, in turn, may help to explain episodic larval settlement in certain times and places, and guide spatial management such as marine protected areas.

Interannual variability of circulation under spring ice in a boreal lake

A small range (∼ 1°C) of under‐ice water temperature is shown to result in remarkably different circulation regimes under spring ice in a deep, oligotrophic boreal lake. With the water column at < 4°C, melting of snow led to deepening vertical convection before ice break and a final depth of convection inversely correlated with earlier deep‐water temperature. We attribute that to the nonlinear dependence of water density on temperature, albeit further affected by stochastic weather factors. In four of nine study years, convection led to complete under‐ice overturn of the lake, indicating that this may not be uncommon in similar lakes with steep topography. River inflow and more intensive warming of water in the littoral zone also created a horizontal density differential, convection that involved flow down the sloping bottom and a lateral intrusion of this sinking water at a depth between the vertical convection and the quiescent deep‐water layers. The vertical and horizontal convection together produced a profile of temperature slightly increasing from the surface to the bottom of the convection layer. The contribution of horizontal convection to under‐ice mixing was interannually variable, and in one of the study years it eventually dominated under‐ice mixing. A thermal bar circulation regime developed occasionally and only in the open water between ice and shoreline. We identified five different under‐ice mixing regimes that form an interannually variable continuum of behavior during the ice melting period. The dependence on a narrow temperature range likely makes the circulation regime sensitive to a warming climate.

Interannual Variability of the Circulation over the Eastern Canadian Shelf

This study examines the main physical processes affecting the interannual variability of circulation over the eastern Canadian continental shelf (ECS) based on numerical circulation results for the period 1988–2004 produced by a regional ocean–ice model. The regional circulation model is applied to the northwest Atlantic and uses a horizontal curvilinear grid with a horizontal resolution of approximately 1/4°. The model is forced by atmospheric reanalysis fields produced by Large and Yeager (2004) and boundary forcing based on the ocean reanalysis data produced by Smith et al. (2010). In comparison with previous observational and numerical results in the literature, the regional circulation model has reasonable skill in simulating the large-scale circulation and associated seasonal and interannual variability over the ECS. Complex Empirical Orthogonal Function (CEOF) analysis is used to examine the interannual variability in the monthly mean temperature and salinity anomalies in the model. The CEOF analysis of model results demonstrates that the interannual variability over the Labrador and northern Newfoundland shelves is significantly affected by the variability at high latitudes which propagates onto these shelves through the northern open boundary. Over the eastern Newfoundland Shelf, the interannual variability is significantly affected by the non-linear interaction of the Labrador Current with the North Atlantic Current and by the variability propagating from the southern Labrador Shelf. The interannual variability of circulation and hydrography over the Slope Water region off the Scotian Shelf is significantly affected by anomalies advected by the Gulf Stream and by the non-linear dynamics taking place in the deep waters to the south of the Tail of the Grand Banks. RÉSUMÉ [Traduit par la rédaction] La présente étude examine les principaux processus physiques ayant une influence sur la variabilité interannuelle de la circulation dans l'est du plateau continental canadien (ECS) d'après les résultats numériques de circulation pour la période 1988–2004 obtenus d'un modèle régional océan–glace. Le modèle de circulation régional est appliqué au nord-ouest de l'Atlantique et utilise une grille curvilinéaire horizontale ayant une résolution horizontale d'approximativement 1/4°. Le modèle est forcé par des champs de réanalyse atmosphériques produits par Large et Yeager (2004) et subit un forçage aux limites basé sur des données de réanalyse océaniques produites par Smith et coll. (2010). Comparativement aux résultats observationnels et numériques précédents que l'on trouve dans la littérature, le modèle de circulation régional possède une habileté raisonnable pour la simulation de la circulation à grande échelle et de la variabilité saisonnière et interannuelle associée sur l'ECS. Nous utilisons l'analyse par fonctions orthogonales empiriques complexes (CEOF) pour examiner la variabilité interannuelle des anomalies mensuelles moyennes de température et de salinité dans le modèle. L'analyse par CEOF des résultats du modèle montre que la variabilité interannuelle sur les plateaux du Labrador et du nord de Terre–Neuve est significativement influencée par la variabilité aux hautes latitudes qui se propage sur ces plateaux à travers la limite nord ouverte. Sur le plateau de l'est de Terre–Neuve, la variabilité interannuelle est significativement influencée par l'interaction non linéaire du courant du Labrador avec le courant de l'Atlantique Nord et par la variabilité se propageant depuis le plateau du sud du Labrador. La variabilité interannuelle de la circulation et de l'hydrographie dans la région du talus continental au large du plateau néo-écossais est significativement influencée par les anomalies advectées par le Gulf Stream et par la dynamique non linéaire se produisant dans les eaux profondes au sud de la queue des Grands Bancs.

Austral Spring Stratospheric and Tropospheric Circulation Interannual Variability

Abstract The relationship between the October (spring) total ozone column (TOC) midlatitude zonal asymmetry over the Southern Hemisphere (SH) and the stratospheric quasi-stationary wave 1 (QSW1) interannual phase variability is analyzed. Once contributions to the TOC from known global predictors, estimated with a multiregression model, are removed, the residual TOC interannual variability is observed to be dynamically coupled to the stratospheric QSW1 phase behavior. The stratospheric QSW1 interannual phase variability, when classified according to specifically designed indices, yields different circulation patterns in the troposphere and stratosphere. High (upper quartile) index values correspond to a westward rotation of the midlatitude ozone trough and the stratospheric QSW1 phase, while low (lower quartile) index values represent their eastward-rotated state. These values can be associated with statistically different tropospheric circulation patterns: a predominantly single poleward tropospheric jet structure for high index values and a predominantly double-jet structure for low index values. For the latter, there is a higher daily probability of double-jet occurrence in the troposphere and a stronger stratospheric jet. These jet structures and their daily behavior are supported by significant synoptic-scale activity anomalies over SH mid- to high latitudes as well as changes in tropospheric quasi-stationary waves 1–3. The wave activity flux (W flux) diagnosis shows the contribution of active quasi-stationary waves in the observed tropospheric anomalies associated with high and low index values. With low index values, the quasi-stationary waves lead to a self-sustaining state of the stratospheric–tropospheric coupled system. With high index values, the overall mid- to high latitude circulation is associated with wave energy propagation from the tropical central Pacific into higher latitudes. Thus, during the austral spring, there are interactions between the troposphere and stratosphere, leading to the locally well-defined upward and downward propagation of wave anomalies, that is, significant upper troposphere (UT)–lower stratosphere (LS) interactions can occur within a spring month itself.

Modeling study of seasonal and inter-annual variability of circulation in the coastal upwelling site of the El Loa River off northern Chile

A three-dimensional numerical model using the Finite Element Method (FEM) was used to diagnose coastal currents off the El Loa River (21° S) in the northern upwelling region of Chile. This site has been recognized as an important spawning zone of the southern anchovy Engraulis ringens. Diagnostic current fields were obtained for summer and winter during the 1997–1998 El Nino conditions and during a “normal” upwelling year. The results show this site as an efficient retention area in the nearshore, because of a reduced cross-shelf flow, a strong alongshore flow and presence of several anticyclonic eddies. A simulated Lagrangian experiment indicated that retention within the nearshore (<10 km) may last for more than 4 days under a steady-state wind condition. Wind regimes and water density fields during 1997–1998 (El Nino) and 1995–1996 (“normal” upwelling) did not cause differences in the general pattern of coastal circulation. However, the magnitudes of both the alongshore and the cross-shelf flows are substantially reduced during El Nino in the nearshore spawning zone, possibly as a consequence of an anomalous water mass in the coastal area. This altered condition may limit the transport and dispersion of anchovy spawning products.

Simulations by the atmospheric model intercomparison project of atmospheric circulation over southern Africa

Seven simulations by the Atmospheric Model Intercomparison Project (AMIP) of mean climate and interannual variability over southern Africa during the 1979-1988 decade are evaluated. The focus of the study is the representation in the models of both regional and hemispheric circulation features which influence the climate of the subcontinent. The AMIP integrations provide an improved simulation of mean circulation around southern Africa when compared with earlier atmospheric general circulation models. Improvements are noted in the representation of the position and intensity of the subtropical anticyclones, the circumpolar trough and the strength of the mid-latitude westerlies. The representation of tropospheric circulation features throughout the Southem Hemisphere, including zonal asymmetries and planetary wave 1 at 500 hPa, appear to be dependent on the spatial resolution of the models. No single model simulates all features of southern African circulation accurately, and model performance varies with the seasons. Several of the AMIP models are capable of simulating the observed features of interannual variability in circulation over southern Africa. Simulated interannual variability in both summer rainfall and circulation is weaker than observed. Simulated circulation adjustments in the subtropics and mid-latitudes are in good agreement with observed adjustments associated with periods of above and below average summer rainfall over the subcontinent. It is recognized that considerable uncertainty is associated with the single integrations using observed sea-surface temperature forcing considered in the present analysis. Ensemble approaches are required to provide a more comprehensive assessment of the simulated response to imposed forcing.

Observing the Fluctuations of Gyre-Scale Ocean Circulation: A Study of the Subtropical South Pacific

Seasonal and interannual variability of the subtropical gyre in the South Pacific Ocean are investigated by means of a time series of expendable bathythermograph (XBT) sections between New Zealand (36°S, 175°E) and Fiji (18°S, 178°E). The experiment spans much of the subtropical gyre and is a prototype for future basin-scale observations. Eddy-resolving transects along the precisely repeating ship track, spanning four years, are used to estimate the mean field and fluctuations of temperature and geostrophic velocity. The mean field dominates on very large spatial scales while the fluctuations dominate on small scales. Mean and fluctuations have equal energy at a horizontal wavelength of about 2000 km. The study region contains three recurring small-scale features. These are the East Auckland Current, flowing eastward along the New Zealand continental slope, a front at about 29°S which is likely an extension of the Tasman Front, and a weaker feature, the Tropical Convergence at about 22°S. At lower latitudes in the study region, the entire thermocline migrates vertically at annual period. This annual oscillation ends near the front at 29°S. Farther poleward, the only substantial subsurface annual variation is in the strength of the East Auckland Current. Interannual variability of circulation during 1986–90 consisted of rapid transitions between two rather steady states. In one state, which persisted through 1987–88 and from mid-1989 to the present (mid-1990), the eastward flowing limb of the gyre was relatively strong and narrow, with a reversal in velocity at the ocean surface south of Fiji.

A strategy for studying shelf circulation in China Seas —Interannual variability of shelf circulation

Circulation in China Seas has been investigated by Chinese oceanographers in some detail for many years. However, owing to data being sparse and scarce, studies were basically concerned in interseasonal (mainly summer and winter) fluctuations and almost none was in the interannual variability of the circulation in China Seas.