The ocean thermal field is often represented in hurricane-ocean interaction by a metric termed upper Ocean Heat Content (OHC), the vertical integral of ocean temperature in excess of 26°C. High values of OHC have proven useful for identifying ocean regions that are especially favorable for hurricane intensification. Nevertheless, it is argued here that a more direct and robust metric of the ocean thermal field may be afforded by a vertical average of temperature. In the simplest version, dubbed T₁₀₀, the averaging is from the surface to 100 m, a typical depth of vertical mixing by a category 3 hurricane. OHC and T₁₀₀ are well correlated over the deep open ocean in the high range of OHC, ≥75 kJ cm⁻². They are poorly correlated in the low range of OHC, ≤50 kJ cm⁻², in part because OHC is degenerate when evaluated on cool ocean regions, ≤26°C. OHC and T₁₀₀ can be qualitatively different also over shallow continental shelves: OHC will generally indicate comparatively low values regardless of the ocean temperature, while T₁₀₀ will take on high values over a shelf that is warm and upwelling neutral or negative. In so far as the ocean thermal field alone is concerned, these warm, shallow continental shelves would appear to be as favorable for hurricane intensification as are warm, deep ocean regions.

On 23 August, 2017, Typhoon Hato rapidly intensified by 10 kt within 3 h just prior to landfall in the city of Macau along the South China coast. Hato’s surface winds in excess of 50 m s⁻¹ devastated the city, causing unprecedented damage and social impact. This study reveals that anomalously warm ocean conditions in the nearshore shallow water (depth < 30 m) likely played a key role in Hato’s fast intensification. In particular, cooling of the sea surface temperature (SST) generated by Hato at the critical landfall point was estimated to be only 0.1–0.5 °C. The results from both a simple ocean mixing scheme and full dynamical ocean model indicate that SST cooling was minimized in the shallow coastal waters due to a lack of cool water at depth. Given the nearly invariant SST in the coastal waters, we estimate a large amount of heat flux, i.e., 1.9k W m⁻², during the landfall period. Experiments indicate that in the absence of shallow bathymetry, and thus, if nominal cool water had been available for vertical mixing, the SST cooling would have been enhanced from 0.1 °C to 1.4 °C, and sea to air heat flux reduced by about a quarter. Numerical simulations with an atmospheric model suggest that the intensity of Hato was very sensitive to air-sea heat flux in the coastal region, indicating the critical importance of coastal ocean hydrography.

The authors have designed and deployed a neutrally buoyant sediment trap (NBST) intended for use in the upper ocean. The aim was to minimize hydrodynamic flow interference by making a sediment trap that drifted freely with the ambient current. The principal design problem was to make the NBST descend to and stay near a prescribed depth. For a variety of reasons, the most success has been with NBSTs that were autoballasted by means of a microprocessor-controlled volume changer. Autoballasting NBSTs has demonstrated an ability to hold a prescribed depth to within 10 m. There have been two successful, concurrent deployments of NBSTs and conventional surface-tethered sediment traps (STSTs) at the Bermuda Atlantic Times Series site. During both periods the observed flow past the STSTs was low, about 0.05 m s−1, so that hydrodynamic effects on the STSTs would have been minimized. Comparisons of the trap results (described in a companion paper by Buesseler et al.) indicate that the total mass of collect... Abstract The authors have designed and deployed a neutrally buoyant sediment trap (NBST) intended for use in the upper ocean. The aim was to minimize hydrodynamic flow interference by making a sediment trap that drifted freely with the ambient current. The principal design problem was to make the NBST descend to and stay near a prescribed depth. For a variety of reasons, the most success has been with NBSTs that were autoballasted by means of a microprocessor-controlled volume changer. Autoballasting NBSTs has demonstrated an ability to hold a prescribed depth to within 10 m. There have been two successful, concurrent deployments of NBSTs and conventional surface-tethered sediment traps (STSTs) at the Bermuda Atlantic Times Series site. During both periods the observed flow past the STSTs was low, about 0.05 m s−1, so that hydrodynamic effects on the STSTs would have been minimized. Comparisons of the trap results (described in a companion paper by Buesseler et al.) indicate that the total mass of collect...