U14B-01 INVITED
Observed Tropical Cyclone Variability and Trends
Historical "best track" data and homogeneously reanalyzed data are explored to identify variability and trends in tropical cyclones, and relationships with sea surface temperature (SST) variability will be presented. I'll argue that the relationship between warming SST and the increasing intensity of the strongest storms can be reconciled with theoretical predictions by considering atmospheric circulation changes associated with SST variability.
U14B-02 INVITED
Proxy Record of Holocene Hurricane Activity from Barbuda in the Northeastern Caribbean
Sediment cores from a coastal salt pond in Barbuda at the northeastern corner of the Caribbean Basin contain multiple overwash sand layers that were deposited over the past 5000 years. The stratigraphy suggests two active periods of more frequent events during ca. 5000-2500 cal yr BP and ca. 1500-0 cal yr BP, separated by a relatively quiet period between 2500 and 1500 cal yr BP. While dating control still needs fine-tuning, the proxy data suggest that paleohurricane activity in Barbuda is broadly out of phase with that reported from the Gulf of Mexico coast, but parallels with that documented from Nobska Pond, Massachusetts. The spatial pattern of Holocene hurricane activity reconstructed from the Caribbean, the Gulf Coast, and the Northeast is consistent with the predictions of the Bermuda High hypothesis. The discrepancy between the Barbuda record and the Puerto Rico record remains to be explained.
U14B-03 INVITED
The influence of natural climate variability on tropical cyclones
We present a review of the relationship of climate and tropical cyclones on various time scales, from intra- seasonal to decadal. The response of tropical cyclone activity to natural modes of variability, such as El Niņo- Southern Oscillation (ENSO) and the Madden Julian Oscillation (MJO) in various regions of the world are discussed. Globally, ENSO and the MJO modulate TC activity.The nature of the modulation depends on the region considered. On a regional scale, other climate modes also affect TC activity. These modes of variability affecting only specific regions, such as the Atlantic Meridional Mode, will also be examined. Genesis location, track types and intensity of tropical cyclones are influenced by these modes of variability. Many of the relationships are well-documented empirically, but our understanding of the physics behind them remains rudimentary due to our incomplete understanding of how the large-scale environment influences TC formation and intensification in general.
U14B-04
Impact of ocean shortwave absorption on tropical cyclogenesis
Because ocean water contains absorbers and scatterers, shortwave radiation is absorbed closer to the surface than would be the case in pure water. In a one-dimensional sense, this process warms the surface and cools the subsurface. However, the three-dimensional ocean circulation brings the cool water to the surface in upwelling regions such as the Equatorial Pacific, leading to the creation of a two-dimensional pattern of warming and cooling. In a coupled climate model, these changes in SST result in changes in the Hadley circulation which change mid- tropospheric humidity, wind shear, and vorticity- all parameters known to impact tropical cyclogenesis. Using both statistical proxies for hurricane formation and a statistical- dynamical downscaling technique, we demonstrate that clearer waters off-equator tend to shift hurricane activity towards the equator. In the extreme case where all chlorophyll is removed from the Pacific subtropical gyres (which are already quite clear) the result is to decrease the number of cyclone days in the Pacific to the north of 10N by upwards of 60%, while channeling about 30- 40% more storms through the Caroline Islands. These large changes suggest that smaller changes in ocean color could still drive significant changes in cyclone activity and re-emphasize the importance of mechanisms distributing heat within the upper ocean for determining such activity.
U14B-05
The effects of tropical cyclone winds on the upper ocean
Tropical cyclone winds efficiently mix the upper ocean. This mixing may be important for regulating heat transported by the oceans [Emanuel, 2001], which is closely linked to the strength of the Merdional Overturning Circulation. Here we present results from a modeling study examining the sensitivity of an ocean general circulation model to tropical cyclone surface winds. We combined 7 years of global tropical cyclone winds from the QuikScat satellite with a standard ocean model bulk-forcing data set. The resulting surface wind data set was used as input to ocean-only climate model simulations using the Parallel Ocean Program (POP v1.4.3) as part of NCAR's Community Climate System Model (CCSM v3.0). Cyclone winds were varied by factors of (1x, 2x, 3x) compared to present-day values, and the response of the model was analyzed. We observe model-scale surface cooling and vertical mixing consistent with observations, and these features were found to be extremely sensitive to wind intensity. Globally integrated surface effects will be compared to observation-based estimates [Sriver and Huber, 2007], and the potential impacts of increased tropical cyclone activity on large-scale ocean dynamics will be explored.
U14B-06 INVITED
The curious hurricane-winter climate relationship: Causation or red-herring?
The existence of wind on Earth largely results from the gradient of temperature from the poles to the equator caused by varying insolation with latitude. It has long been established and recently quantified, that this redistribution of temperature is accomplished through oceanic currents, atmospheric currents, and latent heat release. Implied within all three is the role of tropical cyclones (TCs) that move out of the tropics and poleward of the Hadley cell. It remains a curious climate wildcard that although the pole to equator temperature gradient does not directly produce TCs, TCs weaken this gradient when escaping the Hadley cell. Yet, the magnitude of this TC role has remained nebulous and qualitative. By examining the winter midlatitude climate following varying recurving TC seasons over 50 years, the role of recurving TCs in the redistribution of energy is laterally approximated. The results suggest that a recurving TC, on average, may weaken the subsequent role of winter baroclinic eddies (and, implicitly, the mean pole- to-equator temperature gradient) by approximately one percent. This reduction of meridional eddy temperature flux implies a decrease in the available potential energy (APE) of the atmosphere, either as a direct or indirect result of the anomalous recurving TCs. Simultaneous with the decrease in APE, the static stability (N) of the midlatitudes is decreased as a consequence of the enhanced recurving TCs. These two competing changes result in a shift in winter extratropical cyclone (XC) existence: Following an active recurving TC season, intense XCs are slightly more frequent (owing to the decrease in N), but weak to moderate XCs are far less frequent (owing to the decrease in APE). It remains to be determined conclusively how the accumulated impacts of the TCs, and/or the resulting large-scale circulation changes, retain memory for months following the TC season. Speculation is provided based on preliminary analyses of hemispheric snowcover leading up to the start of the winter. Improved seasonal forecasting of higher latitude winter climate anomalies may be possible when the hemispheric frequency of recurving TCs is anomalous. Unfortunately, with this frequency minimally predictable, some component of winter midlatitude climate may be necessarily unpredictable until the TC season has nearly ended. This study suggests a view wherein TCs may be integral to the variability and forcing of climate, rather than incidental and purely responsive to other forcings, such as ENSO. The talk concludes with a discussion of the possiblity that the relationship presented is entirely a red-herring -- a consequence of teleconnections such as ENSO masterminding both the hurricane season and the subsequent winter.
U14B-07
Causality and contamination in the attribution of Atlantic hurricane activity fluctuations to seasonal atmospheric anomalies
Statistical studies suggest a link between anomalies in tropical seasonally averaged atmospheric wind fields and Atlantic hurricane activity. Here we show that a significant fraction of those hurricane-associated atmospheric seasonal wind anomalies result from the presence of the hurricanes themselves. This is done by assuming a hypothetical hurricane vortex structure whose radial and vertical structure is constrained by observations derived from aircraft probing of tropical cyclones. Seasonal vorticity anomalies associated with Atlantic hurricane activity are accumulated by summing these idealized vorticies along observed tropical cyclone tracks. Winds associated with these seasonal vorticity anomalies explain the bulk of observed hurricane activity-related fluctuations in the seasonally averaged lower tropospheric wind and tropical Atlantic vertical wind shear. Hence, seasonal wind anomalies appear to have limited causal information relevant to understanding why hurricane activity in the Atlantic has fluctuated in the past, and may be of limited value in projecting future hurricane activity.
U14B-08
Semi-Empirical Projections of Future Atlantic Tropical Cyclone Activity
Using a previously validated statistical model relating Atlantic annual tropical cyclone (TC) counts to climate state variables, we investigate the impact of projected future changes in climate on Atlantic TC activity. In contrast to some recent studies, our results suggest, albeit with a wide uncertainty range, the possibility that anthropogenic climate change could lead to a substantial increase in annual TC counts