No abstract

Over the past two decades the primary driver of mass loss from the West Antarctic Ice Sheet (WAIS) has been warm ocean water underneath coastal ice shelves, not a warmer atmosphere. Yet, surface melt occurs sporadically over low-lying areas of the WAIS and is not fully understood. Here we report on an episode of extensive and prolonged surface melting observed in the Ross Sea sector of the WAIS in January 2016. A comprehensive cloud and radiation experiment at the WAIS ice divide, downwind of the melt region, provided detailed insight into the physical processes at play during the event. The unusual extent and duration of the melting are linked to strong and sustained advection of warm marine air toward the area, likely favoured by the concurrent strong El Niño event. The increase in the number of extreme El Niño events projected for the twenty-first century could expose the WAIS to more frequent major melt events.

The authors analyze 105 yr of tropical cyclone strikes at 45 coastal locations from Brownsville, Texas, to Eastport, Maine, with the primary objective of examining spatiotemporal patterns of storm activity. Interpretation of the data suggests that geographically, three focal points for activity are evident: south Florida, the Outer Banks of North Carolina, and the north-central Gulf Coast. Temporally, clusters of hyperactivity are evident in south Florida from the 1920s through the 1950s and then again during the most recent years. North Carolina was a region of enhanced activity in the 1950s and again in the 1990s. A more consistent rate of occurrence was found along the north-central Gulf Coast; the last two years, however, were active in this region. Return periods of tropical storm strength systems or greater range from a frequency of once every 2 yr along the Outer Banks of North Carolina, every three years on average in southeast Texas, southeastern Louisiana, and southern Florida, and about once every 10-15 yr in northern New England. Hurricane return periods range from 5 yr in southern Florida to 105ϩ years at several sheltered portions of the coastline (e.g., near Cedar Key, Florida, Georgia, and the northeastern seaboard), where some locations experienced only one strike, or no strikes through the entire period of record. Severe hurricane (category 3-5) return periods range from once every 15 yr in South Florida to 105ϩ in New England.

Hurricane Gustav (2008) made landfall in southern Louisiana on 1 September 2008 with its eye never closer than 75 km to New Orleans, but its waves and storm surge threatened to flood the city. Easterly tropical-storm-strength winds impacted the region east of the Mississippi River for 12–15 h, allowing for early surge to develop up to 3.5 m there and enter the river and the city’s navigation canals. During landfall, winds shifted from easterly to southerly, resulting in late surge development and propagation over more than 70 km of marshes on the river’s west bank, over more than 40 km of Caernarvon marsh on the east bank, and into Lake Pontchartrain to the north. Wind waves with estimated significant heights of 15 m developed in the deep Gulf of Mexico but were reduced in size once they reached the continental shelf. The barrier islands further dissipated the waves, and locally generated seas existed behind these effective breaking zones. The hardening and innovative deployment of gauges since Hurricane Katrina (2005) resulted in a wealth of measured data for Gustav. A total of 39 wind wave time histories, 362 water level time histories, and 82 high water marks were available to describe the event. Computational models—including a structured-mesh deepwater wave model (WAM) and a nearshore steady-state wave (STWAVE) model, as well as an unstructured-mesh “simulating waves nearshore” (SWAN) wave model and an advanced circulation (ADCIRC) model—resolve the region with unprecedented levels of detail, with an unstructured mesh spacing of 100–200 m in the wave-breaking zones and 20–50 m in the small-scale channels. Data-assimilated winds were applied using NOAA’s Hurricane Research Division Wind Analysis System (H*Wind) and Interactive Objective Kinematic Analysis (IOKA) procedures. Wave and surge computations from these models are validated comprehensively at the measurement locations ranging from the deep Gulf of Mexico and along the coast to the rivers and floodplains of southern Louisiana and are described and quantified within the context of the evolution of the storm.

[1] Wave propagation on the Louisiana inner shelf is studied using concurrent measurements at two shallow water ocean observatory sites, in sedimentary environments dominated by mud at one site, and sand at the other. Contrary to the widely accepted hypothesis that mud-induced wave dissipation is important only for long waves, observations show significant damping of high-frequency, short waves, which interact weakly with the bottom. The mechanism of short wave dissipation is not understood. Numerical simulations show that other dissipative processes such as refraction, or depthlimited breaking, do not account for the magnitude of the observed effects. Independent observations of strong sediment re-working during storms suggest that these effects are related to sediment resuspension processes.