13 July 2010

Maybe not today, maybe not tomorrow, but soon...

...it just might rain here in southern Arizona. We had a trace a few days ago, just around sunset, and there have been storm clouds all but covering the sky on a few days, but now we're back to light high stratus and temperatures in the triple-digits. We've had highs above 100°F today and for ten of the past 12 days (with an official high of only 99°F on those other two days) and on one day in May and 22 days in June. Since the beginning of May, however, we've had less than 1 mm of rain in Tucson.

Sure, it was a good winter. The Salt and Gila Rivers actually flowed in measurable amounts through Phoenix and all the way to the Colorado River when the floodgates at Roosevelt Dam were opened for a while during an early spring melt in the Verde Valley. Reservoirs in Arizona are at comfortable levels, given the long-term drought that retains its grip on the Lower Colorado River Basin. The summer monsoon has turned into a wild-card, however. Today's Arizona Daily Star reports that the monsoon is fizzling even as I write this, though that assessment is based on the apocryphal method by which the monsoon season has "started" when the dewpoint temperature remains above 54°F for three consecutive days, which threshold was technically reached last Friday. Historically, as the Star reports, the monsoon begins in Tucson in the first week of July, and the average start date over the past decade has been 7 July. The National Weather Service has apparently left behind the dewpoint method as an indicator of the monsoon onset in favor of an even more arbitrary declaration: the "official monsoon period" begins on 15 June and ends on 30 September. Well, we've seen no rain above a trace since Friday, or 7 July, or even 15 June. According to all available "official" sources, the monsoon must be a bust like last season...or not...

As reported in various outlets for historical reference, the term "monsoon" comes originally from the Arabic mawsam, or "season," through the Hindi mausam for "weather," and persistent variations in Portugese and Dutch (naturally, given the evolution of sea trade over time) to our present terminology. Technically, in meteorology, "monsoon" refers to the seasonal shift in wind patterns and not specifically to the storms and rain that often result from that shift. The sea-trade routes pioneered by the Portugese and Spanish in the 15th century, and later the Dutch in the 16th century, relied on these shifts to time their transit around Africa to India and Southeast Asia, catching the favorable following winds by their seasonal regularity and, thus, predictability.

Now, however, most of us expect that the "monsoon" is the rainy season in and near the tropics. Since adoption of the term in southern Asia, where the monsoon season is pronounced and sometimes even violent, the monsoon rains are relied upon as the bringer of summer life in otherwise parched landscapes for principally agricultural societies around the Indian Ocean. It is the second, more abundant growing season in the year following the winter rain-fed grain crops. While the winter moisture is adequate for wheat and other dry-agriculture crops, more water-intensive rice and similar crops flourish in the summer monsoon season that extends from Pakistan in the west through India, Bangladesh, and throughout Southeast Asia. A similar, connected, but slightly weaker monsoon system also develops every summer in southern China and around the South China Sea, supporting massive and vital agricultural production (and frequent flood events) in southern China, Vietnam, and the Philippines. That's (literally) tons more rice and grain for the burgeoning populations in Asia; we in the West could still learn a great deal from the agricultural practices that have been refined over thousands of years and now support a population of more than 2.5B in Asia. Analogous climate and surface conditions can be found in many places to provide abundant examples for us to follow, if only we paid closer attention; as efficient as the farming practices in the vegetable fields of California's Imperial Valley and the grain-fields of the northern U.S. Great Plains have become, we still have a lot to learn and a long way to go toward greater agricultural water efficiency and food security in many areas of the world.

But South Asia is not the only monsoon region in the world: the Sahel region of Africa sees a monsoon flood season in the northern summer, areas of northern Australia and northern Brazil see their monsoons in the southern summer, and the North American Monsoon System (NAMS) is a climatological feature of growing interest along the west coast of Mexico and in the Southwest U.S. Several of my colleagues are involved in NAMS research at the University of Arizona (UA), the National Center for Atmospheric Research (NCAR) in Boulder, and at Colorado State University (CSU) in Fort Collins. Although traditional analyses show the extent of the NAMS to reach the Four Corners area by the end of the season, while living in Fort Collins we would see the northernmost extent of the monsoon rains come over the Continental Divide around August of every summer, sometimes leaving scorched forests behind as moist thunderstorms had diminished to lightning spectacles over dry tinder that far from the coast.

It is clear that the monsoon is a tropical phenomenon, extending into the subtropical regions (such as northern India and the SW U.S.) where the configuration of ocean, land and mountains (or other significant topography) are favorable. Studies in the professional journals have shown that the Himalaya ranges provide an elevated heat source that helps organize the monsoon circulation over South Asia, essentially locking the Asian monsoon into a specific position and timing that is most favorable and rather predictable. The schematic at right is Figure 4.5 from The Asian Monsoon by Bin Wang and shows the major circulation and rainfall patterns and other relevant features of the South Asian monsoon.

My own hypothesis regarding the North American monsoon, which may have some elements in common with the findings and theories of other researchers in the community, is that a similar circulation pattern supports the NAMS. Consider the relative positions in the above figure for South Asia of the moisture sources, flow patterns, land areas and elevated topography, and then look at the west side of subtropical North America (figure from Adams and Comrie, 1997: ''The North American Monsoon,'' Bulletin of the American Meteorological Society). Sure, the orientation of features is rotated by a decent amount and so that has some impact on the effectiveness of surface wind patterns to carry moisture on-shore, but we can't deny that the essential elements of the Asian monsoon circulation are also present in the NAMS: the eastern Pacific Ocean and Gulf of California instead of the Indian Ocean, Arabian Sea and Bay of Bengal; Baja and mainland Mexico and the Southwest U.S. instead of the Indian subcontinent; the Sierra Madre, Colorado Plateau and Rocky Mountains instead of the Deccan Plateau and Himalaya ranges. They even share a commonality in the proximity of regions for tropical storm generation, just west of Central America for the NAMS and just east of Sri Lanka for the Asian monsoon, that provide moisture surges into the monsoon regions and often bring about the most devastating flood events of the season.

I certainly recognize, however, that the forcing elements in the NAMS are smaller, of a slightly different relative orientation (and rotated with respect to the typical tropical circulations), and more gentle in slope near the mountainous forcing regions than those for the Asian monsoon region. Overall, that is where I would attribute the diminished predictability of the NAMS relative to the Asian monsoon; where the signals from these influences are stronger, the noise from other influences fades to the background and the overall uncertainty in prediction is reduced. In fact, things are different enough that, to get precipitation in southeastern Arizona, it often takes a "gulf surge" to get the moisture flowing into Tucson. Another figure from Adams and Comrie (1997) at right shows this variation on the NAMS configuration, in which moisture from the Gulf of California (instead of, or in addition to, that from the Pacific Ocean) is forced into the southern Rocky Mountain region where conditions remain otherwise dry through the summer months.

That said, however, it is certainly devastating when the Asian Monsoon fails, as in 2009. Need for food aid in countries around the Indian Ocean, especially in the climatologically drier western parts of the basin over the Horn of Africa and the Arabian Peninsula, is greatest in those years when the Indian and Southeast Asian monsoons end up weakest by the close of the summer season. When the Somali Jet (see figure above) slows, less moisture is transported from the Indian Ocean over eastern Africa, and the dry subtropics there just get drier as a result. Here in the American Southwest, we worry about shortages on the Colorado River far more than crop yields over a few dry summers. We're the lucky ones...

The Somali Jet within the Asian monsoon circulation pattern is, of course, also connected to the topographic influence of the Ethiopian highlands, which exert a strong influence on westward-training storm systems that often bring flooding rains to the Sahel of eastern Africa and sometimes end up as late-season tropical cyclones in the Atlantic Ocean or easterly waves that make it across Central America to generate tropical storms in the eastern Pacific, exactly those that force the gulf surges on which Tucson's perception of the monsoon season lives and dies. It's all connected, around the globe, and especially in the tropics, and to demonstrate that we need to look little further than the often-cited El Niño phenomenon for far-reaching connections and correlations. There are other phenomena and connections, of course, in both the ocean and atmosphere: the quasi-biennial oscillation, the multi-decadal oscillation, the condition of the western Pacific warm pool and the volume of recently-identified warm through-flow to the Indian Ocean. Numerous dissertations and years of post-doctoral work have been spent on just parts of these phenomena, not to mention the scholarship and ingenuity required to put all the pieces together into a coherent concept of the connected tropical ocean - atmosphere system, and then its impacts on extra-tropical regions.

I'll gladly dive into some basics on the impacts of El Niño and the Southern Oscillation (ENSO) on weather patterns that make significant hydrological impact around the world, but it's a long discussion with lots of great scholarship (by others, not my own) behind it and some very cool but nuanced figures to discuss, so it's also a topic best put on the schedule for another post. In the meantime, let's simplify it to a couple of solid observations. Primarily, the impacts of ENSO are best-forecast when the ENSO signal itself is strong, that is, when there is clearly and unambiguously a warm (El Niño) or cold (La Niña) sea-surface temperature anomaly, which seems to peak in the northern winter months, in the tropical eastern Pacific Ocean. That was clearly the case in 1983, one of the strongest El Niño events on record, during which the Rocky Mountains collected snowpack levels that led to record snowmelt floods the following spring and a relatively normal monsoon season and wet early autumn, based on historical rainfall data for Tucson from the National Climatic Data Center. According to historical data from the India Meteorological Department, the Indian monsoon was below average in terms of rainfall (all-India anomaly of -14.5%) the summer prior to the 1983 El Niño but above average (+13.0%) the summer after, as El Niño conditions waned quickly in the eastern Pacific Ocean.

Current conditions, available from the NOAA Climate Prediction Center, show a trend from a moderate El Niño last winter toward La Niña conditions, but we've only made it part of the way there yet; right now, conditions in the East Pacific Ocean are generally in the neutral phase of the ENSO cycle, right where its impacts on other regions are hardest to predict with any certainty. At least the monsoon in India seems to show some correlation: the 2009 Indian monsoon, before this past winter's moderate El Niño, was a distinct failure, with an all-India rainfall anomaly of -21.8% and some provinces exceeding a deficit of 30% in total summer rainfall. This year's Indian monsoon started strong, as the monsoon rainfall set on southern India a day earlier than normal and with early tropical cyclones (though not all good in their effects) that churned both the Bay of Bengal (Cyclone Laila) and the Arabian Sea (Cyclone Phet). The most recent report states that the monsoon has covered India about 10 days earlier than normal, so expectations there for good seasonal rains are hopeful.

The failed monsoons like last year's, in both India and North America, remain a problem, and especially so as they occur ahead of El Niño event peaks, one of our principal seasonal climate indicators. To add to the uncertainty, El Niño is one of the least predictable phenomena on an inter-annual scale, with a cycle that varies erratically. At least we are certainly on a trend out of an El Niño event right now, which seems to lend some predictability to other more regional phenomena. While the forecast looks good for agriculture in India, here in the southwestern U.S. we are still holding out hope against the media's uncertain odds for monsoon rains this season.

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