Review of summer 2009 in North America
Temperature
In the summer of 2009, the average temperature over the contiguous United States was 71.7°F, or 0.4°F below the 20th Century average (Fig. 1). In our forecast the temperature was expected to be above the average. The forecast was based primarily on rules 298 and 363, which link the average US temperature with the circum-global teleconnection (CGT) index and atmospheric pressure in the Tropics, respectively. In the spring of 2009, the CGT index was positive, suggesting above normal temperatures over much of the United States, but it dropped sharply in the following months. Negative summer CGT index was also associated with the worst Indian monsoon in years and a heavy flooding in southern China.
The national Canadian average temperature for the summer of 2009 was 0.4°C above normal, ranking it as the twenty-seventh warmest since nationwide records began in 1948. The warmest summer was in 1998 (1.8°C above normal), and 1968 (0.8°C below normal) remains the coolest. Summer temperatures in Canada have remain at or above normal since 1993.
The spatial distribution of summer 2009 temperature anomalies over North America is shown in Fig. 2. This pattern is generally consistent with what is usually observed during El Niño years, but the current El Niño event is weak and was difficult to predict. Note also that the El Niño signal in North American temperatures is weaker in summer than in winter.
The strongest negative temperature anomalies were observed in the Great Lakes region, where Michigan experienced its fifth coolest summer since the record began in 1895. In Detroit, July 2009 was the coldest July since 1891.
Colder than normal temperatures were also observed in the US Northeast. Thus, June 2009 temperature in Boston was the coldest since 1982 an second coldest since 1916.
The reason for the anomalously cold summer in the Great Lakes region, Northeast and northern Plains States is a strong upper atmospheric trough (Fig. 3), which caused a massive and persistent advection of cold air from the north.
Our forecast was correct regarding temperatures in the Northeast, but obviously underestimated the cold spell in the Great Lakes region. This is partly because the “warm-cold” index in the North Pacific turned out to be negative, while it was expected to be positive (rule 417). Regions of the northern US and central Canada also tend to have warmer summers when a positive Indian Ocean Dipole (IOD) event occurs (Saji and Yamagata, 2003). Although the IOD index this summer was lower than in the previous summer, it still remained positive, suggesting a warmer than normal summer in the Great Lakes region (rule 492). Unfortunately, the relationship has failed.
When summer is anomalously cold in the Midwest, it tends to be anomalously warm in the Pacific Northwest and British Columbia (Fig. 4). This is because an upper atmospheric trough over the Great Lakes is usually accompanied by a ridge over the West Coast. Indeed, 2009 summer temperatures in British Columbia were up to 3°C above the 1971-2000 average (Fig. 2). In Vancouver, June 2009 temperature was warmest on record for that month (since 1896).
Warmer than normal temperatures were also observed in the southern United States. Thus, temperatures in Florida averaged out to be fourth warmest, and Texas experienced its ninth warmest summer on record. Temperature variations in these regions are partially associated with the North Atlantic Oscillation (NAO). When the NAO is negative, summers in the Southeast and Gulf States tend to be warmer than normal (rule 419). This summer was not an exception. The NAO index has been consistently negative since 2003, being below one standard deviation in the past three summers.
Precipitation
Precipitation was below normal for the contiguous U.S. as a whole (Fig. 5). Canada experienced a slightly wetter than normal summer overall, 2.2% above normal, ranked thirtieth wettest out of the 62-years of record (click here for more information).
The distribution of summer precipitation anomalies in the contiguous United States is shown in Fig. 6. Wetter than normal conditions were observed in a band stretching from southeast Oregon to Central Plains. In Colorado, there was a sharp transition from a very dry winter to a very wet period in spring and early summer. Frequent rains there were associated with two mechanisms. First, there was frontal precipitation. A characteristic pattern was a “stalled front” along the Rocky Mountains, which could stay there for several days in a row. Second, a monsoon-like circulation was established in early summer. Moisture was pulled from the Gulf of Mexico and transported northward along the western periphery of a high pressure center.
Regions with anomalously high atmospheric pressure are usually drier and warmer than normal. It was interesting to see how a high pressure center was trying to establish itself in Colorado. But as soon as temperatures increased, so did evaporation, which resulted in rain that knocked the temperature off. And that pattern kept repeating itself day after day.
Later in August (the month when monsoonal rains typically start in Colorado), the monsoon ridge weakened and moved south, which effectively shut down the moisture transport from the Gulf of Mexico. As a result, August 2009 in Colorado was the eighth driest August on record (since 1895). Lack of monsoonal moisture contributed to the near record dryness in the entire Southwest region. Tucson, Arizona experienced its driest August since 1976 and its third driest since 1948.
A “stalled front” mentioned above was a common feature along the Atlantic seaboard as well. As a result, the Northeast Region saw its eighth wettest summer in 115 years of record-keeping.