Modulation of eastern North Pacific hurricanes by the Madden-Julian oscillation. (Maloney, E. D., and D. L. Hartmann, 2000: J. Climate, 13, )

Introduction The Madden-Julian oscillation (MJO) is a dominant mode of variability in the tropical atmosphere with characteristic periods of days The MJO has a baroclinic mixed Kelvin and Rossby wave structure in the India and western Pacific Ocean, where the equatorial wave is strongly coupled to convection. Analyzing tropical cyclone activity (fig1), the region traditionally defined as the eastern North Pacific hurricane basin and the region of local amplification of the MJO signal in the composites of MH98

Genesis locations of eastern Pacific Ocean tropical systems attaining wind speeds of tropical storm force (34kt) or greater during May-Nov

Data and compositing technique Data a. NCEP-NCAR gridded reanalysis data (2.5×2.5) in pentad format were used for winds at 200 and 850mb for the years ( ) b. Microwave Sounding Unit (MSU) gridded pentad precipitation data (2.5×2.5) were available during the same periods. c. Hurricane and tropical storm tracking data for the northeastern Pacific Ocean were obtained form the NOAA/NWS/Tropical Prediction Center. (Brown and Leftwich 1982)

Data and compositing technique Compositing technique a. The index used for compositing is based on the days bandpass-flitered 850-mb zonal wind average from 5°N to 5°S at every longitude. b. EOF analysis on the entire 17-yr time series yields two prominent EOFs. c. Principal components (PCs) were derived by projecting the first two EOFs onto the original filtered zonal wind time series. The index is linear combination of PCs 1and 2 with contribution from PC 2 reflecting that it peaks an average of 2-3 pentad after PC 1.

EOF1 (solid) and EOF2 (dashed) of the 850-mb zonal wind as a function of longitude. Data during in all season are used.

Global life cycle composites Lag correlations of the 850-mb zonal average from 10°N~10°S as a function of longitude Composites of bandpass 850-mb wind anomalies and MSU precipitation anomalies during May- Nov for phase 2,4,6 and 8. Maximum vector are 3.0 m/s. Precipitation contours are at intervals of 0.6 mm/day start at 0.3 mm/day. Negative contours are dashed.

Eastern Pacific composites The 850-mb bandpass wind anomalies and 850-mb relative vorticity anomalies during May-Nov for phase 2 and 6. Maximum vector are 3.0 m/s. Contours are every 1.2×10 -6 s -1 start at 0.6×10 -6 s -1. Negative contours are dashed

MSU precipitation anomalies during May-Nov for phase 2 and 6. Precipitation contours are at intervals of 0.6 mm/day start at 0.3 mm/day. Negative contours are dashed Phase 2 has positive contribution to precipitation coinciding with the regime of tropical systems. Phase 6 has Negative contribution to precipitation coinciding with the regime of tropical systems..

30°N Eq 150W 120W 90W 60W120W 90W 60W The difference between 850-mb relative vorticity anomalies at phase 2 and 6. Contours are every 1.6×10 -6 s -1 starting at 0.8×10 -6 s -1.

Bandpassed 200-mb wind 200-mb relative vorticity anomalies during May-Nov for phase 2 and 6. Maximum wind speed are 7.0 m/s. Contours are every 1.2×10 -6 s -1 starting at 0.6×10 -6 s -1

The 200-mb zonal wind minus 850- mb zonal wind during May-Nov for phase 2 and 6. Values are not anomalies but total shear values. Contours are every 5.0 m/s starting at 0.0 m/s

Tropical cyclone composites (a) (c) (b) Number of hurricanes and tropical storms (a), average strength of hurricanes and tropical storms (b) and Number of hurricanes (c) as a function of MJO phase for the eastern Pacific Ocean hurricane region during May-Nov Error bar represent 95% confidence limits.

Conclusions Periods of westerly equatorial 850-mb wind anomalies over the eastern Pacific account for twice as many hurricanes and tropical storms as do periods of easterly anomalies. Kelvin wave that propagate eastward from western Pacific convective areas enhance convection over the eastern Pacific. Easterly wind periods coincide with anomalously anticyclonic 850-mb relative vorticity and a less favorable vertical shear profile. In fact, some of the strongest wind anomalies in the global MJO composites occur in the eastern Pacific Ocean region. Convection not associated with tropical cyclones, however, may be a important factor in strengthening eastern Pacific wind anomalies.

Equatorial Kelvin waves are often associated with anomalies in surface wind stress. The plot below shows wind stress anomalies (deviations from "normal" winds) on the left and on the right the depth at which the water is 20°C known as the 20°C isotherm depth, which defines the lower limit of the surface layer of warm water). Positive (eastward) anomalies in wind stress in the central Pacific excite positive anomalies in 20°C isotherm depth which propagate to the east as equatorial Kelvin waves