3. Evaluation of the new initialization procedure
b. Comparison to AXBT profiles and another ocean data assimilation technique
1) DATA-ASSIMILATED AND RSMAS HYCOM PROFILE COMPARISON
AXBT #1 is located outside of the eastern branch of the LC (Fig. 1d or 2a). According to Fig. 3a, the initial GDEM climatology has an OML that is ~0.5°C too cold and an upper thermocline (i.e. above 150-m depth) that is ~2-3°C too warm. After data assimilation, however, the OML temperature is nearly identical to the AXBT OML temperature, and the upper thermocline is only ~1-2°C too warm. In both cases, the OML depth is ~10 m too shallow, but this error may be due to the relatively coarse vertical resolution in the climatology (i.e. no levels between 30 m and 50 m depth; see section 2). By comparison, the RSMAS HYCOM OML is nearly identical to the GDEM climatology OML (i.e. ~0.5°C too cold relative to the AXBT OML), and although the RSMAS HYCOM extreme upper thermocline (~50-70-m depth) is quite accurate, the vertical temperature gradient below 70 m is too large, so that the temperature at > 80-m depth is ~3°C too cold.
AXBTs #2-5 progressively approach the LC center line along a path that begins with AXBT #1 and ends near the center line between the two LC branches (Fig. 1d or 2a). For AXBT #2 (Fig. 3b), the climatological and data-assimilated OMLs are accurate, but the upper thermocline is too warm because the temperature gradient at the base of the OML is too gradual. The RSMAS HYCOM OML is also accurate, as is the temperature in the extreme upper thermocline (~40-70-m depth), but again the vertical temperature gradient below 70 m is too large, so the temperature at > 70-m depth is too cold. For AXBT #3 (Fig. 3c), the climatological profile from the surface to 150-m depth is too cold. Data-assimilation improves the profile from climatology, but the OML is still slightly too shallow and cold, and the gradual temperature gradient at the base of the OML causes the temperature below 70-m depth to be ~1°C too warm. The RSMAS HYCOM OML is similar to climatology (too cold and shallow), and the temperature below the OML is even colder than climatology. For AXBT #4 (Fig. 3d), climatology and RSMAS HYCOM are similar to each other, but both are up to 6°C too cold in the upper thermocline. Data-assimilation, however, improves the profile dramatically from climatology, creating an accurate OML depth and upper thermocline temperature. For AXBT #5 (Fig. 3e), climatology is again up to 6°C too cold in the upper thermocline, while RSMAS HYCOM is up to 5°C too cold in the upper thermocline. Data-assimilation produces a relatively accurate profile, but the upper thermocline temperature is up to 2°C too cold below ~100-m depth.
AXBTs #6-10 progressively approach the northwestern edge of a path that begins near the LC center line (Fig. 1d or 2a). For AXBT #6 (Fig. 3f), climatology is up to 6°C too cold in the upper thermocline. Data-assimilation yields an accurate OML depth and temperature, but the temperature gradient is slightly too gradual at the OML base and slightly too large below this level. RSMAS HYCOM’s OML depth is relatively accurate, but the temperature is 1°C too cold, and poor vertical resolution inhibits proper representation of the OML base. For AXBT #7 (Fig. 4a) and AXBT #8 (Fig. 4b), the results are similar to AXBT #6. For AXBT #9 (Fig. 4c), climatology is up to 5°C too cold in the upper thermocline. Data-assimilation yields an accurate profile, except that it is up to ~1°C too cold below 110-m depth. RSMAS HYCOM is ~1°C too cold in the OML and ~1°C too warm below 75-m depth. For AXBT #10 (Fig. 4d), climatology is up to 4°C too cold in the upper thermocline. Data-assimilation yields an OML that is slightly too deep and an upper thermocline temperature that is ~1°C too warm. RSMAS HYCOM’s upper thermocline temperature is ~3°C too warm.
AXBTs #11 and 12 are located near the northwestern edge of an LCE (i.e. WCR1), and AXBT #13 is located near the center of that LCE (Fig. 1d or 2a). For AXBT #11 (Fig. 4e), climatology is up to 6°C too cold in the upper thermocline. Data-assimilation improves the profile so that the upper thermocline is at most 2°C too cold, but the OML is still slightly too shallow. RSMAS HYCOM yields an OML depth that is slightly too deep but a temperature gradient in the upper thermocline that is too large, yielding a profile that is up to 2°C too warm above 110-m depth and up to 3°C too cold below 110-m depth. For AXBT #12 (Fig. 4f), the results are similar to AXBT #11, but the data-assimilated profile is more accurate while the large RSMAS HYCOM temperature gradient in the upper thermocline renders that profile less accurate (> 4°C too cold at 150-m depth). For AXBT #13 (Fig. 5a), climatology is again up to 6°C too cold in the upper thermocline. Data-assimilation yields an accurate profile, except the OML depth is slightly too shallow. RSMAS HYCOM is similar to the data-assimilated profile, except the upper thermocline temperature is up to 2°C too cold.
AXBTs #14 and 15 are located in a CCR (i.e. CCR1) to the south of the aforementioned LCE (Fig. 1d or 2a). For AXBT #14 (Fig. 5b), climatology has an OML that is too cold and deep and an upper thermocline that is up to 5°C too warm. Data-assimilation improves the OML and upper thermocline temperatures, but the OML is still too deep. RSMAS HYCOM has a submerged OML located below an unsubstantiated temperature gradient near the surface and an upper thermocline temperature that is too warm. For AXBT #15 (Fig. 5c), climatology again has an OML that is too cold, but the upper thermocline is only ~1°C too warm. Data-assimilation yields an even more accurate profile. RSMAS HYCOM has an OML that is slightly too cold, a temperature in the ~30-70-m depth layer that is ~2°C too warm, and a temperature below 70-m depth that is ~2°C too cold.
AXBTs #16-18 are located near the southeastern Louisiana coastline, north of the LC and LCE (Fig. 1d or 2a). For AXBT #16 (Fig. 5d), climatology has an OML that is too shallow and an upper thermocline temperature that is up to ~2°C too cold. Data-assimilation improves the upper thermocline temperature, but the OML is still too shallow. RSMAS HYCOM has an OML that is too warm and shallow and a sharp temperature gradient in the upper thermocline that causes the profile to become too cold below ~95-m depth. For AXBT #17 (Fig. 5e), climatology is accurate from the surface to ~65-m depth and is slightly too cold below this depth. Data-initialization slightly degrades the profile by making it up to ~1°C colder than climatology in the upper thermocline. RSMAS HYCOM’s OML is far too deep and warm (up to 4°C), and the temperature gradient in the upper thermocline is again too large, yielding a temperature at ~110-m depth that is nearly 5°C too cold. For AXBT #18 (Fig. 5f), the results are similar to AXBT #17, but the difference between the data-assimilated and AXBT profiles is greater while the difference between the RSMAS HYCOM and AXBT profiles is less.