Brewing Up a Storm: Depth of warm ocean water encourages development of Northeasst Pacific hurricanes accross years
Scientists have struggled to pinpoint the factors that control hurricane activity across years in the Northeast tropical Pacific. Previous studies focused mainly on the temperature at the water’s surface as the key influence on developing hurricanes. The warmer the surface temperature, the more likely hurricanes intensified given the right atmospheric conditions. The Northeast Pacific, however, has some of the coldest water surface temperatures in the tropics yet places second around the world in the number of serious storms per year. Clearly, something else was at work. Now researchers at Pacific Northwest National Laboratory have discovered that the year-to-year variability in the depth of warm ocean water plays an important role in brewing up a storm’s intensity.
Scientists analyzed 28 years of hurricane track observations (1984 to 2011) from the National Hurricane Center and the U.S. Navy’s Joint Typhoon Warning Center. They also looked at previous data and analyses from the National Centers for Environmental Prediction, Geophysical Fluid Dynamics Laboratory, and European Centre for Medium-Range Weather Forecasts for that 28-year period in the same area.
“The depth of the warm water plays a critical role in determining upper-ocean heat content, which ultimately translates into a source of energy for hurricane intensification. Our study looked at the impact of year-to-year variability in the surface warm-water depth, but potential long-term changes in surface warm-water depth due to global warming could also be important.” said Dr. Karthik Balaguru, the PNNL atmospheric researcher who led the scientific team. “Understanding ocean variability and changes will help us target improvements needed in climate models. The ultimate goal is improving projections of future hurricane activities.”
Predicting the timing and path of hurricanes in the Northeast tropical Pacific Ocean has always been tough. While the tracks of these storms rarely fall over land, their remnant moisture contributes to nearly 30 percent of the annual precipitation in the southwestern United States and Mexico. Climate models predict that these regions will experience more prolonged droughts in the future, making it important to be able to predict how Northeast tropical Pacific Ocean hurricanes and their contribution to precipitation may be changing.
Under the blast of vertical mixing induced by hurricanes, the heat content in the upper ocean is converted to thermal flux at the air-sea interface, providing an energy source that intensifies hurricanes. Consequently, it is very important to understand the factors that govern the variability in the ocean heat content. A study by DOE scientists at Pacific Northwest National Laboratory found that thermocline depth–or the depth of the warm upper-ocean water–a parameter that was not included in previous analyses, plays a pivotal role in the interannual variability of the ocean heat content and hurricane activity in the southern part of the Northeast Pacific basin. The prominence of the southern sub-region is further enhanced as it includes the primary hurricane development area. They found that both sea surface temperature and thermocline depth have a significant influence on the Northeast Pacific hurricane activity at interannual timescales. The respective roles of these parameters are more clearly delineated when sub-regions along an approximate north-south demarcation are considered rather than the basin as a whole.
This research was supported by the U.S. Department of Energy Office of Science as part of the Regional and Global Modeling Analysis program.