ORIGINAL PAPER Impact of ocean warm layer thickness on the intensity of hurricane Katrina in a regional coupled model Hyodae Seo • Shang-Ping Xie Received: 30 November 2012 / Accepted: 20 August 2013 / Published online: 3 September 2013 Ó Springer-Verlag Wien 2013 Abstract The effect of pre-storm subsurface thermal structure on the intensity of hurricane Katrina (2005) is examined using a regional coupled model. The Estimating Circulation and Climate of Ocean (ECCO) ocean state estimate is used to initialize the ocean component of the coupled model, and the source of deficiencies in the sim- ulation of Katrina intensity is investigated in relation to the initial depth of 26 °C isotherm (D26). The model under- estimates the intensity of Katrina partly due to shallow D26 in ECCO. Sensitivity tests with various ECCO initial fields indicate that the correct relationship between intensity and D26 cannot be derived because D26 variability is under- estimated in ECCO. A series of idealized experiments is carried out by modifying initial ECCO D26 to match the observed range. A more reasonable relationship between Katrina’s intensity and pre-storm D26 emerges: the inten- sity is much more sensitive to D26 than to sea surface temperature (SST). Ocean mixed layer process plays a critical role in modulating inner-core SSTs when D26 is deep, reducing mixed layer cooling and lowering the center pressure of the Katrina. Our result lends strong support to the notion that accurate initialization of pre-storm subsur- face thermal structure in prediction models is critical for a skillful forecast of intensity of Katrina and likely other intense storms. 1 Introduction The amount of upper ocean thermal energy, hereafter referred to as the upper ocean heat content (UOHC, Leipper and Vol- genau 1972), is the primary energy source term for the development of hurricanes. The UOHC is determined by the temperature integrated from the surface to the depth of the 26 °C isotherm (D26), i.e., UOHCðx; yÞ¼ q o C p R sfc D26 ðT ðx; y; zÞ 26Þ dz, where, D26 represents an approximate thickness of the upper ocean warm layer, q o is the density of seawater (1,025 kg m -3 ) and C p is the specific heat at constant pressure (4 9 10 3 J kg -1 °C -1 ). The reference temperature, 26 °C, is the typical near-surface air temperature in the sub- tropical atmosphere (Price 2009). Since tropical cyclones mostly form over surface water with temperature of 26 °C or higher, T(sfc) - 26 °C, the upper bound of the integration, represents a thermal disequilibrium between the air and sea, resulting in an enthalpy transfer to the hurricane. Conse- quently, the higher equivalent potential temperature (h e ) in the lower atmosphere reduces the storm’s central pressure (Kleinschmidt 1951; Riehl and Malkus 1961; Riehl 1963). During the typical hurricane seasons in the Gulf of Mexico (GoM), the temperatures at the sea surface and subsurface are rather distinct, making it difficult to detect the latter from the former (e.g., Goni and Trinanes 2003). As suggested from the recent studies, information on the pre-storm spatial distribution of the subsurface thermal structure has an important implication to the prediction of storm intensity, whereby in situ ocean mixed layer (OML) dynamics bridge these two (e.g., Halliwell et al. 2008; Lin Responsible editor: J. Fasullo. H. Seo (&) Physical Oceanography Department, Woods Hole Oceanographic Institution, 266 Woods Hole Road, MS#21, Woods Hole, MA 02543, USA e-mail: hseo@whoi.edu S.-P. Xie Scripps Institution of Oceanography, University of California San Diego, 9500 Gilman Drive # 0206, La Jolla, CA 92093, USA 123 Meteorol Atmos Phys (2013) 122:19–32 DOI 10.1007/s00703-013-0275-3