Upper-ocean Thermal Structure Research Articles

Coral record of variability in the upstream Kuroshio Current during 1953–2004

AbstractThe Kuroshio Current (KC), one of the most important western boundary currents in the North Pacific Ocean, strongly affects regional hydroclimate in East Asia and upper ocean thermal structure. Limited by few on‐site observations, the responses of the KC to regional and remote climate forcings are still poorly understood. Here we use monthly coral δ18O data to reconstruct a KC transport record with annual to interannual resolution for the interval 1953–2004. The field site is located in southern Taiwan on the western flank of the upstream KC. Increased (reduced) KC transport would generate strong (weak) upwelling, resulting in relatively high (low) local coral δ18O. The upstream KC transport and downstream transport, off Tatsukushi Bay, Japan, covary on interannual and decadal time scales. This suggests common forcings, such as meridional drift of the North Equatorial Current bifurcation, or zonal climatic oscillations in the Pacific. The intensities of KC transport off southeastern and northeastern Taiwan are in phase before 1990 and antiphase after 1990. This difference may be due to a poleward shift of the subtropical western boundary current as a response to global warming.

Upper-Ocean Contribution to Short-Term Regional Coastal Sea Level Variability along the United States

Abstract The upper ocean thermal structure is largely influenced by natural internal variability, and it modulates global surface temperature as well as regional sea level anomalies over decadal time scales. The internal variability of some important oceanographic processes, such as the Pacific decadal oscillation, can cause differences of 10 to 20 cm relative to the global mean in many coastal locations. These differences are sufficient to alter decisions between soft and/or large protective measures on every coast. Yet, current indicators of sea level changes are based on either surface variables or subsurface ocean measurements through the entire water column. There is also a lack of focus on short-term predictions of sea level change and concrete applications of these predictions at regional scale, which clouds local decision-making. The results herein confirm that the information contained within the top 300 m of the ocean is essential to capture decadal, regional relative sea level variability, whereas depths well below the thermocline do not appear to be closely tied to these large oscillations. Hence, the authors propose a regionally scoped indicator based on upper-ocean temperature as a first step toward identifying trend changes in short-term sea level rise over large coastal regions of the United States. The proposed indicator is a promising new tool that could help close the gaps mentioned above and improve the utility of existing local sea level records.

Tropical cyclone‐ocean interaction in Typhoon Megi (2010)—A synergy study based on ITOP observations and atmosphere‐ocean coupled model simulations

AbstractA mesoscale model coupling the Weather Research and Forecasting model and the three‐dimensional Price‐Weller‐Pinkel ocean model is used to investigate the dynamical ocean response to Megi (2010). It is found that Megi induces sea surface temperature (SST) cooling very differently in the Philippine Sea (PS) and the South China Sea (SCS). The results are compared to the in situ measurements from the Impact of Typhoons on the Ocean in the Pacific (ITOP) 2010 field experiment, satellite observations, and ocean analysis field from Eastern Asian Seas Ocean Nowcast/Forecast System of the U.S. Naval Research Laboratory. The uncoupled and coupled experiments simulate relatively accurately the track and intensity of Megi over PS; however, the simulated intensity of Megi over SCS varies significantly among the experiments. Only the experiment coupled with three‐dimensional ocean processes, which generates rational SST cooling, reasonably simulates the storm intensity in SCS. Our results suggest that storm translation speed and upper ocean thermal structure are two main factors responsible for Megi's distinct different impact over PS and over SCS. In addition, it is shown that coupling with one‐dimensional ocean process (i.e., only vertical mixing process) is not enough to provide sufficient ocean response, especially under slow translation speed (~2–3 m s−1), during which vertical advection (or upwelling) is significant. Therefore, coupling with three‐dimensional ocean processes is necessary and crucial for tropical cyclone forecasting. Finally, the simulation results show that the stable boundary layer forms on top of the Megi‐induced cold SST area and increases the inflow angle of the surface wind.

Monsoons to Mixing in the Bay of Bengal: Multiscale Air-Sea Interactions and Monsoon Predictability

Skillful prediction of the ``active'' and ``break'' spells of monsoon intraseasonal oscillations during the South Asian monsoon season is crucial for the socio-economic fate of one-sixth of the world's population, yet it remains a grand challenge problem. The limited skill of our coupled weather and climate models is largely due to our inability to represent the complex multiscale interactions of the north Indian Ocean and the atmosphere. Air-sea interactions are at the heart of not only the climatological mean annual cycle of the South Asian monsoon but also its synoptic, subseasonal, interannual, and decadal variability. With high local monsoon precipitation and discharge from major rivers (Ganges-Brahmaputra, Irrawaddy), the Bay of Bengal (BoB) exhibits the lowest surface salinities in the tropics as well as unique thermal stratification, making it a natural laboratory for studying multiscale interactions ranging from planetary-scale monsoons to submesoscale mixing in freshwater pools. The current ocean component of coupled models is inadequate for simulating the BoB's upper-ocean thermal structure with fidelity. To further improve monsoon forecasts on intraseasonal and interannual time scales, we need new high-resolution and high-frequency observations over the BoB to fill the gap in our understanding of how the ocean mixes in highly fresh regions, and we need modeling of processes that will convert this understanding to parameterizations of mixing that can be used to improve large-scale ocean models.

Observed ocean thermal response to Hurricanes Gustav and Ike

AbstractThe 2008 Atlantic hurricane season featured two hurricanes, Gustav and Ike, crossing the Gulf of Mexico (GOM) within a 2 week period. Over 400 airborne expendable bathythermographs (AXBTs) were deployed in a GOM field campaign before, during, and after the passage of Gustav and Ike to measure the evolving upper ocean thermal structure. AXBT and drifter deployments specifically targeted the Loop Current (LC) complex, which was undergoing an eddy‐shedding event during the field campaign. Hurricane Gustav forced a 50 m deepening of the ocean mixed layer (OML), dramatically altering the prestorm ocean conditions for Hurricane Ike. Wind‐forced entrainment of colder thermocline water into the OML caused sea surface temperatures to cool by over 5°C in GOM common water, but only 1–2°C in the LC complex. Ekman pumping and a near‐inertial wake were identified by fluctuations in the 20°C isotherm field observed by AXBTs and drifters following Hurricane Ike. Satellite estimates of the 20° and 26°C isotherm depths and ocean heat content were derived using a two‐layer model driven by sea surface height anomalies. Generally, the satellite estimates correctly characterized prestorm conditions, but the two‐layer model inherently could not resolve wind‐forced mixing of the OML. This study highlights the importance of a coordinated satellite and in situ measurement strategy to accurately characterize the ocean state before, during, and after hurricane passage, particularly in the case of two consecutive storms traveling through the same domain.

Effect of Upper-Ocean Evolution on Projected Trends in Tropical Cyclone Activity

Abstract Recent work has highlighted the possible importance of changing upper-ocean thermal and density stratification on observed and projected changes in tropical cyclone activity. Here seven CMIP phase 5 (CMIP5)-generation climate model simulations are downscaled under IPCC representative concentration pathway 8.5 using a coupled atmosphere–ocean tropical cyclone model, generating 100 events per year in the western North Pacific from 2006 to 2100. A control downscaling in which the upper-ocean thermal structure is fixed at its monthly values in the year 2006 is compared to one in which the upper ocean is allowed to evolve, as derived from the CMIP5 models. As found in earlier work, the thermal stratification generally increases as the climate warms, leading to increased ocean mixing–induced negative feedback on tropical cyclone intensity. While trends in the frequency of storms are unaffected, the increasing stratification of the upper ocean leads to a 13% reduction in the increase of tropical cyclone power dissipation over the twenty-first century, averaged across the seven climate models. Much of this reduction is associated with a moderation of the increase in the frequency of category-5 storms.

Improving climate model simulation of tropical Atlantic sea surface temperature: The importance of enhanced vertical atmosphere model resolution

AbstractA long‐standing problem in climate modeling is the inaccurate simulation of tropical Atlantic (TA) sea surface temperature (SST), known as the TA SST bias. It has far‐reaching consequences for climate prediction in that area as it goes along, among others, with erroneous precipitation patterns. We show that the TA SST bias can be largely reduced by increasing both the atmospheric horizontal and vertical resolutions in a climate model. At high horizontal resolution, enhanced vertical resolution is indispensable to substantially improve the simulation of TA SST by enhancing surface wind stress. This also reduces biases in the upper ocean thermal structure and precipitation patterns. Although, enhanced horizontal resolution alone leads to some improvement in the mean climate, typical bias patterns characterized by a reversed zonal SST gradient at the equator and too warm SST in the Benguela upwelling region are mostly unchanged at a coarser vertical resolution.

Satellite derived upper ocean thermal structure and its application to tropical cyclone intensity forecasting in the Indian Ocean

Upper ocean heat content is a factor critical to the intensity change of tropical cyclones (TCs). Because of the inhomogeneity of in situ observations in the North Indian Ocean, gridded temperature/salinity (T/S) profiles were derived from satellite data for 1993–2012 using a linear regression method. The satellite derived T/S dataset covered the region of 10°S–32°N, 25°–100°E with daily temporal resolution, 0.25°×0.25° spatial resolution, and 26 vertical layers from the sea surface to a depth of 1 000 m at standard layers. Independent Global Temperature Salinity Profile Project data were used to validate the satellite derived T/S fields. The analysis confirmed that the satellite derived temperature field represented the characteristics and vertical structure of the temperature field well. The results demonstrated that the vertically averaged root mean square error of the temperature was 0.83 in the upper 1 000 m and the corresponding correlation coefficient was 0.87, which accounted for 76% of the observed variance. After verification of the satellite derived T/S dataset, the TC heat potential (TCHP) was verified. The results show that the satellite derived values were coherent with observed TCHP data with a correlation coefficient of 0.86 and statistical significance at the 99% confidence level. The intensity change of TC Gonu during a period of rapid intensification was studied using satellite derived TCHP data. A delayed effect of the TCHP was found in relation to the intensity change of Gonu, suggesting a lag feature in the response of the inner core of the TC to the ocean.

Spatial trend patterns in the Pacific Ocean sea level during the altimetry era: the contribution of thermocline depth change and internal climate variability

This study investigates the spatial trend patterns and variability of observed sea level and upper ocean thermal structure in the Pacific Ocean during the altimetry era (1993–2012), and the role of thermocline depth changes. The observed sea level trend pattern in this region results from the superposition of two main signals: (1) a strong broad-scale V-shaped positive trend anomaly extending to mid-latitudes in the central Pacific and (2) another very strong positive trend anomaly located in the western tropical Pacific within about 120° E–160° E and 20° S–20° N latitude. In this study, we focus on the tropical Pacific (20° N–20° S) where the strongest trends in sea level are observed. By making use of in situ observational data, we study the impact of thermocline depth changes on steric sea level between the surface and 700 m and its relation with the altimetry-based observed sea level changes. This is done by calculating the time-varying thermocline depth (using the 20 °C isotherm depth as a proxy) and estimating the sea level trend patterns of the thermocline-attributed individual steric components. We show that it is essentially the vertical movement of the thermocline that governs most of the observed sea level changes and trends in the tropical Pacific. Furthermore, we also show that in the equatorial band, the changes in the upper ocean thermal structure are in direct response to the zonal wind stress. Away from the equatorial band (say, within 5°–15° latitude), the changes in the upper ocean thermal structure are consistent with the wind stress-generated Rossby waves. We also estimate the contribution of the Interdecadal Pacific Oscillation (IPO) on the vertical thermal structure of the tropical Pacific Ocean. Removing the IPO contribution to the upper layer steric sea level provides a non-negligible residual pattern, suggesting that IPO-related internal ocean variability alone cannot account for the observed trend patterns in the Pacific sea level. It is likely that the residual signal may also reflect non-linear interactions between different natural modes like El Nino Southern Oscillation (ENSO), IPO, etc.

Enthalpy and Momentum Fluxes during Hurricane Earl Relative to Underlying Ocean Features

Abstract Using dropsondes from 27 aircraft flights, in situ observations, and satellite data acquired during Tropical Cyclone Earl (category 4 hurricane), bulk air–sea fluxes of enthalpy and momentum are investigated in relation to intensity change and underlying upper-ocean thermal structure. During Earl’s rapid intensification (RI) period, ocean heat content (OHC) variability relative to the 26°C isotherm exceeded 90 kJ cm−2, and sea surface cooling was less than 0.5°C. Enthalpy fluxes of ~1.1 kW m−2 were estimated for Earl’s peak intensity. Daily sea surface heat losses of , , and kJ cm−2 were estimated for RI, mature, and weakening stages, respectively. A ratio of the exchange coefficients of enthalpy (CK) and momentum (CD) between 0.54 and 0.7 produced reliable estimates for the fluxes relative to OHC changes, even during RI; a ratio overestimated the fluxes. The most important result is that bulk enthalpy fluxes were controlled by the thermodynamic disequilibrium between the sea surface and the near-surface air, independently of wind speed. This disequilibrium was strongly influenced by underlying warm oceanic features; localized maxima in enthalpy fluxes developed over tight horizontal gradients of moisture disequilibrium over these eddy features. These regions of local buoyant forcing preferentially developed during RI. The overall magnitude of the moisture disequilibrium (Δq = qs − qa) was determined by the saturation specific humidity at sea surface temperature (qs) rather than by the specific humidity of the atmospheric environment (qa). These results support the hypothesis that intense local buoyant forcing by the ocean could be an important intensification mechanism in tropical cyclones over warm oceanic features.

Impacts of nonbreaking wave‐stirring‐induced mixing on the upper ocean thermal structure and typhoon intensity in the South China Sea

AbstractTo investigate the effect of nonbreaking wave‐induced mixing caused by surface wave stirring on the upper ocean thermal structure (UOTS) and the typhoon intensity, a simple nonbreaking wave‐stirring‐induced mixing parameterization (WMP) scheme is incorporated into a regional coupled atmosphere‐ocean model for the South China Sea (SCS), which couples the Princeton Ocean Model (POM) to the Weather Research and Forecasting (WRF) model using the OASIS3 coupler. The results of simulating two selected typhoon cases indicate that the nonbreaking wave‐stirring‐induced mixing has significant impacts on UOTS and the typhoon intensity, and the incorporation of the simple WMP scheme in the coupled model helps to improve the simulation of UOTS and thus the typhoon intensity. In the case that the typhoon intensity is underestimated by the atmosphere model alone, the improvement of initial UOTS by the ocean model with the WMP included can deepen the initial thermocline depth, reduce the effect of SST cooling, and prevent the typhoon intensity from undesired weakening. In the case that the typhoon intensity is overestimated (with strong winds), including the WMP in the ocean model significantly enhances the total vertical mixing rate in the upper ocean, which in turn enhances the SST cooling and thus reduces the typhoon intensity as desired. The results obtained in this study make a contribution to the ongoing efforts of improving the typhoon intensity forecast using a regional atmosphere‐ocean coupled model by worldwide researchers and forecasters, especially for the typhoons in the SCS regions.

THE IMPACT OF SURFACE HEAT FLUXES ON THE SIMULATION OF THE BRAZIL CURRENT

The impact of different formulations of surface heat fluxes (no fluxes, climatological fluxes, restoring of SST towards climatology, climatological fluxes plus SST restoring, and model-computed fluxes via bulk formulas) on the modeling of the Brazil Current off southeast Brazil is investigated in numerical simulations performed with the Regional Ocean Model (ROMS). While mechanical forcing may be dominant in this region, it is shown that correct upper ocean currents and thermal structure can only be obtained when heat fluxes are implemented, even in regions of strong horizontal advection, and that some kind of feedback of the ocean state upon the fluxes is also necessary. This results are of particular importance for ocean modeling developed having operational oceanography in view.

Development and Analysis of the Systematically Merged Atlantic Regional Temperature and Salinity Climatology for Oceanic Heat Content Estimates

Abstract An oceanic climatology to calculate upper-ocean thermal structure was developed for application year-round in the North Atlantic Ocean basin. The Systematically Merged Atlantic Regional Temperature and Salinity (SMARTS) Climatology is used in a two-layer model to project sea surface height anomalies (SSHA) into the water column at ¼° resolution. SMARTS blended monthly temperature and salinity fields from the World Ocean Atlas 2001 (WOA01) and Generalized Digital Environmental Model (GDEM) version 3.0 based on their performance compared to in situ measurements. Daily mean isotherm depths of 20°C (D20) and 26°C (D26) (and their mean ratio), reduced gravity, and mixed layer depth (MLD) were estimated from the climatology. This higher-resolution climatology resolves features in the Gulf of Mexico (GOM), including the Loop Current (LC) and eddy shedding region. Using SMARTS with satellite-derived SSHA and SST fields, daily values of isotherm depths, mixed layer depths, and ocean heat content (OHC) were calculated from 1998 to 2012. OHC is an important scalar when determining the ocean’s impact on tropical cyclone intensification, because it is a better predictor of SST cooling during hurricane passage. Airborne- and ship-deployed expendable bathythermographs (XBT), long-term moorings, and Argo profiling floats provided over 50 000 thermal profiles to assess satellite retrievals of isotherm depths and OHC using the SMARTS Climatology. The OHC calculation presented in this document reduces errors basinwide by 20%, with a 35% error reduction in the GOM.

Holocene centennial-scale changes of the Indonesian and South China Sea throughflows: Evidences from the Makassar Strait

The Indonesian throughflow (ITF), as one of the key links of the global thermohaline circulation in the tropics, influences the large-scale redistribution of ocean heat and freshwater between the Pacific and the Indian Oceans. The ITF interacts with the low-salinity South China Sea throughflow (SCSTF) in the Makassar Strait at the pace of El Niño Southern Oscillation (ENSO). In this paper, variation of the throughflows via the Makassar Strait over the past 12ka is reconstructed according to the seawater δ18Osw differences (indicating salinity gradients), which were determined based on paired measurements of δ18O and magnesium/calcium ratio (Mg/Ca) of Globigerinoides ruber obtained from two sediment cores from the northern and the southern ends of the Makassar Strait respectively. Furthermore, thermal structure variation of the upper water column in the upstream ITF is retrieved on the basis of the temperature difference between the sea surface (G. ruber) and the thermocline (Mg/Ca of Pulleniatina obliquiloculata). It is shown that the surface ITF might have become stronger during the intervals 0.6–1.2, 3–3.6 and 7.2–8ka BP. It also turns out that the SCSTF/ITF system and the upper ocean thermal structure co-vary with each other, and are likely linked with ENSO-like variation of the tropical Pacific. During El Niño-like periods, such as 1.2–3, 3.8–4.6, 5.2–7, and 8–9.6ka BP, the depth of thermocline in the Celebes Sea shoaled and the transport of SCSTF was enhanced, meanwhile the surface warm water transport of the ITF was reduced. An opposite evolution was reconstructed during La Niña-like periods.

Ocean barrier layers’ effect on tropical cyclone intensification

Improving a tropical cyclone's forecast and mitigating its destructive potential requires knowledge of various environmental factors that influence the cyclone's path and intensity. Herein, using a combination of observations and model simulations, we systematically demonstrate that tropical cyclone intensification is significantly affected by salinity-induced barrier layers, which are "quasi-permanent" features in the upper tropical oceans. When tropical cyclones pass over regions with barrier layers, the increased stratification and stability within the layer reduce storm-induced vertical mixing and sea surface temperature cooling. This causes an increase in enthalpy flux from the ocean to the atmosphere and, consequently, an intensification of tropical cyclones. On average, the tropical cyclone intensification rate is nearly 50% higher over regions with barrier layers, compared to regions without. Our finding, which underscores the importance of observing not only the upper-ocean thermal structure but also the salinity structure in deep tropical barrier layer regions, may be a key to more skillful predictions of tropical cyclone intensities through improved ocean state estimates and simulations of barrier layer processes. As the hydrological cycle responds to global warming, any associated changes in the barrier layer distribution must be considered in projecting future tropical cyclone activity.

Sensitive study of the long and short surface wave-induced vertical mixing in a wave-circulation coupled model

The previous studies by the MASNUM research team have shown the effectiveness of the wave-induced mixing (Bv) in improving the simulation of upper-ocean thermal structure. The mechanisms of Bv are further investigated by incorporating different Bv products into the MASNUM wave-circulation coupled model. First, experiments were designed to explore the effects of Bv, which contain the contributions at different wave lengths (l). The results of three experiments, the non-Bv case, the short-wave case (l 300 m) are compared, and it is found that the long waves are the most important component for Bv to generate mixing in the upper ocean. As the swell plays dominant role in mixing, the parameterization of Bv into wind may be not a proper way. Second, Bv effects at different time-scales, including daily and monthly, were examined. The results show that the monthly averaged Bv has larger impact than the daily averaged Bv, especially in summer.

Impact of upper-ocean thermal structure on the intensity of Korean peninsular landfall typhoons

The roles of oceanic factors in the intensity of typhoons which made landfall on the Korean peninsula (KP) are investigated using various meteorological and oceanic measurements and the best track data. Special attention is given an investigation of upper-ocean thermal conditions necessary for becoming the most destructive typhoons on the KP. Based on hydrographic measurements and ocean modeling results, the subsurface thermal structure (STS) along the track of the KP-landfall typhoons reveals two distinct features: the deep subsurface warm layer regions influenced by the Kuroshio Branch Current West of Kyushu (KBCWK) and the Tsushima Current (TSC) and the shallow subsurface warm layer regions influenced by the Yellow Sea Bottom Cold Water (YSBCW). Jeju Island is a border dividing the two regions.Statistical analysis shows that 70–90% of the 10 strongest typhoons on the KP passed through the eastern side of Jeju Island where the warm ocean currents exist at the subsurface. In contrast, the majority of typhoons which passed over the YSBCW regions where cold water exists at the subsurface, along the western side of Jeju Island, experience a large surface temperature cooling due to the storm’s self-induced vertical mixing and upwelling, resulting in a significant decrease in intensity. These results demonstrate that the intensity of the KP-landfall typhoons is very sensitive to the storm’s track and how the difference in the upper ocean thermal structures in small areas can affect the typhoon intensity change.

Reconstructing the upper ocean thermal profiles using one-dimensional numerical model

The observation data for 5 d at a station in the South China Sea is presented. After brief analysis of the wind speed, air temperature from the ship-borne meteorological instruments and temperature and salinity profiles from the CTD (conductivity, temperature, depth recorder) data, the authors find that the CTD casts are too sparse for us to understand the diurnal evolution of the thermal structure in the upper ocean. A one-dimensional (1D) numerical code based on Mellor-Yamada turbulence closure model is used to reconstruct the upper ocean thermal structure, utilizing the atmospheric forcing data from ship-borne weather station. The simulation results show good agreement with the observational data; the significance of breaking waves is also briefly discussed. The evolution of turbulence kinetic energy (TKE) and the contribution from shear production and buoyancy production are discussed respectively. Finally, several possible factors which might influence the numerical results are briefly analyzed.

Improving simulations of the upper ocean by inclusion of surface waves in the Mellor-Yamada turbulence scheme

[1] The Mellor-Yamada turbulence closure scheme, used in many ocean circulation models, is often blamed for overly high simulated surface temperature and overly low simulated subsurface temperature in summer due to insufficient vertical mixing. Surface waves can enhance turbulence kinetic energy and mixing of the upper ocean via wave breaking and nonbreaking-wave-turbulence interaction. The influences of wave breaking and wave-turbulence interaction on the Mellor-Yamada scheme and upper ocean thermal structure are examined and compared with each other using one-dimensional and three-dimensional ocean circulation models. Model results show that the wave-turbulence interaction can effectively amend the problem of insufficient mixing in the classic Mellor-Yamada scheme. The behaviors of the Mellor-Yamada scheme, as well as the simulated upper ocean thermal structure, are significantly improved by adding a turbulence kinetic energy production term associated with wave-turbulence interaction. In contrast, mixing associated with wave breaking alone seems insufficient to improve significantly the simulations as its effect is limited to the very near-surface layers. Therefore, the effects of wave-turbulence interaction on the upper ocean are much more important than those of wave breaking.

Variability of nutrient and thermal structure in surface waters between New Zealand and Antarctica, October 2004–January 2005

We describe the upper ocean thermal structure and surface nutrient concentrations between New Zealand and Antarctica along five transects that cross the Subantarctic Front, the Polar Front and the southern Antarctic Circumpolar Current front. The surface water thermal structure is coupled with variations in surface nutrient concentrations, making water masses identifiable by both temperature and nutrient ranges. In particular, a strong latitudinal gradient in orthosilicate concentration is centred at the Polar Front. On the earlier sections, which extend south-west from the Campbell Plateau, orthosilicate increases sharply southward from 10-15 to 50-55µmol l-1, between 58°S and 60°S, while surface temperature drops from 7°C to 2°C. Nitrate increases more regularly toward the south, with concentrations ranging from 10-12µmol l-1 at 54°S to 25-30µmol l-1 at 66°S. The same features are observed during the later transects between New Zealand and the Ross Sea, but the sharp silica and surface temperature gradients are shifted between 60°S and 64°S. Both temporal and spatial factors may influence the observed variability. The January transect suggests an uptake of silica, orthophosphate and nitrate between 63°S and 70°S over the intervening month, with an average depletion near 37%, 44% and 29%, respectively. An N/P apparent drawdown ratio of 8.8±4.1 and an Si/N apparent drawdown ratio >1 suggests this depletion results from a seasonal diatom bloom. A southward movement of the oceanic fronts between New Zealand and the Ross Sea relative to prior measurements is consistent with reports of recent warming and changes in the Antarctic Circumpolar Current.Keywords: Southern Ocean, nutrients, silica belt, Antarctic Circumpolar Current, expendable bathythermograph (Published: 13 April 2011) Citation: Polar Research 2011, 30, 7064, DOI: 10.3402/polar.v30i0.7064