Barite accumulation rates throughout Pliocene-Pleistocene in Eastern Equatorial Pacific at ODP Site 138-849

Export production is an important component of the carbon cycle, modulating the climate system by transferring CO2 from the atmosphere to the deep ocean via the biological pump. Here we use barite accumulation rates to reconstruct export production in the eastern equatorial Pacific over the past 4.3 Ma. We find that export production fluctuated considerably on multiple time scales. Export production was on average higher (51 g C/m**2/yr) during the Pliocene than the Pleistocene (40 g C/m**2/yr), decreasing between 3 and 1 Ma (from more than 60 to 20 g C/m**2/yr) followed by an increase over the last million years. These trends likely reflect basin-scale changes in nutrient inventory and ocean circulation. Our record reveals decoupling between export production and temperatures on these long (million years) time scale. On orbital time scales, export production was generally higher during cold periods (glacial maxima) between 4.3 and 1.1 Ma. This could be due to stronger wind stress and higher upwelling rates during glacial periods. A shift in the timing of maximum export production to deglaciations is seen in the last ~1.1 million years. Results from this study suggest that, in the eastern equatorial Pacific, mechanisms that affect nutrient supply and/or ecosystem structure and in turn carbon export on orbital time scales differ from those operating on longer time scales and that processes linking export production and climate-modulated oceanic conditions changed about 1.1 million years ago. These observations should be accounted for in climate models to ensure better predictions of future climate change.

Oligocene to early Miocene benthic foraminifera in the eastern Equatorial Pacific

We investigated Oligocene and early Miocene benthic foraminiferal faunas (> 105 µm in size) from Ocean Drilling Program (Leg 199) Site 1218 (4826 m water depth and ~3300 to ~4000 m paleo-water depth) and Site 1219 (5063 m water depth and ~4200 to ~4400 m paleo-water depth) to understand the response of abyssal benthic foraminifera to mid-Oligocene glacial events in the eastern Equatorial Pacific Ocean. Two principal factor assemblages were recognized. The Factor 1 assemblage (common Nuttallides umbonifer) is related to either an influx of the Southern Component Water (SCW), possibly carbonate undersaturated, or a decrease in seasonality of the food supply from the surface ocean. The Factor 2 assemblage is characterized by typical deep-sea taxa living under variable trophic conditions, possibly with a seasonal component in food supply. The occurrence of abyssal benthic foraminifera faunas during the mid-Oligocene depends on either the effect of SCW or the seasonality of food resources. The Factor 1 assemblage was most common near 76Ol-C11r, 73Ol-C10rn and 67Ol-C9n (ca. 30.2, 29.1 and 26.8 Ma respectively by Pälike et al. (2006, doi:10.1126/science.1133822)). This indicates that the effect of SCW increased or the seasonal input of food from the surface ocean to benthic environments was weakened close to these glacial events. In contrast, the huge export flux of small biogenic carbonate particles close to these glacial events might be responsible for carbonate-rich sediments buffering carbonate undersaturation. Changes in deep-water masses or the periodicity of food supply from the surface ocean and variation in surface carbonate production affected by orbital forcing had an impact on the mid-Oligocene faunas of abyssal benthic foraminifera around the intervals of glacial events in the eastern Equatorial Pacific Ocean. The Factor 1 assemblage decreased sharply at ? 30 Ma (29.8 Ma by Pälike et al. (2006), 30.0 Ma by CK95) and returned to dominance after ? 29 Ma (28.6 Ma by Pälike et al. (2006), 28.8 Ma by CK95). It is likely that the effect of SCW (possibly carbonate undersaturated) has intensified since the late Oligocene. The faunal transition of benthic foraminifera in the eastern Equatorial Pacific Ocean at ~29 Ma might be attributable to the influence of Northern Component Water (NCW) input to the Southern Ocean and the subsequent formation of SCW at about that time.

(Table S2) The reconstructed sea surface temperature (SST) over the eastern equatorial Pacific

Abundant hydroclimatic evidence from western Amazonia and the adjacent Andes documents wet conditions during Heinrich Stadial 1 (HS1, 18-15 ka), a cold period in the high latitudes of the North Atlantic. This precipitation anomaly was attributed to a strengthening of the South American summer monsoon due to a change in the Atlantic interhemispheric sea surface temperature (SST) gradient. However, the physical viability of this mechanism has never been rigorously tested. We address this issue by combining a thorough compilation of tropical South American paleorecords and a set of atmosphere model sensitivity experiments. Our results show that the Atlantic SST variations alone, although leading to dry conditions in northern South America and wet conditions in northeastern Brazil, cannot produce increased precipitation over western Amazonia and the adjacent Andes during HS1. Instead, an eastern equatorial Pacific SST increase (i.e., 0.5-1.5 °C), in response to the slowdown of the Atlantic Meridional Overturning Circulation during HS1, is crucial to generate the wet conditions in these regions. The mechanism works via anomalous low sea level pressure over the eastern equatorial Pacific, which promotes a regional easterly low-level wind anomaly and moisture recycling from central Amazonia towards the Andes.

The role of the Indian Ocean sector and sea surface salinity for prediction of the coupled Indo-Pacific system

The purpose of this dissertation is to evaluate the potential downstream influence of the Indian Ocean (IO) on El Niño/Southern Oscillation (ENSO) forecasts through the oceanic pathway of the Indonesian Throughflow (ITF), atmospheric teleconnections between the IO and Pacific, and assimilation of IO observations. Also the impact of sea surface salinity (SSS) in the Indo-Pacific region is assessed to try to address known problems with operational coupled model precipitation forecasts. The ITF normally drains warm fresh water from the Pacific reducing the mixed layer depths (MLD). A shallower MLD amplifies large-scale oceanic Kelvin/Rossby waves thus giving ~10% larger response and more realistic ENSO sea surface temperature (SST) variability compared to observed when the ITF is open. In order to isolate the impact of the IO sector atmospheric teleconnections to ENSO, experiments are contrasted that selectively couple/decouple the interannual forcing in the IO. The interannual variability of IO SST forcing is responsible for 3 month lagged widespread downwelling in the Pacific, assisted by off-equatorial curl, leading to warmer NINO3 SST anomaly and improved ENSO validation (significant from 3-9 months). Isolating the impact of observations in the IO sector using regional assimilation identifies large-scale warming in the IO that acts to intensify the easterlies of the Walker circulation and increases pervasive upwelling across the Pacific, cooling the eastern Pacific, and improving ENSO validation (r ~ 0.05, RMS~0.08C). Lastly, the positive impact of more accurate fresh water forcing is demonstrated to address inadequate precipitation forecasts in operational coupled models. Aquarius SSS assimilation improves the mixed layer density and enhances mixing, setting off upwelling that eventually cools the eastern Pacific after 6 months, counteracting the pervasive warming of most coupled models and significantly improving ENSO validation from 5-11 months. In summary, the ITF oceanic pathway, the atmospheric teleconnection, the impact of observations in the IO, and improved Indo-Pacific SSS are all responsible for ENSO forecast improvements, and so each aspect of this study contributes to a better overall understanding of ENSO. Therefore, the upstream influence of the IO should be thought of as integral to the functioning of ENSO phenomenon.

The phylogeography and connectivity of the latitudinally widespread scleractinian coral Plesiastrea versipora in the Western Pacific

Whereas terrestrial animal populations might show genetic connectivity within a continent, marine species, such as hermatypic corals, may have connectivity stretching to all corners of the planet. We quantified the genetic variability within and among populations of the widespread scleractinian coral, Plesiastrea versipora along the eastern Australian seaboard (4145 km) and the Ryukyu Archipelago (Japan, 681 km) using sequences of internal transcribed spacers (ITS1-2) from ribosomal DNA. Geographic patterns in genetic variability were deduced from a nested clade analysis (NCA) performed on a parsimony network haplotype. This analysis allowed the establishment of geographical associations in the distribution of haplotypes within the network cladogram, therefore allowing us to deduce phylogeographical patterns based under models of restricted gene flow, fragmentation and range expansion. No significant structure was found among Ryukyu Archipelago populations. The lack of an association between the positions of haplotypes in the cladogram with geographical location of these populations may be accounted for by a high level of gene flow of P. versipora within this region, probably due to the strong Kuroshio Current. In contrast, strong geographical associations were apparent among populations of P. versipora along the south-east coast of Australia. This pattern of restricted genetic connectivity among populations of P. versipora on the eastern seaboard of Australia seems to be associated with the present surface ocean current (the East Australian Current) on this side of the south-western Pacific Ocean.

Oceanic interchange and nonequilibrium population structure in the estuarine dependent Indo-Pacific tasselfish, Polynemus sheridani

We assayed mtDNA haplotype [300 base pairs (bp) control region] geography and genealogy in the Indo-Pacific tasselfish, Polynemus sheridani from its contiguous estuarine distribution across northern Australia (n = 169). Eight estuaries were sampled from three oceanographic regions (Timor Sea, Gulf of Carpentaria and the Coral Sea) to assess the impact of Pleistocene sea level changes on the historical connectivity among P. sheridani populations. Specifically, we investigated the genetic consequences of disruption to Indian-Pacific Ocean connectivity brought about by the closure of the Torres Strait. Overall there was significant population subdivision among estuaries (F-ST = 0.161, (Phi(ST) = 0.187). Despite a linear distribution, P. sheridani did not show isolation by distance over the entire sampled range because of genetic similarity of estuaries greater than 3000 km apart. However, significant isolation by distance was detected between estuaries separated by less than 3000 km of coastline. Unlike many genetic studies of Indo-Pacific marine species, there was no evidence for an historical division between eastern and western populations. Instead, phylogeographical patterns were dominated by a starlike intraspecific phylogeny coupled with evidence for population expansion in both the Gulf of Carpentaria and the Coral Sea but not the Timor Sea. This was interpreted as evidence for recent west to east recolonization across of northern Australia following the last postglacial marine advance. We argue that although sufficient time has elapsed postcolonization for populations to approach gene flow-drift equilibrium over smaller spatial scales (< 3000 km), the signal of historical colonization persists to obscure the expected equilibrium pattern of isolation by distance over large spatial scales (> 3000 km).

On the origin of the North Pacific arcs

We present a new hypothesis that relates global plate tectonics to the formation of marginal basins, island arcs, spreading ridges and arc-shaped mountain belts around the North Pacific Ocean. According to our model, the ellipsoidal-shaped Paleogene basins of the South China Sea, Parece-Vela Basin, Shikoku Basin, Sea of Japan and the Sea of Okhotsk in addition to those of the North American Cordillera can be attributed to the change in plate convergence direction at 42 Ma between the Indoaustralian and Eurasian plates. The new direction of convergence was parallel to the eastern continental margin of Asia and resulted in widespread extension perpendicular to this margin and to the western margin of North America. Both margins form part of a circle parallel to the Indoaustralian-Eurasian direction of convergence.

Late mesozoic to neogene radiolarian biostratigraphy and palaeoceanography in the Caribbean and East pacific region

^Raduolarians constitute a good tool for contributing to the biostratigraphy of accreted terranes and in deep-sea sediment sequences. The use of radiolarians is also proven to be valuable as a palaeoceanographic indicator. The present study evaluates radiolarians in three different geological settings, in order to better constrain the age of the sites and to try to understand their palaeoenvironmental situation at different periods, particularly in the Caribbean-Central America area. On the Jarabacoa Block, in Central Dominican Republic, a hundred meters of siliceous mudstones (Pedro Brand section in the Tireo Group) was dated as Turonian- Coniancian in age using radiolarians. A 40Ar-39Ar whole rock age of 75.1±1.1 Ma (Campanian), obtained in a basalt dyke crosscutting the radiolarian bearing rocks, a consistent minimum age for the pelagic-hemipelagic Pedro Brand section. The Jarabacoa Block is considered as the most complete outcrop section of Pacific ocean crust overlain by a first Aptian-Albian phase of Caribbean Large Igneous Province-type activity (CLIP), followed by the development of a Cenomanian-Santonian intraoceanic arc, which is in turn overlain by a late Campanian-Maastrichtian CLIP-phase. The Tireo Group records an episode of pelagic to hemi-pelagic and intermediate to acidic arc-derived sedimentation, previous to the youngest magmatic phase of the CLIP. Thus, the section of Pedro Brand has been interpreted in this study as being part of the intraoceanic arc. In northern Venezuela, a greenish radiolarite section from Siquisique Ophiolite (basalts, gabbros and some associated cherts) in Guaparo Creek has been studied. In previous studies, the Ophiolite unit (Petacas Creek section) has been dated as Bajocian-Bathonian, based on ammonites present in interpillow sediments from basalt blocks. New dating of the present study concluded in an Aptian?-Albian-Cenomanian age for the Guaparo creek section (middle Cretaceous), based on radiolarian assemblage associated to basalts-gabbros rocks of the unit. Previous plagioclase 40Ar-39Ar ages from the Siquisuique Ophiolite may be slightly younger (94-90 Ma.) and may, therefore, represent younger dykes that intruded onto a well-developed sheeted dyke complex of the Siquisique. The geochemistry of these rocks and the palaeotectonic reconstruction of the Caribbean area during this period suggest that these rocks were derived from a mid-ocean ridge with an influence of deep mantle plume. The Siquisique Ophiolite most probably represents a fragment of the proto-Caribbean basin. The Integrated Ocean Drilling Program Expedition 344 drilled a transect across the convergent margin off Costa Rica. Two sites of this expedition were chosen for radiolarian biostratigraphy and palaeoceanographic studies. Both sites (U1381C and U1414A) are located in the incoming Cocos plate, in the eastern Equatorial Pacific. The succession of U1381C yields a Middle Miocene to Pleistocene age, and presents an important hiatus of approximately 10 Ma. The core of U1414A exposes a continuous sequence that deposited during Late Miocene to Pleistocene (radiolarian zones RN6-RN16). The ages were assigned based on radiolarians and correlated with nannofossil zonation and tephra 40Ar-39Ar datation. With those results, and considering the northward movement of the Cocos plate motion (about 7 cm/year), deduction is made that the sites U1381C and U1414A were initially deposited during the Miocene, several hundreds of kilometres from the current location, slightly south of the Equator. This suggests that the faunas of these sites have been subjected to different currents, first influenced by the cold tongue of the South Equatorial Current and followed by the warm Equatorial Countercurrent. At last, coastal upwelling influenced faunas of the Pleistocene. -- Les radiolaires sont considérés comme un outil utile à la biostratigraphie des terrains accrétés et des sédiments profonds. Leur utilité est aussi prouvée comme étant remarquable au niveau des reconstructions paléocéanographiques. La présente étude évalue l'importance et la présence des radiolaires de trois localités géologiquement différentes d'Amérique Centrale-Caraïbes, dans le but d'améliorer les model d'âges et de mieux comprendre la situation paléoenvironnementale à travers le temps. Dans le Bloque de Jarabacoa, au centre de la République Dominicaine, une section de cent mètres (section de Pedro Brand, Groupe de Tireo) a été datée comme faisant partie du Turonien-Santonien, en utilisant les radiolaires. Une datation 40Ar-39Ar sur roche totale de 75±1.1 Ma (Campanien) a été obtenu pour vin dyke traversant les sédiments riches en radiolaires, en cohérence avec l'âge minimum accordé à la section de Pedro Brand. Aux Caraïbes, le Bloque de Jarabacoa est considéré comme l'affleurement le plus complet présentant une succession de croûte océanique d'origine Pacifique recouverte d'une première phase d'activité volcanique de type CLIP (Caribbean Large Igneous Province) d'âge Aptien- Albien, de dépôts d'arc volcanique intra-océanique d'âge Cénomanien-Santonien, puis d'une seconde phase de type CLIP d'âge Campanien-Maastrichtien. Le Groupe de Tireo enregistre un épisode de dépôt pélagiques-hémipélagiques et d'arc volcanique, antérieur à la plus jeune phase de type CLIP. Cette étude place donc la formation de la section de Pedro Brand au moment du développement de l'arc intra-océanique. A Guaparo Creek (nord du Vénézuela), une section de radiolarite verdâtre faisant partie des ophiolites de Siquisique (basaltes, gabbros, cherts) a été étudiée. Dans des études précédentes, sur la localité de Petacas Creek, l'unité ophiolitique a été daté d'âge Bajocien- Bathonien (Jurassique) sur la base d'ammonites trouvées dans des sédiments intercalés entre des laves en coussins. Les nouvelles datations de notre étude, basées sur des assemblages à radiolaires de l'unité à basaltes-gabbros, donnent un âge Aptien?-Albien-Cénomanien (Crétacé moyen). Les âges de l'Ophiolite de Siquisique, précédement calculés par la méthode sur plagioclases, pourraient être légèrement plus jeune (94-90 Ma) et donc représenter des intrusions plus récentes de dykes dans le complexe filonien déjà bien dévelopé. La géochimie de ces roches magmatiques, ainsi que les reconstructions paléotectoniques de la zone Caraïbes durant cette période, suggèrent que ces formations sont dérivées d'une ride médio-océanique associée à l'influence d'un panache mantellique. L'ophiolite de Siquisique représente très probablement un fragment du bassin de proto¬Caraïbe. L'expédition 344 du programme IODP (Integrated Ocean Drilling Program) a eu lieu dans l'optique de forer et dresser une coupe de la marge convergente au large du Costa Rica. Deux sites de cette expédition ont été choisis pour les besoins des études de biostratigraphie et de reconstruction paléocéanographique. Ces deux sites (U1381C et U1414A) sont situés sur la plaque subductante de Cocos, dans la zone Pacifique est-équatoriale. La carotte U1381C expose une séquence s'étalant du Miocène moyen au Pléistocène, et présente un important hiatus d'environ 10 Ma. La carotte U1414A expose une séquence continue s'étalant du Miocène tardif au Pléistocène (zone à radiolaires RN6-RN16). Les âges ont été assignés sur la base des radiolaires et corrélés avec les zones à nanofossiles et les datations 40Ar-39Ar sur téphras. Avec ces résultats, et en considérant le mouvement nord de la plaque de Cocos (environ 7 cm/an), déduction est faite que les deux sites étaient initialement situés, au cours du Miocène, à plusieurs centaines de kilomètres de leur location actuelle, au sud de l'équateur. Cela suggère que les faunes de ces sites ont été sujettes à différents courants; premièrement influencées par la langue froide du SEC (South Equatorial Current), puis par les eaux chaudes du ECC (Equatorial Countercurrent). Pour terminer, les remontées d'eau côtières ont influencées les faunes Pléistocène.

POST-MONSOON SEA SURFACE TEMPERATURE AND CONVECTION ANOMALIES OVER INDIAN AND PACIFIC OCEANS

We have studied sea surface temperature (SST) anomalies over the Indian and Pacific Oceans (domain 25 °S to 25°N and 40 °E to 160 °W) during the three seasons following the Indian summer monsoon for wet monsoons and also for dry monsoons accompanied or not by El Ni˜no. A dry monsoon is followed by positive SST anomalies in the longitude belt 40 to 120 °E, negative anomalies in 120 to 160 °E and again positive anomalies east of 160 °E. In dry monsoons accompanied by El Ni˜no the anomalies have the same sign, but are much stronger. Wet monsoons have weak anomalies of opposite sign in all three of the longitude belts. Thus El Ni˜no and a dry monsoon have the same types of association with the Indian and Pacific Ocean SSTs. In the sector 40 to 120 °E SST anomalies first appear over the western part of the Indian Ocean (June to September) followed by the same sign of anomalies over its eastern part and China Sea (October to March). By March after a dry monsoon or El Ni˜no the Indian Ocean between 10 °N and 10 °S has a spatially large warm SST anomaly. Anomalies in deep convection tend to follow the SST anomalies, with warm SST anomalies producing positive convection anomalies around the seasonal location of the intertropical convergence zone

Why the western Pacific subtropical high has extended westward since the late 1970s

The western Pacific subtropical high (WPSH) is closely related to Asian climate. Previous examination of changes in the WPSH found a westward extension since the late 1970s, which has contributed to the inter-decadal transition of East Asian climate. The reason for the westward extension is unknown, however. The present study suggests that this significant change of WPSH is partly due to the atmosphere's response to the observed Indian Ocean-western Pacific (IWP) warming. Coordinated by a European Union's Sixth Framework Programme, Understanding the Dynamics of the Coupled Climate System (DYNAMITE), five AGCMs were forced by identical idealized sea surface temperature patterns representative of the IWP warming and cooling. The results of these numerical experiments suggest that the negative heating in the central and eastern tropical Pacific and increased convective heating in the equatorial Indian Ocean/ Maritime Continent associated with IWP warming are in favor of the westward extension of WPSH. The SST changes in IWP influences the Walker circulation, with a subsequent reduction of convections in the tropical central and eastern Pacific, which then forces an ENSO/Gill-type response that modulates the WPSH. The monsoon diabatic heating mechanism proposed by Rodwell and Hoskins plays a secondary reinforcing role in the westward extension of WPSH. The low-level equatorial flank of WPSH is interpreted as a Kelvin response to monsoon condensational heating, while the intensified poleward flow along the western flank of WPSH is in accord with Sverdrup vorticity balance. The IWP warming has led to an expansion of the South Asian high in the upper troposphere, as seen in the reanalysis.

A model study of oceanic mechanisms affecting Equatorial Pacific sea surface temperature during the 1997-98 El Niño

In this study, the processes affecting sea surface temperature variability over the 1992–98 period, encompassing the very strong 1997–98 El Niño event, are analyzed. A tropical Pacific Ocean general circulation model, forced by a combination of weekly ERS1–2 and TAO wind stresses, and climatological heat and freshwater fluxes, is first validated against observations. The model reproduces the main features of the tropical Pacific mean state, despite a weaker than observed thermal stratification, a 0.1 m s−1 too strong (weak) South Equatorial Current (North Equatorial Countercurrent), and a slight underestimate of the Equatorial Undercurrent. Good agreement is found between the model dynamic height and TOPEX/Poseidon sea level variability, with correlation/rms differences of 0.80/4.7 cm on average in the 10°N–10°S band. The model sea surface temperature variability is a bit weak, but reproduces the main features of interannual variability during the 1992–98 period. The model compares well with the TAO current variability at the equator, with correlation/rms differences of 0.81/0.23 m s−1 for surface currents. The model therefore reproduces well the observed interannual variability, with wind stress as the only interannually varying forcing. This good agreement with observations provides confidence in the comprehensive three-dimensional circulation and thermal structure of the model. A close examination of mixed layer heat balance is thus undertaken, contrasting the mean seasonal cycle of the 1993–96 period and the 1997–98 El Niño. In the eastern Pacific, cooling by exchanges with the subsurface (vertical advection, mixing, and entrainment), the atmospheric forcing, and the eddies (mainly the tropical instability waves) are the three main contributors to the heat budget. In the central–western Pacific, the zonal advection by low-frequency currents becomes the main contributor. Westerly wind bursts (in December 1996 and March and June 1997) were found to play a decisive role in the onset of the 1997–98 El Niño. They contributed to the early warming in the eastern Pacific because the downwelling Kelvin waves that they excited diminished subsurface cooling there. But it is mainly through eastward advection of the warm pool that they generated temperature anomalies in the central Pacific. The end of El Niño can be linked to the large-scale easterly anomalies that developed in the western Pacific and spread eastward, from the end of 1997 onward. In the far-western Pacific, because of the shallower than normal thermocline, these easterlies cooled the SST by vertical processes. In the central Pacific, easterlies pushed the warm pool back to the west. In the east, they led to a shallower thermocline, which ultimately allowed subsurface cooling to resume and to quickly cool the surface layer.

Impact of the Atlantic Ocean on the multidecadal fluctuation of El Nino–Southern Oscillation–South Asian monsoon relationship in a coupled general circulation model

The multidecadal variability of El Niño–Southern Oscillation (ENSO)–South Asian monsoon relationship is elucidated in a 1000 year control simulation of a coupled general circulation model. The results indicate that the Atlantic Multidecadal Oscillation (AMO), resulting from the natural fluctuation of the Atlantic Meridional Overturning Circulation (AMOC), plays an important role in modulating the multidecadal variation of the ENSO-monsoon relationship. The sea surface temperature anomalies associated with the AMO induce not only significant climate impact in the Atlantic but also the coupled feedbacks in the tropical Pacific regions. The remote responses in the Pacific Ocean to a positive phase of the AMO which is resulted from enhanced AMOC in the model simulation and are characterized by statistically significant warming in the North Pacific and in the western tropical Pacific, a relaxation of tropical easterly trades in the central and eastern tropical Pacific, and a deeper thermocline in the eastern tropical Pacific. These changes in mean states lead to a reduction of ENSO variability and therefore a weakening of the ENSO-monsoon relationship. This study suggests a nonlocal mechanism for the low-frequency fluctuation of the ENSO-monsoon relationship, although the AMO explains only a fraction of the ENSO–South Asian monsoon variation on decadal-multidecadal timescale. Given the multidecadal variation of the AMOC and therefore of the AMO exhibit decadal predictability, this study highlights the possibility that a part of the change of climate variability in the Pacific Ocean and its teleconnection may be predictable.

Interdecadal enhancement of the Walker circulation over the Tropical Pacific in the late 1990s

The Walker circulation is one of the major components of the large-scale tropical atmospheric circulation and variations in its strength are critical to equatorial Pacific Ocean circulation. It has been argued in the literature that during the 20th century the Walker circulation weakened, and that this weakening was attributable to anthropogenic climate change. By using updated observations, we show that there has been a rapid interdecadal enhancement of the Walker circulation since the late 1990s. Associated with this enhancement is enhanced precipitation in the tropical western Pacific, anomalous westerlies in the upper troposphere, descent in the central and eastern tropical Pacific, and anomalous surface easterlies in the western and central tropical Pacific. The characteristics of associated oceanic changes are a strengthened thermocline slope and an enhanced zonal SST gradient across the tropical Pacific. Many characteristics of these changes are similar to those associated with the mid-1970s climate shift with an opposite sign. We also show that the interdecadal variability of the Walker circulation in the tropical Pacific is inversely correlated to the interdecadal variability of the zonal circulation in the tropical Atlantic. An enhancement of the Walker circulation in the tropical Pacific is associated with a weakening zonal circulation in the tropical Atlantic and vise versa, implying an inter-Atlantic-Pacific connection of the zonal overturning circulation variation. Whether these recent changes will be sustained is not yet clear, but our research highlights the importance of understanding the interdecadal variability, as well as the long-term trends, that influence tropical circulation.

Simulations of the impact of orbital forcing and ocean on the Asian summer monsoon during the Holocene

The importance of orbital forcing and ocean impact on the Asian summer monsoon in the Holocene is investigated by comparing simulations with a fully coupled ocean–atmosphere general circulation model (FOAM) and with the atmospheric component of this model (FSSTAM) forced with prescribed modern sea-surface temperatures (SSTs). The results show: (1) the ocean amplifies the orbitally-induced increase in African monsoon precipitation, makes somewhat increase in southern India and damps the increase over the southeastern China. (2) The ocean could change the spatial distribution and local intensity of the orbitally-induced latitudinal atmospheric oscillation over the southeastern China and the subtropical western Pacific Ocean. (3) The orbital forcing mostly enhances the Asian summer precipitation in the FOAM and FSSTAM simulations. However, the ocean reduces the orbitally-induced summer precipitation and postpones the time of summer monsoon onset over the Asian monsoon region. (4) The orbital forcing considerably enhances the intensity of upper divergence, which is amplified by ocean further, over the eastern hemisphere. But the divergence is weaker in the FOAM simulations than in the FSSTAM simulations when the orbital forcing is fixed. (5) The orbital forcing can enhance the amplitude of precipitation variability over the subtropical Africa, the southeastern China and northwestern China, inversely, reduce it over central India and North China in the FOAM and FSSTAM simulations. The ocean obviously reduces the amplitude of precipitation variability over most of the Asian monsoon regions in the fixed orbital forcing simulations. (6) The areas characterized by increased summer precipitation in the long-term mean are mostly characterized by increased amplitude of short-term variability, whereas regions characterized by decreased precipitation are primarily characterized by decreased amplitude of short-term variability. However, the influences of orbital forcing or dynamical ocean on regional climate depend on the model.