Showing papers by "Sarah T. Gille published in 2012"

In Situ Observations of Madden–Julian Oscillation Mixed Layer Dynamics in the Indian and Western Pacific Oceans

Abstract: The boreal winter response of the ocean mixed layer to the Madden–Julian oscillation (MJO) in the Indo-Pacific region is determined using in situ observations from the Argo profiling float dataset. Composite averages over numerous events reveal that the MJO forces systematic variations in mixed layer depth and temperature throughout the domain. Strong MJO mixed layer depth anomalies (>15 m peak to peak) are observed in the central Indian Ocean and in the far western Pacific Ocean. The strongest mixed layer temperature variations (>0.6°C peak to peak) are found in the central Indian Ocean and in the region between northwest Australia and Java. A heat budget analysis is used to evaluate which processes are responsible for mixed layer temperature variations at MJO time scales. Though uncertainties in the heat budget are on the same order as the temperature trend, the analysis nonetheless demonstrates that mixed layer temperature variations associated with the canonical MJO are driven largely by anoma...

Mean dynamic topography in the Southern Ocean: Evaluating Antarctic Circumpolar Current transport

Abstract: [1] Mean Dynamic Ocean Topography (MDT) is the difference between the time-averaged sea surface height and the geoid. Combining sea level and geoid measurements, which are both attained primarily by satellite, is complicated by ocean variability and differences in resolved spatial scales. Accurate knowledge of the MDT is particularly difficult in the Southern Ocean as this region is characterized by high temporal variability, relatively short spatial scales, and a lack of in situ gravity observations. In this study, four recent Southern Ocean MDT products are evaluated along with an MDT diagnosed from a Southern Ocean state estimate. MDT products differ in some locations by more than the nominal error bars. Attempts to decrease this discrepancy by accounting for temporal differences in the time period each product represents were unsuccessful, likely due to issues regarding resolved spatial scales. The mean mass transport of the Antarctic Circumpolar Current (ACC) system can be determined by combining the MDT products with climatological ocean density fields. On average, MDT products predict higher ACC transports than inferred from observations. More importantly, the MDT products imply an unrealistic lack of mass conservation that cannot be explained by the a priori uncertainties. MDT estimates can possibly be improved by accounting for an ocean mass balance constraint.

Seasonal variability of upper ocean heat content in Drake Passage

Abstract: [1] Mixed-layer depth (MLD) is often used in a mixed-layer heat budget to relate air-sea exchange to changes in the near-surface ocean temperature. In this study, reanalysis heat flux products and profiles from a 15 year time series of high-resolution, near-repeat expendable bathythermograph/expendable conductivity-temperature-depth (XBT/XCTD) sampling in Drake Passage are used to examine the nature of MLD variations and their impact on a first-order, one-dimensional heat budget for the upper ocean in the regions north and south of the Polar Front. Results show that temperature and density criteria yield different MLD estimates, and that these estimates can be sensitive to the choice of threshold. The difficulty of defining MLD in low-stratification regions, the large amplitude of wintertime MLD (up to 700 m in Drake Passage), and the natural small-scale variability of the upper ocean result in considerable cast-to-cast variability in MLD, with changes of up to 200 m over 10 km horizontal distance. In contrast, the heat content over a fixed-depth interval of the upper ocean shows greater cast-to-cast stability and clearly measures the ocean response to surface heat fluxes. In particular, an annual cycle in upper ocean heat content is in good agreement with the annual cycle in heat flux forcing, which explains ∼24% of the variance in heat content above 400 m depth north of the Polar Front and ∼63% of the variance in heat content south of the Polar Front.

High-latitude ocean and sea ice surface fluxes

Abstract: Polar regions have great sensitivity to climate forcing; however, understanding of the physical processes coupling the atmosphere and ocean in these regions is relatively poor. Improving our knowledge of high-latitude surface fluxes will require close collaboration among meteorologists, oceanographers, ice physicists, and climatologists, and between observationalists and modelers, as well as new combinations of in situ measurements and satellite remote sensing. This article describes the deficiencies in our current state of knowledge about air–sea surface fluxes in high latitudes, the sensitivity of various high-latitude processes to changes in surface fluxes, and the scientific requirements for surface fluxes at high latitudes. We inventory the reasons, both logistical and physical, why existing flux products do not meet these requirements. Capturing an annual cycle in fluxes requires that instruments function through long periods of cold polar darkness, often far from support services, in situations subject to icing and extreme wave conditions. Furthermore, frequent cloud cover at high latitudes restricts the availability of surface and atmospheric data from visible and infrared (IR) wavelength satellite sensors. Recommendations are made for improving high-latitude fluxes, including 1) acquiring more in situ observations, 2) developing improved satellite-flux-observing capabilities, 3) making observations and flux products more accessible, and 4) encouraging flux intercomparisons.

High-latitude ocean and sea ice surface fluxes: requirements and challenges for climate research

Abstract: Improving knowledge of air-sea exchanges of heat, momentum, fresh water, and gases is critical to understanding climate, and this is particularly true in high-latitude regions, where anthropogenic climate change is predicted to be exceptionally rapid. However, observations of these fluxes are extremely scarce in the Arctic, the Southern Ocean, and the Antarctic marginal seas. High winds, high sea state, extreme cold temperatures, seasonal sea ice, and the remoteness of the regions all conspire to make observations difficult to obtain. Annually averaged heat-flux climatologies can differ by more than their means, and in many cases there is no clear consensus about which flux products are most reliable. Although specific flux accuracy requirements for climate research vary depending on the application, in general fluxes would better represent high-latitude processes if wind stresses achieved 0.01Nm accuracy at high wind speed and if heat fluxes achieved 10 W m accuracy (averaged over several days) with 25 km grid spacing. Improvements in flux estimates will require a combination of efforts, including a concerted plan to make better use of ships of opportunity to collect meteorological data, targeted efforts to deploy a few flux moorings in high-wind regions, and improved satellite retrievals of flux-related variables. Short summary: Air-sea fluxes at high latitudes are critical for evaluating climate processes, but they have been sparsely measured in the past and the physics is not yet well understood.

Diurnal variability of upper ocean temperatures from microwave satellite measurements and Argo profiles

Abstract: [1] Diurnal temperature variability in the top 50 m of the ocean is assessed by pairing Argo temperature profiles with geographically colocated Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) sea surface temperatures (SSTs) collected within ±24 h of each other. Data pairs with time separations of up to ±3 h are used to evaluate systematic differences between the two data sets. Daytime SSTs are warmer than Argo 5 m temperatures in low-wind conditions, as expected due to diurnal surface warming effects. SSTs also tend to be warmer than Argo 5 m temperatures when columnar water vapor is less than ∼7 mm. These effects are removed empirically. For Argo data collected within ±24 h of satellite overpass times, temperature differences between Argo and AMSR-E show evidence of a diurnal cycle detectable at 5 m depth and below. The diurnal amplitude decreases with increasing latitude and increasing depth to the base of the mixed layer and is stronger in summer than in winter. At 5 m depth, the amplitude of the summer diurnal cycle ranges from about 0.1°C at the equator to 0.05°C near 60° latitude. At latitudes where the diurnal amplitude exceeds about 0.04°C, maximum temperatures occur at about 16:50 ±0:40 local time, and minimum temperatures occur at about 07:50 ±0:40 local time. Above the base of the mixed layer, the time of the diurnal maximum increases with depth, consistent with downward propagation of the diurnal signal, while the time of the minimum implies an upward propagation.

Spatial Variation in Turbulent Heat Fluxes in Drake Passage

Abstract: High-resolution underway shipboard atmospheric and oceanic observations collected in Drake Passage from 2000 to 2009 are used to examine the spatial scales of turbulent heat fluxes and flux-related state variables.Themagnitudeof the seasonalcycle ofsea surface temperature (SST) southof the Polar Frontis found to be twice that north of the front, but the seasonal cycles of the turbulent heat fluxes show no differences on either side of the Polar Front. Frequency spectra of the turbulent heat fluxes and related variables are red, with no identifiable spectral peaks. SST and air temperature are coherent over a range of frequencies corresponding to periods between ;10 h and 2 days, with SST leading air temperature. The spatial decorrelation length scales of the sensible and latent heat fluxes calculated from two-day transects are 65 6 6 km and 80 6 6 km, respectively. The scale of the sensible heat flux is consistent with the decorrelation scale for air‐sea temperature differences (70 6 6 km) rather than either SST (153 6 2 km) or air temperature (138 6 4 km) alone. These scales are dominated by the Polar Front. When the Polar Front region is excluded, the decorrelation scales are 10‐20 km, consistent with the first baroclinic Rossby radius. These eddy scales are often unrepresented in the available gridded heat flux products. The Drake Passage shipmeasurementsarecomparedwithfourrecentlyavailablegriddedturbulentheatfluxproducts:theEuropean Centre for Medium-Range Weather Forecasts high-resolution operational product in support of the Year of Coordinated Observing Modeling and Forcasting Tropical Convection (ECMWF-YOTC), ECMWF interim reanalysis (ERA-Interim), the Drake Passage reanalysis downscaling (DPRD10) regional product, and the objectively analyzed air‐sea fluxes (OAFlux). The decorrelation length scales of the air‐sea temperature difference, wind speed, and turbulent heat fluxes from these four products are significantly larger than those determined from shipboard measurements.

The Diapycnal and Isopycnal Mixing Experiment: A First Assessment

Abstract: The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) was designed as a multi-pronged US and UK CLIVAR effort to measure and to better understand diapycnal mixing and along-isopycnal eddy transport in the Antarctic Circumpolar Current (ACC), because these processes together appear to play a key role in the Meridional Overturning Circulation (MOC) (Gille et al, 2007). The project represents an unusual effort to evaluate simultaneously the roles of diapycnal and isopycnal mixing, and the program has benefited from close collaboration between observationalists, theoreticians and modelers. Fieldwork for DIMES began in early 2009, and the initial phase of the field observations is now wrapping up. This article provides a brief preliminary summary of early DIMES findings.

Wind-Driven Variability of the Subtropical North Pacific Ocean

Abstract: The Argo array provides a unique dataset to explore variability of the subsurface ocean interior. This study considers the subtropical North Pacific Ocean during the period from 2004 to 2011, when Argo coverage has been relatively complete in time and space. Two distinct patterns of Argo dynamic height transport function () are observed: in 2004/05, the gyre is stronger, and in 2008/09 it is weaker. The orientation of the subtropical gyre also shifts over time: the predominantly zonal major axis shifts to a more northwest–southeast orientation in 2004/05 and to a more southwest–northeast orientation in 2008/09.The limited temporal range of the Argo observations does not allow analysis of the correlation of ocean transport and wind forcing in the basin for the multiyear time scale (6–8-yr period) typical of the dominant gyre patterns. The meridional geostrophic transport anomaly between 180° and 150°E is computed both from Argo data (0–2000 db) and from the Sverdrup relation (using reanalysis winds...