Figure 1. The pre-Maemi SSHA map (based on the 30 August-8 September 2003 cycle of TOPEX/Poseidon and Jason-1 measurements) showing the warm ocean eddy (characterized by a positive SSHA of 10-45 cm). Maemi's track, intensity (on the Saffir-Simpson scale, illustrated as color bullets) and radius of maximum wind are also shown. The storm position is denoted every 6h. (After Lin et al., 2005.)

Using a simple coupled typhoon-ocean model, i.e. the CHIPS (Coupled Hurricane Intensity Prediction System) model (Emanuel, 1999; Emanuel et al., 2004), numerical experiments were conducted to assess the influence of the warm eddy on the intensity evolution of Maemi. Numerical experiments are run with and without the input of the eddy information derived from the satellite SSHA field. The run without the eddy input (denoted as CTRL) uses monthly climatological ocean mixed layer depth (Levitus, 1982). The one with the eddy input (denoted as EDDY) uses one cycle (ten days) of the observed pretyphoon satellite SSHA measurement as input to an algorithm developed by

Shay et al. (2000) to estimate a new mixed layer depth (Emanuel et al., 2004) (Fig. 2). The besttrack intensity data from the JTWC are also shown in Fig. 2(a). All the runs were initialized and tuned according to the best-track data for the first 24 h.

As in Fig. 2(a), it is evident that the intensity evolutions, including the eddy-adjusted mixed layer depth, are much closer to the best-track intensity than the run without the eddy (i.e. CTRL) in both the magnitude and timing of the peak intensity. The peak intensity is 68ms_1 (i.e. category 4) for the CTRL run, but the observed peak is 77ms_1, occurring at 0300 UTC, 10 September [Fig. 2(a)]. Maemi

Figure 2. Results of the CHIPS runs showing the two primary experiments, i.e. CTRL (a controlled run using standard CHIPS input) and EDDY (a run incorporating the warm ocean eddy information in SSHA). JTWC's besttrack intensity is shown in black. (a) The intensity (ms_1 ) evolution. (b) The typhoon-induced sea surface temperature anomaly at the storm center. (After Lin et al., 2005.)

Figure 2. Results of the CHIPS runs showing the two primary experiments, i.e. CTRL (a controlled run using standard CHIPS input) and EDDY (a run incorporating the warm ocean eddy information in SSHA). JTWC's besttrack intensity is shown in black. (a) The intensity (ms_1 ) evolution. (b) The typhoon-induced sea surface temperature anomaly at the storm center. (After Lin et al., 2005.)

reaches its maximum intensity in CTRL at 1800 UTC, 9 September, 9 h earlier than the OBS. When the warm eddy is included to initialize a deeper mixed layer in CHIPS, an improvement in the intensity hindcast is evident. The peak intensity for the EDDY run reaches the category 5 scale of 75ms_1, well matched by the observed best-track intensity peak. The timing for peak intensity is also correctly captured [Fig. 2(a)].

Further analysis of the CHIPS results finds that the reason why the EDDY run can match well with the observed intensity is that the upper ocean thermal structure in the warm eddy is correctly represented by the satellite SSHA in the EDDY run. As can be seen in Fig. 3, in the eddy region, warm water of ^ 26°C extends well downward to about 120-130 m. In contrast, outside the eddy region the subsurface warm layer is much shallower, so that the warm water of ^ 26°C extends only to 40 m or so (i.e. the background). Also, it is noted that the difference between the eddy and the background is not in the SST, but in the subsurface thermal structure. As can be seen in Fig. 3, both background and eddy profiles show similar SST

Ocean Eddy Structure
Figure 3. The ocean's depth-temperature profiles (based on the US Naval Research Lab's NPACNFS ocean model; Ko et al., 2003) in the warm ocean eddy and the reference background region. (After Lin et al., 2005.)

values of around 28.2°C. In other words, what makes the difference is the thickness of the subsurface warm layer, but not the SST.

Figure 2(b) shows that with the very deep warm mixed layer in the warm eddy, typhoon-induced ocean surface cooling is significantly suppressed. The typhoon-induced ocean cooling is only < 0.5°C [dashed curve in Fig. 2(b)] throughout the intensification period before the storm reaches peak intensity at 0300 UTC, 10 September. As a consequence, the storm is able to intensify without being hampered by the cooler sea surface temperature induced by the typhoon, as the negative feedback mechanism (Emanuel, 1999, Bender and Ginis, 2000; Emanuel et al., 2004) is reduced in the presence of the warm eddy. In contrast, in the CTRL run (solid curve), the self-induced ocean cooling is much stronger, with the SST anomaly -1.5-2.5°C [Fig. 2(b)], during the intensification period (0000 UTC, 5 September, to the peak at 0300 UTC, 10 September) without the warm eddy and its deep warm mixed layer. The increased cooling of the sea surface in the CTRL experiment contributes to the reduction of the maximum wind speed by —10 ms-1 of the simulated storm, as is evident in Fig. 2(a) (solid curve).

In the current literature, three other cyclone-warm ocean feature interaction cases in the Atlantic (i.e. hurricanes Opal, Mitch, and Bret) have been studied using atmosphere-ocean coupled models (Hong et al., 2000; Emanuel et al., 2004). Opal (1995) was simulated by Hong et al. (2000) using the U.S. Naval Research Laboratory's Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) as the atmospheric component and the Geophysical Fluid Dynamics Laboratory's Modular Ocean Model, version 2 (MOM2) as the ocean component. The other two cases, Mitch (1998) and Bret (1999), were run using the CHIPS model (Emanuel et al., 2004). Table 1 compares the results of these three Atlantic cases with the Maemi case.

In Table 1, it is clear that in all cases without the warm ocean eddy information, the intensity is too low as compared to the besttrack intensity. With the addition of the eddy information in the numerical experiments, the predicted intensity is evidently improved. In the case of Opal, the peak of the without-eddy simulation reaches only category 4 (i.e. 932-hPa). When the eddy is included, the peak intensity reaches 916 hPa, in agreement with the besttrack peak at category 5 (Hong et al., 2000). Consistent results are also found in Mitch, Bret, and Maemi: without the inclusion of the ocean feature in the simulation, the intensity is about 26-31% lower (typically one category lower) as compared to the best-track peak (Table 1). With the inclusion of the eddy information in the simulation, the peak intensity can be correctly simulated (Table 1).

3. The Ocean's Biogeochemical Response to the Typhoon (Based on Lin et al., 2003b)

When passing over land, tropical cyclones can affect human lives and activities. Over the ocean, they can also affect other forms of life, i.e. ocean primary production. As introduced in Sec. 1, ocean primary production plays a significant role in the Earth's ecological and environmental system, especially because it affects the uptake of the important greenhouse gas carbon dioxide (Eppley and Peterson, 1979; Eppley, 1989; Behrenfeld and Falkowski, 1997; Bates et al., 1998; McGillicuddy et al., 1998; Uz et al., 2001; Babin et al., 2004). Primary production takes place mainly in the euphotic zone of the ocean, i.e. the top 50-150 m of the water column where there is abundant light for photosynthesis and when nutrients are available. Marine nutrients, however, are mostly located in the deeper ocean. Therefore, the vertical entrainment due to mixing and the induced upwelling in the ocean, caused by tropical cyclone winds, are crucial mechanisms

Table 1. Comparison of the intensification parameters based on coupled model results for Maemi (2003), Opal (1995), Bret (1999), and Mitch (1998).

Was this article helpful?

0 0

Post a comment