The only named tropical system so far this season has been Arthur which formed off the southeast US coastline earlier this month and reached hurricane status (category 2). There is now a pretty healthy-looking tropical wave way out in the Atlantic Ocean and this could eventually become the first “Atlantic-type” tropical storm of the season, but it does have some dry air to penetrate in the near term.
The latest Saharan air layer tracking product (courtesy University of Wisconsin – CIMSS/NOAA-HRD) shows a large expanse of dry air off the west coast of Africa (“oranges and yellows”) that extends well out into the Atlantic Ocean and “in front” of the generally westward-moving new tropical disturbance. This tropical system will encounter some of this African dry air mass in the near term, but it also will be moving into a region with favorable sea surface temperatures of greater 80 degrees. Should the tropical wave survive its encounter with the dry air and develop into a named tropical storm, it would become known as Bertha. We’ll monitor its progress over the next few days here at thesiweather.com.
10:45 AM | The sun has gone quiet…solar cycle 24 continues to rank as one of the weakest cycles more than a century
Ten days ago, the sun was quite active and peppered with several large spots. Now the sun has gone quiet and it is nearly completely blank. It appears that the solar maximum phase for solar cycle 24 may have been reached and it is not very impressive. It looks as if this solar cycle is “double-peaked” (see below) which is not all that uncommon; however, it is somewhat rare that the second peak in sunspot number during the solar max phase is larger than the first. In fact, this solar cycle continues to rank among the weakest on record which continues the recent trend for increasingly weaker cycles. The current predicted and observed size makes this the smallest sunspot cycle since Cycle 14 which had a maximum of 64.2 in February of 1906. Going back to 1755, there have been only a few solar cycles in the previous 23 that have had a lower number of sunspots during its maximum phase. For this reason, many solar researchers are calling this current solar maximum a “mini-max”. Solar cycle 24 began after an unusually deep solar minimum that lasted from 2007 to 2009. In fact, in 2008 and 2009, there were almost no sunspots, a very unusual situation during a solar minimum phase that had not happened for almost a century.
Consequences of a weak solar cycle
First, the weak solar cycle has resulted in rather benign “space weather” in recent times with generally weaker-than-normal geomagnetic storms. By all Earth-based measures of geomagnetic and geoeffective solar activity, this cycle has been extremely quiet. However, there is some evidence that most large events such as strong solar flares and significant geomagnetic storms tend to occur in the declining phase of the solar cycle. In other words, there is still a chance for significant solar activity in the months and years ahead.
Second, it is pretty well understood that solar activity has a direct impact on temperatures at very high altitudes in a part of the Earth’s atmosphere called the thermosphere. This is the biggest layer of the Earth’s atmosphere which lies directly above the mesosphere and below the exosphere. Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation and are highly dependent on solar activity.
Finally, if history is a guide, it is safe to say that weak solar activity for a prolonged period of time can have a negative impact on global temperatures in the troposphere which is the bottom-most layer of Earth’s atmosphere - and where we all live. There have been two notable historical periods with decades-long episodes of low solar activity. The first period is known as the “Maunder Minimum”, named after the solar astronomer Edward Maunder, and it lasted from around 1645 to 1715. The second one is referred to as the “Dalton Minimum”, named for the English meteorologist John Dalton, and it lasted from about 1790 to 1830. Both of these historical periods coincided with below-normal global temperatures in an era now referred to by many as the “Little Ice Age”. In addition, research studies in just the past couple of decades have found a complicated relationship between solar activity, cosmic rays, and clouds on Earth. This research suggests that in times of low solar activity where solar winds are typically weak; more cosmic rays reach the Earth’s atmosphere which, in turn, has been found to lead to an increase in certain types of clouds that can act to cool the Earth.
The increasingly likely outcome for an historically weak solar cycle continues the recent downward trend in sunspot cycle strength that began over twenty years ago during solar cycle 22. If this trend continues for the next couple of cycles, then there would likely be more talk of another “grand minimum” for the sun. Some solar scientists are already predicting that the next solar cycle, #25, will be even weaker than this current one. However, it is just too early for high confidence in these predictions since some solar scientists believe that the best predictor of future solar cycle strength involves activity at the sun’s poles during a solar minimum and the next solar minimum is still likely several years away.
[12Z Euro 500 mb height anomaly forecast map for next Tuesday, July 15th; map courtesy Weather Bell Analytics at weatherbell.com]
Last summer, the hottest weather in the Mid-Atlantic region took place in the week between July 14th and July 20th with Philadelphia, for example, averaging an impressive 95 degrees for high temperatures over the 7-day period. The same time period this summer - which happens to be next week - looks like it may have a far different result and the upper level weather maps may actually resemble the “polar vortex” pattern made famous this past winter.
Today's European computer forecast model indicates there will be an anomalously strong upper level low situated over the central Great Lakes in much the same manner that occurred frequently during the past winter (see above; 12Z Euro forecast map courtesy Weather Bell Analytics at weatherbell.com). Also, in much the same manner as this past winter, the coldest air relative-to-normal is likely to occur over the Great Lakes and Upper Midwest, but cooler-than-normal air is likely to extend all the way to the Mid-Atlantic coastline. Normal high temperatures have now climbed to 87 degrees in Philly, 89 in DC, and 84 at Central Park, NY and these levels are more than likely not going to be reached for at least a portion of next week. The cold front that will usher in this cool air mass is likely to arrive in the Mid-Atlantic region in the Monday/Monday night time frame - perhaps associated with numerous showers and thunderstorms.
[Sea surface temperature (SST) Anomalies before Arthur]
[Sea surface temperature (SST) Anomalies after Arthur]
In their very essence, tropical storms are “heat machines” that are fueled by warm sea surface temperatures and they are one of mother nature’s most efficient methods of transporting heat from the tropics to the mid and high latitudes in her ceaseless attempt at balancing out the atmosphere. Often, there can be an immediate impact in this perpetual balancing act of nature with respect to sea surface temperatures in areas near the path of a tropical storm. In fact, Hurricane Arthur, which moved on Friday from the Outer Banks of North Carolina to off the Mid-Atlantic coastline, generated a dramatic drop in sea surface temperatures at the Jersey Shore. As the tropical storm moved over nearby ocean waters, it acted to churn up the water at the Jersey Shore so that colder water from lower levels in the ocean rose to the surface in a process caused “upwelling”. In the same manner that warm air rises and cold air sinks in the atmosphere, cold water will typically sink to lower levels in the ocean as it is more dense than warm water. The “upwelling” generated by Arthur helped to cause sea surface temperatures to plunge at the Jersey Shore from the lower 70’s on Thursday to the upper 50’s on Friday – they have since recovered some back to the lower 60’s. In fact, sea surface temperatures dropped significantly from “pre-to-post Arthur” off the southeast US coastline where Arthur first formed and hovered for a few days (see “before and after” SST maps above). While the change in sea surface temperatures is usually just a short-term phenomenon on the order of days and not weeks, it can act to suppress any new tropical activity until sea surface temperatures climb back to warmer-than-normal levels.