Excitation of equatorial Kelvin and Yanai waves by tropical cyclones in an ocean general circulation model

R L Sriver,L Chafik,M Huber

doi:10.5194/esd-4-1-2013

Abstract

Abstract. Tropical cyclones (TCs) actively contribute to the dynamics of Earth's coupled climate system. They influence oceanic mixing rates, upper-ocean heat content, and air–sea fluxes, with implications for atmosphere and ocean dynamics on multiple spatial and temporal scales. Using an ocean general circulation model with modified surface wind forcing, we explore how TC winds can excite equatorial ocean waves in the tropical Pacific. We highlight a situation where three successive TCs in the western North Pacific region, corresponding to events in 2003, excite a combination of Kelvin and Yanai waves in the equatorial Pacific. The resultant thermocline adjustment significantly modifies the thermal structure of the upper equatorial Pacific and leads to eastward zonal heat transport. Observations of upper-ocean temperature by the Tropical Atmosphere Ocean (TAO) buoy array and sea-level height anomalies using altimetry reveal wave passage during the same time period with similar properties to the modeled wave, although our idealized model methodology disallows precise identification of the TC forcing with the observed waves. Results indicate that direct oceanographic forcing by TCs may be important for understanding the spectrum of equatorial ocean waves, thus remotely influencing tropical mixing and surface energy budgets. Because equatorial Kelvin waves are closely linked to interannual variability in the tropical Pacific, these findings also suggest TC wind forcing may influence the timing and amplitude of El Niño events.

Highlights

Tropical cyclones (TCs) are important contributors to the dynamics of Earth’s coupled system
It is possible that thermodynamic effects associated with vertical mixing and wake recovery may play a role in wave formation, but our methodology cannot adequately account for these effects due to lack of ocean–atmosphere coupling
The modeled waves are consistent with observations from the Tropical Atmosphere Ocean (TAO) array and sea-level anomalies (SLA) derived from altimetry, providing evidence that transient wind forcing by TCs can influence the zonal redistribution of heat in the upper equatorial Pacific ocean via Kelvin wave excitation

Summary

Introduction

Tropical cyclones (TCs) are important contributors to the dynamics of Earth’s coupled system. Because trends in TC activity are generally linked to basin-scale surface temperature variability (Emanuel, 2005; Sriver and Huber, 2006) and inter-basin surface temperature differences (Vecchi et al, 2008), feedbacks may exist in the coupled system. These feedbacks have been shown to be potentially important in maintaining past climates with characteristics much different than present conditions, such as permanent El Nino-like conditions of the early Pliocene (Fedorov et al, 2010) or the unusually warm conditions during the Eocene (Korty et al, 2008). While recent observational studies indicate TCs are important contributors to upper-ocean tropical mixing budgets

Methods

Results

Conclusion