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2017
Alberty, MS, Sprintall J, MacKinnon J, Ganachaud A, Cravatte S, Eldin G, Germineaud C, Melet A.  2017.  Spatial patterns of mixing in the Solomon Sea. Journal of Geophysical Research-Oceans. 122:4021-4039.   10.1002/2016jc012666   AbstractWebsite

The Solomon Sea is a marginal sea in the southwest Pacific that connects subtropical and equatorial circulation, constricting transport of South Pacific Subtropical Mode Water and Antarctic Intermediate Water through its deep, narrow channels. Marginal sea topography inhibits internal waves from propagating out and into the open ocean, making these regions hot spots for energy dissipation and mixing. Data from two hydrographic cruises and from Argo profiles are employed to indirectly infer mixing from observations for the first time in the Solomon Sea. Thorpe and finescale methods indirectly estimate the rate of dissipation of kinetic energy (E) and indicate that it is maximum in the surface and thermocline layers and decreases by 2-3 orders of magnitude by 2000 m depth. Estimates of diapycnal diffusivity from the observations and a simple diffusive model agree in magnitude but have different depth structures, likely reflecting the combined influence of both diapycnal mixing and isopycnal stirring. Spatial variability of E is large, spanning at least 2 orders of magnitude within isopycnal layers. Seasonal variability of E reflects regional monsoonal changes in large-scale oceanic and atmospheric conditions with E increased in July and decreased in March. Finally, tide power input and topographic roughness are well correlated with mean spatial patterns of mixing within intermediate and deep isopycnals but are not clearly correlated with thermocline mixing patterns. Plain Language Summary In the ocean, a number of physical processes move heat, salt, and nutrients around vertically by mixing neighboring layers of the ocean together. This study investigates the strength and spatial patterns of this mixing in the Solomon Sea, which is located in the tropical west Pacific Ocean. Estimates of the strength of mixing are made using measurements of temperature, salinity, and velocity taken during two scientific cruises in the Solomon Sea. Measurements of temperature and salinity from a network of floats that move up and down through the ocean and travel with ocean currents were also used to estimate the strength and patterns of mixing. This research finds three key results for mixing in the Solomon Sea: (1) Mixing is strongest near the surface of the Solomon Sea and less strong at deeper depths. (2) Mixing varies horizontally, with stronger mixing above underwater ridges and seamounts, and with weaker mixing above smooth and flat seafloor. (3) The strength of mixing changes with the seasons, possibly related to the monsoonal winds which also change in strength over the seasons.

Cuypers, Y, Pous S, Sprintall J, Atmadipoera A, Madec G, Molcard R.  2017.  Deep circulation driven by strong vertical mixing in the Timor Basin. Ocean Dynamics. 67:191-209.   10.1007/s10236-016-1019-y   AbstractWebsite

The importance of deep mixing in driving the deep part of the overturning circulation has been a long debated question at the global scale. Our observations provide an illustration of this process at the Timor Basin scale of similar to 1000 km. Long-term averaged moored velocity data at the Timor western sill suggest that a deep circulation is present in the Timor Basin. An inflow transport of similar to 0.15 Sv is observed between 1600 m and the bottom at 1890 m. Since the basin is closed on its eastern side below 1250 m depth, a return flow must be generated above 1600 m with a similar to 0.15 Sv outflow. The vertical turbulent diffusivity is inferred from a heat and transport balance at the basin scale and from Thorpe scale analysis. Basin averaged vertical diffusivity is as large as 1 x 10(-3) m(2) s(-1). Observations are compared with regional low-resolution numerical simulations, and the deep observed circulation is only recovered when a strong vertical diffusivity resulting from the parameterization of internal tidal mixing is considered. Furthermore, the deep vertical mixing appears to be strongly dependent on the choice of the internal tide mixing parameterization and also on the prescribed value of the mixing efficiency.

Hu, SJ, Sprintall J.  2017.  Observed strengthening of interbasin exchange via the Indonesian seas due to rainfall intensification. Geophysical Research Letters. 44:1448-1456.   10.1002/2016gl072494   AbstractWebsite

A proxy of the Indonesian Throughflow (ITF) transport, developed using in situ hydrographic measurements along with assimilations, shows a significant strengthening trend during the past decade. This trend is due to a freshening and subsequent increase in the halosteric component of the ITF transport associated with enhanced rainfall over the Maritime Continent over the same period. The strengthening of the ITF transport leads to a significant change in heat and freshwater exchange between the Pacific and Indian Oceans and contributes to the warming and freshening of the eastern Indian Ocean. The combined effect of the ITF transport of mass and freshwater along with tropical rainfall plays a very important role in the climate system.

2016
Erickson, ZK, Thompson AF, Cassar N, Sprintall J, Mazloff MR.  2016.  An advective mechanism for deep chlorophyll maxima formation in southern Drake Passage. Geophysical Research Letters. 43:10846-10855.   10.1002/2016gl070565   AbstractWebsite

We observe surface and subsurface fluorescence-derived chlorophyll maxima in southern Drake Passage during austral summer. Backscatter measurements indicate that the deep chlorophyll maxima (DCMs) are also deep biomass maxima, and euphotic depth estimates show that they lie below the euphotic layer. Subsurface, offshore and near-surface, onshore features lie along the same isopycnal, suggesting advective generation of DCMs. Temperature measurements indicate a warming of surface waters throughout austral summer, capping the winter water (WW) layer and increasing off-shelf stratification in this isopycnal layer. The outcrop position of the WW isopycnal layer shifts onshore, into a surface phytoplankton bloom. A lateral potential vorticity (PV) gradient develops, such that a down-gradient PV flux is consistent with offshore, along-isopycnal tracer transport. Model results are consistent with this mechanism. Subduction of chlorophyll and biomass along isopycnals represents a biological term not observed by surface satellite measurements which may contribute significantly to the strength of the biological pump in this region.

Germineaud, C, Ganachaud A, Sprintall J, Cravatte S, Eldin G, Alberty MS, Privat E.  2016.  Pathways and water mass properties of the thermocline and intermediate waters in the Solomon Sea. Journal of Physical Oceanography. 46:3031-3049.   10.1175/jpo-d-16-0107.1   AbstractWebsite

The semienclosed Solomon Sea is the final passage in the equatorward transit of the South Pacific western boundary currents (WBCs) that play a key role in heat and mass budgets of the equatorial Pacific. The Solomon WBCs and their associated water properties are examined using data from two oceanographic cruises undertaken during the contrasting trade wind seasons: July 2012 and March 2014. The mean circulation and associated transports with uncertainties is determined from the cruise data using a unique configuration of an inverse box model formulated based on measured shipboard acoustic Doppler current profiler velocities. An intense inflow of 36 Sv is found entering the Solomon Sea in July-August 2012 that falls by 70% to 11 Sv in March 2014. Large differences are also found in the total transport partitioning through each of the major exit passages during each season. Different water masses are found in the WBC stream northeast of the Solomon Islands that are likely related to a northern stream of the South Equatorial Current. Within the Solomon Sea, isopycnal salinity gradients are gradually stronger than within the subtropical Pacific, likely induced by stronger diapycnal mixing processes. WBC pathways exhibit distinct water mass signatures in salinity, oxygen, and nutrients that can be traced across the Solomon Sea, associated with significant water mass modifications at the northern exit straits and south of the Woodlark Island.

Delman, AS, Sprintall J, McClean JL, Talley LD.  2016.  Anomalous Java cooling at the initiation of positive Indian Ocean Dipole events. Journal of Geophysical Research: Oceans.   10.1002/2016JC011635   AbstractWebsite

Anomalous sea surface temperature (SST) cooling south of Java, initiated during May–July, is an important precursor to positive Indian Ocean Dipole (pIOD) events. As shown previously, the Java SST anomalies are spatially and temporally coincident with seasonal upwelling induced locally by southeasterly trade winds. However, we confirm earlier findings that interannual variability of the Java cooling is primarily driven by remote wind forcing from coastal Sumatra and the equatorial Indian Ocean (EqIO); we also find an influence from winds along the Indonesian Throughflow. The wind forcing in the EqIO and along coastal Sumatra does not initiate SST cooling locally due to a deep thermocline and thick barrier layer, but can force upwelling Kelvin waves that induce substantial surface cooling once they reach the seasonally shallower thermocline near the coast of Java. Satellite altimetry is used to obtain a Kelvin wave coefficient that approximates Kelvin wave amplitude variations along the equator. All pIOD years in the satellite record have anomalous levels of upwelling Kelvin wave activity along the equator during April–June, suggesting that upwelling waves during this season are necessary for pIOD event development. However, a change to wind-forced downwelling Kelvin waves during July–August can abruptly terminate cool Java SST anomalies and weaken the pIOD event. Upwelling Kelvin wave activity along the equator and wind stress anomalies west of Sumatra are both robust predictors of the IOD index later in the calendar year, while values of the Kelvin wave coefficient are the most reliable predictor of pIOD events specifically.

Cheng, LJ, Abraham J, Goni G, Boyer T, Wijffels S, Cowley R, Gouretski V, Reseghetti F, Kizu S, Dong SF, Bringas F, Goes M, Houpert L, Sprintall J, Zhu J.  2016.  XBT science: Assessment of instrumental biases and errors. Bulletin of the American Meteorological Society. 97:923-934.   10.1175/bams-d-15-00031.1   AbstractWebsite

Expendable bathythermograph (XBT) data were the major component of the ocean temperature profile observations from the late 1960s through the early 2000s, and XBTs still continue to provide critical data to monitor surface and subsurface currents, meridional heat transport, and ocean heat content. Systematic errors have been identified in the XBT data, some of which originate from computing the depth in the profile using a theoretically and experimentally derived fall-rate equation (FRE). After in-depth studies of these biases and discussions held in several workshops dedicated to discussing XBT biases, the XBT science community met at the Fourth XBT Science Workshop and concluded that XBT biases consist of 1) errors in depth values due to the inadequacy of the probe motion description done by standard FRE and 2) independent pure temperature biases. The depth error and temperature bias are temperature dependent and may depend on the data acquisition and recording system. In addition, the depth bias also includes an offset term. Some biases affecting the XBT-derived temperature profiles vary with manufacturer/probe type and have been shown to be time dependent. Best practices for historical XBT data corrections, recommendations for future collection of metadata to accompany XBT data, impact of XBT biases on scientific applications, and challenges encountered are presented in this manuscript. Analysis of XBT data shows that, despite the existence of these biases, historical XBT data without bias corrections are still suitable for many scientific applications, and that bias-corrected data can be used for climate research.

Hu, SJ, Sprintall J.  2016.  Interannual variability of the Indonesian Throughflow: The salinity effect. Journal of Geophysical Research-Oceans. 121:2596-2615.   10.1002/2015jc011495   AbstractWebsite

The Indonesian Throughflow (ITF) region possesses strong mixing and experiences significant freshwater input, but the role of salinity variability in the Indonesian Seas remains unclear. The goal of this study is to understand how salinity variability influences the ITF transport on interannual time scales. The ITF transport is calculated using observations and assimilation data sets and verified using direct ITF transport estimates. We find that the halosteric component of the ITF transport contributes (36 +/- 7)% of the total ITF variability, in contrast to (63 +/- 6)% by the thermosteric component. Thus, while not dominant, this result nonetheless implies that the salinity variability in the Indonesian Seas is of remarkable importance in determining the interannual variability of ITF transport. Correlation analysis indicates that the interannual variability of the total ITF transport is mainly influenced by the El Nino-Southern Oscillation (ENSO) rather than the Indian Ocean Dipole. Under the ENSO cycle, the Walker Circulation shifts longitudinally resulting in fluctuations in precipitation over the Indonesian Seas that modulates salinity and subsequently influences the interannual variability of ITF transport. This result signals the importance of precipitation and the subsequent salinity effect in determining the interannual variability of the ITF transport. The role of wind forcing and oceanic planetary waves is also revisited using this newly calculated ITF transport series. ENSO-related wind forcing is found to modulate the ITF transport via Rossby waves through the wave guide in the Indonesian Seas, which is in agreement with previous studies.

2015
Hu, DX, Wu LX, Cai WJ, Sen Gupta A, Ganachaud A, Qiu B, Gordon AL, Lin XP, Chen ZH, Hu SJ, Wang GJ, Wang QY, Sprintall J, Qu TD, Kashino Y, Wang F, Kessler WS.  2015.  Pacific western boundary currents and their roles in climate. Nature. 522:299-308.   10.1038/nature14504   AbstractWebsite

Pacific Ocean western boundary currents and the interlinked equatorial Pacific circulation system were among the first currents of these types to be explored by pioneering oceanographers. The widely accepted but poorly quantified importance of these currents-in processes such as the El Nino/Southern Oscillation, the Pacific Decadal Oscillation and the Indonesian Throughflow-has triggered renewed interest. Ongoing efforts are seeking to understand the heat and mass balances of the equatorial Pacific, and possible changes associated with greenhouse-gas-induced climate change. Only a concerted international effort will close the observational, theoretical and technical gaps currently limiting a robust answer to these elusive questions.

Delman, AS, McClean JL, Sprintall J, Talley LD, Yulaeva E, Jayne SR.  2015.  Effects of eddy vorticity forcing on the mean state of the Kuroshio Extension. Journal of Physical Oceanography. 45:1356-1375.   10.1175/jpo-d-13-0259.1   AbstractWebsite

Eddy-mean flow interactions along the Kuroshio Extension (KE) jet are investigated using a vorticity budget of a high-resolution ocean model simulation, averaged over a 13-yr period. The simulation explicitly resolves mesoscale eddies in the KE and is forced with air-sea fluxes representing the years 1995-2007. A mean-eddy decomposition in a jet-following coordinate system removes the variability of the jet path from the eddy components of velocity; thus, eddy kinetic energy in the jet reference frame is substantially lower than in geographic coordinates and exhibits a cross-jet asymmetry that is consistent with the baroclinic instability criterion of the long-term mean field. The vorticity budget is computed in both geographic (i. e., Eulerian) and jet reference frames; the jet frame budget reveals several patterns of eddy forcing that are largely attributed to varicose modes of variability. Eddies tend to diffuse the relative vorticity minima/maxima that flank the jet, removing momentum from the fast-moving jet core and reinforcing the quasi-permanent meridional meanders in the mean jet. A pattern associated with the vertical stretching of relative vorticity in eddies indicates a deceleration (acceleration) of the jet coincident with northward (southward) quasi-permanent meanders. Eddy relative vorticity advection outside of the eastward jet core is balanced mostly by vertical stretching of the mean flow, which through baroclinic adjustment helps to drive the flanking recirculation gyres. The jet frame vorticity budget presents a well-defined picture of eddy activity, illustrating along-jet variations in eddy-mean flow interaction that may have implications for the jet's dynamics and cross-frontal tracer fluxes.

Pena-Izquierdo, J, van Sebille E, Pelegri JL, Sprintall J, Mason E, Llanillo PJ, Machin F.  2015.  Water mass pathways to the North Atlantic oxygen minimum zone. Journal of Geophysical Research-Oceans. 120:3350-3372.   10.1002/2014jc010557   AbstractWebsite

The water mass pathways to the North Atlantic Oxygen Minimum Zone (naOMZ) are traditionally sketched within the cyclonic tropical circulation via the poleward branching from the eastward flowing jets that lie south of 10 degrees N. However, our water mass analysis of historic hydrographic observations together with numerical Lagrangian experiments consistently reveal that the potential density level of sigma=26.8 kg m(-3) (sigma 26.8, approximately 300 m depth) separates two distinct regimes of circulation within the Central Water (CW) stratum of the naOMZ. In the upper CW (above sigma 26.8), and in agreement with previous studies, the supply of water mainly comes from the south with a predominant contribution of South Atlantic CW. In the lower CW (below sigma 26.8), where minimal oxygen content is found, the tropical pathway is instead drastically weakened in favor of a subtropical pathway. More than two thirds of the total water supply to this lower layer takes place north of 10 degrees N, mainly via an eastward flow at 14 degrees N and northern recirculations from the northern subtropical gyre. The existence of these northern jets explains the greater contribution of North Atlantic CW observed in the lower CW, making up to 50% of the water mass at the naOMZ core. The equatorward transfer of mass from the well-ventilated northern subtropical gyre emerges as an essential part of the ventilation of the naOMZ.

Munro, DR, Lovenduski NS, Stephens BB, Newberger T, Arrigo KR, Takahashi T, Quay PD, Sprintall J, Freeman NM, Sweeney C.  2015.  Estimates of net community production in the Southern Ocean determined from time series observations (2002-2011) of nutrients, dissolved inorganic carbon, and surface ocean pCO(2) in Drake Passage. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 114:49-63.   10.1016/j.dsr2.2014.12.014   AbstractWebsite

In remote regions such as the open Southern Ocean, satellite observations often provide the only available tool with which to evaluate large-scale biogeochemical processes. However, these observations need to be carefully evaluated with in situ measurements. With an average of 20 crossings per year from 2002 to 2011, the Drake Passage Time-series (DP) represents one of the most complete datasets of biogeochemical measurements in the open Southern Ocean. This dataset offers a unique opportunity to validate satellite-based productivity algorithms and to improve understanding of the role of this region in the global carbon cycle. Net community production (NCP) was estimated using discrete measurements of total dissolved inorganic carbon (TCO2) and phosphate (PO43-), and high-frequency underway measurements of the partial pressure of carbon dioxide in the surface ocean (pCO2(surf)) from the DPT, combined with estimates of gas exchange, Elcman transport wind stress curl, and vertical entrainment We estimate annual NC!' using seasonal PO43- (NCPPO43-) and TCO2 (NCPTCO2,) budgets of 12 +/- 0.7 and 1.6 +/- 0.4 mol C m(-2) yr(-1), respectively. Budget terms for gas exchange, entrainment, and advective supply indicate that a closed system seasonal-drawdown approach that does not consider additional terms may underestimate NCP in this region by nearly 35%. NCP estimates are compared to satellite algorithms commonly used to estimate both net primary production (NPP) and organic carbon export Budget-based NCP approaches indicate high rates of NCP during austral spring with little additional NCP over austral summer. In contrast, satellite approaches suggest a more gradual increase and decline in NCP rates over the growing season with approximately 40% of NCP accumulating during austral summer. (C) 2015 Elsevier Ltd. All rights reserved.

2014
Ganachaud, A, Cravatte S, Melet A, Schiller A, Holbrook NJ, Sloyan BM, Widlansky MJ, Bowen M, Verron J, Wiles P, Ridgway K, Sutton P, Sprintall J, Steinberg C, Brassington G, Cai W, Davis R, Gasparin F, Gourdeau L, Hasegawa T, Kessler W, Maes C, Takahashi K, Richards KJ, Send U.  2014.  The Southwest Pacific Ocean circulation and climate experiment (SPICE). Journal of Geophysical Research-Oceans. 119:7660-7686.   10.1002/2013jc009678   AbstractWebsite

The Southwest Pacific Ocean Circulation and Climate Experiment (SPICE) is an international research program under the auspices of CLIVAR. The key objectives are to understand the Southwest Pacific Ocean circulation and the South Pacific Convergence Zone (SPCZ) dynamics, as well as their influence on regional and basin-scale climate patterns. South Pacific thermocline waters are transported in the westward flowing South Equatorial Current (SEC) toward Australia and Papua-New Guinea. On its way, the SEC encounters the numerous islands and straits of the Southwest Pacific and forms boundary currents and jets that eventually redistribute water to the equator and high latitudes. The transit in the Coral, Solomon, and Tasman Seas is of great importance to the climate system because changes in either the temperature or the amount of water arriving at the equator have the capability to modulate the El Nino-Southern Oscillation, while the southward transports influence the climate and biodiversity in the Tasman Sea. After 7 years of substantial in situ oceanic observational and modeling efforts, our understanding of the region has much improved. We have a refined description of the SPCZ behavior, boundary currents, pathways, and water mass transformation, including the previously undocumented Solomon Sea. The transports are large and vary substantially in a counter-intuitive way, with asymmetries and gating effects that depend on time scales. This paper provides a review of recent advancements and discusses our current knowledge gaps and important emerging research directions. Key Points Southwest Pacific WBCs transport large volumes toward the equator and the pole Pathways are complex; water properties tend to erode during the transit Variations due to seasons, ENSO and the SPCZ modulate the relative WBC strengths

Drushka, K, Gille ST, Sprintall J.  2014.  The diurnal salinity cycle in the tropics. Journal of Geophysical Research-Oceans. 119:5874-5890.   10.1002/2014jc009924   AbstractWebsite

Observations from 35 tropical moorings are used to characterize the diurnal cycle in salinity at 1 m depth. The amplitude of diurnal salinity anomalies is up to 0.01 psu and more typically approximate to 0.005 psu. Diurnal variations in precipitation and vertical entrainment appear to be the dominant drivers of diurnal salinity variability, with evaporation also contributing. Areas where these processes are strong are expected to have relatively strong salinity cycles: the eastern Atlantic and Pacific equatorial regions, the southwestern Bay of Bengal, the Amazon outflow region, and the Indo-Pacific warm pool. We hypothesize that salinity anomalies resulting from precipitation and evaporation are initially trapped very near the surface and may not be observed at the 1 m instrument depths until they are mixed downward. As a result, the pattern of diurnal salinity variations is not only dependent on the strength of the forcing terms, but also on the phasing of winds and convective overturning. A comparison of mixed-layer depth computed with hourly and with daily averaged salinity reveals that diurnal salinity variability can have a significant effect on upper ocean stratification, suggesting that representing diurnal salinity variability could potentially improve air-sea interaction in climate models. Comparisons between salinity observations from moorings and from the Aquarius satellite (level 2 version 3.0 data) reveal that the typical difference between ascending-node and descending-node Aquarius salinity is an order of magnitude greater than the observed diurnal salinity anomalies at 1 m depth.

Sprintall, J, Gordon AL, Koch-Larrouy A, Lee T, Potemra JT, Pujiana K, Wijffels SE.  2014.  The Indonesian seas and their role in the coupled ocean-climate system. Nature Geoscience. 7:487-492.   10.1038/ngeo2188   AbstractWebsite

The Indonesian seas represent the only pathway that connects different ocean basins in the tropics, and therefore play a pivotal role in the coupled ocean and climate system. Here, water flows from the Pacific to the Indian Ocean through a series of narrow straits. The throughflow is characterized by strong velocities at water depths of about 100 m, with more minor contributions from surface flow than previously thought. A synthesis of observational data and model simulations indicates that the temperature, salinity and velocity depth profiles of the Indonesian throughflow are determined by intense vertical mixing within the Indonesian seas. This mixing results in the net upwelling of thermocline water in the Indonesian seas, which in turn lowers sea surface temperatures in this region by about 0.5 degrees C, with implications for precipitation and air-sea heat flux. Moreover, the depth and velocity of the core of the Indonesian throughflow has varied with the El Nino/Southern Oscillation and Indian Ocean Dipole on interannual to decadal timescales. Specifically, the throughflow slows and shoals during El Nino events. Changes in the Indonesian throughflow alter surface and subsurface heat content and sea level in the Indian Ocean between 10 and 15 degrees S. We conclude that inter-ocean exchange through the Indonesian seas serves as a feedback modulating the regional precipitation and wind patterns.

Griesel, A, McClean JL, Gille ST, Sprintall J, Eden C.  2014.  Eulerian and Lagrangian isopycnal eddy diffusivities in the Southern Ocean of an eddying model. Journal of Physical Oceanography. 44:644-661.   10.1175/jpo-d-13-039.1   AbstractWebsite

Lagrangian isopycnal diffusivities quantify the along-isopycnal mixing of any tracer with mean gradients along isopycnal surfaces. They are studied in the Southern Ocean of the 1/10 degrees Parallel Ocean Program (POP) model using more than 50 000 float trajectories. Concurrent Eulerian isopycnal diffusivities are estimated directly from the eddy fluxes and mean tracer gradients. Consistency, spatial variation, and relation to mean jets are evaluated. The diffusivities are calculated in bins large enough to reduce contributions from the rotational components that do not lead to net tracer mixing. Because the mean jets are nonzonal and nonparallel, meridional dispersion includes standing eddies and is significantly different from cross-stream dispersion. With the subtraction of the local Eulerian mean, the full Lagrangian diffusivity tensor can be estimated. Along-stream diffusivities are about 6 times larger than cross-stream diffusivities. Along-streamline averages of Eulerian and Lagrangian isopycnal diffusivities are similar in that they are larger north of the Antarctic Circumpolar Current (ACC) and smaller in the ACC in the upper 500 m. Eulerian diffusivities are often twice as large as the Lagrangian diffusivities below 500 m. There is large longitudinal variability in the diffusivities and in their relation to the mean flow. In bins with one prominent jet, diffusivities are reduced at the surface in the jet and increased to the north and south of the jet. There is a local maximum at depths of 500-1000 m. In other bins where mean jets merge and diverge because of topography, there is no consistent relation of the diffusivities with the mean flow. Eulerian fluxes are upgradient in about 15% of the bins.

Sprintall, J, Revelard A.  2014.  The Indonesian Throughflow response to Indo-Pacific climate variability. Journal of Geophysical Research-Oceans. 119:1161-1175.   10.1002/2013jc009533   AbstractWebsite

The Indonesian Throughflow (ITF) is the only open pathway for interocean exchange between the Pacific and Indian Ocean basins at tropical latitudes. A proxy time series of ITF transport variability is developed using remotely sensed altimeter data. The focus is on the three outflow passages of Lombok, Ombai, and Timor that collectively transport the entire ITF into the Indian Ocean, and where direct velocity measurements are available to help ground-truth the transport algorithm. The resulting 18 year proxy time series shows strong interannual ITF variability. Significant trends of increased transport are found in the upper layer of Lombok Strait, and over the full depth in Timor Passage that are likely related to enhanced Pacific trade winds since the early 1990s. The partitioning of the total ITF transport through each of the major outflow passage varies according to the phase of the Indian Ocean Dipole (IOD) or El Nino-Southern Oscillation (ENSO). In general, Pacific ENSO variability is strongest in Timor Passage, most likely through the influence of planetary waves transmitted from the Pacific along the Northwest Australian shelf pathway. Somewhat surprisingly, concurrent El Nino and positive IOD episodes consistently show contradictory results from those composites constructed for purely El Nino episodes. This is particularly evident in Lombok and Ombai Straits, but also at depth in Timor Passage. This suggests that Indian Ocean dynamics likely win out over Pacific Ocean dynamics in gating the transport through the outflow passages during concurrent ENSO and IOD events.

van Sebille, E, Sprintall J, Schwarzkopf FU, Sen Gupta A, Santoso A, England MH, Biastoch A, Boning CW.  2014.  Pacific-to-Indian Ocean connectivity: Tasman leakage, Indonesian Throughflow, and the role of ENSO. Journal of Geophysical Research-Oceans. 119:1365-1382.   10.1002/2013jc009525   AbstractWebsite

The upper ocean circulation of the Pacific and Indian Oceans is connected through both the Indonesian Throughflow north of Australia and the Tasman leakage around its south. The relative importance of these two pathways is examined using virtual Lagrangian particles in a high-resolution nested ocean model. The unprecedented combination of a long integration time within an eddy-permitting ocean model simulation allows the first assessment of the interannual variability of these pathways in a realistic setting. The mean Indonesian Throughflow, as diagnosed by the particles, is 14.3 Sv, considerably higher than the diagnosed average Tasman leakage of 4.2 Sv. The time series of Indonesian Throughflow agrees well with the Eulerian transport through the major Indonesian Passages, validating the Lagrangian approach using transport-tagged particles. While the Indonesian Throughflow is mainly associated with upper ocean pathways, the Tasman leakage is concentrated in the 400-900 m depth range at subtropical latitudes. Over the effective period considered (1968-1994), no apparent relationship is found between the Tasman leakage and Indonesian Throughflow. However, the Indonesian Throughflow transport correlates with ENSO. During strong La Ninas, more water of Southern Hemisphere origin flows through Makassar, Moluccas, Ombai, and Timor Straits, but less through Moluccas Strait. In general, each strait responds differently to ENSO, highlighting the complex nature of the ENSO-ITF interaction.

Drushka, K, Sprintall J, Gille ST.  2014.  Subseasonal variations in salinity and barrier-layer thickness in the eastern equatorial Indian Ocean. Journal of Geophysical Research-Oceans. 119:805-823.   10.1002/2013jc009422   AbstractWebsite

The barrier layer, the layer between the bottom of the density-defined mixed layer and the isothermal layer in the upper ocean, may play a role in air-sea dynamics. In the present study, data from Argo profiling floats in the tropical Indian Ocean and a mooring at 90 degrees E, 0 degrees N are used to examine subseasonal variations in upper ocean salinity and barrier-layer thickness (BLT) during boreal winter. In the eastern equatorial Indian Ocean, subseasonal variations in BLT are energetic. However, composites used to isolate the Madden-Julian Oscillation (MJO) component of the subseasonal signal reveal that, on average, the MJO anomaly in BLT is negligible despite large swings in both the mixed-layer depth and the isothermal-layer depth. This discrepancy is likely due to (a) noise from other subseasonal processes; and (b) the diversity of individual MJO events: the thickness of the mixed layer and the isothermal layer are sensitive to wind and rain forcing, so even subtle differences in the phasing and strength of MJO-related atmospheric anomalies can produce a very different effect on upper ocean stratification and hence on the thickness of the barrier layer. The effect of the barrier layer on the upper ocean response to MJO forcing is also evaluated. When the barrier layer is thick, entrainment cooling during the MJO is reduced, so the MJO drives a weaker sea surface temperature anomaly. This suggests that modulation of BLT can have significant consequences for the response of the upper ocean to the MJO, and hence, potentially, for feedbacks of the ocean onto the atmosphere on MJO time scales.

Jiang, CL, Gille ST, Sprintall J, Sweeney C.  2014.  Drake Passage oceanic pCO(2): Evaluating CMIP5 coupled carbon-climate models using in situ observations. Journal of Climate. 27:76-100.   10.1175/jcli-d-12-00571.1   AbstractWebsite

Surface water partial pressure of CO2 (pCO(2)) variations in Drake Passage are examined using decade-long underway shipboard measurements. North of the Polar Front (PF), the observed pCO(2) shows a seasonal cycle that peaks annually in August and dissolved inorganic carbon (DIC)-forced variations are significant. Just south of the PF, pCO(2) shows a small seasonal cycle that peaks annually in February, reflecting the opposing effects of changes in SST and DIC in the surface waters. At the PF, the wintertime pCO(2) is nearly in equilibrium with the atmosphere, leading to a small sea-to-air CO2 flux.These observations are used to evaluate eight available Coupled Model Intercomparison Project, phase 5 (CMIP5), Earth system models (ESMs). Six ESMs reproduce the observed annual-mean pCO(2) values averaged over the Drake Passage region. However, the model amplitude of the pCO(2) seasonal cycle exceeds the observed amplitude of the pCO(2) seasonal cycle because of the model biases in SST and surface DIC. North of the PF, deep winter mixed layers play a larger role in pCO(2) variations in the models than they do in observations. Four ESMs show elevated wintertime pCO(2) near the PF, causing a significant sea-to-air CO2 flux. Wintertime winds in these models are generally stronger than the satellite-derived winds. This not only magnifies the sea-to-air CO2 flux but also upwells DIC-rich water to the surface and drives strong equatorward Ekman currents. These strong model currents likely advect the upwelled DIC farther equatorward, as strong stratification in the models precludes subduction below the mixed layer.

2013
Stephenson, GR, Gille ST, Sprintall J.  2013.  Processes controlling upper-ocean heat content in Drake Passage. Journal of Geophysical Research-Oceans. 118:4409-4423.   10.1002/jgrc.20315   AbstractWebsite

A 16 year record of expendable bathythermograph transects across Drake Passage is used to examine variability in upper-ocean heat content that is not associated with the annual cycle. Links between upper-ocean heat content and anomalous heat fluxes, winds, two large-scale climate indices, and mesoscale eddies and meanders are examined. Results suggest that interannual variations in surface heat fluxes explain approximate to 5 to 10% of the variance in upper-ocean heat content. Anomalous surface heat fluxes are linked to meridional wind anomalies upstream of Drake Passage, which in turn are linked to forcing by El Nino/Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). ENSO and SAM are correlated with upper-ocean heat content at near-zero lags, and statistically significant correlations occur at longer time lags as well. The impact of mesoscale eddies and meanders on upper-ocean heat content is explored with the use of a tracked eddy database. An empirical relationship is constructed relating upper-ocean heat content anomalies to eddy length scales and amplitudes. Eddies and meanders are estimated to account for more than one third of the nonannual cycle variance in Drake Passage upper-ocean heat content.

Polton, JA, Lenn YD, Elipot S, Chereskin TK, Sprintall J.  2013.  Can Drake Passage observations match Ekman's classic theory? Journal of Physical Oceanography. 43:1733-1740.   10.1175/jpo-d-13-034.1   AbstractWebsite

Ekman's theory of the wind-driven ocean surface boundary layer assumes a constant eddy viscosity and predicts that the current rotates with depth at the same rate as it decays in amplitude. Despite its wide acceptance, Ekman current spirals are difficult to observe. This is primarily because the spirals are small signals that are easily masked by ocean variability and cannot readily be separated from the geostrophic component. This study presents a method for estimating ageostrophic currents from shipboard acoustic Doppler current profiler data in Drake Passage and finds that observations are consistent with Ekman's theory. By taking into account the sampling distributions of wind stress and ageostrophic velocity, the authors find eddy viscosity values in the range of 0.08-0.12 m(2) s(-1) that reconcile observations with the classic theory in Drake Passage. The eddy viscosity value that most frequently reconciles observations with the classic theory is 0.094 m(2) s(-1), corresponding to an Ekman depth scale of 39 m.

Brannigan, L, Lenn YD, Rippeth TP, McDonagh E, Chereskin TK, Sprintall J.  2013.  Shear at the base of the oceanic mixed layer generated by wind shear alignment. Journal of Physical Oceanography. 43:1798-1810.   10.1175/jpo-d-12-0104.1   AbstractWebsite

Observations are used to evaluate a simple theoretical model for the generation of near-inertial shear spikes at the base of the open ocean mixed layer when the upper ocean displays a two-layer structure. The model predicts that large changes in shear squared can be produced by the alignment of the wind and shear vectors. A climatology of stratification and shear variance in Drake Passage is presented, which shows that these assumptions are most applicable to summer, fall, and spring but are not highly applicable to winter. Temperature, salinity, and velocity data from a high spatial resolution cruise in Drake Passage show that the model does not predict all large changes in shear variance; the model is most effective at predicting changes in shear squared when it arises owing to near-inertial wind-driven currents without requiring a rotating resonant wind stress. The model is also more effective where there is a uniform mixed layer above a strongly stratified transition layer. Rotary spectral and statistical analysis of an additional 242 Drake Passage transects from 1999 to 2011 confirmed the presence of this shear-spiking mechanism, particularly in summer, spring, and fall when stratification is stronger.

Pujiana, K, Gordon AL, Sprintall J.  2013.  Intraseasonal Kelvin wave in Makassar Strait. Journal of Geophysical Research-Oceans. 118:2023-2034.   10.1002/jgrc.20069   AbstractWebsite

Time series observations during 2004-2006 reveal the presence of 60-90days intraseasonal events that impact the transport and mixing environment within Makassar Strait. The observed velocity and temperature fluctuations within the pycnocline reveal the presence of Kelvin waves including vertical energy propagation, energy equipartition, and nondispersive relationship. Two current meters at 750 and 1500m provide further evidence that the vertical structure of the downwelling Kelvin wave resembles that of the second baroclinic wave mode. The Kelvin waves derive their energy from the equatorial Indian Ocean winds, including those associated with the Madden-Julian oscillations, and propagate from Lombok Strait to Makassar Strait along the 100-m isobath. The northward propagating Kelvin waves within the pycnocline reduce the southward Makassar Strait throughflow by up to 2Sv and induce a marked increase of vertical diffusivity.

Frants, M, Damerell GM, Gille ST, Heywood KJ, MacKinnon J, Sprintall J.  2013.  An assessment of density-based finescale methods for estimating diapycnal diffusivity in the Southern Ocean. Journal of Atmospheric and Oceanic Technology. 30:2647-2661.   10.1175/jtech-d-12-00241.1   AbstractWebsite

Finescale estimates of diapycnal diffusivity are computed from CTD and expendable CTD (XCTD) data sampled in Drake Passage and in the eastern Pacific sector of the Southern Ocean and are compared against microstructure measurements from the same times and locations. The microstructure data show vertical diffusivities that are one-third to one-fifth as large over the smooth abyssal plain in the southeastern Pacific as they are in Drake Passage, where diffusivities are thought to be enhanced by the flow of the Antarctic Circumpolar Current over rough topography. Finescale methods based on vertical strain estimates are successful at capturing the spatial variability between the low-mixing regime in the southeastern Pacific and the high-mixing regime of Drake Passage. Thorpe-scale estimates for the same dataset fail to capture the differences between Drake Passage and eastern Pacific estimates. XCTD profiles have lower vertical resolution and higher noise levels after filtering than CTD profiles, resulting in XCTD estimates that are, on average, an order of magnitude higher than CTD estimates. Overall, microstructure diffusivity estimates are better matched by strain-based estimates than by estimates based on Thorpe scales, and CTD data appear to perform better than XCTD data. However, even the CTD-based strain diffusivity estimates can differ from microstructure diffusivities by nearly an order of magnitude, suggesting that density-based fine-structure methods of estimating mixing from CTD or XCTD data have real limitations in low-stratification regimes such as the Southern Ocean.