Topic 4: Climate Variability and Seasonal Prediction of Tropical Cyclone Activity/Intensity Topic 4: Climate Variability and Seasonal Prediction of Tropical Cyclone Activity/Intensity National Hurricane Center, Miami Chris Landsea 27 November, 2006 IWTC-6 – San Jose, Costa Rica

Topic 4.0: The 2004 and 2005 Atlantic Hurricane Seasons (Max Mayfield – Rapporteur) 1.Extraordinarily busy for both overall activity (especially in 2005) and U.S. landfalling hurricanes (both seasons) 2.The combination of these years in conjunction with prominent papers published has helped to push to the forefront for the public issues of natural climate variability and anthropogenic climate change with regards to hurricanes.

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Topic 4.1: Variability of Tropical Cyclone Activity/Intensity on Intraseasonal and Interannual Scale (Chang-Hoi Ho – Rapporteur) 1.Well-established relationships between Madden-Julian Oscillation and TC activity (primarily genesis) in most global basins. 2.Also observed are significant intraseasonal alterations on the order of days in Northwest Pacific – possibly linked to monsoon trough development/migration.

Topic 4.1: Variability of Tropical Cyclone Activity/Intensity on Intraseasonal and Interannual Scale (Chang-Hoi Ho – Rapporteur) 1. El Nino-Southern Oscillation is dominant interannual variability factor globally (genesis, intensity, and tracks), though it is weak in the Indian Ocean. 2. The influence of the stratospheric Quasi-Biennial Oscillation has been linked to TC activity in three basins, but is secondary to ENSO. 3. Other phenomenon (e.g., Arctic Oscillation, Antarctic Oscillation, and North Atlantic Oscillation) have also been identified in individual basins.

Topic 4.3: Short-term Climate (Seasonal and Intraseasonal) Predictions of Tropical Cyclone Activity/Intensity (Suzana Carmago – Rapporteur) 1. Intra-seasonal tropical cyclone forecasts –Statistical forecasts 2. Seasonal tropical cyclone forecasts –Statistical forecasts (whole basin) –Statistical forecasts (Landfall probability) –Dynamical forecasts

ENSO SST loop here… BMRC

Dr. Bill Gray Colorado State University - Discovered link between El Nino and Atlantic Hurricanes in 1984

El Nino versus La Nina Hurricane Probabilities

El Nino-Southern Oscillation forecasts – Issued in November 2005 El Nino La Nina

El Nino-Southern Oscillation forecasts – Issued in May 2006

Atlantic Seasonal Tropical Cyclone Forecasts for 2006 IRI

Evolution of the 2006 El Nino NOAA/ESRL

2006 Atlantic Hurricane Season Weather Underground

El Nino-Southern Oscillation forecasts – Issued in November 2006

1.A century-scale SST increase in Atlantic and Pacific tropical cyclogenesis regions is likely due in large part to increased greenhouse gases (akin to global mean SST). 2.Several recent studies have found increasing trends in TC intensity and frequency in individual ocean basins, which have been related to independent SST increases that are likely due to greenhouse warming. However, there are concerns that data problems may have greatly affected the TC trend estimates. 3.There is continued debate over the cause of tropical Atlantic SST variability and multidecadal variability of Atlantic major hurricane activity since the 1940sin particular on the relative roles of internal variability (the Atlantic Multidecadal Oscillation) vs radiative forcing (greenhouse gases, aerosols, volcanoes, etc). 4.Future TC intensity projections: increase in TC intensities is likely (~4 to 5% wind speed increase per degree Celsius CO2-induced warming is current estimate). A recent study implies that TC destruction potential could then increase by ~30% per degree Celsius. Increased rainfall rate from TCs is also likely. There is debate as to whether changes in TC geographical distribution could increase these sensitivities. 5.Future TC frequency projection: undetermined. Several recent models show global decreases with increases in a few basins, though confidence in these projections is low. Topic 4.2: Possible Relationships between Climate Change and Tropical Cyclone Activity (Tom Knutson – Rapporteur)

Webster et al. (2005)

Coverage Today of Meteorological/Oceanographic Satellites

A globally consistent satellite-based reanalysis of hurricane intensity trends… Excellent agreement in the Atlantic and East Pacific! New record Old record Kossin et al. (in review)

Terrible agreement everywhere else! Kossin et al. (in review)

In Atlantic, the frequency of storms is well correlated with tropical Atlantic SSTs Emanuel (2006); Mann and Emanuel (2006)

Open Atlantic Ocean Differences 1933 Hurricane Season 2005 Hurricane Season

Satellite era - 58% strike land

Pre-satellite era - 77% strike land

Considerations/Recommendations for Topic 4.1 (Intraseasonal/Interannual Associations) Little work has been done on intraseasonal/interannual variations and controls of tropical cyclone track. There is a big difference in impact between, say, the 1995 and 2004 Atlantic seasons because of steering changes. Proposed links of the hemispheric Annual Modes (Arctic Oscillation [including the North Atlantic Oscillation] and Antarctic Oscillation) to tropical cyclone activity need better physical foundation than is available currently. Near-global relationships have been identified between the stratospheric QBO and tropical cyclone activity. However, a verified physical connection has not been identified. Also why has the Atlantic tropical cyclone-QBO relationship dried up? Do western and central Pacific tropical cyclones play a meaningful active role in El Nino-Southern Oscillation formation/development or merely a passive, secondary association? Is the new hypothesis of El Nino and Atlantic tropical cyclone activity (via changes in the moist static stability) viable?

Considerations/Recommendations for Topic 4.3 (Intraseasonal/Interannual Forecasting) Intra-seasonal operational tropical cyclone forecasts are beginning to be produced in real-time. Verifications and skills for real-time seasonal forecasts should be made readily available from all forecasts. Skill analysis (in hindcasts and real time) should be published in peer review papers, if possible with a common metric for all forecasts. Improvements could be possible with new homogeneous datasets for TC (e.g. new dataset by Jim Kossin). Combination of statistical and dynamical methods should be used for improvement in landfall prediction. Desperate need for improved forecasting of El Nino- Southern Oscillation.

Considerations/Recommendations for Topic 4.2 (Climate Variability and Change) Re-analyses of current tropical cyclone database should be highest priority, including creative homogeneous record creation (a la Kossin et al.). This would also include efforts on a more generalized wind-pressure relationship (e.g., Knaff & Zehr) as well as better understanding of tropical cyclone wind structure (e.g., Kepert, Powell, Franklin, etc.). Extension of records farther back in time either from historical observations (e.g., Perez, Chenoweth, Mock) or via paleotempestology methods should be strongly encouraged. Modeling/theory efforts on the effect of anthropogenic greenhouse gases should focus upon how tropical cyclone intensity, duration, frequency can vary both today and in future. Modeling work should address both the vortices in the model as well as how environmental fields are altered. Continued research into the Atlantic Multidecadal Mode and its causes are needed to more fully delineate the roles of natural and manmade forcing.

Topic 4: Climate Variability and Seasonal Prediction of Tropical Cyclone Activity/Intensity Topic 4: Climate Variability and Seasonal Prediction of Tropical Cyclone Activity/Intensity National Hurricane Center, Miami Chris Landsea 27 November, 2006 IWTC-6 – San Jose, Costa Rica