NOWCAST: PAPERS OF NOTE

INTENSIFICATION OF OCEAN FRONTS

Many fronts in the ocean, such as the Gulf Stream and the fronts comprising the Antarctic Circumpolar Current, experience strong local atmospheric forcing by down-front winds (i.e., winds blowing parallel to the front, in the direction of the frontal jet). An analytic theory and nonhydrostatic numerical simulations demonstrate the mechanism by which spatially uniform, down-front winds lead to frontogenesis. The frontogenesis mechanism does not require that the wind stress has negative curl (or any curl, for that matter), as is the case with classic Ekman frontogenesis. Nor does it require a horizontal, confluent flow field, and thus is distinct from frontogenesis mechanisms based on the frontal model of Hoskins and Bretherton.

Frontal intensification by down-front winds is a consequence of nonlinear Ekman dynamics arising from advection of momentum and density by Ekman flow. When wind blows down a front, Ekman flow advects dense water over lighter water and destabilizes the water column, which leads to convection along the front. Mixing of buoyancy by this convection drives an ageostrophic secondary circulation (ASC), consisting of slantwise overturning cells, that accelerates the frontal jet. The vorticity contrast of the jet induces Ekman pumping, which strengthens the destabilizing density advection, subsequent convective mixing, and jet-accelerating ASC. Repetition of this process leads to frontal intensification, with stronger winds producing faster frontogenesis. For mixed layers with negative potential vorticity (PV), in which two-dimensional disturbances trigger instabilities rather than the inertia-gravity waves common in mixed layers with positive PV, the most frontogenetic ASCs do not translate with the Ekman transport, but instead remain stationary. Vertical circulation is characterized by down-welling on the dense side of the front and upwelling along the frontal interface.

Cross-front sections of density, potential vorticity, and velocity at the subpolar front of the Japan/ East Sea suggest that frontogenesis by down-front winds is active during cold-air outbreaks and could result in strong vertical circulation. Along fronts forced by down-front winds, Ekman-driven ASCs and convective mixing could play an important role in the formation and downward transport of mode waters-waters with low PV that retain properties reflective of the climatic conditions at the time of their formation.-LEIF N. THOMAS (UNIVERSITY OF WASHINGTON) AND CRAIG M. LEE. "Intensification of Ocean Fronts by Down-Front Winds," in the June Journal of Physical Oceanography.

SEA SPRAY EFFECTS ON MIDLATITUDE CYCLONES

Air-sea transfers of momentum and enthalpy have long been recognized as important elements for generating and maintaining hurricanes. Turbulent transfer processes over the ocean are commonly parameterized using Monin-Obukhov similarity theory, which assumes horizontal homogeneity, steady flow, and no flux divergence in the boundary layer. However, high-wind hurricane conditions produce large amounts of sea spray when air bubbles burst in the whitecaps and spume tears from the wave crests. Consequently, both turbulence and sea spray provide routes by which moisture, heat, and momentum cross the air-sea interface.

Although the question as to whether or how sea spray affects the evolution of hurricanes has been around for a long time, the answer has remained elusive. Most recent studies of sea spray focus on tropical cyclones because of their high wind speeds, high sea surface temperatures beneath them, and the role of sea surface fluxes. Our study instead simulates extratropical Hurricanes Earl (1998) and Danielle (1998), and an intense winter cyclone from January 2000, denoted Superbomb, using a mesoscale atmospheric model coupled to a recent sea spray parameterization. Results suggest that sea spray can increase the sea surface heat flux, especially the latent heat flux, in a midlatitude cyclone. They also suggest that the effect of the sea spray on cyclone intensity depends on storm structure and development, and is strongest for cyclones with high winds.

The figure compares simulations with and without sea spray for the three storms. In each case, sea spray proliferates within a few hours after the simulations begin, and ultimately deepens the central pressure, resulting in an increase in winds. Ongoing research considers the influence of surface waves, and sea surface temperature changes induced by the hurricane, in competition with sea spray.-WILL PERRIE (BEDFORD INSTITUTE OF OCEANOGRAPHY AND DALHOUSIE UNIVERSITY), EDGAR L. ANDREAS, WEIQING ZHANG, WEIBIAO LI, JOHN GYAKUM, AND RON MCTAGGARTCOWAN. "Sea Spray Impacts on Intensifying Midlatitude Cyclones," in the June Journal of the Atmospheric Sciences.

HUMIDITY OVER THE WEST FLORIDA SHELF

Latent heat flux from the ocean to the atmosphere fuels the Earth's climate engine, and humidity is a primary factor in determining this flux. To better understand the humidity variability in a subtropical coastal ocean environment, we describe the annual cycle in relative humidity (RH) using four years of meteorological measurements from an array of moorings on the West Florida Continental Shelf (WFS). In determining the cycle, we found that the two different meteorological packages used to collect the data exhibit an offset in the monthly average RH and that the RH in winter sometimes exceeds 100%.

Despite considerable daily and synoptic variability within seasons, the monthly mean values of RH in the eastern Gulf of Mexico are nearly constant at about 75%. Summertime specific humidity is twice that during winter, suggesting that high air temperatures are responsible for the low summer monthly mean RH. Winter and early spring have the greatest RH variability; values range from less than 50% to over 100% as extratropical fronts move over the WFS. High values are observed ahead of slow-moving or stationary fronts, as southerly winds advect warm, moist air over colder water. During this time the RH can exceed 100%, and we observe supersaturation values of up to 3%. Dense fog may form in this dynamically stable atmospheric boundary layer. As the front passes, veering winds become northerly, bringing cold, dry air into the region, and RH decreases by 40%-50%.

Two different RH sensors, mounted on multiple moorings, make the observations. The monthly mean RH values from the Rotronics sensor are consistently higher than the Hygrometrix sensor. While this may partially be due to sensor differences, a contributing cause appears to be the locations chosen for sensor deployment on the WFS. The Rotronics sensors are positioned farther north and closer to shore than the Hygrometrix sensors and are in different air-sea regimes. Therefore, air-sea fluxes over the WFS are sensitive to small spatial variability in the coastal ocean and atmosphere. The coarse grid spacing of the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) Reanalysis does not resolve this variability. The lack of coastal ocean data for assimilation biases the NCEP-NCAR Reanalysis RH fields toward land-based measurements where a seasonal cycle is more prominent.

Increased spatial coverage of measurements is required to fully understand coastal ocean air-sea interactions, and hence to correctly model the coastal ocean, thereby justifying a need for Coastal Ocean Observing Systems.-J. I. VIRMANI (UNIVERSITY OF SOUTH FLORIDA) AND R. H. WEISBERG. "Relative Humidity over the West Florida Continental Shelf," in the June Monthly Weather Review.

"EAST GREENLAND SPILL JET" DISCOVERED

One of the fundamental components of the global ocean circulation is the so-called meridional overturning cell, which strongly impacts Earth's climate. In the cell, surface water flows poleward and sinks due to buoyancy loss to the atmosphere, then returns equatorward as deep flow. The newly transformed dense water enters the North Atlantic through the shallow Denmark Strait, between Iceland and Greenland, and subsequently sinks to the base of the continental slope in the Irminger Sea. However, not all of the water flowing through Denmark Strait sinks; some of it remains on the shelf east of Greenland. A new finding reveals that a portion of this dense shelf water subsequently cascades over the shelf break and forms a gravity current that sits atop the overflow water along the upper continental slope.

This feature has been dubbed the "East Greenland spill jet." It was discovered during a shipboard survey of the shelf and slope in August 2001 using conductivity/ temperature/depth measurements and acoustic Doppler velocity profiles. While the spill jet is shallower and lighter than the well-known overflow current, it transports roughly 2 million m^sup 3^ of water per second-comparable to the volume flux of overflow water through Denmark Strait. The spill jet is extremely narrow (less than 10 km wide, which partly explains why it has gone undetected until now), and the large lateral gradients in velocity and density suggest that it is prone to instability. Hence, the jet may offer clues to the mechanisms by which nearshore water is transferred into the interior of the basin.

Future research will investigate what causes the spilling-including the role of the atmospheric barrier winds east of Greenland-as well as the dynamics of the spill jet and its role in the overturning circulation of the subpolar North Atlantic.-ROBERT S. PICKART (WOODS HOLE OCEANOGRAPHIC INSTITUTION), DANIEL J. TORRES, AND PAULA S. FRATANTONI. "The East Greenland Spill Jet," in the June Journal of Physical Oceanography.

NEW LOOK AT SOIL CARBON STORAGE

After four growing seasons, soil below white oak trees exposed to elevated atmospheric carbon dioxide levels (ambient + 300 ppm) had an average of 14% more soil carbon than soil below trees exposed to ambient levels of carbon dioxide. The results were obtained using new methods for collecting and processing soil samples. These techniques yielded improved accuracy in our soil carbon measurements.

The heterogeneity inherent in soil makes quantifying the amount of carbon stored in soil due to CO2 fertilization an elusive problem. Other studies have failed to find statistically significant increases in soil carbon storage because the experiments did not remove litter and fine root fragments that increase soil carbon heterogeneity in the samples.

To overcome these challenges, we developed two new methods for sampling and processing soil. For sampling, we modified an existing soil probe to collect soil without compacting the sample. Our processing technique used flotation to remove litter and root fragments from the mineral soil. Using these improved methods, we measured soil carbon inventories in five soil cores collected from chambers with ambient carbon dioxide levels and in six soil cores collected from chambers with elevated carbon dioxide levels (ambient + 300 ppm) at the Global Change Field Research Site in Oak Ridge, Tennessee. The observed increase in soil carbon storage in the chambers exposed to elevated carbon dioxide levels could help explain why atmospheric carbon dioxide concentrations are increasing more slowly than expected. Results also could help improve predictions of future atmospheric carbon dioxide levels.

Future research includes examining how other ecosystems' soil carbon inventories change when their vegetation is exposed to elevated carbon dioxide levels and finding a way to compare the results of carbon dioxide enrichment experiments having different soil carbon turnover times, different levels of CO enrichment, and different lengths of exposure to elevated carbon dioxide levels. -KEVIN G. HARRISON (NORTHEASTERN UNIVERSITY), RICHARD J. NORBY, WILFRED M. POST, AND EMILY L. CHAPP. "Soil C Accumulation in a White Oak CO2-Enrichment Experiment via Enhanced Root Production," in Earth Interactions, vol. 8 (2004; available online at http:// earthinteractions.org).