Jay A. Austin

Rapid and highly variable warming of lake surface waters around the globe

In this first worldwide synthesis of in situ and satellite-derived lake data, we find that lake summer surface water temperatures rose rapidly (global mean = 0.34°C decade−1) between 1985 and 2009. Our analyses show that surface water warming rates are dependent on combinations of climate and local characteristics, rather than just lake location, leading to the counterintuitive result that regional consistency in lake warming is the exception, rather than the rule. The most rapidly warming lakes are widely geographically distributed, and their warming is associated with interactions among different climatic factors—from seasonally ice-covered lakes in areas where temperature and solar radiation are increasing while cloud cover is diminishing (0.72°C decade−1) to ice-free lakes experiencing increases in air temperature and solar radiation (0.53°C decade−1). The pervasive and rapid warming observed here signals the urgent need to incorporate climate impacts into vulnerability assessments and adaptation efforts for lakes.

The Inner Shelf Response to Wind-Driven Upwelling and Downwelling*

Abstract A two-dimensional numerical model is used to study the response to upwelling- and downwelling-favorable winds on a shelf with a strong pycnocline. During upwelling or downwelling, the pycnocline intersects the surface or bottom, forming a front that moves offshore. The characteristics of the front and of the inner shelf inshore of the front are quite different for upwelling and downwelling. For a constant wind stress the upwelling front moves offshore at roughly a constant rate, while the offshore displacement of the downwelling front scales as t because the thickness of the bottom layer increases as the front moves offshore. The geostrophic alongshelf transport in the front is larger during downwelling than upwelling for the same wind stress magnitude because the geostrophic shear is near the bottom in downwelling as opposed to near the surface in upwelling. During upwelling, weak stratification is maintained over the inner shelf by the onshore flux of denser near-bottom water. This weak stratif...

A global database of lake surface temperatures collected by in situ and satellite methods from 1985–2009

Global environmental change has influenced lake surface temperatures, a key driver of ecosystem structure and function. Recent studies have suggested significant warming of water temperatures in individual lakes across many different regions around the world. However, the spatial and temporal coherence associated with the magnitude of these trends remains unclear. Thus, a global data set of water temperature is required to understand and synthesize global, long-term trends in surface water temperatures of inland bodies of water. We assembled a database of summer lake surface temperatures for 291 lakes collected in situ and/or by satellites for the period 1985–2009. In addition, corresponding climatic drivers (air temperatures, solar radiation, and cloud cover) and geomorphometric characteristics (latitude, longitude, elevation, lake surface area, maximum depth, mean depth, and volume) that influence lake surface temperatures were compiled for each lake. This unique dataset offers an invaluable baseline perspective on global-scale lake thermal conditions as environmental change continues.

The relationship between synoptic weather systems and meteorological forcing on the North Carolina inner shelf

A strong relationship is observed between synoptic weather systems and atmospheric forcing of the ocean as estimated from buoy measurements made on the North Carolina inner shelf during August and October-November 1994 as part of the Coastal Ocean Processes (CoOP) Inner Shelf Study. Synoptic variation (timescales of days to weeks) in the meteorological time series was primarily associated with the passage of atmospheric frontal systems. The most common synoptic weather pattern observed was the passage of a low-pressure center to the north of the study site, which caused the associated cold front to pass over the study region. Before passage of the cold front, warm, moist northeastward winds increased the heat flux into the ocean, whereas after the cold front passed, cold, dry southwestward winds decreased the heat flux into the ocean. In addition, in the presence of oceanic stratification, northeastward winds drove coastal upwelling, bringing colder water to the surface, further increasing the air-sea temperature contrast and hence the heat flux into the ocean inshore of the surface front between cool upwelled water and warmer water offshore. The decrease in surface heat flux during the passage of a cold front was of order 400 W m−2, due primarily to a decrease in latent heat flux. Although other synoptic patterns were observed, including one warm front passage and two tropical storm systems, the dominance of cold fronts as a source of variability resulted in a strong positive correlation between the along-shelf component of wind stress and the surface heat flux. To address the issue of spatial variation in the surface heat fluxes, data from several different sources located along a cross-shelf transect were analyzed. This analysis suggests that the temperature of the atmospheric boundary layer undergoes adjustment when warm air blows over cold water but not when cold air blows over warm water. This produces cross-shelf gradients in the bulk estimates of turbulent heat fluxes during offshore winds but not during onshore winds.

Wind-forced Variations in Sea Surface Height in the Northeast Pacific Ocean

Abstract Sea surface height (SSH) anomalies from the Geosat altimeter for the northeast Pacific Ocean were analysed to determine their annual and interannual fluctuations over a 2.5-year period. The interannual anomalies suggested large-scale changes in the intensity of the California and Alaska currents, with a weak California Current for the first year (1986–1987), which strengthened during the second year, partly by a diversion of flow from the Alaskan gyre into the California Current and partly by a decrease in SSH along the coast. In the California Current between about 36° and 46°N, the annual fluctuations in SSH showed westward phase propagation. These observations were modeled using a linearized potential vorticity equation with one active layer, forced by realistic wind stress curl, which resembled a standing wave. The annual fluctuations in SSH were produced primarily by Ekman pumping, because Rossby waves are coastally trapped poleward of about 37°N. The predicted response had excellent phase a...

The Design and Testing of Small, Low-cost GPS-tracked Surface Drifters

We have designed and constructed drifters appropriate for use in regions characterized by limited horizontal extent. The drifters follow the upper 30 cm of surface currents, but can be modified to follow water at any depth. The drifters store their latitude and longitude internally, and transmit their current latitude and longitude to a handheld Global Positioning System (GPS)-radio receiver, making their location and subsequent retrieval straight forward A field test of six drifters in Hog Island Bay, Virginia, United States, in August 2003 was successful and led to several design improvements. With simple construction and a total materials cost of under U.S.

Drifter behavior on the Oregon-Washington Shelf during downwelling-favorable winds

200 this design will make drifters an accessible part of interdisciplinary experiments, provide a potentially valuable educational tool, and make experiments that require large numbers of drifters more cost-effective.

The role of the alongshore wind stress in the heat budget of the North Carolina inner shelf

Abstract Drifters released offshore of Oregon during predominantly downwelling favorable alongshore winds during three different deployments (October 1994, January 1998, and September 1998) display similar behavior: after being advected around in the offshore eddy field, they move onshore to a particular isobath and are advected poleward alongshore, without coming ashore. Numerical modeling results suggest that this may be due to downwelling circulation creating a marginally stable density gradient on the shelf inshore of the downwelling front, thereby increasing the vertical eddy diffusivity, which reduces the effective cross-shelf Ekman transport to nearly zero. The downwelling front itself is accompanied by a poleward jet, which carries drifters rapidly to the north. This behavior is consistent with previous modeling results.

Entrainment of Shelf Water by a Bifurcating Continental Boundary Current

The heat budget of a cross-shelf section extending 16 km offshore of the outer banks of North Carolina is studied during two time periods: August 1994 and October 1994, using data collected as part of the Coastal Ocean Processes (CoOP) Inner Shelf Study. Heat budgets are computed on two different timescales: monthly averages over August and October, which reflect seasonal variations, and a fluctuation budget, which reflects variation on daily to weekly timescales. During August, a period of strong stratification, the increase in the area-averaged water temperature (approximately 3.2°C) was due primarily to the surface heat flux. Fluctuations in temperature during August were caused primarily by the cross-shelf heat flux, due to wind-driven upwelling and downwelling circulation. In October, the area-averaged shelf temperature dropped by approximately 3.5°C due to both surface heat loss and the alongshore transport of heat. Weak vertical stratification in October led to small cross-shelf heat fluxes, and temperature fluctuations in October were due primarily to fluctuations in the surface and alongshore heat fluxes. In both August and October, variation on daily to weekly timescales of the area-averaged temperature of the shelf was strongly correlated with the alongshore component of the wind stress. In August, alongshore poleward winds caused upwelling and the area-averaged temperature decreased; conversely, equatorward winds caused downwelling and warming. In October, although the variations in temperature were smaller, alongshore winds were positively correlated with alongshore currents and the surface heat flux (for reasons discussed by Austin and Lentz [this issue]), so that poleward winds resulted in warming; conversely, equatorward winds resulted in cooling. Therefore the dependence of the change in heat content on the alongshore wind stress changed sign between August and October. A simple dynamical model was constructed to relate changes in heat content to the alongshore wind stress. The model results were compared to 12 years of meteorological records from the Coastal Engineering Research Centers Field Research Facility, directly onshore of the experimental site. The results suggest a seasonal cycle in the dominant fluctuating heat balance, consistent with the field results found for August and October 1994. In May through August, cross-shelf flux dominates variation in the heat content. In October through March, the surface heat flux and alongshore heat flux dominate the variation.

A mechanical treatment of the leaky faucet experiment

Abstract At the northeast corner of Taiwan the direction of the continental slope isobaths changes rapidly relative to the oncoming Kuroshio, so that the inertia of a small inshore fraction of this current causes it to cross the slope, while the main branch follows the isobaths. It is suggested that the portion of the bifurcated current entering the shelf displaces ambient water of relatively high potential vorticity as a countercurrent, which flows across the slope. The vortex stretching and subsequent entrainment of this water into the main branch of the Kuroshio increases its maximum cyclonic vorticity and helps to maintain the inshore shear of the western boundary current. This Idea is supported by simple initial value and steady-state models, and also by dye observations of the flow from a source on the wall of a rotating tank.