Record Low Arctic Sea Ice for July; Quiet tropics
Last month, Arctic sea ice extent was the lowest ever recorded for any July in the 1979 to 2011 satellite record, according to the National Snow and Ice Data Center. Most of the ice loss occurred in the first half of the month when high pressure made for clear skies and melting sunshine, and warm air blew into the Arctic from the south. In the first two weeks of July, air temperature over the North Pole was 11 to 14 degrees Fahrenheit above average. During the last two weeks of July, low pressure took over and brought cooler temperatures, although it appears this also acted to push the ice around, which resulted in a larger but thinner area of ice. New research shows that old ice continues to decline as well, which is problematic because older ice is more stable and tends to grow thicker over multiple seasons, and new ice is thin and more susceptible to melting. According to the University of Washington Polar Science Center, Arctic sea ice volume was 51% lower than average and 62% lower than the maximum (which was seen in 1979 at the beginning of the record).
Figure 1. Monthly July ice extent from 1979 to 2011 from the National Snow and Ice Data Center shows a 6.8% decline per decade.
The low amount of sea ice along Siberia has opened up the Northern Sea Route early (figure 2), and some companies are already taking advantage. It doesn't appear possible to get through the whole passage without the aid of an ice breaker or two around the East Siberian Sea, but compared to the normal route south through the Mediterranean and Indian Ocean, it's a deal. From Yokohama, Japan to the Rotterdam port in the Netherlands, the route through the Arctic is around 8,500 miles. If the Arctic is impassable, the route through the Pacific, Indian, and Atlantic oceans is around 13,000 miles.
Figure 2. Sea ice concentration from the University of Illinois Polar Research Group. Image modified to highlight the Northern Sea Route in light blue.
After a brief reformation over the weekend as a tropical depression, Emily finally dissipated for good on Sunday, and the National Hurricane Center issued its last advisory on the system. The 7-day precipitation accumulation for July 30th through August 5th (figure 3) shows us that most of Hispaniola and eastern Cuba dodged the widespread, extreme rain that could have fallen had Emily stayed organized. Locally high accumulation of 5 to 15 inches fell in the Dominican Republic, which was probably aided by topography, but it does not appear that heavy, widespread rain fell in Haiti. The southeast Bahamas also might have also seen some relatively heavy rain (1.5 to 5 inches) from the system as it redeveloped thunderstorm activity north of Cuba late last week.
Figure 3. Satellite-estimates precipitation in millimeters for the 7 days preceding Friday, August 5th. Data provided by the Climate Prediction Center.
Beyond Emily, we're seeing a typical upswing in African easterly wave activity for August. Out in the main development region of the Atlantic, between Africa and the Caribbean, we have two waves, one near 50°W and the other around 25°W. Neither of these waves are forecast by any models to develop into tropical cyclones at this point, but the National Hurricane Center did invest the eastern wave over the weekend as 92L. They're no longer updating that invest as of yesterday afternoon, since satellite presentation degraded and the GFS stopped developing the wave. Even though they aren't favored to develop, what these systems might provide is a primer for waves that have yet to leave Africa. The next two waves, scheduled to enter the Atlantic around August 11th and August 15th, are looking slightly more favorable, although model support has waned since last week. This is expected though—its hard to get consensus and consistency from models on waves that have yet to enter open water. We'll know more at the end of this week, for sure.