An Examination of the Intensification of Storms over the Western North Atlantic Using QuikSCAT Winds

Building on previous work of Hoskins and Valdes (1990) and Lee and Kim (2003), Nakamura et al (2004) proposed that storm tracks are anchored to the oceanic frontal zone and that there is a feedback from the wind-controlled ocean fronts that drives the low-level baroclinicity in the atmosphere and the storm tracks. Using 7 years of QuikSCAT data we have examined storms and their intensification in the Gulf Stream region, which has been shown to be a local maximum in storm generation density (along with the Kuroshio Extension region in the North Pacific), as well as a global maximum in storm growth rate (Hoskins and Hodges (2002). This region of explosive storm growth has strong SST gradients, strong currents, deep ocean mixed layers, large air-sea fluxes, and large ocean heat transports, all of which may play a role in the air-sea interaction. The large air-sea heat fluxes are generally attributed in the literature to ``cold-air outbreaks," the passing of cold, dry continental air over the warm ocean. However, analyses by Nakamura (2004), and Kelly and Dong (2004) show that SST (or upper ocean heat content) is an important factor in the magnitude of the fluxes on interannual time scales.

To characterize storms we use and compare several metrics to define storm intensity, as in Hoskins and Hodges (2002): central pressure, pressure difference, and vorticity. A storm tracking algorithm is used to identify, follow, and quantify the storms from successive sea level pressure (SLP) fields. The SLP fields are derived from QuikSCAT winds using a boundary layer model (Pataux et al, 2007) and are then merged with SLP from ECMWF. Good correspondence was found between all of the intensity measures. These analyses confirm that the region of greatest storm intensification coincides with the warm core of the Gulf Stream. In addition, the greatest impact of the Gulf Stream occurs when the storm center is slightly north of the warmest water. Storms tend to move northeastward from the generation region near the Gulf Stream, toward Iceland. A secondary region of storm intensification occurs over the relatively warm North Atlantic Current.

A study goal is to determine the extent to which changes in the ocean state affect storm intensification, and therefore, the predictability of storm statistics. The relatively small number of storms in each year preclude a direct examination of interannual variations in storm intensification following a storm. However, following Nakamura and co-workers, we examine interannual variations in the variance of high-frequency meridional wind stress as a proxy for storm intensity. In the scatterometer data there is a strong variance maximum directly over the Gulf Stream mean path.

References

Hoskins, B. J. and Hodges, K. I., 2002: New perspectives on the Northern Hemisphere winter storm tracks, J. Atmos. Sci., 59, 1041--1061.

Hoskins, B. J., and P. J. Valdes, 1990: On the existence of storm tracks, J. Atmos. Sci., 47, 1854-1864.

Kelly, K.A., and S. Dong, 2004: The relationship of western boundary current heat transport and storage to mid-latitude ocean-atmosphere interaction, in Earth's Climate: The Ocean-Atmosphere Interaction, edited by C. Wang, S.-P. Xie, and J. A. Carton, pp. 347-363, American Geophysical Union Geophysical Monograph 147.

Lee, S., and H.-K. Kim, 2003: The dynamical relationship between subtropical and eddy-driven jets, J. Atmos. Sci., 60, 1490-1503.

Nakamura, H. T. Sampe, Y. Tanimoto, and A. Shimpo, 2004: Observed associations among storm tracks, jet streams and midlatitude oceanic fronts, Earth's Climate: the Ocean-Atmosphere Interaction, Geophysical Monograph Series 147, 329-345, 10.1029/147GM18.

Pataux, J., R. Foster, and R.A. Brown 2007: An evaluation of scatterometer pressure fields, J. Appl. Meteor. & Clim., in revision.>