RSS Feed
Ocean Oscillations and Hurricanes

By Joe D'Aleo
Monday, May 19, 2008

The activity in the Atlantic Basin during hurricane season is influenced by sea surface temperature patterns in both oceans. In recent weeks we have posted stories about the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation. Both factors influence the relative frequency of storms, the number of strong storms and the most likely storm tracks and areas affected. Their current state portends an enhanced risk for landfalling storms along both the Gulf and  Atlantic Coasts this season. 



The AMO is a measure of the mean sea surface temperature from the equator to 70N in the Atlantic. The AMO undergoes cycles of about 70 years.

When the Atlantic is warmer than normal, the number of hurricanes and major hurricanes (CAT 3-5) approximately double in frequency. This is because the oceans reach critical temperature thresholds sooner in the eastern areas where the biggest storms often form and they stay warm longer.

The positive AMO also has an affect on surface pressures that favor more storms.  Purple is lower than normal pressure during the warm Atlantic mode.

Note surprisingly the number of east coast and Florida hurricanes increases.



The Pacific Decadal Oscillation also plays a role in Atlantic activity by affecting the relative frequency of El Nino and La Nina.



Note in the chart of the monthly Multivariate ENSO Index, La Ninas (blue spikes negative MEI) were more frequent and stronger than El Ninos (red spikes positive MEI) during the cold modes of the PDO while El Ninos were more frequent and stronger and la Ninas weaker and less common during the PDO warm (positive) phase.




During El Ninos with warm water in the eastern and central tropical Pacific, more frequent eastern Pacific hurricanes occur. Sometimes they affect the southwestern United States in a weakened state. They have another effect. They energize the westerlies at high levels that cross into the Atlantic basin. These westerlies act to shear thunderstorm clusters or disrupt tropical systems.



Once storms form, the biggest challenge for forecasters is where they will recurve.  Tropical storms will turn to the north at first opportunity. A key determining factor in where the storms recurve is the position and strength and western extent of the subtropical high (in the Atlantic called the Bermuda High). If there is a weakness in the high in the Atlantic a storm may recurve well out at sea. If it is stronger and extends further west, the storm will track further west and become more of a landfall threat to the mainland.


The latitude of the system is also important. The further south it tracks the less likely it is to be picked by a trough or escape through a weakness in the high to the north and the more likely it is to enter the Caribbean and maybe the Gulf of Mexico. El Ninos tend to either get torn up out at sea of recurve early or make it along the southern route into the Gulf of Mexico. You can see that on the landfall chart from NOAA (1950-2002). Most of the landfalls were in the Gulf of Mexico. Only Gerda (1969) snuck into northern Maine and Andrew (1992) and Floyd (1987) into south Florida.



In La Nina years, there is a wide scatter of storm tracks with a cluster in the western Gulf of Mexico, one over Florida, another in the Mid-Atlantic and the last in New York and New England.



This is because there tends to be weaknesses in the upper level heights at steering flow levels through the Gulf and east coast states.



Not surprisingly in La Ninas with a favorable upper level and positive AMO years with below normal pressures favored, more storms threatened Florida and the east coast. Indeed in New York and New England, the greatest number of storms occurred during the period of negative PDO with more La Ninas and when the Atlantic was also warm (1950s to 1960s)



When we look at all 9 years with a La Nina and warm Atlantic, there were 16 landfalling storms somewhere from Florida to New England, 11 of which were major hurricanes.  9 affected the northeast directly or on second or third landfall.






Dr. William Gray and Dr. Philip Klotzbach in their April Atlantic Tropical Season Outlook predicted a well above-average Atlantic basin tropical cyclone season in 2008. They anticipate an above-average probability of United States major hurricane landfall. NOAA will shortly issue their outlook.


The Pacific is in its cold mode and we have had a strong La Nina. The La Nina is weakening now as most stronger La Ninas do in the spring. If it holds on or like most first year strong La Ninas regenerates, there would be enhanced Atlantic Basin activity and increased landfall probability of landfall as Drs. Gray and Klotzbach have forecasted.


The AMO has weakened in recent months. In April it was the coolest it has been since 1994, before the number of storms increased. It is still positive and similar to the value in 1955 at this time of year. That year the AMO increased by summer and Connie and Diane wrecked brought havoc for both the Mid-Atlantic and New England.  


The AMO may rebound as it has been only warm for a dozen years when most warm phases last at least two decades.  If it does, this season should be active and one folks along the east coast should be wary of.