SSTs
ENSO played a major role in climate extremes in 2010 and 2011 (TF12). El Niño conditions persisted through April 2010 but rapidly gave way to La Niña conditions by June. The SST anomalies for the northern summer (JJA) of 2010 (Fig. 1) reveals the La Niña conditions in the Pacific and hence the cooler than normal conditions mean that this was the region where the thunderstorms, tropical storms, and other convective activity were not occurring. However, as shown in TF12, very high SST anomalies from 0.5 to 1.5 °C, indeed record high SSTs in many instances (Fig. 1), occurred in the Indian Ocean and Indonesian region as well as throughout the tropical Atlantic (relative to a 1951–1970 normal that precedes most anthropogenic warming), regions that are normally very warm anyway (TF12). The total SSTs exceeded 29 °C over broad regions and were at an all time high in May 2010 (30.4 °C) in the northern Indian Ocean encompassing the Arabian Sea and Bay of Bengal (TF12). SSTs were also very high (second highest on record) north of Australia for September to November 2010, and by December they were the highest on record for that month. SST anomalies were also highest on record in the Gulf of Mexico in August 2010 and in the Caribbean in September 2010 (TF12). In 2011, SSTs were well above normal in the Gulf of Mexico in April but had cooled off by May. However, SSTs were still very high in the tropical Atlantic.
Because the water holding capacity of the atmosphere increases exponentially with temperature (e.g., Trenberth et al. 2003), a positive anomaly on top of already high SSTs has much greater effect than if located elsewhere. Indeed, the high SSTs were accompanied by very high water vapor amounts. The high SSTs provide ample moisture to the atmosphere and the resulting evaporative cooling of the ocean dropped the subsequent SST values down, but meanwhile heavy rains, often record breaking in intensity, occurred nearby to where the winds carried the moisture. This happened in China, India and Pakistan (June to early August 2010); Queensland, Australia (December 2010 and January 2011), and Colombia (October to December 2010) (Fig. 1). It also seems to have been a factor from 19 to 25 April 2011 when exceptionally heavy rains, exceeding 300 mm, occurred over southern Missouri, parts of Arkansas, eastern Oklahoma, and southern Illinois, and extended along the Ohio River Valley http://earthobservatory.nasa.gov/IOTD/view.php?id=50243, as a prelude to the flooding in the Mississippi.
La Niña and the Americas
La Niña conditions are well known to be associated with major anomalies in the Americas, and precipitation and flooding risk increase substantially in northern South America, such as in Colombia (Poveda et al. 2011). In La Niña summer and autumn the hurricane season is more active owing to a more favorable tropical circulation that allows storms to form in an environment of reduced wind shear and stability (Vecchi et al. 2008).
The SSTs (Fig. 1) in the Atlantic sector throughout the region north of Colombia were above 29 °C from July to September, and August 2010 was the warmest on record in both the Caribbean and in the Gulf of Mexico: anomalies exceeded 0.5–1.5 °C relative to the 1971–2000 base period (Fig. 1) (TF12). SST anomalies were especially large off the Colombian coast. The much cooler conditions to the west of the Central American isthmus both in absolute and anomaly terms understandably focused convective activity as a whole into the Atlantic and away from the Pacific. North of the equator, the result was a much above normal Atlantic hurricane season, in which there were 19 named storms, and 12 hurricanes, of which 4 were category 4 or 5, likely making it the second most active year after 2005. These aspects related to specific extremes are documented in TF12, including links between the heavy rains and the Russian heat wave of 2010, and the Colombian rains and the drought in the Amazon.
When La Niña is present, it strongly influences where the storms track across the United States, and the storms track in such a way as to miss the South. Consequently, Texas and surrounding areas (especially parts of Arizona, New Mexico and Oklahoma) suffered severe drought, and subsequently heat waves and wild fires in the northern spring and summer 2011. Nevertheless in spring, the storms crossing the central Midwest were able to link up with the warm moist air from the Gulf of Mexico, creating extra instability and buoyancy for the air that was entrained into the storms. This led to extensive heavy rains, flooding and tornado outbreaks. The pattern of rainfall in the spring is characteristic of La Niña although the extreme nature of the changes is not. The intense heat wave and “exceptional drought” continued in Texas through August. Many of these events are described in detail on line at the NOAA National Climatic Data Center, State of the Climate, Global Hazards site: http://www.ncdc.noaa.gov/sotc/hazards/2010/m (or 2011/m) where m is the month.
In spring, when land-sea contrasts transition to zero, strong westerly winds blow from the Pacific Ocean across the United States. Because the Rockies block the wind at low levels, the result is a strong westerly jet stream aloft while at low levels the air east of the Rockies comes from elsewhere including the Gulf of Mexico when there is a pronounced southerly component ahead of cold fronts. Both the change in wind speed and direction with height (southerlies at low levels, strong westerlies aloft) create wind shear, which sets the stage for super-cell thunderstorms to form tornadoes as the shear gets converted into rotation. According to NOAA, there were 539 deaths from over 1075 (actual count) tornadoes in April and May 2011 in the United States, the most deadly on record. Trends in the tornado record are not reliable, as increases in population over previously rural areas lead to more reporting of tornadoes, but the exceptional nature of the 2011 spring is not in doubt.
Global warming does not contribute directly to tornadoes themselves, but it does contribute to the vigor of the thunderstorms that host them through the increased warmth and moisture content (moist static energy) of the low level air flow. The increase in buoyancy of the air flowing through the Gulf of Mexico helps fuel the storms. Similarly, the extra moisture provided incremental amounts to the heavy rains that ultimately led to flooding along the Mississippi and later, farther north, heavy rains and melting snows contributed to extensive flooding of the Missouri River.
The Asian sector
The heavy rains and flooding in China, India, and Pakistan in JJA 2010 were associated with the very high SSTs to the south (Fig. 1) that provided extra moisture for the monsoon rains. The strong monsoon circulation then played a role in the Russian heat wave from mid-June to mid-August 2010 (Barriopedro et al. 2011; TF12), perhaps not unlike that in 2003 (Black and Sutton 2007) although influences from the Atlantic likely also played a role. The drought and famine in East Africa was also related to the high Indian Ocean SSTs (Williams and Funk 2011). Very large anomalies also existed at this time in Arctic sea ice and, in conjunction with positive Arabian Sea SST anomalies, connections to the events in Eurasia are suggested (Sedláček et al. 2011).
In the Asian sector, as the northern monsoon faded in late August of 2010, activity began to pick up in Australia, which switched to become very wet in September, continent wide, again reflecting the very high SSTs to the north (second highest on record), abundant moisture and the La Niña conditions. This was a fore-runner to the exceptionally heavy rains in Queensland in December 2010, and January 2011 where the southern monsoon rains kicked in with the presence of record high SSTs. Category 5 hurricane Yasi made landfall in Queensland in early February 2011.