AOML
NOAA

National Oceanic and Atmospheric Administration

Atlantic Oceanographic and Meteorological Laboratory

Physical Oceanography Division

Study Highlights Out-of-phase Relationship Between Tropical Cyclones in the North Atlantic and Eastern Pacific

By Chunzai Wang and Sang-Ki Lee

In articles published in the scientific journal of Geophysical Research Letters and the AGU Newspaper of EOS, AOML’s scientists Chunzai Wang and Sang-Ki Lee show that tropical cyclone activity in the North Atlantic varies out-of-phase with that in the eastern North Pacific on both interannual and multidecadal timescales. That is, when tropical cyclone activity in the North Atlantic increases (decreases), tropical cyclone activity in the eastern North Pacific decreases (increases). Both vertical wind shear and convective instability contribute to the out-of-phase relationship, whereas relative humidity and vorticity variations at the lower troposphere do not seem to cause the relationship.

Wang, C., and S.-K Lee, 2009: Covariability of tropical cyclones in the North Atlantic and the eastern North Pacific. GRL.

Wang, C., and S.-K Lee, 2010: Is Hurricane Activity in One Basin Tied to Another? Eos.

In the Western Hemisphere, tropical cyclones (TCs) can form and develop in both the tropical North Atlantic and eastern North Pacific oceans, which are separated by the narrow landmass of Central America. In comparison with TCs in the North Atlantic, TCs in the eastern North Pacific have received less attention although TC activity is generally greater in the eastern North Pacific than in the North Atlantic. Needless to say, few studies have focused on the relationship between TCs in the North Atlantic and eastern North Pacific oceans. A new study, published in the journal Geophysical Research Letters by AOML principle investigators Chunzai Wang and Sang-Ki Lee, shows that TC activity in the North Atlantic varies out-of-phase with that in the eastern North Pacific on both interannual and multidecadal time scales. That is, when TC activity in the North Atlantic increases (decreases), TC activity in the eastern North Pacific decreases (increases). An important implication is that seasonal hurricane outlooks could be improved by considering the North Atlantic and eastern North Pacific together.

This study investigates what causes the out-of-phase relationship between TC variability in the North Atlantic and eastern North Pacific. The regression analyses show that both vertical wind shear and convective stability in the main development region of the North Atlantic are opposite to those in the eastern North Pacific. That is, when vertical wind shear and atmospheric convective stability in the hurricane main development region of the North Atlantic are reduced, the wind shear and atmospheric convective stability in the eastern North Pacific are enhanced. These opposite wind shear and convective stability patterns in the main development regions of the North Atlantic and eastern North Pacific result in the out-of-phase relationship between North Atlantic and eastern North Pacific tropical cyclones. Relative humidity and vorticity variations in the lower troposphere do not appear to cause the out-of-phase relationship.

Time series of accumulated cyclone energy (ACE; depicted on the y-axis in units of 104 square knots) in the North Atlantic (NA) and eastern North Pacific (ENP) from 1949-2007. Shown are the (a) total, (b) multidecadal, and (c) interannual variations. The multidecadal variability is obtained by performing a seven-year running mean after the linear trend is removed from the ACE indices. The interannual variability is calculated by subtracting the multidecadal variability from the detrended ACE indices.

It is also shown that oceanic conditions contribute to the out-of-phase relationship: an active (inactive) North Atlantic TC year is associated with a cold (warm) tropical eastern Pacific and a warm (cold) tropical North Atlantic. A further calculation shows that the distribution of sea surface temperatures on interannual timescales result in a La Niña-like pattern in the Pacific and a warm tropical North Atlantic or a large Atlantic warm pool. On multidecadal timescales, the regression pattern of sea surface temperatures resembles the distribution of the Atlantic Multidecadal Oscillation (AMO), with a warming of the North Atlantic and eastern tropical Pacific. Thus, a combination of a La Niña (El Niño) year, a warm (cold) year in the tropical North Atlantic, and a warm (cold) phase of the AMO will greatly increase the probability of an active (inactive) hurricane season in the North Atlantic and an inactive (active) season in the eastern North Pacific.