Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 10. elokuuta 2007 klo 14:29 (GMT)
Thunderstorm activity in association with a surface trough of low pressure near Jamaica has diminished this morning. However, we will have to watch this area this weekend, as most of the computer models forecast that low levels of wind shear will develop over the western Caribbean and southern Gulf of Mexico. Two computer models, the NOGAPS and ECMWF, predict a tropical depression could form in the region as early as Tuesday. The GFS and UKMET models do not. As there will be plenty of moisture and low wind shear, the limiting factor would seem to be lack of a trigger to get things spinning. We'll have to wait and see on this.
An area of disturbed weather off the North Carolina/Virginia coast is associated with the tail end of a cold front that pushed off the East Coast. Sea surface temperatures are warm enough (80-84 F) and wind shear is low enough (10-20 knots) to allow some slow development over the next few days. This disturbance is expected to slide off to the northeast away from land, and I don't expect a tropical depression will form.
Thunderstorm activity has picked up on the west coast of Africa, and it appears that one or two strong African waves will push off the coast over the coming week. Most of the computer models forecast that one of these waves will develop into a tropical depression. We are into mid-August, when these waves traditionally start to develop, and the dry air and dust associated with the Saharan Air Layer (SAL) does not appear to be a major impediment at present. Thus, I think it likely a tropical depression will form off the coast of Africa 3-7 days from now.
Hurricane Flossie in the Eastern Pacific remains a threat to the Big Island of Hawaii, and could bring rains there as early as Wednesday.
Record July Arctic sea ice loss
Sea ice extent in the Arctic in July 2007 set a record low, posting a large 7% decline compared to July 2006. July marked the third month this year that a record monthly low was set. Arctic sea ice coverage in July has declined by about 26% since measurements began in 1979 (Figure 1). The trailing end of Figure 1 shows a very striking drop, so it's worth investigating this decline in more detail.
Figure 1. Arctic sea ice extent for July, for the years 1979-2007. July 2007 had the lowest Arctic sea ice extent since satellite measurements began in 1979. July sea ice coverage has declined about 26% since 1979. Image credit: National Snow and Ice Data Center.
What caused the July sea ice loss?
Sea ice in July over the past 28 years has shown a steady decline, punctuated by ups and downs characteristic of year-to-year natural variability in the weather patterns over the Arctic. The steady decline is largely due to increasing temperatures in the Arctic from global warming, but a significant portion is due to changing wind patterns. As I discussed in detail in a blog earlier this year, much of the 1990s saw lower than average pressure over the Arctic, which drove stronger than average westerly winds along the north coast of Canada. These west-to-east winds acted to push ice out of the Arctic through Fram Strait, the region between Greenland and Europe. Was a similar wind pattern responsible for the July 2007 decline in ice?
Figure 2. Surface wind for the Arctic averaged for July 2007.
A plot of the surface wind speed for July 2007 (Figure 2) shows that the meteorology of July 2007 led to a wind pattern the opposite of the one in the early 1990s that pushed so much ice out of the Arctic. In July 2007, surface winds blew from east to west along the north shore of Canada, rotating clockwise around a high pressure system over the North Pole. It is more difficult to flush ice out of the Arctic with this kind of wind pattern. There were very strong north-to-south winds over Fram Strait--in excess of 7 m/s (about 14 mph). The wind is normally nearly calm in this region in July, so these strong winds did account for a small portion of the July 2007 record sea ice loss.
By comparing the sea ice coverage in July 2006 versus 2007 (Figure 3), we can identify areas along the northern coasts of Russia and Canada where most of the melting in 2007 occurred. A plot of the temperature anomalies (how far temperature differed from average) for July 2007 (Figure 4) show that the greatest ice loss in July 2007 occurred where temperatures much above average occurred. Thus, it appears that warming, and not wind patterns, was primarily responsible for the record July 2007 sea ice loss.
I asked Dr. Mark Serreze, a Arctic sea ice expert at the National Snow and Ice Data Center, about the record July sea ice loss. His comments:
We see a strongly anticyclonic atmospheric circulation at sea level in July over the Arctic Ocean. This seems to be fostering rather clear skies, promoting strong melt. Also, if you look at the temperature anomalies for July, there is an area of very warm conditions along eastern Siberia, on the west side of the anticyclone where winds have a southerly component. Southerly winds will also "push" the ice away from shore, helping to reduce ice extent along the Siberian coast. Having said this, we are also strongly seeing "memory" of past conditions. We started out on a bad footing with ice extent in May 2007 well below norms. There also seems to be very little thick ice in the Arctic Ocean--as the ice is thinner, large areas can melt out in summer. At the current rate of loss, it's a good bet that we will exceed the record 2005 September ice minimum. However, last year, we were on track to set a new record, until it got colder and stormier in August. In essence, we were "saved by the bell". Hence, we'll just have to wait and see.
Figure 3. Comparison of Sea ice extent for July 2006 and July 2007. Major sea ice loss in July 2007 compared to July 20006 occurred along the north coast of Russia and Canada.
Figure 4. Temperature anomalies (how far temperature differed from average) for July 2007. Much warmer than average temperatures were observed over the land areas adjacent to where the major sea ice losses occurred.
This July's major loss of sea ice will amplify sea ice loss the remainder of the summer, due to a positive feedback loop. As sea ice melts in response to rising temperatures, more of the dark ocean is exposed, allowing it to absorb more of the sun's energy. This further increases air temperatures, ocean temperatures, and ice melt in a process know as the "ice-albedo feedback" (albedo means how much sunlight a surface reflects). The July 2007 ice loss may mean that a runaway "ice-albedo feedback" has taken hold, which will amplify until the Arctic Ocean is entirely ice-free later this century. Other scientists will disagree, but I believe that such a runaway ice-albedo feedback has taken hold.
The melting of the Arctic sea ice will not raise ocean levels appreciably, since the ice is already floating in the ocean. However, it will bring warmer temperatures to the Arctic, which will accelerate the melting of the Greenland Ice Cap. This ice sheet holds enough water to raise sea level 20 feet--though much less melting is expected this century, with only a 0.6-1.9 foot sea level rise predicted. Loss of Arctic sea ice will also dramatically change the global weather and precipitation patterns. For example, the jet stream should move further north, bringing more precipitation to the Arctic, and more frequent droughts over the U.S. In any case, the reduced Artic sea ice should give us another delayed start to winter in the Northern Hemisphere this year.
Beginning today, the National Snow and Ice Data Center has begun a blog providing expert analysis of this summer's record Arctic sea ice loss. Expect weekly updates from now until Arctic sea ice reaches its minimum extent in September.
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