3. Evaluation of the new initialization procedure
b. Comparison to AXBT profiles and another ocean data assimilation technique
3) TCHP COMPARISON AND RMSE
Figure 8 presents the TCHP calculated at each AXBT location (1) from the GDEM climatology, (2) after data-assimilation, (3) after data2-assimilation, (4) from RSMAS HYCOM, and (5) from the AXBT itself. At both locations east of the LC (i.e. AXBTs #1 and #2), all five TCHP values are within 10 kJ cm-2 of each other. Inside the LC (i.e. AXBT locations 3-9), the differences increase dramatically. The GDEM values inside the LC remain between 43 and 52 kJ cm-2. Data-assimilation, however, yields values inside the LC as high as 124 kJ cm-2, which is supported by the AXBT profiles (e.g. AXBT #5 TCHP = 128 kJ cm-2). Data2-assimilated values inside the LC are ~2-5 kJ cm-2 lower than the corresponding data-assimilated values. RSMAS HYCOM yields LC values as high as 106 kJ cm-2, but this maximum occurs further west than the AXBTs and the data-assimilated profiles suggest. Considering now AXBT locations 1-13, which includes WCR1, it is apparent that the TCHP has only one peak in RSMAS HYCOM, whereas the AXBT and data-assimilated TCHP values are bimodal. Focusing on the RSMAS HYCOM overestimate of TCHP at the location of AXBT #10 suggests that RSMAS HYCOM fails to adequately separate WCR1 from the LC. In fact, while the data-assimilated TCHP reveals a coherent LC and ring structure (Fig. 9a), RSMAS HYCOM indicates a more complex spatial variation of TCHP in the GoM (Fig. 9b).
Recall that AXBTs #14 and #15 are located in CCR1. Although neither of these AXBTs is near the center of CCR1, AXBT #14 is used as CCR1PROFILE because it is the best data available. Hence, the data-assimilated profile overestimates the TCHP at the location of AXBT #14 by 18 kJ cm-2, whereas the GDEM climatology and RSMAS HYCOM only overestimate the TCHP at this location by 7 kJ cm-2. Although the RSMAS HYCOM TCHP is more accurate than the data-assimilated TCHP at this location, recall that the shape of the RSMAS HYCOM profile is inconsistent with the shape suggested by the AXBT (Fig. 5b). To decrease the data-assimilated TCHP at the location of AXBT #14, CCR1PROFILE could be adjusted by decreasing the upper ocean temperature during the data-assimilation phase (i.e. modifying the CCR1PROFILE values given in Table 2).
In the northern GoM off the southeastern Louisiana coast (i.e. locations of AXBTs #16-18), RSMAS HYCOM overestimates the TCHP. In the case of AXBT #17, RSMAS HYCOM’s TCHP is 46 kJ cm-2 too large, while GDEM’s TCHP and the data-assimilated TCHP are only 5 kJ cm-2 and 1 kJ cm-2 too large, respectively. Further investigation is needed to determine why RSMAS HYCOM overestimates the TCHP in this region and whether this result is case-specific or systematic. Figure 9b reveals that this RSMAS HYCOM TCHP overestimate may be due to a spurious small-scale WCR that is partially attached to the northern end of a poorly-defined LC.
To obtain a crude but concise picture of the overall error associated with each initialization technique in the GoM, it is helpful to calculate the TCHP root-mean-squared error (rmse) based on the temperature profiles at all 18 AXBT locations (Table 4). Taking the AXBT profiles to be truth, the TCHP rmse associated with the GDEM climatology, after data assimilation, after data2-assimilation, and with RSMAS HYCOM is 44.8, 9.5, 10.2, and 26.0 kJ cm-2, respectively. Thus, the data-assimilated simulations provide considerably more accurate initialization of the main mesoscale features in the GoM than either the GDEM climatology or RSMAS HYCOM does.