OS41B-0529
Barotropic zonal jets induced by islands in the southwest Pacific
The oceanic circulation entering the tropical southwest Pacific (SWP) is dominated by the broad westward flow of the South Equatorial Current (SEC), forced by the trade winds. It has been argued that the numerous islands of the SWP are able to restructure the SEC into a series of deep and narrow zonal jets, which control important pathways connecting equatorial and extra-equatorial signals. The primary objective of this work is to improve our understanding of the structure and dynamics of SWP zonal jets, giving special attention to topographic effects. Our study is based on the use of a high-resolution regional oceanic model, whose solution is compared with observations, as well as with solutions from global models and the Sverdrup relation. Our model indicates that the regional topography drives a general equatorward shift of the SEC, which is beneficial to the North Fiji, North Vanuatu and North Caledonian Jets. A depth-integrated vorticity budget shows that this topographic effect is considerably attenuated by baroclinicity and advection processes, but not to the point of total compensation as often admitted for the interior ocean. The effect of non-linear advection is to allow flow rectification of the jets fluctuations, consistent with the Rhines effect, which takes the form of zonally elongated dipole circulations in the leeward side of the islands.
OS41B-0531
Flows entering the Solomon Sea: observed features during the FLUSEC-1 cruise in August 2007
The properties of water masses transported into the equatorial band by the subtropical gyre in the thermocline and below are of primary importance for long-term climate variations. At decadal timescales, the circulation of the southwest Pacific Ocean is suspected to play a major role in that variability. However, the region remains undersampled because of its remote location and the dynamics of the circulation is still poorly understood. Within the context of the Southwest Pacific ocean Circulation and Climate Experiment (SPICE), an oceanographic cruise has been staged from the IRD center of Nouméa on the R/V Alis in August 2007. The main section has been conducted at the entrance of the Solomon Sea from the Louisade archipelago to Guadalcanal Island, roughly along 10°S from 154°E to 160°E. Climatology across this section suggests that waters originating from the South Equatorial Current are transported by two main pathways through the Solomon Sea before joining the equatorial band. The main objective of that cruise is to quantify the respective contributions of the western boundary current in the west, and of the direct flow of the SEC in the east, to the circulation in the Solomon Sea. Conductivity- Temperature-Depth and 02 concentration profiles have been collected from the surface down to 2000 m depth in order to discriminate these different flows and the corresponding water mass properties. Currents measured by L-ADCP have also provided direct insights on the dynamical description of the deep flows entering this region. In addition to these in situ observations, outputs of a regional ocean circulation model dedicated to the dynamics of the Solomon Sea will be jointly analyzed and discussed.
OS41B-0532
Sensitivity of ENSO events to the decadal salinity changes and wind bursts of the SouthWest Pacific
This study is based on the ENSO simulations obtained with an Intermediate Coupled Model (ICM) of the tropical Pacific Ocean/Atmosphere system. Using FSU wind and Sea Level data to partially control the ICM, we have shown that among the events observed between 1980 and 1998, some were sensitive to Westerly Wind bursts, others were not, because in addition to WWB, the ICM forecasts are sensitive to decadal changes of the tropical Pacific sea level average. First, we examine the sea level variations observed by the TOPEX-Poseidon-Jason (TPJ) satellites since 1992. Using the ECCO model outputs, we determine the changes that are due to heat and those to salt content in the upper ocean. It is found that the dynamic topography of the South West Pacific above 750m has been rising monotously by as much as 4 cm in 15 years. By contrast the dynamic topography due to thermal changes is mostly affected by the ENSO events, the biggest signal being 16cm drop between 1996 and 1998. These values are averaged on the Pacific region west of the dateline and between 5S and 20S. They are largely explained by the changes of the wind stress curl of the SPCZ. The variations of the basin-averaged sea level are explained in terms of equatorial zonal wind stress, wind stress curls in the North and South, and the mass adjustments between the tropical Indian and Pacific Oceans. Second, we force the ICM by observed winds: the ICM reproduces well the ENSO events observed since 1992, the model Nino3 SST and Nino4 zonal wind indices are correlated with observations by more than 0.7. When in addition to the winds forcing the ICM, sea level data are added to the equations to modify the closed boundary conditions of the model and take into account the connection with the Indian ocean, we find that the model Sea Level is improved, mostly in the western Pacific, but also on average over the basin. Third, initialized with the various conditions described above, the ICM is used to deliver a series of forecasts centered on the 3 cases: 1997-1998, 2001-2002, 2006-2007. Only the 1997-98 event can be predicted after the addition of the observed WWBs in winter 1996-1997. Even though strong WWBs were observed prior to the other 2 events, the ICM fails to predict these events which were very unusual. It appears necessary to control the mass and salinity changes in the West Pacific during the forecast experiment to be able to make progress.
OS41B-0533
Southwest Pacific Subtropical Mode Water: a Climatology
The large scale distribution and changes in southwest Pacific Subtropical Mode Water (STMW) are investigated and discussed. The paper presents geographic maps showing the spatial distribution of STMW thicknesses, with a vertical temperature gradient <2.0°C/100m occupying the 14°-20°C range below the mixed layer depth, across the entire southwest Pacific region. STMW changes in areal thickness extent, vertical cross-sectional area along selected transects, and total volume, are examined on seasonal and interannual time scales between 1973 and 1988. We find that STMW extends across the entire width of the Tasman Sea in a very broad swath between the Tropical Convergence in the north (just to the south of New Caledonia), the southeast Australian coast in the west to as far south as 39°S (likely due to the southward extension of the EAC), and eastwards along the southern STMW boundary in a meandering pathway that broadly follows the Tasman Front. The total STMW volume across the region (i.e., west of 180°) varies seasonally by a factor of more than three between the estimated maximum of 6.6 \( ± 0.5 \) × 1014 m3 in October and minimum of 1.9 \( ± 0.4 \) × 1014 m3 in May. Interannual variations O(± 0.5 × 1014 m3) are also observed in the spatial extent of the thick Mode Water and its total volume. El Niño composite maps show an anomalous thickening of the STMW during the El Niño year with October positive thickness anomalies in excess of +20m (total volume anomaly of +0.6 × 1014 m3) manifested throughout the subtropical gyre interior as far north as New Caledonia. Total volume anomalies tend to be positive from January of the El Niño year through to the July following (18 months). The maximum correlation coefficient r = -0.3 between three-monthly STMW volume anomalies and the Southern Oscillation index is statistically significant at the 95% confidence level. We conclude that during the anomalous cooling of the upper southwest Pacific Ocean in the El Niño year, winter-time convection and STMW formation is enhanced across the region resulting in an El Niño - Southern Oscillation climate signal that is identifiable below the mixed layer by the increased STMW volume which persists through to the following winter.
OS41B-0534
What are the Physical Processes Driving Sea Surface Temperature and Salinity Changes at Interannual Timescales in the Southwestern Tropical Pacific Ocean?
Although the most prominent signature of the El Nino-Southern Oscillation is seen in the equatorial Pacific Ocean, significant interannual variability correlated with the equatorial signal is also observed in the southwestern subtropical Pacific. The sea surface is saltier and colder during El Nino than during La Nina events. In the course of El Nino events, the South Pacific Convergence Zone is displaced northeastward, producing interannual variability of the wind speed, wind curl, and the atmospheric heat and freshwater fluxes in the southern subtropics. The mean currents and thermocline depth of the southern gyre are also modified, and the oceanic advection terms are changed in consequence. To diagnose the relative importance of the physical processes driving the sea surface changes, the outputs of an ocean general circulation model are used and validated against the available data. The mixed layer heat and salt balances at interannual timescales are estimated. It appears that the deepening of the mixed layer during El Nino events is of primary importance to explain the surface modifications.
OS41B-0535
Shipboard Measurements of the Distribution of Methane Mixing Ratio and 13CH4 in the Western Pacific: The Impact of Tropical Meteorology
Trace gas transport in the troposphere from the Northern to Southern Hemisphere is profoundly influenced by the position and strength of two meteorological zones: the Inter-Tropical and South Pacific Convergence Zones (ITCZ and SPCZ). In the mid-Pacific these zones are well separated, vary with season and El Nino Southern Oscillation events, and are usually well defined. In the Western Pacific the situation is more complicated with the zones often joining in a wedge east of Indonesia or disappearing altogether. The situation is further complicated by complex tropical meteorology involving deep convection. Here we present the first results of a joint project aimed at examining the impact of the ITCZ and SPCZ on the transport of atmospheric trace gases from the Northern to the Southern Hemispheres in the tropical Western Pacific. The project (developed between the New Zealand National Institute of Water and Atmospheric Research - NIWA, and the Japanese National Institute of Environmental Studies - NIES) uses high precision measurements of methane mixing ratio and the stable methane carbon isotopic species 13CH4 as atmospheric tracers supplemented by measurements of related species such as CO, 13CO, and 14CO. The measurements are made on large clean air samples (up to 1~m3) collected aboard bulk carrier ships operated by the Japanese shipping company Toyofuji on voyages between New Zealand and Japan. These air samples provide excellent snapshots of trace gas mixing ratio and isotopic gradients between the hemispheres as well as in-depth information on changes through the convergence zones. We show the initial results from 6 voyages between May 2004 and January 2007 illustrating the clearly defined latitudinal gradients in mixing ratio and isotopic composition, and their seasonal variations. One particular voyage (December 2005) showcases the distinct "zones" in trace gas distribution as the ship moved from south to north through the SPCZ and ITCZ, and highlights the influence of Asian outflow on methane mixing ratio and isotopes.
OS41B-0536
Variability of the South Pacific Convergence Zone and its Influence on the General Atmospheric Circulation
Intense atmospheric convection associated with the South Pacific Convergence Zone (SPCZ) significantly impacts basin-scale circulation patterns over the Pacific. We explore dynamical processes which foster changes in convection along the convergence zone. These forcings include strong moisture convergence and accumulation of wave energy in the boundary layer, as well as dynamical instability associated with moderate cross-equatorial wind bursts. Focus is applied to observing the dominant modes of variability on synoptic to intraseasonal timescales using a combination of satellite observations and NCEP reanalysis data. Accumulation of energy, due to negative stretching deformation, comes from both tropical and extratropical modes suggesting that the SPCZ is an artifact of wide ranging modes. Signals of the dominant modes (inferred from fields of outgoing longwave radiation: OLR) are isolated using bandpass filtering techniques which are then mapped in space and time using Principal Components from Empirical Orthogonal Function analyses. Variability of convective systems in the SPCZ is believed to be significantly correlated with changes in the regional Hadley Circulation and the Pacific Walker cell. This presents the possibility of important teleconnection routes between the tropical West and East Pacific, as well as with the mid-latitude regions of the Northern and Southern Hemispheres. We test these interaction hypotheses by developing composites of the circulation patterns using dates of maximum convection events (regions of minimum OLR) in the SPCZ. Intensities of the large-scale circulations are measured using observations of stream function mass fluxes. Results suggest that deep convection maximums (minimums) are associated with an increase (decrease) in the Walker Circulation. Additional research is presented showing how off-equatorial convection anomalies in the subtropical portion of the SPCZ may cause changes to the Hadley Circulation. Interactions with the zonal (Walker) and meridional (Hadley) circulations appear to have important consequences on the ability for wave energy to propagate through the tropical Pacific atmosphere. Examples include Northern Hemisphere cross-equatorial teleconnections through the Westerly Wind Duct in the upper branch of the Walker circulation and Rossby wave trains in the SPCZ, which may be partially governed by characteristics of the regional Hadley circulation.
OS41B-0537
SPICE: South PacIfic Circulation and Climate Experiment
South Pacific thermocline waters are transported from the subtropical gyre center in the westward flowing South Equatorial Current, towards the southwestern Pacific Ocean-a major circulation pathway that redistributes water from the subtropics to the equator and southern ocean. The transit in the Coral Sea is potentially of great importance to tropical climate prediction because changes in either the temperature or the amount of water arriving at the equator have the capability to modulate the ENSO cycle and thereby produce basin-scale climate feedbacks. The Southwest Pacific is a region of complex ocean circulation. The South Equatorial Current is split in strong zonal jets upon encountering the island archipelago. Those jets partition on the Australian eastern boundary to feed the East Australian Current for the southern branch and the North Queensland Current and eventually the Equatorial Undercurrent for the northern branch. This climatological view of the current system is subject to substantial seasonal and inter-annual modulation. This circulation, and its influence on remote and regional climate, is poorly understood due to the lack of appropriate measurements. Ocean and atmosphere scientists from Australia, France, New Zealand and the United States initiated an international research project under the auspices of CLIVAR to comprehend the Southwest Pacific ocean circulation and its direct and indirect influence on the climate and environment. The outline of a feasible, regionally-coordinated experiment to measure, study and monitor the ocean circulation, to validate and improve numerical models, and to integrate with assimilating systems is presented. This project, named SPICE, reflects a strong sense that substantial progress can be made through collaboration among South Pacific national research groups, with full coordination with broader South Pacific projects. http://www.pmel.noaa.gov/people/ganachaud/spice/SPICEscienceplan_bkgnd.pdf
OS41B-0538
Variability In The Solomon Sea From Altimetric Sea Level Data
In the southwest tropical Pacific, subtropical waters from the SEC flow in the Solomon Sea, mainly through the western boundary New Guinea Coastal Undercurrent, and join the equatorial western Pacific by three narrow straits. The NGCU transports part of the spiciness anomalies generated in the South East Pacific and subducted in the thermocline. Because the NGCU is a primary source of the EUC, variations of its characteristics are expected to play a role in the equatorial thermocline features and more generally on decadal climate variability. Therefore, the study of the Solomon Sea is a key issue of the SPICE program. In this study, we focus on the variability of the Solomon Sea in term of sea level. The Solomon Sea is semi closed with a complex topography and numerous islands. Thus, the use of classical gridded altimetric products is inadequate. Consequently, this work is based on original along track Topex/Poseidon data. New data processing (CTOH/LEGOS) has been applied to recover proper data and to gain more information on the altimetric signal in this region. A track-by-track specific and customized post processing has been used to finalize the dataset. These new altimetric data have been assessed against tide gauge data. The analysis of the resulting sea level anomalies exhibits the highest variability observed in the tropical Pacific in an area centred near 8°S and expanding from each side of the Solomon Islands, outside of the WBC. Sea level variability presents a wide temporal spectrum, from intraseasonal to interannual ranges with the notable influence of the monsoon and of ENSO. In the Solomon Sea, three frequencies emerge : 60, 365 and 2000 days. The 60-days frequency seems particularly important in the Solomon Sea compared with the surrounding waters and an EOF analysis is used to understand its features. We also depict the signature of the New Guinea Coastal Current (NGCC), the western boundary current flowing north along the eastern coast of Papua New Guinea, which is particularly important for the connection to the equator. In parallel to this study, a hierarchy of high resolution (1/4°, 1/12°) regional models is also being built. The altimetric dataset will be used to perform a realistic simulation of the region circulations through data assimilation.
OS41B-0539
Transport in the Solomon Sea Measured by an Ocean Glider in Aug-Nov 2007
A coast-to-coast glider mission in the Solomon Sea measured the temperature, salinity and absolute velocity in August-November 2007. The Spray glider, built at the Scripps Institution of Oceanography, was launched about 3km from the reef edge at the southeast tip of Papua New Guinea. The glider first crossed the 200-km wide eastern mouth of Milne Bay between the two arms of the Papua Peninsula and again came close to land, then proceeded east across the remainder of the Sea to the Solomons coast for recovery. This is the first measurement of the complete low-latitude western boundary current (LLWBC) in the Solomon Sea, which has been a missing element of the Pacific shallow overturning cell. Vertically-averaged speeds in the LLWBC were 30-50 cm/s over the upper 500m. Surprisingly, about 8Sv of the LLWBC (perhaps half the total) flowed into Milne Bay rather than through the open Solomon Sea, and must exit through narrow, shallow channels on the northwest corner of the Bay. Beyond Milne Bay, recirculating, apparently permanent eddies were observed both east and west of the main stream. The geostrophic shear showed that the western boundary current had a subsurface maximum near 2-300m depth, as has been observed in the equatorial Pacific along the north coast of New Guinea. The mission will be repeated 3 more times in the next year to sample the annual cycle, with a view towards ongoing monitoring of the transport from the South Pacific to the equator.
OS41B-0540
Spectral estimates of the first few Rossby wave baroclinic modes in the South Pacific Ocean from satellite altimeters and testing of theories against these observations
Previous literature has suggested that multiple peaks in sea level anomalies (SLA) detected by two-dimensional Fourier transform (2D-FT) analysis are spectral components of multiple propagating signals which may correspond to different baroclinic Rossby wave modes. We test this hypothesis in the South Pacific Ocean by applying a 2D-FT analysis to the long Rossby wave signal determined from filtered TOPEX/Poseidon and ERS- 1/2 satellite altimeter derived SLA. The first four baroclinic mode dispersion curves for the classical linear wave theory and the Killworth and Blundell extended theory are used to determine the spectral signature and energy contributions of each mode. South of 17°S, the first two extended theory modes explain up to 60% more of the variance in the observed power spectral energy than their classical linear theory counterparts. The second mode contributes significantly over most of the basin. The third mode is also evident in some localised regions of the South Pacific but may be ignored at the large scale. Examination of a selection of case study sites suggest that bathymetric effects may dominate at longer wavelengths, or permit higher order mode solutions but mean flow tends to be the more influential factor in the extended theory. This study also examines the prevalence and characteristics of multiple propagating signals in the South Pacific SLA using the two-dimensional Radon Transform (2D-RT). Primary Radon Transform (RT) and Fourier Transform (FT) peaks generally compared well to each other and to the extended theory first baroclinic mode for most of the domain. A comparison to the energy ratios for the first four FT baroclinic modes showed that while the number of modes in their FT and peaks in the RT analysis coincided, the actual spatial distribution and relative contribution of these was not as consistent. Strong similarities existed in the spatial location and energy contribution between RT peaks 1 and 2 and FT modes 1 and 2. We conclude that the first and second peaks in the RT are most likely to be the signature of long Rossby waves but the dynamics behind subsequent peaks is not as clear.
OS41B-0541
Argo Float Trajectories at Mid-Depth in the Southwest Pacific
Trajectories from Argo floats have been used for a preliminary estimate of the mean ocean circulation of the southwest Pacific at 1000 m and to study zonal jets in the South Equatorial Current (SEC) where it encounters island chains and ridges in that region. Since 2004, 240 Argo floats have passed through the southwest Pacific, producing over 9800 trajectories. Most floats stay submerged for 9 days before ascending to telemeter their data and record their position at the sea surface. The World Ocean Circulation Experiment (WOCE) also deployed floats in this region and data from these floats are being added to the dataset. Accuracy of the submerged velocities depends on knowing the position and time of the float sinking and rising to the surface again. An extrapolation method was used to estimate the rise and fall locations and times from the series of position fixes and the known residence time on the sea surface. Near Fiji and New Caledonia, zonal jets appear on the north side of these islands. The mean velocity of the jet at 1000 m around the north of Fiji is 5 cm/s with a maximum velocity of approximately 18cm/s. The mean velocity of the jet around the north of New Caledonia is 5cm/s with a maximum of near 15cm/s. At present, not many floats have passed near the southern side of these islands, and it is difficult to confirm whether zonal jets are present at these locations as well. Other island chains also show trajectories that are steered by the topography. A map of dynamic height from Argo profile data shows reasonable agreement of geostrophic velocity with the mean velocity vectors calculated from the trajectories in the SEC region. The present Argo dataset shows the main circulation features of the southwest Pacific, and a strong influence of topography on the intermediate-depth circulation, but there is not yet enough data to produce a detailed description of the flow field.