According to the Drought Monitor, drought conditions are relatively unchanged in the past two weeks. As of Jan. 29, 2013, 57.7% of the contiguous US is experiencing moderate or worse drought (D0-D4). The percentage area experiencing extreme to exceptional drought increased from 19.3% to 19.4%. Percentage areas experiencing drought across the West stayed mostly the same at the end of January as they were at in the middle. Drought across the Southwest decreased slightly. Meanwhile, drought across the Southeast grew due to relative lack of precipitation.
Figure 1 – US Drought Monitor map of drought conditions as of the 29th of January.
Figure 2 – US Drought Monitor map of drought conditions in Western US as of the 29th of January. Some small relief is evident in the past week, but note the lack of change of drought conditions across the regions, despite recent snows throughout the mountains. Mountainous areas and river basins will have to wait until spring for snowmelt to help start to alleviate drought conditions.
Figure 3 – US Drought Monitor map of drought conditions in Colorado as of the 29th of January. Drought conditions held steady across the state in the past week. 100% of Colorado experienced Severe or worse drought conditions for the past three weeks.
Figure 4 – US Drought Monitor map of drought conditions in Southeast US as of the 29th of January. As mentioned above, drought conditions expanded and worsened in the past couple of weeks. The worst hit area, in central Georgia, has experienced the longest duration drought conditions on this map. Drought has expanded and contracted around this area during that time.
The latest seasonal (three-month) outlook from the National Weather Service predicts enhanced chances for above-average temperature and below-average precipitation for the central US. This means that drought conditions are likely to continue for at least another three months and probably longer if prevailing conditions do not change. One of the major weather stories of 2012 was drought; 2013 is shaping up to have the same story.
What is causing this? A combination of factors: the Arctic Oscillation (AO), the Madden-Julian Oscillation (MJO), the El-Nino and Southern Oscillation (ENSO), the Interdecadal Pacific Oscillation (IPO), and background climate warming.
As I discussed in my last drought post:
The lack of sea ice in the Arctic back in September is part of what caused the negative phase of the AO. The Arctic Ocean absorbed solar radiation instead of reflecting it back to space. The ocean then slowly released that heat to the atmosphere before new ice could form. That extra heat in the atmosphere changed how and where the polar jet stream established this winter. Instead of a tight loop near the Arctic Circle, the jet stream has grown in North-South amplitude, allowing cold air to pour to latitudes more southerly than usual and warm air to move over northern latitudes. The large amplitude jet has kept the normal type of storms from moving over locations that used to see them regularly during the winter.
An active MJO is keeping trade winds stronger than they otherwise would be, which piles up warm ocean water in the western tropical Pacific Ocean. This causes cool, deep ocean water to rise in the eastern Pacific, as seen in Figure 5.
Figure 6 – ENSO conditions as of 2 Feb 2013 from NOAA-CPC.
Cooler than normal sea-surface temperatures (SSTs) are present in the eastern Pacific due to the current MJO and ENSO data. Additionally, eastern Pacific SSTs are cooler than the climatic average due to the current negative phase of the IPO. This in turn is due in part to global warming, which is causing western Pacific and Indian Ocean SSTs warmer than usual. The cool SSTs in the eastern Pacific initiate and reinforce air circulations that generally keep precipitation away from the Southwest and Midwest US. This doesn’t mean that drought will be ever-present; only that we are potentially forcing the climate system toward more frequent drought conditions in these regions. Some years will still be wet or normal; other years (increasing in number) will be dry. This is a counter to skeptics who claim that more CO2 and warmer temperatures are necessarily better for plants. If there is no precipitation, plants cannot take advantage of longer growing seasons. Moreover, we will experience years with food pressure. These conditions’ extent in the future is up to us and our climate policy (or lack thereof).