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History of OHC Research at the Rosenstiel School

Based on deliberations of the Prospectus Development Team 5 tasked by the National Oceanic and Atmospheric Administration (NOAA) and the National Science Foundation (NSF), improving our understanding of hurricane intensity requires knowledge of the

  1. atmospheric circulation;

  2. inner core and eyewall processes; and,

  3. upper-ocean circulation and ocean heat transport.

The Upper Ocean Dynamics Lab focuses on the air-sea interactions during atmospheric forcing events, particularly tropical cyclones (TC). While the oceanic energy source for TC has largely been known for more than half of a century, subsequent studies indicate that the maximum TC intensity is constrained by sea surface temperature (SST). Research on the SST response to TC was largely focused on the “negative” feedback aspects of how a cooled upper ocean decreases air-sea fluxes as the oceanic and lower atmospheric boundary layer temperatures approach equilibrium. Winds induce more stress on the upper ocean surface causing strong turbulent mixing across the base of oceanic mixed layer and to a lesser extent upwelling of the thermocline due to net wind-driven current transport away from the storm center. Shear-induced mixing effects deepen and cool the oceanic mixed layer (OML) as colder thermocline water is entrained from below. This entrainment heat flux subsequently causes the OML temperature (and by proxy the SST) to decrease, limiting air-sea heat and moisture fluxes that reduce TC intensity. This negative feedback mechanism is particularly effective when the OML depths are shallow or when storms become stationary for a few days. By contrast, in regimes where the OML (and the depth of 26°C water is deep), the OHC can be quite large.As this deeper OML has greater vertical extent, more turbulence is required to overturn and cool the deeper layers. A well-studied example of this effect is the response of Hurricane Opal (1995) that intensified rapidly as it crossed a warm core ring in the Gulf of Mexico under favorable atmospheric conditions. When Opal encountered this deeper, warmer oceanic regime, the storm unexpectedly intensified from a Category 1 to a Category 4 hurricane status in 14 hours, as atmospheric conditions were favorable. Sensitivity studies with a coupled ocean-atmosphere model showed that the central pressure of Opal was more than 10 hPa higher when the WCR was removed in the pre-storm state. Using a hurricane season (June through November) climatology, Mainelli (2000) extended the Opal investigations across the Atlantic Ocean basin including the Caribbean Sea and the Gulf of Mexico.

More recent examples included both hurricanes Katrina and Rita encountering the Loop Current and warm core ring in the central Gulf of Mexico in 2005. For both hurricanes, Shay (2008) showed that the sea-level pressure decreases were directly correlated to large values of the 26°C isotherm depth and OHC than just SSTs, which were essentially flat. Using the Statistical Hurricane Intensity Prediction Scheme (SHIPS: DeMaria et al. 2005), Mainelli et al. (2008) found that the OHC parameter contributed about 5 to 6% to the reduction in intensity forecasting using the seasonal climatological approach. Lin et al. (2009) showed that the translation speeds of typhoons may impact its intensity in that for fast movers (>6 m s-1) only 60 kJ cm-2 are necessary for intensification to severe status in the Western Pacific Ocean basin compared to more than 100 kJ cm-2 for slower moving typhoons. These findings support the premise that oceanic regimes with high OHC contribute to TC intensification where SST cooling is reduced (e.g., less negative feedback) beneath the storm and maintaining or enhancing surface sensible and latent heat fluxes.

Recent improvements to SHIPS have shown that OHC parameter has reduced intensity forecast errors in the Atlantic Ocean Basin. These studies show that a seasonal OHC climatology reduced forecast errors in intensity by an average of 2% when averaged over all storms between 1995 and 2003 compared to less than 1% from an annual analysis. In some cases, the reduction in intensity errors is considerably more dramatic as observed during hurricane Ivan in 2004. SHIPS with seasonal OHC showed as much as a 22% reduction in forecast intensity errors. This result points to the importance of OHC variability in the warm pool of the Caribbean Sea and the Gulf of Mexico’s Loop Current (LC) and warm core rings (WCR) as well as other eddy-rich regimes such as the western Pacific Ocean.

The 20°C isotherm depth is used as the level for a two-layer model to estimate reduced gravities (i.e., density differences between the upper and lower ocean layers). Radar altimetry data are merged and blended each day when new SHA data become available. Two or three sets of altimeter data are then objectively analyzed to the same grid as the hurricane season climatology derived from US Navy’s Generalized Digital Environmental Model (GDEM) for the two-layer model application. The resultant isotherm depths (particularly the 26°C isotherm) are used to estimate OHC when combined with sea surface temperature (SST) and a climatological OML depth over the hurricane season. The isotherm depths and OHC estimates are carefully compared to temperature structure measurements from the Tropical Atmosphere Ocean (TAO) mooring data spanning the Eastern Pacific Ocean equatorial wave guide, EPIC, and Volunteer Ship of Opportunity XBT transects in building an evaluated daily climatology.