Ocean heat content variability in the Eastern Pacific Ocean for intensity forecasting

Recent evidence supports the premise that sub-surface ocean structure plays an important role in modulating air-sea fluxes during tropical cyclone (TC) passage which in turn affects intensity change. Given the generally sparse in situ data, it has been difficult to provide region to basin-wide estimates of isotherm depths and upper ocean heat content (OHC). In this broader context, satellite-derived sea surface height anomalies (SHA) from multiple platforms carrying radar altimeters are blended, objectively analyzed and combined with a hurricane season climatology to estimate isotherm depths and OHC within the context of a reduced gravity model at 0.25º spatial intervals in the Eastern Pacific Ocean where TC intensity change occurs.

Measurements from the Eastern Pacific Investigation of Climate in 2001, long-term Tropical Ocean Atmosphere mooring network and Volunteer Observing Ship deploying eXpendable Bathythermographs (XBTs) profilers are used to carefully evaluate satellite-based measurements of upper ocean variability. Regression statistics reveal small biases with slopes of 0.8 to 0.9 between the subsurface measurements compared with isotherm depths (20ºC and 26ºC) and OHC fields derived from objectively analyzed SHA field. Root-mean-square differences in OHC range between 10 to 15 kJ cm^-2 or roughly 10 to 15% of the mean signals. Similar values are found for isotherm depth differences between in situ and inferred satellite-derived values. Blended daily values are used in the Statistical Hurricane Intensity Prediction Scheme (SHIPS) forecasts as are OHC estimates for the Atlantic Ocean Basin. Equivalent oceanic heat content variability incorporates the strength of the thermocline at the base of the oceanic mixed layer using a stratification parameter which seems better correlated to hurricane intensity change than just anomalies as observed in Hurricane Juliette in 2001.