Atlantic Hurricane Activity 2007

2 Composites of the WH Warm Pool ( ) Interannual variability of the AWP is large Large AWPs are almost three times larger than small ones 5 Largest WPs5 Smallest WPs Dark contour ==> SST = 28.5°C

3  The warm pool area follows closely the tropical North Atlantic SST  Interannual variability is large, some related to Pacific ENSO Index of the WHWP

4  2 = 40%  2 = 95% ==> we need skillful numerical model forecasts!!

5 54 Years of Atlantic Hurricanes ( ) Busy hurricane years = years for which the number of late-season hurricanes fall within the top tercile of all years Of the 18 years with small warm pools 3 busy years, 23 storms Of the 18 years with large warm pools 11 busy years, 82 storms

6 ENSO: shear is increased but moist instability (CAPE) also increases Warm pool: moist instability (CAPE) and shear work together

7 Summary of Summer Responses to AWP

8 Slight warming of IAS, normal SST over the TNA & MDR. Dry over Caribbean & ENP regions. Warming in Gulf of Guinea implies a weak West African Monsoon, south-shifted ITCZ. Tendency toward a negative North Atlantic tripole pattern. North Atlantic: SST, SST Tendency, OLR, 850mb Wind 8

9 Recent Evolution of Equatorial Pacific SST (shaded, º C), Surface Zonal Wind (contour, m/s) and Upper Ocean (0-300m) Heat Content Anomaly (shaded, º C) and MJO Activities Since mid-May 2007, two oceanic Kevin wave episodes were evident in heat content (HC) anom., which have brought HC in the central-eastern Pacific from below-normal to near normal from May to June, and from near normal to below-normal from June to July. Corresponding to the recent negative HC surge, SST has transitioned from near normal to below-normal between 140W and 110W in mid-July. The intraseasonal variability in the ocean was apparently associated with the intraseasonal surface wind variability related to the Madden Julian Oscillation, which was represented by 200mb velocity potential anom. 9

10 CFS Nino3.4 Forecasts from Different Initial Months Underestimated the amplitude of the 2006/07 El Nino. Missed the fast termination of the 2006/07 El Nino. Missed the onset of the 2006/07 El Nino. Forecast La Nina to be developed during late summer to autumn of

But: Atlantic 2007 least active in 50 years! 1)Weak West African monsoon 2)Normal TNA SST (MDR) 3)Weak and/or late La Niña 4)AMO negative tendency 2007 season

2007 season ACE activity index