Examining Tropical Cyclone — Kelvin Wave Interactions using Adjoint Diagnostics

Adjoint-based tools can provide valuable insight into the mechanisms that influence the evolution and predictability of atmospheric phenomena. An adjoint model can be used for the efficient and rigorous computation of forecast sensitivity to changes in the initial state, as well as for the calculation of leading singular vectors, which are the fastest growing perturbations to a given forecast trajectory. We apply adjoint-based tools from the non-hydrostatic Coupled Atmosphere/Ocean Mesoscale Prediction System (COAMPS) to explore the initial-state sensitivity and interactions between a tropical cyclone and atmospheric equatorial waves associ-ated with the Madden Julian Oscillation (MJO) in the Indian Ocean during the DYNAMO field campaign. The development of Tropical Cyclone 5 (TC05) coincided with the passage of an equatorial Kelvin wave and westerly wind burst associated with an MJO that developed in the Indian Ocean in late November 2011, but it was unclear if and how one affected the other. COAMPS 24-h and 36-h adjoint sensitivities are analyzed for both TC05 and the equatorial waves to understand how the evolution of each system is sensitive to the other. The sensitivity of equatorial westerlies in the western Indian Ocean on 23 November shares characteristics with the classic Gill (1980) Rossby and Kelvin wave response to symmetric heating about the equator, including symmetric cyclonic circulations to the north and south of the westerlies, and enhanced heating in the area of convergence between the equatorial westerlies and easterlies. In addition, there is sensitivity in the Bay of Bengal associated with the cyclonic circulation that eventually develops into TC05. At the same time, the developing TC05 system shows strongest sensitivity to local wind and heating perturbations, but sensitivity to the equato-rial westerlies is also clear. On 24 November, when the Kelvin wave is immediately south of the developing tropical cyclone, located off the southern tip of India, both phenomena are sensitive to each other, which is not unexpected, given their proximity. On 25 November, when the first Kelvin wave has moved east into the Bay of Bengal, and TC05 has moved west into the Arabian Sea, TC05 no longer shows sensitivity to the Kelvin wave, while the Kelvin Wave still exhibits some weak sensitivity to TC05. In summary, it appears that one system does not dominate the evolution of the other, but that both work synergistically to strengthen each other before and dur-ing their colocation. The similarity of the sensitivity pattern to the classic simple-model equato-rial wave pattern supports the utility of adjoint diagnostics applied to tropical phenomena.