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ENSO and short-term variability of the
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South Equatorial Current entering the Coral Sea
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To be submitted to the Journal of Physical Oceanography
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William S. Kessler
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NOAA Pacific Marine Environmental Laboratory, Seattle WA USA
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Sophie Cravatte
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Institut de Recherche pour le Développement, Nouméa, New Caledonia
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Abstract
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Historical section data extending to 1985 are used to estimate the interannual variability
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of transport entering the Coral Sea between New Caledonia and the Solomon Islands.
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Typical magnitudes of this variability are ±5-8 Sv on a mean of about 30 Sv. Transport
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increases a few months after an El Niño event, and decreases following a La Niña. Most
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of the interannual transport variability is well-simulated by a reduced-gravity long
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Rossby wave model. Vigorous westward-propagating mesoscale eddies can yield
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substantial aliasing (under- or over-estimates comparable to the mean transport) on
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individual ship or glider surveys. Since the transport variability is surface-intensified and
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highly correlated with satellite-derived surface geostrophic currents, a simple index of
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South Equatorial Current transport based on satellite altimetry is developed.
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1. Introduction
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The South Equatorial Current carries the westward limb of the South Pacific
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subtropical gyre to the Coral Sea where it bifurcates into poleward and equatorward
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branches (REFs). It has been long known, and is expected from simple dynamical ideas
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(REFs), that the incoming transport varies annually (Kessler and Gourdeau 20NN) and
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with the ENSO cycle (REFs), but developing time series of this transport variability has
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been difficult because much of the historical data consists of occasional ship surveys that
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are hard to put in context. With the advent of the Argo program (REF), this problem will
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be rectified for the future, but Argo sampling of the Coral Sea became sufficiently dense
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only in 2005 or 2006. The purpose of this paper is to make use of older data to extend the
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transport time series back before the Argo era. We would also like to develop indices that
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could allow satellite altimetry to be used as a proxy for interannual transport variations;
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the [rapid and regular] satellite sampling gives the opportunity to evaluate high-frequency
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signals that could alias ship surveys.
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Several estimates of the transport entering the Coral Sea based on ship surveys
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have been published, and these vary by about a factor of two (REFs), confusing the
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interpretation of these data. There are several factors that cause such estimates to differ:
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sampling locations (especially obtaining adequate resolution of the narrow zonal jets and
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boundary currents), sampling depths, choices of reference level, timing relative to the
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seasonal or ENSO cycles, and short-term eddy aliasing. By extending the existing time
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series back before Argo, and making use of weekly satellite altimetry, we hope to
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evaluate these estimates.
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2. Data and methods
Three sources of geostrophic current information provide time series of the transport
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entering the Coral Sea: First, the SIO/ORSTOM Volunteer Observing Ship XBT program
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(REF Donguy-Meyers) deployed XBTs along a regularly-repeated merchant ship track
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from Auckland, New Zealand to Solomon Strait, passing just west of New Caledonia
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(Fig.AAA), from 1985 through 2001. 3917 temperature profiles, usually to 400m, were
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made on this track and quality controlled by the Coriolis Data Centre; only profiles with
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quality flags indicating good data were considered. 1346 of these profiles were within the
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20°S-11°S region considered here, for an average of about 1.5 profiles per degree latitude
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per month. For each XBT temperature value, a mean T-S relation from the CARS gridded
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compilation (Dunn and Ridgway 2002; http://www.cmar.csiro.au/cars) was used to
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associate a salinity. The resulting temperatures and salinities were interpolated to regular
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5m depth intervals, then gridded in (y,t) using a Gaussian mapping procedure to a 0.5°
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latitude by 3-month grid, using mapping scales 0.75° latitude and 3 months. Dynamic
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height relative to 400m was found from these, and crosstrack (perpendicular,
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approximately southwestward) geostrophic velocity estimated by centered differencing.
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Second, the Argo Atlas (Roemmich and Gilson 2009) provides gridded temperature
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and salinity profiles to 2000m on a global 1° by 1° monthly grid. The Argo Atlas fields
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were sampled along the XBT track at 1° latitude intervals to produce a time series
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comparable to the XBT data from 2004 through 2011. Crosstrack and zonal geostrophic
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currents were found relative to 2000m or the deepest common level.
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Third, weekly AVISO satellite altimetric sea surface height (SSH) data (REF) on a
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roughly 1/3° grid from August 1992 through 2011, was sampled along the same track and
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used to find surface geostrophic velocity anomalies.
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In all three cases, anomalies were found as differences from the average annual cycle
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over the record length of each time series, then filtered with a 7-month triangle. This
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filter has a half-power point of about 8 months, and passes more than 80% of the
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variability with timescales longer than 13 months; the result is here referred to as
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"interannual".
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The South Equatorial Current (SEC) can have ambiguous and fluctuating boundaries
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(REFs). In this work, however, we are interested in anomalies to the flow entering the
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Coral Sea, not to a specifically-defined SEC. Thus we have chosen a simple metric:
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transport perpendicular to the XBT line between 20°S and 11°S, which approximates the
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transport entering the Coral Sea between New Caledonia and the Solomon Islands
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(Fig.1). The anomalous transports are not sensitive to the actual boundaries of the current
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at a given time because interannual dynamic height anomalies are highly correlated over
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latitude in this range.
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3. Results
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3.1 Mean transports and representativeness of shallow transport anomalies
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Although here we are concerned with anomalies, especially since we consider
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disparate measures of flow, one motivation is to place historical cruise data into its ENSO
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context, so total transport values are relevant. Additional benefits of doing this are to
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show what is and is not sampled by the 400m XBT profiles, to compare perpendicular
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transport across the slanted XBT track (Fig.1) to meridional transport sections, and to
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evaluate the magnitudes of ENSO and other anomalies relative to mean transport values.
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Crosstrack speed across the XBT track (Fig.BBBa) shows two well-known westward
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branches of the SEC: the narrow North Caledonian Jet (NCJ; Gourdeau et al. 2007;
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REFs) at 19°S, with a mean speed relative to 400m of about 1 cm s–1 and transport of
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about 0.5 Sv, and the North Vanuatu Jet (NVJ; REFs) from about 15°S to 9.5°S with
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speeds to about 6 cm s–1 and transport of about 7 Sv. Between them the shallow eastward
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Coral Sea Countercurrent (CSCC; Qiu et al. 2010) is centered at 15.5°S. Comparable
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mean crosstrack speed relative to 400m from the Argo Atlas is very similar, with slightly
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weaker velocities (Fig.BBBb).
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Relative to the 2000m reference level available from the Argo Atlas data, the NVJ is
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found to be stronger, with a similar structure, but the NCJ extends much deeper and its
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speeds are at least five times larger (Fig.BBBc), as has been previously described
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(Gourdeau et al. 2007). The CSCC is weaker and narrower. NCJ transport relative to
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2000m from the Argo Atlas is about 11 Sv, and NVJ transport about 15 Sv. Thus the
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relative-to-400m transports omit most of the mean transport of the NCJ, and about half
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the mean transport of the NVJ.
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The Argo Atlas provides a further check on the representativeness of transport
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variability relative to 400m. Time series of interannual SEC transport relative to 400m
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and 1000m are very highly correlated with that relative to 2000m (r = 0.98 and 0.93
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respectively) . The magnitude of transport variance relative to 1000m is about 46% larger
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than to 400m, and that relative to 2000m is about 54% larger. Regression of the 0/400m
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onto 0/2000m transport gives a slope of 0.61 and intercept of 0.02 Sv (essentially zero).
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The high correlations are consistent with the idea that ENSO variability is focused in the
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thermocline, and suggests that the XBT 400m transport gives a good representation of the
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temporal variance pattern, but the interannual magnitude of 20°S-10°S transport
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variability is larger by about 50-60% than measured by the XBT line.
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The representativeness of perpendicular velocities across the slanted section can be
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checked using a meridional section at 160°E from the Argo Atlas. Relative to 2000m, the
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three currents on the purely meridional section have an almost identical structure to that
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on the track-sampled field, with slightly stronger velocities and total (20°S-10°S) zonal
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transport of 28 Sv (Fig.BBBd). This correspondence is expected since the vector flow is
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nearly zonal so the crosstrack velocity component gives the same integral. A similar
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calculation at 160°E from the CARS climatology relative to 2000m gives a transport of
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just under 30 Sv.
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3.2 Internannual transport anomalies
Unfortunately the XBT and Argo time series do not overlap so no direct comparison
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can be made. We note that the 1985-2001 period covered by the XBT data included the
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strong El Niño events of 1986-87, 1991-92, 1994-95 and 1997-98, as well as the strong
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La Niña of 1988, while the Argo time series has been made during a period of generally
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weaker El Niño events. Two other ENSO-relevant time series span these in situ data
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sources: The Southern Oscillation Index (SOI;
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http://www.cpc.ncep.noaa.gov/data/indices/soi), and crosstrack surface geostrophic
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currents derived from AVISO satellite SSH (Fig.CCC). All the estimates of the SEC are
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well-correlated with the SOI with a few-month lag consistent with Rossby wave
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propagation from the mid-basin focus of ENSO wind variability (REFs). The 16-year
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XBT transport correlates with the SOI at 0.88 with a 4-month lag, the 7-year Argo
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transport at 0.67 with a 2-month lag, and the 19-year AVISO surface velocity at 0.75 with
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a 3-month lag. The shorter recent lag times could reflect the change from the east-Pacific-
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centered El Niños during the 1980s and 1990s sampled by the XBT and AVISO data, to
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the late 2000s sampled by Argo when central Pacific “Modoki” El Niños (Ashok et al.
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2007) produced wind anomalies further west.
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The ENSO signal in crosstrack velocity is clearly seen in examples of El Niños and
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La Niñas on the track. Anomalies of the zonal flow were strongest at the surface and
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toward the north end of the New Caledonia-Solomons gap (Fig.DDD). Near-surface NVJ
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speeds increased by about 50% following the El Niño of 2009-10, and decreased about
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the same following the La Niña of 2008-09, while NCJ changes were much smaller
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(Fig.DDD). Very similar anomalies were seen during the other ENSO events, both in the
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XBT and Argo data. This weakening of interannual variance with depth is similar to that
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seen in glider data across the mouth of the Solomon Sea (Davis et al. 2012).
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Since ENSO anomalies are surface-intensified, it is not surprising that AVISO surface
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geostrophic velocities are highly correlated with transport anomalies: the correlation of
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AVISO speeds (averaged along the XBT line from 20°S to 11°S) with XBT transport is
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0.93, and with Argo transport is 0.94, as is obvious from Fig.CCC. Thus AVISO could be
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used as a proxy for interannual transport anomalies, with the factor of about 1.3 Sv
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(transport relative to 400m) per cm/s (surface speed) given by the regression slope. To
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approximate Argo transport variability relative to 2000m, the factor would be about 2.1.
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Published estimates of the transport at or near 160°E from synoptic sampling were
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coincidentally taken at times when the ENSO cycle was nearly neutral, and thus do not
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provide a useful test of the SEC response to ENSO. Scully-Power (1973) found 30 Sv
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from a cruise at 162°E in Aug-Sep 1970, while Gourdeau et al. (2008) found 32 Sv
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absolute transport above 600m from glider data and 28 Sv from cruise data based on
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geostrophy relative to 2000m, both in Aug-Oct 2005. These values are all
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indistinguishable from the mean transport from the Argo Atlas and CARS climatological
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means (28 and 30 Sv respectively). A somewhat larger estimate, again from Scully-
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Power (1973) based on data taken during mid-1968 (very weak La Niña conditions), was
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37 Sv.
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There is one outlier to these values: Sokolov and Rintoul’s (2000) SEC transport
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estimate of 54 Sv westward, relative to 2000db, from the WOCE P11S cruise data taken
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during June-July 1993, a value well above any others noted before or since. This cruise
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took place following the relatively weak El Niño of 1993, and the AVISO proxy index
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suggests that westward transport would have therefore been about 6-7 Sv greater than
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normal during this period; not nearly enough to account for the large P11S value. Since
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this apparently cannot be explained based on ENSO conditions, we use the weekly
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AVISO data to ask if higher-frequency variability could have aliased the sampling.
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3.3 An example of eddy aliasing?
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The standard deviation of high-passed AVISO crosstrack velocity averaged along the
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XBT section is about 1.4 cm/s, which is about 60% the size of the interannual RMS, and
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about 3 times larger than the annual cycle RMS. Inspection of weekly maps shows that
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much of this variability is due to westward-moving mesoscale eddies with typical
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diameters of 100-300km and anomalous surface geostrophic velocities of 20 cm/s or
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more (e.g. Fig.EEE). These features translate west across the Coral Sea with speeds
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decreasing from about 17 cm/s at 12°S to 12 cm/s at 20°S; that is, with somewhat smaller
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meridional speed gradient than would be expected from linear Rossby wave propagation
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(Chelton; Maharaj et al. 2009; REF). At these speeds, an individual eddy affects a section
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line for 15-25 days. However, most of the time such an eddy will produce oppositely-
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directed velocity anomalies on its flanks that do not result in a correspondingly-large
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transport anomaly when integrated over a section as we consider here.
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Nevertheless, during the WOCE P11S cruise in June-July 1993, such mesoscale
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eddies lined up so as to produce eastward surface velocity anomalies within 100km of the
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coast of Papua New Guinea, where the mean flow is the eastward boundary current (the
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Gulf of Papua Current; GPC), and westward anomalies across the background SEC from
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13°S to 19°S, thus temporarily augmenting both mean currents. In the GPC, geostrophic
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crosstrack surface currents relative to 2000db measured by P11S were about 50 cm/s, but
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the short-term eddy anomalies seen by AVISO at this time were about 20 cm/s. Across
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the SEC, P11S surface currents averaged about 13 cm/s (westward), while AVISO
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anomalies were about 8 cm/s, thus more than half the westward surface velocity
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measured during P11S was transient. We note that if the cruise had been conducted 2
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months earlier, the AVISO anomalies along the P11S section would have had the
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opposite sign, emphasizing the short-term nature of these eddy signals.
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It is impossible to determine the depth extent of the eddy anomalies, but it is
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reasonable to think that they represent fluctuations of the upper thermocline, and an
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exponential decay scale of 400m would be a conservative estimate of the eddy vertical
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structure. If that is the case, the SEC eddy anomalies on the P11S section amount to about
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23 Sv of transient westward transport. Thus, the background SEC seen in the P11S cruise
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could then be estimated at about 31 Sv, a value consistent with other samples and
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climatologies. This has an important implication for the redistribution of incoming flow
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from the SEC. Sokolov and Rintoul (2000) noted that the 54 Sv SEC transport across
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P11S implied that about 29 Sv of the arriving westward flow turns south into the East
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Australian Current. If the typical transport is instead only about 30 Sv, then the required
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southward flow would be small, and the bulk of the SEC would be found to turn north
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into the Solomon Sea, as most other studies have found (REFs).
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4. A simple Rossby wave model
The simplest representation of low-frequency wind-driven dynamics is the linear,
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reduced-gravity long Rossby wave model as has been frequently used to study tropical
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ocean variability (Meyers 1979; Kessler 1990; Chen and Qiu 2004). The model is:
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  
h
h
 cr
 Rh  Curl ,
t
x
f 
(1)
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where h is the pycnocline depth anomaly (positive down). The long Rossby speed is
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cr=c2/f 2, (c is the internal long gravity wave speed, f is the Coriolis parameter and  its

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meridional derivative), R is a damping timescale and  is the wind stress (data sources are
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given below). The two parameters to be chosen are c and R, which we take to be
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c = 2.5 m s–1 and R = (24 months)–1. In this case, the results are only weakly sensitive to
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these choices because the largest interannual forcing is in the western half of the basin
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(Fig.FFF, bottom panels), so differences in propagation have little time to manifest
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themselves. We ignore signals that might emanate from the eastern boundary (Kessler
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1990; Minobe and Takeuchi 1995; Vega et al. 2003; Fu and Qiu 2002), so solutions
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represent purely the linear response to interior-ocean wind stress curl.
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The model was forced with a sequence of interannually-filtered monthly wind
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stresses: from January 1981 to July 1991 with ship-derived winds from the Center for
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Ocean-Atmospheric Predictions Studies at Florida State University (Bourassa et al.
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2005), then until July 1999 with the scatterometer winds from ERS 1 and 2 (European
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Space Agency Remote Sensing) satellites, then until November 2009 with QuikSCAT
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winds [REF]. We ignored the effects of islands: the stresses  were linearly interpolated
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in x across all the islands east of Australia-New Guinea, then we did not consider any
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effects of island blocking (Island Rule; Firing et al. 1999).
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Geostrophic zonal transports analogous to the XBT and Argo estimates (Fig.3) were
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found from the solution h(x,y,t) of (1), using U=(c2/f )dh/dy, then similarly integrated
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from 20°S to 11°S.
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The Rossby transport timeseries is very clearly correlated with the 16-year XBT
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transport (r=0.86), but does less well during the 5-year period when the winds and Argo
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data overlap (r=0.53; Fig.FFFa). We note that Argo sampling of the southwest Pacific
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was sparse until at least mid-2005, when the Rossby and Argo transport time series
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appear dissimilar, which perhaps contributes to the weaker overall relation. The
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magnitude of the Rossby-predicted transport anomalies is comparable to the observed as
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well (Fig.FFFa, where the RMS of XBT anomalies is 4.2 Sv, of Argo 3.1 Sv, and of the
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Rossby model 5.1 Sv). As with the XBT, Argo and Aviso time series, the Rossby
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solution has a strong ENSO signal, with a lag relation to the SOI similar to what was seen
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for the XBT and Argo transport (section 3.2; Fig.FFFa,b).
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The genesis of the ENSO transport signals can be seen by comparing average wind
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stress and curl over peaks of the ENSO cycle: defining La Niña as those times when the
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SOI was greater than 1, and El Niño when it was less than 1 (blue and red highlighting
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in Fig.FFFb). Because ENSO changes character after 1999, this means that El Niño
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mainly samples the period before 1999, while La Niña samples the time after, so this
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method does not distinguish between interannual and longer-timescale variations.
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La Niña periods have easterly anomalies spanning the equator to about 10°S west of
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about 150°W, then northwesterlies south of about 15°S. Downwelling curl (red shades in
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Fig.FFFc,d) north of 15°S deepens the thermocline there and to the west, while upwelling
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curl from 15°-25°S shoals it; the result is an anomalous tilt down towards the equator and
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consequent weakening of the SEC. Similar but opposite-sign anomalies occur during El
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Niño, with the entire pattern shifted a few degrees south (Fig.FFFd; Harrison and Vecchi
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1999). The curl patterns south of 20°S suggest that a second reversal of anomalies occurs
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at 25°-30°S, with anomalous westward flow during La Niña and eastward during El Niño
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(Fig.FFFc,d); thus La Niña can be seen as shifting the subtropical gyre to the south and El
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Niño to the north (Wyrtki and Wenzel 1984).
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5. Conclusion
Historical section data extending back to 1985 are used to estimate the interannual
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variability of transport entering the Coral Sea between New Caledonia and the Solomon
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Islands. Typical magnitudes of this variability are ±5-8 Sv on a mean of about 30 Sv.
13
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Westward transport increases a few months after an El Niño event, and decreases
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following a La Niña. A linear, reduced-gravity Rossby model reproduces the observed
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transport variability well, especially during the period before 2001, and shows that much
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of the ENSO transport anomalies are due to wind changes extending south to at least
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20°S. These fluctuations are consistent with ENSO transport anomalies measured across
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the mouth of the Solomon Sea by Davis et al. (2012, submitted). Interannual fluctuations
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are about 50% larger than the annual cycle transport variability on a similar section
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estimated from the CARS climatology by Kessler and Gourdeau (2007; their Fig.7).
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Vigorous westward-propagating mesoscale eddies produce strong transient current
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fluctuations on scales of 100-300km, but usually these average out over a long section as
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considered here. Occasionally these eddies transiently be aligned to yield substantial
282
aliasing (under- or over-estimates comparable to the mean transport) on individual ship or
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glider surveys.
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Since the transport is highly correlated with AVISO surface geostrophic currents, and
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its variability is surface-intensified, a simple index of SEC transport based on AVISO
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currents predicts its anomalies to be about 1.3 Sv (transport relative to 400m) per cm/s
287
(section-average surface speed). A factor of 2.1 Sv per cm/s estimates the total (relative
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to 2000m) transport.
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Acknowledgements
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This paper was possible because of the data collection, compilation and quality control
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efforts of several international programs. The quality-controlled XBT profiles were made
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available by the Coriolis Data Centre of IFREMER (http://www.coriolis.eu.org). The
14
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Argo profiling float data were collected and made freely available by the International
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Argo Program and the national programs that contribute to it (http://www.argo.ucsd.edu).
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The Aviso altimeter products were produced by Ssalto/Duacs and distributed by Aviso,
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with support from CNES (http://www.aviso.oceanobs.com/duacs/). Winds: COAPS/FSU,
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ERS, Quikscat.
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Wyrtki, K. and J. Wenzel, 1984: Possible gyre-gyre interaction in the Pacific Ocean.
Nature, 309, 538-540.
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Figure captions
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Figure 1. IFREMER XBT profiles in the southwest Pacific, showing the selected track
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from Auckland to Solomon Strait.
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Figure 2. Mean crosstrack geostrophic speed on the XBT track. Positive values (red)
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cross the track to the east. a) XBT, b) Argo 400m, c) Argo 2000m, d) Meridional section
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at 160°E .
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Figure 3. Time series of interannual anomalies on the XBT track between 20°S and 11°S
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from the XBT data (green, Sv), Argo data (red, Sv), Aviso surface speed (black, cm s–1).
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The SOI is overlaid in blue dots (scale at right).
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Figure 4. Example of crosstrack currents during mean (top), El Niño (middle) and La
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Niña (bottom) conditions. Positive values (red) cross the track to the east.
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Figure 5. AVISO sea level anomalies (colors) and surface geostrophic velocity during
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the WOCE P11S cruise in June-July 1993. The P11S shiptrack is shown in green, roughly
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along 154.5°E.
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Figure 6. a) Time series of transport anomalies from the Rossby model (black), XBT
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data (green) and Argo (red). The XBT and Argo time series are the same as in Fig.3.
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b) Southern Oscillation Index, where red and blue shading indicates El Niño (SOI less
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than −1) and La Niña (SOI greater than +1) respectively. c) Mean wind stress and curl for
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the La Niña periods shown in panel (b). Units given at bottom. d) As panel c but for El
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Niño periods.