High-Resolution Underway Upper Ocean and Surface Atmospheric Observations in Drake Passage : Synergistic Measurements for Climate Science

Recent and ongoing studies are highlighted that analyze the in situ underway upper ocean and surface atmospheric observations from frequently-repeated transects by the Antarctic Research and Supply Vessel Laurence M. Gould (LMG) in Drake Passage. High-resolution measurements of upper ocean temperature, salinity and velocity, along with concurrent shipboard meteorological, surface water CO2 and nutrient sampling have been routinely acquired aboard the LMG since the late 1990s. There are significant benefits and synergy of air-sea observations when they are measured at similar time and space scales from the same platform. The multi-year measurements have been used to examine seasonal and spatial variability in upper ocean heat content, Antarctic Circumpolar Current transport variability, eddy heat and momentum fluxes, frontal variability, validation of satellite and model-based air-sea fluxes, and the upper ocean response to climate variability. At present the LMG provides the only year-round shipboard air-sea measurements in the Southern Ocean. Collectively the measurements in Drake Passage have provided much insight as to the characteristics, mechanisms and impacts of the processes and changes that are occurring within the Southern Ocean.


Introduction
Drake Passage, lying between Cape Horn at the tip of the Tierra del Fuego archipelago and the western Antarctic Peninsula (Figure 1), has long had a reputation as one of the roughest stretches of water in the global oceans.Strong westerly winds and currents that flow without interruption around the Southern Ocean are directly funneled into the relatively narrow Drake Passage choke-point.As part of the homeward stretch of the circumpolar "Clipper Route" for trading vessels between Europe and the southern Antipodes, the fierce winds and huge waves gave Drake Passage a notorious reputation among sailors.In fact, sailors eponymously referred to any gigantic waves encountered during their sea voyages as "Cape Horners".Even the intrepid sailor Sir Francis Chichester, who in 1966 was the first solo circumnavigator in his 16-m ketch the Gypsy Moth IV, implored after his adventure in 15-plus meter waves that flooded his cockpit five times while in Drake Passage, "Wild horses could not drag me down to Cape Horn and that sinister Southern Ocean again in a small boat.There is something nightmarish about deep breaking seas and screaming winds …" While Chichester's sentiment may still ring true for many modern day sailors who venture into the Southern Ocean -whether for recreational, commercial or research purposes -for oceanographers, the Drake Passage fills a unique role to help in our understanding of the connections of the large-scale global circulation.Drake Passage provides direct linkages for the inter-ocean exchange of waters between the Pacific and Atlantic Oceans.This inter-ocean pathway via Drake Passage is known as the "cold-water" route for the surface to intermediate depth water that, upon entering the South Atlantic, flows northward to ultimately participate in the Atlantic meridional overturning circulation.The leakage of waters from the Indian Ocean via the Agulhas Current system south of South Africa forms the "warm-water" route that also contributes to the overturning circulation.Drake Passage is the narrowest constriction (~800 km) through which the Antarctic Circumpolar Current (ACC) must pass on its global journey around the Southern Ocean.This feature has historically made Drake Passage an ideal location for monitoring the full meridional width of the ACC transport, water masses and properties.In particular, the pioneering International Southern Ocean Studies (ISOS) program that began in the mid-1970s deployed a picket-fence mooring array and conducted numerous hydrographic surveys designed to resolve the structure and transport of the ACC (Nowlin et al., 1977).
Analyses of the ISOS data lead to the canonical ACC mean volume transport of 134± 11.2 Sv (Whitworth and Peterson, 1985), which has subsequently remained fairly robust (Cunningham et al., 2003).
In the modern-era, the logistical convenience of the Antarctic Peninsula has led to the establishment of many international bases and frequent visits by supply ships has fostered many opportunities for oceanographic surveys of Drake Passage.In 1993, the UK initiated sampling on the near-repeat WOCEdesignated SR1b transect that crosses Drake Passage, making use of the crossings to or from British Antarctic Survey bases.A review of scientific studies of these full-depth SR1b hydrographic sections (some with lowered acoustic Doppler current profiles) can be found in Meredith et al. (2011).Similarly, underway in situ measurements of upper-ocean temperature, salinity and velocity, along with concurrent shipboard meteorological and CO 2 sampling, have been routinely acquired aboard U.S. Antarctic Research and Supply Vessels (ARSVs) since the late 1990s.The ARSVs are the principal supply ships for the US base of Palmer Station, Antarctica, and cross Drake Passage on average twice a month, thus providing underway measurements at high temporal and spatial resolution on a year-round basis.
Our paper will highlight the results from some recent analyses of the in situ underway shipboard observations from the near-repeat transects in Drake Passage.Our motivation is to demonstrate the significant benefits and synergy of air-sea observations when they are measured at similar time and space scales from the same platform.As we will show, the multi-year high-resolution measurements have been used to examine seasonal and spatial variability in upper ocean heat content, surface ocean CO 2 , ACC transport, eddy heat and momentum fluxes, and frontal variability.The simultaneous, comprehensive suite of air-sea shipboard data have enabled one of the very few data-based evaluations of the air-sea heat fluxes in the Southern Ocean, as estimated from satellites, models and the reanalysis flux products.This long history of measurements makes Drake Passage the most observed region in the Southern Ocean and has provided much insight as to the characteristics and mechanisms of the changes that are occurring within the Southern Ocean.

Underway Measurement Systems on the ARSV Laurence M. Gould
Fittingly, it was Ray Peterson (Scripps Institution of Oceanography) who in 1996 had the foresight to first suggest the inclusion of an expendable bathythermograph (XBT) sampling program in Drake Passage as part of the existing broader Pacific-wide network of high-resolution XBT transects.Ray had undertaken his Ph.D. at Texas A&M University with Worth Nowlin measuring the transport and dynamics of the ACC through Drake Passage based on the ISOS observations (Whitworth and Peterson, 1985;Peterson, 1988) and had a fine appreciation for the high value of well-managed, sustained measurement programs.The near-bimonthly, high-resolution XBT sampling transect of upper ocean temperature in Drake Passage was initiated in September 1996 aboard the then US ARSV Polar Duke, and continued when the ARSV Laurence M. Gould (LMG) came on line in September 1999.
XBTs are free-falling instruments that measure temperature profiles of the upper ~850-m depth.They are readily deployed from a moving ship making them ideal for operation in the high sea states of the Southern Ocean.Typically 6-7 XBT transects are completed per year in Drake Passage and there have been 102 XBT transects undertaken to date (December 2011).The nature of the ARSV means that the track is not always exactly repeating (Figure 1).During the 2-3 day Drake Passage crossing, about 70 XBTs are dropped between the coastal boundary 200-m isobaths, with probe separations of about 6-10 km across the Subantarctic Front (SAF) and Polar Front (PF), and 10-15 km elsewhere.The dense sampling is designed to resolve the structure of frontal systems and mesoscale eddies that are predicted by theory to have minimum length scales of ~10 km (Chelton et al., 1998).In fact, the sharp transition at the PF often occurs within 2-3 XBT drops (~10-20 km).The XBT data are corrected for the systematic fall rate error following Hanawa et al. (1995).Since 2001, most XBT transects have also included 10-12 expendable conductivity-temperature-depth (XCTD) profiles, spaced 25-50 km apart, that return temperature and conductivity (and hence allow calculation of salinity and density) to ~1100 m.Small spikes in temperature and conductivity, characteristic of the XCTD profiles, are removed through filtering (Gille et al., 2009).The individual XBT and XCTD profiles are carefully quality-controlled to allow proper representation of the subtle property fluctuations within Drake Passage.Fine scale structure of interleaving water masses is evident in many of the profiles, particularly in the frontal regions.All profile data are archived at NODC for public distribution, and are also available at an SIO maintained website (www-hrx.ucsd.edu) on a transect-by-transect basis.
Continuous upper ocean current profiling from a 150-kHz shipboard acoustic Dopper current profiler (ADCP) commenced in September 1999 on the LMG, with 274 transects of Drake Passage completed as of December 2011.A ship-mounted ADCP measures the velocity of the water relative to the ADCP's transducer by transmitting a pulse of high-frequency sound along multiple acoustic beams and estimating the Dopper shift in the frequency of the sound reflected from scatterers in the water.Estimating the Doppler shift over successive time intervals, corresponding to increasing vertical distance from the transducer, results in a vertical profile of measured currents.The navigation of the ship over the ground must be estimated with high accuracy in order to estimate absolute ocean currents.A basic assumption of the technique is that scatterers are present and are passively advected by the currents.Scatterers in the ocean are not uniformly distributed; they vary geographically and with depth.For ADCPs operating at 150 kHz, the main scatterers of the sound are zooplankton and other small particles.Although the primary use of the ADCP is to measure ocean currents, the measured acoustic backscatter has provided valuable insights into the depth distributions, vertical migration behaviors and even life cycles of dominant biological scatterers (e.g., Flagg and Smith, 1989;Zhou et al., 1994;Chereskin and Tarling, 2007).The LMG's 150-kHz ADCP provides velocity and backscatter measurements at 8-m vertical resolution over a 300-m depth range.A second deep-profiling 38-kHz ADCP was added in late 2004, with 147 transects completed as of December 2011.It provides velocity and backscatter measurements at 24-m vertical resolution over a 1000-m depth range.Typical processing for both ADCPs includes editing of profiles, averaging, and removal of the barotropic tide (Firing et al., 2012).The horizontal resolution of the final data set is about 5-km along-track, and the accuracy of absolute ocean currents is about 1 cm s -1 (Chereskin and Harding, 1993).The LMG underway ADCP measurements are made on all crossings (about 20 per year).Public access to the data is provided through the NODC Joint Archive for shipboard ADCP data and through a project website (adcp.ucsd.edu).
Starting in March 2002, underway measurements of the partial pressure of CO 2 (pCO 2 ) have been made for surface waters and the overlying atmosphere collected through the uncontaminated sea water and atmospheric air intake lines, respectively, on the LMG.The pCO 2 in sea water is measured in a 30L air-tight equilibration chamber which allows the pCO 2 of surface waters flowing at ~5 L min -1 to equilibrate with a head-space of air (15L) in a thermally isolated (within 0.3°C of SST) environment.
Every 3 minutes ~150 cc of an equilibrated air sample is extracted from the head space, dried and measured with an infrared gas analyzer (LICOR 6262).These measurements are directly compared to 5 different standards traceable to the World Meteorological Organization (WMO) that have known CO 2 mixing ratios (mol CO 2 /mol Air) every two hours.The LMG pCO 2 measurements can be found at www.ldeo.columbia.edu\CO2.In addition to the continual sampling of pCO2, discrete samples for total CO 2 , salinity, NO 3 , SiO 4 , PO 4 and 13 C of TCO 2 are also taken from the uncontaminated sea water at 15 different locations across the Drake Passage during each XBT transect.
Regular underway atmospheric and near-surface oceanic measurements began on the LMG in 2000.
The complete meteorological package consists of dual sensors, including R.M. Young anemometers on both port and starboard sides for wind speed and direction, barometers, humidity/wet temperature sensors, and PAR (Photosynthetically Available Radiation), PIR (Precision Infrared Radiometer, to measure longwave radiation) and PSP (Precision Spectral Pyranometer, to measure shortwave radiation) sensors.
A hull-mounted thermosalinograph and fluorometer provide continuous measurements of surface temperature, salinity and fluorescence (chlorophyll-a proxy measurement).

Upper Ocean Variability in Drake Passage
The shipboard underway measurements in Drake Passage provide concurrent transect information of air-sea variability at high temporal and spatial resolution on a year-round basis -an unmatched achievement in the Southern Ocean.An example of the data collected during a typical transect in March 2002 is provided in Figure 2.During this late summer transect the PF, defined by the northern extent of the 2°C isotherm at 200-m depth from the XBT temperature section (Figure 2d), is located at 59°S.A surface salinity gradient is also evident at the PF location (Figure 2b).The PF separates the colder, fresher Antarctic water masses to the south from the warmer, saltier Subantarctic waters masses of the north (Nowlin et al., 1977;Sprintall, 2003).
Just south of the PF lies the sub-zero Antarctic Surface Water (Sprintall, 2007) that forms a temperature minimum layer centered at ~ 100 m and is capped by warmer surface waters during spring and summer (Figure 2d).In winter, air-sea fluxes erode the summer cap, and the Antarctic Surface Water extends from ~150 m depth up to the surface.Below the surface water lies the upper Circumpolar Deep Water (uCDW) that is characterized by temperatures of 1.8°-2°C and exhibits the least temporal variability in temperature of all the upper-ocean water masses found in Drake Passage (Sprintall, 2003).The Southern ACC Front (SACCF) represents the southern-most eastward core of the ACC, and is better defined by velocity than temperature (Lenn et al., 2007).For example, the temperature-defined SACCF (Orsi et al., 1995) lies at 61°S, while the strongest velocity core is centered at ~62°S (Figure 2e).Clearly the velocity-defined SACCF also corresponds to gradients in underway ∆pCO 2 , fluorescence and salinity (Figure 2).Two deep cores of relatively warm water lie north of the PF (Figure 2).The northern most core of the warmest water is found north of the SAF located at 55.6°S on this transect.The southern core, centered on 58°S, is separated from the SAF by a deeper core of colder water that is capped by warmer waters at the surface.This cold-core feature corresponds to a slight elevation in ∆pCO 2 , a decrease in fluorescence and a decrease in surface salinity (Figure 2), properties that are more characteristic of levels found south of the PF.The slight increase in surface pCO 2 could be driven by warming of surface waters as the cold-core moves north, and suggests that these cold core rings help move CO 2 from south to north of the PF and further enhance the flux of CO 2 from the surface ocean to the atmosphere.The ADCP vector velocities clearly show the jets associated with the main ACC fronts, with peak speeds of 85 cm s -1 in the SAF, 65 cm s -1 in the PF and 40 cm s -1 in the SACCF (Figure 2e).Contours of sea surface height anomaly (SSHA) from altimetry show good correspondence with ADCP velocity vectors (Figure 2e).The ship track crosses a northward meandering PF and slices through the eastern side of the deep cold core eddy found between the SAF and PF.High mesoscale variability north of 60°S is evident in the SSHA field.
Distinct bands of alternating cores of warm and cool waters that correspond to eddies and frontal meandering are ubiquitous in the XBT transects (Sprintall, 2003) and confirmed in the ADCP velocity measurements (Lenn et al., 2007).Geostrophic velocity anomalies estimated from altimetric sea surface height anomalies (SSHA) tend to underestimate directly observed currents due in part to the lack of a mean geostrophic reference and also due to the coarser resolution of the altimetry (Lenn et al., 2007), but the patterns generally show good agreement.An Eulerian gridded mean updated from Lenn et al. (2008) and based on a longer 11.5 year interval (262 transects) clearly shows the three main frontal jets of the ACC relative to climatology (Figure 3a).The apparent northward displacement by about 50 km of the SAF and PF from their climatological positions (Orsi et al., 1995) agrees with front locations determined from XBTs (Sprintall, 2003) and is likely due to uncertainty in determining front locations from the coarser sampling (~50 km station spacing) of the climatology.In this region of Drake Passage, the eddies seem to be exclusively confined to the Antarctic Polar Frontal Zone between the PF and the SAF (Sprintall, 2003), and standard deviation ellipses show elevated eddy kinetic energy in this zone (Fig. 3b; Lenn et al., 2007).The combined data set contributes useful information on the mesoscale eddy features in Drake Passage and for computations such as estimating eddy heat fluxes (Lenn et al., 2011).
Direct velocity observations are useful in determining total transport over the profiling range of the ADCPs.Only a subset of transects are considered suitable for transport calculation, using the criterion that the Drake crossing needs to be at least 700 km in length and 90% good coverage.Gaps smaller than the above criterion are interpolated.A slab layer extrapolates to the surface.The mean observed transport of the ACC in the upper 1000 m based on 51 38-kHz ADCP transects over 4.5 years is 95 ± 2 Sv, or 71% of the canonical full-depth value of 134 Sv (Firing et al., 2011).The remarkable stability of the 5-year transport estimate is also seen in the 16-year mean from the SR1b transect (Meredith et al., 2011); additionally, the LMG near-bi-monthly sampling avoids potential aliasing of the annual signal and thus is suitable for examining variability at seasonal to annual time scales..It has been postulated that the ACC is presently in an almost eddy-saturated state (see Meredith et al., 2011), whereby strengthening winds change the eddy intensity more than ACC transport.The LMG transport time series provides an extremely important baseline for comparing the response of the ACC to any future changes in wind forcing, whether due to natural or anthropogenic impacts on the environment.
In the Southern Ocean, the zonally-integrated Ekman transport is estimated to be 25-30 Sv and constitutes the shallowest limb of the meridional overturning circulation, a key component of the coupled ocean/atmosphere climate system.Wind-driven Ekman currents have been difficult to observe directly because, even when forced by strong winds as in the Southern Ocean, their magnitudes are small compared to the background geostrophic circulation.While Ekman currents can be inferred from the wind, direct measurements are required in order to determine the Ekman layer depth, mean temperature, eddy viscosity and associated Ekman layer heat fluxes.The shipboard ADCP has been a valuable tool in describing Ekman currents because it provides velocity profiles that span the ocean surface-layer from the level of direct wind influence down into the geostrophic interior.Lenn and Chereskin (2009) describe the characteristics of the Ekman layer in Drake Passage.The mean Ekman currents decay in amplitude and rotate anticyclonically with depth, penetrating to 100 m, above the base of the annual mean mixed layer at 120 m.The rotation depth scale exceeds the e-folding scale of the speed by about a factor of 3, resulting in a current spiral that is compressed relative to predictions from theory.The mean temperature of the Ekman layer is not distinguishable from temperature at the surface.Turbulent eddy viscosities estimated from the time-averaged stress are O(100-1000) m 2 s -1 .The eddy viscosity is not constant; it decreases in magnitude with depth, and the time-averaged stress is not parallel to the time-averaged vertical shear.
These differences from the simplest theory are likely due to nonconstant eddy viscosity and to timeaveraging over the cycling of the stratification in response to diurnal buoyancy fluxes, although the action of surface waves and the oceanic response to high frequency wind variability may also contribute.
Many of the processes most relevant to climate change occur in the oceanic surface layer where direct heat, freshwater and gas exchange occurs with the atmosphere.In many studies of air-sea exchange, the limit of the upper ocean is defined by the mixed layer; the low stratification in Drake Passage, however, makes identification of the mixed layer depth problematic (Stephenson et al., 2012).Consequently, mixed layers are often ill-defined and display considerable variability over very short distances.In contrast, upper ocean heat content has been shown to provide a much more robust measure of upper ocean variability, with a first-order heat balance between heat content and air-sea heat fluxes matching in both phase and amplitude over the seasonal cycle (Stephenson et al., 2012).A similar agreement between the mixed layer integrated heat content and air-sea fluxes could not be achieved, and in particular the seasonal phasing differed significantly.However, the simple one-dimensional heat budget balancing the upper ocean heat content and surface heat fluxes (Stephenson et al., 2012) is not the complete picture of the processes governing upper-ocean variability in the Southern Ocean -for example, frontal meanders and eddies frequently found north of the PF (Figure 2) can redistribute heat through horizontal and vertical advection and so complicate the 1-D heat exchange balance.Nonetheless, the relationship between upper ocean heat content and surface forcing accounts for most of the variance on seasonal time scales.

Evaluation of Southern Ocean Air-Sea Fluxes
The sensitivity of the Southern Ocean to the climate system is largely dependent on the exchanges of heat, freshwater and momentum that occur at the air-sea interface.Southern Ocean air-sea fluxes are particularly important because of their influence on water mass formation and transformation, and also through the oceanic uptake of heat.Significant changes have occurred in observed Southern Ocean heat content and water mass properties over the past few decades (e.g.Gille, 2002;2008;Purkey and Johnson, 2010).Yet the magnitude and variations of air-sea fluxes in the Southern Ocean are poorly known, and this has led to great uncertainties in Southern Ocean heat budgets and the climate system (Dong et al., 2007;Cerovecki et al., 2012).For many physical (e.g.high winds and sea states) and logistical (e.g.remoteness) reasons, routine meteorological measurements are problematic in the Southern Ocean (see Bourassa et al., 2012 for a recent complete review).This has made the multi-year, year-round, highresolution shipboard measurements of flux-related parameters from Drake Passage LMG transects both a unique and invaluable data set for assessing remotely-sensed and National Weather Prediction (NWP) airsea flux products (such as NCEP/NCAR, ECMWF etc.).
The May 2002 launch of the NASA Earth Observing System (EOS) Aqua satellite offered new possibilities for obtaining a suite of remotely-sensed flux-related state variables.However validation of the satellite retrievals of these state variables, mainly relevant to the latent and sensible heat components of the net air-sea heat flux, are mostly based on data that has been collected in the tropics or mid-latitudes.Dong et al. (2006a) presented one of the first attempts to evaluate the performance of the EOS AMSR-E microwave sea surface temperatures using the LMG ship-based SST measurements from the XBT and TSG.Compared to the EOS MODIS infrared retrievals of SST, the AMSR-E provides SST measurements with little bias relative to the in situ observations.The distinction is important since unlike the infrared measurements, the AMSR-E microwave sensor is capable of penetrating through cloud cover, which is perpetual in the Southern Ocean, and hence provides better temporal and spatial coverage.This improves the application of the AMSR-E measurements for Southern Ocean heat budget studies (Dong et al., 2007), frontal variability (Dong et al., 2006b), and potentially for use in determining the Ekman heat transport (Lenn and Chereskin, 2009).
The EOS AIRS surface air temperature and specific humidity sensors were also assessed by comparison to the LMG shipboard meteorological measurements in Drake Passage (Dong et al., 2010).
The objective was to evaluate whether the AIRS retrievals, in conjunction with the AMSR-E SST, could provide sufficiently accurate parameters to estimate the sensible and latent heat fluxes in the data-poor Southern Ocean.The co-located data show that both AIRS-derived SST and air temperature are colder and the specific humidity measurements lower than the shipboard measurements.Nonetheless, compared to several NWP products, the remotely sensed variables showed the small-scale spatial structure that is typical of the LMG observations.Consequently, the turbulent fluxes derived from the AIRS and AMSR-E data using bulk algorithms were better able to represent the full range of flux values estimated from the shipboard data compared to the NWP products (Dong et al., 2010).
In a recent paper, Jiang et al. (2012) used the underway LMG measurements to examine the spatial scales of the turbulent heat fluxes and the flux-related state variables.The spatial decorrelation scales of sensible and latent heat fluxes calculated from the 2-day transects were respectively 65± 6 km and 80± 6 km.However, when the PF region was excluded from the calculations, the decorrelation scales reduced to 10-20 km, consistent with the first baroclinic Rossby radius (Chelton et al., 1998).These eddy scales are often unrepresented in the available NWP gridded heat flux products.To gain a better understanding of air-sea exchange processes and their relevance to the climate system, it is important that the small-scale resolving skills and the response time to mesoscale forcing are improved in the NWP products.

Long-term Changes in Drake Passage
Many recent studies have documented changes in Southern Ocean water mass properties, the coupled ice-ocean system, the marine ecosystem and atmospheric forcing on time scales of relevance to climate (e.g.Yuan and Martinson, 2000;le Quere et al., 2002;Gille, 2002;2008;Vaughan et al., 2003;Thompson and Wallace, 2000).The relatively extensive long-term hydrographic data coverage in Drake Passage means the number of available observations greatly exceed those used in other studies of climate change in the Southern Ocean.Using data from Drake Passage starting in 1963, Sprintall (2008) found statistically significant trends in upper ocean temperature using linear robust regression analysis.Because most of the winter observations are associated with the more recent year-round, high-resolution XBT program, Sprintall (2008) also presented trends determined only from summer transects (November -March) to avoid any seasonal sampling bias.As for the full data set, using the seasonally adjusted data resulted in statistically significant trends of ~ 0.02°C yr -1 are observed north of the PF below the surface with smaller significant trends of ~0.005°C yr -1 observed south of the PF.The time series of temperature anomalies were highly correlated with climate indices of the Antarctic Oscillation (AAO) and ENSO.The annual temperature anomalies both north and south of the PF lagged El Niño variability in the Pacific with a phasing consistent with the cyclical patterns in SST and sea ice associated with the Antarctic Circumpolar Wave or the Antarctic Dipole Mode (White and Peterson, 1996;Yuan and Martinson, 2000).
The temperature anomalies north of the PF were primarily coincident with the AAO, and largely consistent with a southward shift in the PF due to the strengthening and poleward shift of the band of westerly winds in the Southern Ocean (Thompson and Wallace, 2000;Dong et al., 2006b).
Changes in the stratification and properties of the water column in the Southern Ocean are also likely to have important consequences to primary production and the structure of the marine ecosystem in Antarctic waters.The 150-kHz ADCP data has been used to look at variability in backscattering in Drake Passage (Chereskin and Tarling, 2007), a measurement that is correlated with planktivore biomass (e.g.Zhou et al., 1994).Diel vertical migration in the upper 150 m was the dominant variability observed in any single transect.When averaged over depth, there was a well-defined annual cycle in backscattering strength, with a factor of four increase from a late-winter minimum to a spring-summer maximum over a period of four months.The diel and seasonal variability suggest that the scattering is dominated by biological scatterers.A significant decline in backscatter was observed south of the PF over the six years examined (1999)(2000)(2001)(2002)(2003)(2004), peaking in 1999 and dropping to the lowest levels in 2004.Most notably, the largest decline was seen south of the SACCF, where the biomass is dominated by Antarctic krill (Euphausia superba), the keystone species in Southern Ocean food webs (Siegel, 1988).Also notable in this regard is that populations of planktivorous higher-predators (e.g.Adelie penguins, Pygoscelis adeliae) in nearby islands have been declining over a number of years (Forcada et al, 2006).
Long-term declines in population size point to a more significant environmental shift.The fact that both planktivores and acoustic backscatter have declined over similar periods suggests that the wider zooplankton community is currently in a phase of decline in this region of the Southern Ocean.This is especially true in regions where Antarctic krill dominates, south of the SACCF, for example.Such a decline may be a result of recent warming trends in the surface waters of this region (Meredith and King, 2005) as well as the changing ice dynamics (Vaughan et al., 2003).
Finally, the changes in the marine ecosystem structure and in the ice-ocean-atmosphere system will likely also have significant implications for the role of the Southern Ocean as a sink for the increasing anthropogenic atmospheric carbon dioxide (CO 2 ).Recent modeling studies and observations made on the LMG have suggested that the stronger westerlies in the Southern Ocean will lead to increased upwelling and subsequent degassing of the carbon-rich uCDW found at depth south of the PF (e.g.Lovenduski et al., 2008;Sweeney et al., 2012).The models further suggest that the response of the seawater pCO 2 to wind-driven long-term changes in temperature and photosynthesis are relatively small (Lovenduski et al., 2008;Le Quéré et al., 2007).The relationship between the underway LMG surface carbon dioxide and temperature measurements sheds light on some of the processes that may be controlling this variability (Figure 4).Here we examine the correlation of the time series of underway measurements of the ∆pCO 2 (Figure 4a) with the concurrent time series of XBT temperature profiles.We use the ∆pCO 2 (i.e. the difference between the underway measured atmospheric pCO 2 and the measured ocean surface pCO 2 ) to account for the observed trends in atmospheric pCO 2 .Both XBT and surface ∆pCO 2 time series have been binned to the same 10 km along track grid in winter (Figure 4b) and spring (Figure 4c).A significant negative correlation of the surface ∆pCO 2 (air-sea) and the concurrent XBT temperature profiles is found just south of the PF in winter (Figure 4b).A negative correlation corresponds to higher temperatures and lower ∆pCO 2 such that the flux is from the ocean to the atmosphere, or conversely that lower temperatures enhance the flux from atmosphere to ocean.In the Southern Ocean, the pCO 2 is strongly controlled the thermodynamic relationship between temperature and pCO 2 such that every 1°C increase in temperature is equivalent to a 4.23% increase in pCO 2 .The negative correlation pattern is therefore in agreement with the thermodynamic relationship and furthermore, is consistent with the winter upwelling of the warmer and higher oceanic pCO 2 uCDW waters that drive the ∆pCO 2 (and air-sea flux) to be more negative.During the spring months, the positive correlation found in the near surface layer north of the PF suggests that temperature is driving a biological relationship for the increased ∆pCO 2 (Figure 4a,c).The warmer waters of spring enhance the near-surface stratification and the trapping of increased sunlight will drive biological production and thus lower the oceanic pCO 2 and enhance the air to sea flux.

Conclusions
Our review has highlighted some new and ongoing scientific studies of the upper ocean processes in the Drake Passage using the suite of underway air-sea measurements from the LMG.With decades of data now available, the Drake Passage air-sea measurement programs have reached a point where they can be maintained with a minimum of personnel and resources, generating high-quality data that is publicly available in a prompt manner.Oceanographic time series such as this are carefully considered prior to their implementation because their usefulness is in their longevity.Interrupting or disbanding them seriously degrades the potential usefulness of the data already collected.The programs involve a small financial outlay for a huge scientific benefit.Automated underway observations on supply ships provide an extremely cost-effective method for obtaining high-quality data at the air-sea interface that has benefits for a broad range of climate-related research questions.Simultaneous high-resolution repeat transect information of the upper ocean temperature and velocity, the air-sea gas fluxes and meteorological measurements cannot presently be obtained in any other way.At present, the LMG provides the only year-round repeat shipboard air-sea measurements in the Southern Ocean.Encouraging the routine collection of underway concurrently measured air-sea data from vessels operating in the Southern Ocean is a critical recommendation of the community supported Southern Ocean Observing System (Rintoul et al. 2012).Future observation systems would benefit from expanding vessel recruitment in this region of importance to global climate.(Orsi et al., 1995) are shown as thick gray lines.Updated from Lenn et al. (2008).

Figure 1 :Figure 2 :
Figure 1: Bathymetry (m) of Drake Passage with Orsi et al (1995) climatological front positions: from north to south, the Subantarctic Front (SAF), Polar Front (PF), and the Southern Antarctic Circumpolar Current Front (SACCF).The ARSV Laurence M. Gould Drake Passage transect sampling of underway ADCP, carbon and meteorological measurement are indicated by the light gray lines, and those transects that coincide with XBT/XCTD sampling are indicated by the dark gray lines.

Figure 3 .
Figure 3. (a) Gridded (25km x 25km), depth-averaged (26-298 m), time-averaged currents from 262 Drake Passage transects between 9/1999 and 4/2011 and (b) standard deviation ellipses.Gridded velocities are shown in each grid box crossed 3 times or more; standard deviation ellipses are shown for each grid box crossed 15 times or more.Climatological locations of the SAF, PF, and SACCF (Orsi et al.,

Figure 4
Figure 4: a) Mean ∆pCO 2 (µatm) during winter (blue) and spring (red) from the ARSV Laurence M. Gould underway measurements in Drake Passage.Correlation of the time series of surface ∆pCO 2 measurements and the XBT-temperature sections at concurrent along track bins (10 km) in b) winter and c) spring.Only those correlations that are significant at the 95% confidence level are colored.Background solid contours show the mean XBT temperature structure during the corresponding seasons.