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Shota Katsura and Janet Sprintall

Abstract

Seasonality and formation of barrier layers (BLs) and associated temperature inversions (TIs) in the eastern tropical North Pacific Ocean were investigated using raw and gridded Argo profiling float data, satellite data, and various sea surface flux data. BLs were observed frequently in boreal summer and autumn along the sea surface salinity (SSS) front south of the eastern Pacific fresh pool. TIs were found within the gap between the western and eastern Pacific warm pools in autumn when BLs were thickest. A mixed layer salinity budget was constructed to determine the formation mechanism responsible for BLs with TIs. This budget revealed that Ekman advection works to both freshen and cool the eastern tropical North Pacific in autumn and contributes to the formation of the thickest BLs with the warmest TIs through the tilting of the SSS front. Precipitation is a secondary contributor to BL formation in autumn. The BLs are also prevalent during summer but are thinner, are without associated TIs, and are primarily formed through precipitation. The largest rainfall associated with the intertropical convergence zone mostly occurred north of the band of thickest BLs in both summer and autumn. The geostrophic advection of salinity did not coherently contribute to the formation of BLs or TIs. The idea that Ekman advection contributes most to the formation of the thickest BLs with warm TIs was further corroborated because the horizontal salinity gradient was the dominant contributor to the density gradient and so is favorable for BL and TI formation.

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Shenfu Dong, Sarah T. Gille, and Janet Sprintall

Abstract

The mixed layer heat balance in the Southern Ocean is examined by combining remotely sensed measurements and in situ observations from 1 June 2002 to 31 May 2006, coinciding with the period during which Advanced Microwave Scanning Radiometer-Earth Observing System (EOS) (AMSR-E) sea surface temperature measurements are available. Temperature/salinity profiles from Argo floats are used to derive the mixed layer depth. All terms in the heat budget are estimated directly from available data. The domain-averaged terms of oceanic heat advection, entrainment, diffusion, and air–sea flux are largely consistent with the evolution of the mixed layer temperature. The mixed layer temperature undergoes a strong seasonal cycle, which is largely attributed to the air–sea heat fluxes. Entrainment plays a secondary role. Oceanic advection also experiences a seasonal cycle, although it is relatively weak. Most of the seasonal variations in the advection term come from the Ekman advection, in contrast with western boundary current regions where geostrophic advection controls the total advection. Substantial imbalances exist in the regional heat budgets, especially near the northern boundary of the Antarctic Circumpolar Current. The biggest contributor to the surface heat budget error is thought to be the air–sea heat fluxes, because only limited Southern Hemisphere data are available for the reanalysis products, and hence these fluxes have large uncertainties. In particular, the lack of in situ measurements during winter is of fundamental concern. Sensitivity tests suggest that a proper representation of the mixed layer depth is important to close the budget. Salinity influences the stratification in the Southern Ocean; temperature alone provides an imperfect estimate of mixed layer depth and, because of this, also an imperfect estimate of the temperature of water entrained into the mixed layer from below.

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Shenfu Dong, Janet Sprintall, and Sarah T. Gille

Abstract

The location of the Southern Ocean polar front (PF) is mapped from the first 3 yr of remotely sensed Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) sea surface temperature (SST) measurements. In agreement with previous studies, the mean path of the Antarctic PF and its standard deviation are strongly influenced by bottom topography. However, the mean PF path diverges slightly from previous studies in several regions where there is high mesoscale variability. Although the SST and SST gradient at the PF show spatially coherent seasonal variations, with the highest temperature and the lowest temperature gradient during summer, the seasonal variations in the location of the PF are not spatially coherent. The temporal mean SST at the PF corresponds well to the mean PF path: the temperature is high in the Atlantic and Indian Ocean sections and is low in the Pacific Ocean section where the PF has a more southerly position. The relationship of the wind field with the Antarctic PF location and proxies for the zonal and meridional PF transports are examined statistically. Coherence analysis suggests that the zonal wind stress accelerates the zonal transport of the PF. The analysis presented herein also suggests that the meridional shifts of the Antarctic PF path correspond to the meridional shifts of the wind field.

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Doron Nof, Thierry Pichevin, and Janet Sprintall

Abstract

The outflow from the Indonesian seas empties approximately 5–7 Sv of surface warm (and low salinity) Indonesian Throughflow water into the southern Indian Ocean (at roughly 12°S). Using a nonlinear 1½-layer model with a simple geometry consisting of a point source (of anomalous water) situated along a meridional wall on a β plane, the spreading of these waters is examined. An analytical solution is constructed with the aid of the “slowly varying” approach, and process-oriented numerical simulations are performed.

It is found that, immediately after emptying into the ocean, the outflow splits into two branches. One branch carries approximately 13% of the source mass flux and forms a chain of high amplitude anticyclonic eddies (lenses) immediately to the west of the source. These eddies drift westward and penetrate into the interior of the Indian Ocean. The second branch carries the remaining 87% of the mass flux via a coastal southward flowing current. Ultimately, this second branch separates from the coast and turns westward. (A detailed examination of this second branch separation is, nevertheless, beyond the scope of this study.)

It is suggested that the eddies recently observed to the west of the Island of Timor are a result of the above eddy generation process, which is not related to the classical eddy generation process associated with instabilities (i.e., the breakdown of a known steady solution). It is also suggested that this new nonlinear process explains why some of the Indonesian Throughflow water forms the source of the southward flowing coastal Leeuwin Current.

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ChuanLi Jiang, Sarah T. Gille, Janet Sprintall, and Kei Yoshimura
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Kyla Drushka, Janet Sprintall, Sarah T. Gille, and Irsan Brodjonegoro

Abstract

The subsurface structure of intraseasonal Kelvin waves in two Indonesian Throughflow (ITF) exit passages is observed and characterized using velocity and temperature data from the 2004–06 International Nusantara Stratification and Transport (INSTANT) project. Scatterometer winds are used to characterize forcing, and altimetric sea level anomaly (SLA) data are used to trace the pathways of Kelvin waves east from their generation region in the equatorial Indian Ocean to Sumatra, south along the Indonesian coast, and into the ITF region.

During the 3-yr INSTANT period, 40 intraseasonal Kelvin waves forced by winds over the central equatorial Indian Ocean caused strong transport anomalies in the ITF outflow passages. Of these events, 21 are classed as “downwelling” Kelvin waves, forced by westerly winds and linked to depressions in the thermocline and warm temperature anomalies in the ITF outflow passages; 19 were “upwelling” Kelvin waves, generated by easterly wind events and linked to shoaling of the thermocline and cool temperature anomalies in the ITF. Both downwelling and upwelling Kelvin waves have similar vertical structures in the ITF outflow passages, with strong transport anomalies over all depths and a distinctive upward tilt to the phase that indicates downward energy propagation. A linear wind-forced model shows that the first two baroclinic modes account for most of the intraseasonal variance in the ITF outflow passages associated with Kelvin waves and highlights the importance of winds both in the eastern equatorial Indian Ocean and along the coast of Sumatra and Java for exciting Kelvin waves.

Using SLA as a proxy for Kelvin wave energy shows that 37% ± 9% of the incoming Kelvin wave energy from the Indian Ocean bypasses the gap in the coastal waveguide at Lombok Strait and continues eastward. Of the energy that continues eastward downstream of Lombok Strait, the Kelvin waves are split by Sumba Island, with roughly equal energy going north and south to enter the Savu Sea.

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ChuanLi Jiang, Sarah T. Gille, Janet Sprintall, and Colm Sweeney

Abstract

Surface water partial pressure of CO2 (pCO2) variations in Drake Passage are examined using decade-long underway shipboard measurements. North of the Polar Front (PF), the observed pCO2 shows a seasonal cycle that peaks annually in August and dissolved inorganic carbon (DIC)–forced variations are significant. Just south of the PF, pCO2 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 pCO2 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 pCO2 values averaged over the Drake Passage region. However, the model amplitude of the pCO2 seasonal cycle exceeds the observed amplitude of the pCO2 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 pCO2 variations in the models than they do in observations. Four ESMs show elevated wintertime pCO2 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.

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Yueng-Djern Lenn, Teresa K. Chereskin, and Janet Sprintall

Abstract

Accurately resolving the mean Antarctic Circumpolar Current (ACC) is essential for determining Southern Ocean eddy fluxes that are important to the global meridional overturning circulation. Previous estimates of the mean ACC have been limited by the paucity of Southern Ocean observations. A new estimate of the mean surface ACC in Drake Passage is presented that combines sea surface height anomalies measured by satellite altimetry with a recent dataset of repeat high-resolution acoustic Doppler current profiler observations. A mean streamfunction (surface height field), objectively mapped from the mean currents, is used to validate two recent dynamic height climatologies. The new streamfunction has narrower and stronger ACC fronts separated by quiescent zones of much weaker flow, thereby improving on the resolution of ACC fronts observed in the other climatologies. Distinct streamlines can be associated with particular ACC fronts and tracked in time-dependent maps of dynamic height. This analysis shows that varying degrees of topographic control are evident in the preferred paths of the ACC fronts through Drake Passage.

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Janet Sprintall, Sean Kennan, Yoo Yin Kim, and Peter Niiler

Abstract

Observations of horizontal velocity from two shipboard acoustic Doppler current profilers (ADCPs), as well as wind, temperature, and salinity observations from a cruise during June–July 2001, are used to compute a simplified mean meridional momentum balance of the North Equatorial Countercurrent (NECC) at 95°W. The terms that are retained in the momentum balance and derived using the measurements are the Coriolis and pressure gradient forces, and the vertical divergence of the turbulent stress. All terms were vertically integrated over the surface turbulent layer. The K-profile parameterization (KPP) prescribed Richardson number (Ri) is used to determine the depth of the turbulent boundary layer h at which the turbulent stress and its gradient vanish. At the time of the cruise, surface drifters and altimeter data show the flow structure of the NECC was complicated by the presence of tropical instability waves to the south and a strong Costa Rica Dome to the north. Nonetheless, a consistent, simplified momentum balance for the surface layer was achieved from the time mean of 19 days of repeat transects along 95°W with a 0.5° latitude resolution. The best agreement between the ageostrophic transport determined from the near-surface cruise measurements and the wind-derived Ekman transport was obtained for an Ri of 0.23 ± 0.05. The corresponding h ranges from ∼55 m at 4°N to ∼30 m within the NECC core (4.5°–6°N) and shoaling to just 15 m at 7°N. In general, the mean ageostrophic and Ekman transports decreased from south to north along the 95°W transect, although within the core of the NECC both transports were relatively strong and steady. This study underscores the importance of the southerly wind-driven eastward Ekman transport in the turbulent boundary layer before the NECC becomes fully developed later in the year through indirect forcing from the wind stress curl.

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James T. Potemra, Susan L. Hautala, Janet Sprintall, and Wahyu Pandoe

Abstract

Recent measurements from six bottom-mounted gauges are used with numerical model results to study the exchange of water between the Indonesian seas and the Indian Ocean via the Lesser Sunda Islands known collectively as Nusa Tengarra. The observations are approximately three years in length, from late 1995 to early 1999, and include measurements of bottom pressure, temperature, and salinity. The locations of the gauges are at the boundaries of three straits connecting the southern Indonesian seas with the eastern Indian Ocean: the Lombok Strait, the Ombai Strait, and the Timor Passage. The magnitude of intraseasonal variations in the pressure data dominates over that of the seasonal cycle. Intraseasonal variability appears most frequently and largest in magnitude at the westernmost strait (Lombok) and decreases along the coastline to the Timor Passage. Comparison to wind data shows these intraseasonal variations to be due to Kelvin wave activity in the Indian Ocean, forced by two distinct wind variations: semiannual monsoon reversals and Madden–Julian oscillation (MJO) activity. Sea level variations from both forcing mechanisms are then adjusted by local, alongshore winds. Longer-duration model results show the observation period (1996 through early 1999) to be a time of increased ENSO-related interannual variability and of suppressed annual cycle. MJO activity is also increased during this time. These factors explain the dominance of the higher frequency signals in the pressure data and the relative lack of a distinct annual cycle. An optimal fit of model sea level variations to model through-strait transport variations is used to estimate transport variability from the observed pressure records. At each strait the optimal fit is consistent with a cross-strait geostrophic balance for transport variations in the upper 250 m.

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