a state-of-the-art gridded monthly full-depth climatology of temperature and salinity and their standard deviations (Teague et al., 1990; Carnes, 2009; Carnes et al., 2010). GDEM represents the monthly climatological averages of ocean temperature and salinity as a function of depth and location around the globe and is based on analyses of quality-controlled in situ profile observations throughout the historical record. These measurements rely primarily on expendable bathythermographs (XBTs, instruments that measure temperature at depth), conductivity-temperature-depth (CTD) data, and Argo float data sets. Moderate Resolution Imaging Spectroradiometer (MODAS) satellite records may also provide a better source for estimates of the sea surface temperature (SST) climatology than HYCOM, as the MODAS SST is a daily analysis of infrared satellite estimates of surface temperature (Barron and Kara, 2006; Kara and Barron, 2007; and Kara et al., 2009).
As an alternative to the methodology put forth by the assessment group, the committee used GDEM and MODAS to construct fields of the mean monthly temperatures averaged over all years from 1993 to 2010. The combined GDEM and MODAS data sets can be queried to find not only mean monthly SSTs over regions with subsurface temperatures below 4°C, but also average 4°C isotherm depth in regions where monthly mean SST exceeds certain thresholds (21°C, 24°C, and 27°C, for example). Performance predictions on monthly and seasonal timescales could be done with HYCOM (Hurlburt et al., 2008; Chassignet et al., 2009). However, the OTEC assessment group has not made such statistics accessible in its GIS.
The committee feels that the resource assessment should include an investigation of temperature variability that accounts for tidal variations, seasonal variations, and ENSO timescales. The assessment group’s database currently contains only a summer and winter season contrast that was constructed from averaging two anomalous (El Niño/La Niña) years. Including more years in the model run, especially years without El Niño or La Niña, would allow for a better representation of the ocean environment. El Niño and La Niña occur on 3- to 6-year timescales, so approximately a decade of data would be needed to catch both instances. It is these longer-term signals that a planner would need for evaluation of a site beyond the seasonal cycle, and the 2-year average from the assessment group does not even allow exploration of the seasonal signal. The committee also notes that variations in isotherm depth due to internal tides can be significant near islands. For example, deep isotherm displacements of as much as 50 or even 100 m are common near the Hawaiian Islands (Klymak et al., 2008), which could induce a 5-10 percent variation in power output over the tidal cycle from an OTEC plant situated there. In addition, areas with strong internal tides will also impose strong