![]() ![]() The integral spans from time t = 0 to 2P. Here, γ is the mixing efficiency taking into account that only a fraction of the wind work goes into mixing the upper ocean (e.g., ). ![]() Previous studies indicate that TC-induced mixing can penetrate as deep as ~100 m below the sea surface (e.g., ). The 20 ☌ isotherm for h 2 is convenient (but quite arbitrary) since h 2 ≈ h 1 over most of the region ( Figure 1). The choice of the 26 ☌ for h 1 seems reasonable since TCs rarely survive over SST < 26 ☌. (So that they can more precisely correspond to the chosen TC period, h 1 and h 2 are shown as weighted means with weights based on the monthly number of TC locations = (20, 2, 40, 33, 26, 29, 9, and 7) for 2015 (May, June, July, August, September, October, November, and December)). Shading and contours are h 1 and h 2 on the basis of the EN4 reanalysis data ( accessed on 23 September 2022). The TC data are from the IBTrACS dataset. The blue dots in Figure 1 show the TC locations in the western North Pacific where the maximum sustained speed V o ≥ 33 m/s (i.e., Category 1 and above on the Saffir–Simpson scale accessed on 2 January 2023), from May–December 2015. I take h 1 = Z 26 (>0), the 26 ☌ isothermal depth, and choose h 2 to be the thickness between the 26 ☌ and 20 ☌ isotherms, i.e., h 2 = Z 20 − Z 26, where Z 20 (>0) is the 20 ☌ isothermal depth. An inert, deep layer of infinite thickness is assumed to underlie Layer 2, but it does not participate in the mixing process. The model’s upper ocean consists of a near-surface Layer 1 of thickness h 1 and a lower Layer 2 of thickness h 2. However, as the TC has long passed, any wake cooling has little impact on intensity. In the wake, the SST may also be changed by upper-ocean convergences and divergences, as well as by mixing through the turbulence produced by the shears and the breaking of near-inertial internal waves. (In this manuscript, I shall omit the contribution of surface enthalpy loss). The drop in SST under the TC core, δT, is then caused more by vertical mixing rather than by upwelling. Moving at ~5 m/s, the direct and most violent impact on a fixed location of the sea from the TC core lasts for a few hours, generally less than the local inertial period of approximately 1 day. At a typical translation speed U h ≈ 5 m/s, a TC is moving supercritically, as far as the ocean response is concerned, such that U h/c > 1, where the ocean’s mode-1 phase speed c ≈ 3 m/s. The surface conditions, including the SST, directly under the TC core, say, over an area scaled approximately with the square of the radius of maximum wind (r mw ≈ 50 km), has the most impact on the storm’s intensity. Contributions from other parameters are less but not negligible. Tests show that the cube of the TC maximum wind and the ocean’s Z 26 account for 46% and 7%, respectively, of the observed variance, indicating their predominant influence on TC-induced cooling. ![]() The result yields a best-fit, linear relation between δT o and Ψ that explains ~60% of the observed variance: r 2 ≈ 0.6 (99% confidence). The modelled δT is validated against sea-surface cooling observed by satellites, δT o, for typhoons during the May–December 2015 period in the western North Pacific. ![]() The Ψ can be estimated from observations. Here, the author shows that δT (non-dimensionalized by some reference temperature) is linearly related to Ψ, a dimensionless (nonlinear) function of TC and ocean parameters: the TC maximum wind, radius, and translation speed, as well as the ocean’s 26 ☌ and 20 ☌ isothermal depths (Z 26 and Z 20). The decreased SST (δT) under the TC (rather than the cooled water in the wake after the storm has passed) modifies the storm’s intensity and is of interest to TC intensity studies. A major ocean response to tropical cyclone (TC) wind is the mixing of warm sea-surface water with cool subsurface water, which decreases the sea-surface temperature (SST). ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |