T he record-setting 2005 hurricane season has highlighted the urgent need for a better understanding of the factors that contribute to hurricane intensity, and for the development of corresponding advanced hurricane prediction models to improve intensity forecasts. The lack of skill in present forecasts of hurricane intensity may be attributed, in part, to deficiencies in the current prediction models-insufficient grid resolution, inadequate surface and boundary-layer formulations, and the lack of full coupling to a dynamic ocean. The extreme high winds, intense rainfall, large ocean waves, and copious sea spray in hurricanes push the surface-exchange parameters for temperature, water vapor, and momentum into untested regimes.The Coupled Boundary Layer Air-Sea Transfer (CBLAST)-Hurricane program is aimed at developing improved parameterizations using observations from the CBLAST-Hurricane field program (described by Peter Black and colleagues elsewhere in this issue) that will be suitable for the next generation of hurricane-prediction models. The most innovative aspect of the CBLAST-Hurricane modeling effort is the development and testing of a fully coupled 1 atmo- AIR-SEA INTERACTION AND HURRI- CANES.Hurricanes rarely reach their maximum potential intensity (MPI, as defined by Kerry Emanuel and Greg Holland). Many factors can prevent a given storm from reaching MPI, including environmental vertical wind shear, distribution of troposphere water vapor, hurricane internal dynamics, and air-sea interactions. The effect of air-sea interactions on hurricane structure and intensity change is the main focus of the CBLAST-Hurricane program. Intensification of a hurricane depends upon two competing processes at the air-sea interface-the heat and moisture fluxes that fuel the storm and the dissipation of kinetic energy associated with wind stress on the ocean surface. Air-sea interaction is especially important within the extremely high winds (up to 75 m s -1 ) and strong gradient zones of temperature and pressure located in the inner core (eye and eyewall) of a hurricane. The enthalpy and momentum exchange coefficients under the extreme high-wind conditions are, of course, very difficult to determine in precisely the regions where they are most important. The stress is supported mainly by waves in the wavelength range of 0.1-10 m, which are an unresolved "spectral tail" in present wave models. In the November 1995 Journal of the Atmospheric Sciences, Emanuel proposed that storm intensity is largely controlled by the ratio of the air-sea enthalpy MARCH 2007 BAPIS^ | 15 els-are noted in a schematic in Fig. 1. A specific issue we emphasize here is the determination and parameterization of the air-sea momentum and enthalpy fluxes in conditions of extremely high and timevarying hurricane winds. FIG. I.Schematics of a coupled atmosphere-wave-ocean modeling system with the component atmosphere, surface wave, and ocean circulation models, as well as the coupling parameter exchanges between each of the component models.and mome...

Oceanic mesoscale eddies with horizontal scales of 50–300 km are the most energetic form of flows in the ocean. They are the oceanic analogues of atmospheric storms and are effective transporters of heat, nutrients, dissolved carbon, and other biochemical materials in the ocean. Although oceanic eddies have been ubiquitously observed in the world oceans since 1960s, our understanding of their three-dimensional (3D) structure, generation, and dissipation remains fragmentary due to lack of systematic full water-depth measurements. To bridge this knowledge gap, we designed and conducted a multi-months field campaign, called the South China Sea Mesoscale Eddy Experiment (S-MEE), in the northern South China Sea in 2013/2014. The S-MEE for the first time captured full-depth 3D structures of an anticyclonic and cyclonic eddy pair, which are characterized by a distinct vertical tilt of their axes. By observing the eddy evolution at an upstream versus downstream location and conducting an eddy energy budget analysis, the authors further proposed that generation of submesoscale motions most likely constitutes the dominant dissipation mechanism for the observed eddies.

Profiles of current velocity, temperature, and salinity were obtained in the Internal Wave and Mixing Experiment in the South China Sea (SCS), the Luzon Strait, and the North Pacific. The observations are examined for evidence of enhanced diapycnal mixing in the SCS, which reaches O(10 23 m 2 s 21 ) in magnitude. Results from independent casts reveal that diapycnal diffusivity in the SCS and the Luzon Strait is elevated by two orders of magnitude over that of the smooth bathymetry in the North Pacific, which are typical of background values in an open ocean. The vertical distribution of diapycnal diffusivity is nonuniform in the SCS, exhibiting higher values at depths greater than about 1000 m. This result compares favorably with the direct microstructure measurements at four stations in the SCS. Velocity and density profiles are combined to estimate the internal tide energy flux generated in the Luzon Strait and directed into the SCS. The energy amounts to 10 GW, most of which is rationalized to be the potential energy source for enhanced mixing in the SCS.

Sheddings of Kuroshio Loop Current (KLC) eddies in the northeastern South China Sea (SCS) are investigated using mooring arrays, multiple satellite data, and data-assimilative HYCOM products. Based on altimeter sea surface heights between 1992 and 2014, a total of 19 prominent KLC eddy shedding (KLCES) events were identified, among which four events were confirmed by the concurrent moored and satellite observations. Compared to the leaping behavior of Kuroshio, KLCES is a relatively short-duration phenomenon that primarily occurs in boreal autumn and winter. The KLC and its shedding anticyclonic eddy (AE) trap a large amount of Pacific water with high temperature–salinity and low chlorophyll concentration in the upper layer. The corresponding annual-mean transport caused by KLCES reaches 0.24–0.38 Sv (1 Sv ≡ 106 m3 s−1), accounting for 6.8%–10.8% of the upper-layer Luzon Strait transport. Altimeter-based statistics show that among ~90% of the historical KLCES events, a cyclonic eddy (CE) is immediately generated behind the AE southwest of Taiwan. Both energetics and stability analyses reveal that because of its large horizontal velocity shear southwest of Taiwan, the northern branch of KLC is strongly unstable and the barotropic instability of KLC constitutes the primary generation mechanism for the CE. After CE is generated, it quickly grows and gradually migrates southward, which in turn facilitates the detachment of AE from KLC. The intrinsic relationship between KLC and CE explains well why eddy pairs are commonly observed in the region southwest of Taiwan.