Assessing the Climate Footprint of Tropical Cyclones: Pertinent Players or Irrelevant Pawns?

One of the great remaining unanswered questions in tropical meteorology is why there are approximately 90 tropical cyclones (TCs) globally, on average, per year as opposed to 10, 1000, or 10000 TCs. In contrast to extratropical cyclones whose annual frequency can be roughly calculated given the large scale characteristics of the mid-latitudes, there is no equivalent theory that even justifies the order of magnitude of TCs that occur globally each year. In spite of this, there appears to be a preferential spacing of approximately 1500-2000 km between TCs during multiple TC episodes in the Eastern North Pacific, North Atlantic, and Western North Pacific possibly suggesting that the number of storms in each basin is constrained energetically by the environment. Reconciling these issues is fundamentally rooted in determining the role of TCs within the climate. Building upon previous research (e.g. Sobel and Camargo 2005, Hart et al. 2007), the following study seeks to take a preliminary step in addressing these questions by quantifying the spatiotemporal scales over which TCs and the large scale environment interact. Four-dimensional, storm-relative composites of raw variables, raw anomalies, and normalized anomalies for Western North Pacific TCs are utilized in the analysis presented here.

Preliminary results show that the passage of a TC may initially be responsible for cooling and drying their environment both on local and large scales. Specifically, the passage of a major TC excites positive normalized mean sea-level pressure anomalies of over 0.3σ for upwards of a week stretching nearly 6000 km zonally and over 3000 km meridionally. The location of these anomalies near the expected core of the intertropical convergence zone in the composite domain together with a weakening of the trade winds suggests that these anomalies are indicative of a slackening of the Hadley Cell following TC passage. The reduced vigor of the Hadley Cell may be in direct response to potentially significant poleward moisture and heat transports by TCs during a time of year in which contributions from baroclinic eddies are relatively weak.

Within the region through which the TC directly passed, anomalies strongly manifest themselves as a drying of the lower and middle tropospheric environment. The spatial distribution of the moisture and temperature anomalies immediately surrounding the TC track suggests that the suppression of convection, potentially due to the underlying sea surface temperature cold wake induced by the TC, is the predominant factor in anomaly maintenance. Furthermore, the periodic pulsation in the magnitude of the dry anomalies, approximately every 10 days following TC passage, appears to be in response to the passage of the TC itself that further serves to increase the stabilization of the atmospheric environment. In their totality, these results suggest that TCs serve as an efficient mechanism for regulating atmospheric instability locally within the tropics and globally through substantial moist static energy transports from the tropics to the mid-latitudes.