Assimilating mesoscale oceanic features and understanding their impact on hurricanes

Richard Michael Yablonsky, University of Rhode Island

Abstract

A hurricane develops and is maintained by heat energy it receives from the ocean surface. If the sea surface temperature (SST) decreases within the storm core, so does the heat energy available to the storm. Wind stress imposed on the upper ocean by the storm can limit the storm's intensity primarily via shear-induced vertical mixing of the upper ocean and subsequent SST cooling. Since vertical mixing is a one-dimensional (1-D) process, some recent studies suggest that coupling a 1-D ocean model to a hurricane model may be sufficient for capturing the SST cooling beneath the storm core. Using a 1-D and 3-D version of the same ocean model, it is shown that neglecting upwelling, which can only be captured by a 3-D ocean model, underestimates the storm-core SST cooling when the storm's translation speed is <5 m>s-1. Historically, the majority of hurricanes in the western tropical North Atlantic Ocean have translated at <5 m>s-1, underscoring the importance of 3-D ocean models for coupled hurricane-ocean model forecasting, even in the absence of mesoscale oceanic features. ^ An oceanic warm core ring (WCR) has a deeper surface mixed layer than the surrounding environment. Therefore, previous studies generally assume that the presence of a WCR is favorable for hurricane intensification because a deeper surface mixed layer should restrict SST cooling. After developing a feature-based procedure to assimilate WCRs (and other mesoscale oceanic features) into the initialization of a coupled hurricane-ocean model, the aforementioned assumption regarding the impact of WCRs is tested. One significant result, using the 3-D version of the ocean model, indicates that when a WCR is located to the right (left) of the storm track, the background anticyclonic circulation in the WCR can advect the storm's cold wake along (opposite) the storm track, thereby increasing (decreasing) the storm-core SST cooling and potentially decreasing (increasing) hurricane intensity relative to the case where no WCR is present. Like upwelling, horizontal advection cannot be captured by a 1-D ocean model, and its impact on storm-core SST cooling and hurricane intensity varies with storm translation speed. Therefore, a 1-D ocean model is inadequate for coupled hurricane-ocean model forecasts of slow-moving hurricanes and/or hurricanes encountering a WCR. ^

Subject Area

Physical Oceanography|Atmospheric Sciences

Recommended Citation

Richard Michael Yablonsky, "Assimilating mesoscale oceanic features and understanding their impact on hurricanes" (2009). Dissertations and Master's Theses (Campus Access). Paper AAI3368008.
http://digitalcommons.uri.edu/dissertations/AAI3368008

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