The storm systems that moved over the US in the past month alleviated some of the drought conditions across the US, according to the Drought Monitor. As of Jan 15, 2013, 58.9% of the contiguous US is experiencing moderate or worse drought (D0-D4). The percentage area experiencing extreme to exceptional drought decreased from 21.3% to 19.4%. Percentage areas experiencing drought across the West stayed mostly the same in the middle of January as they were at the end of December. Drought across the High Plains expanded slightly during the same period. Meanwhile, drought across the Southeast and Midwest shrank due to the aforementioned storm systems.
Figure 1 – US Drought Monitor map of drought conditions as of the 15th of January.
Figure 2 – US Drought Monitor map of drought conditions in Western US as of the 15th of January. 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 Midwest US as of the 15th of January. This region also has not seen any meaningful shift in drought conditions recently. The Plains will likely have to wait until spring and summer for drought relief. This sector of the country does plant a significant amount of crops. The winter wheat crop has already been devastated.
Figure 4 – US Drought Monitor map of drought conditions in Colorado as of the 15th of January. Drought conditions worsened slightly across the state in the past week. Now, 100% of Colorado is experiencing Severe or worse drought conditions. The percentage area with Extreme drought conditions is 5% higher than last week. There was no significant difference in Exceptional drought area since last week.
Figure 5 – US Drought Monitor map of drought conditions in Colorado as of July 31, 2012. This figure shows how extensive the current drought is – both in space and time. Severe or worse drought has afflicted close to 100% of the state for almost six months now. While specific regions of the state have received some rain or snow, it hasn’t been enough to break the drought yet. The percent area with Extreme or worse drought has decreased from 73.67% on July 24th to 65.35% on July 31st to 58.64% on January 15th. The southeast part of the state has seen the worst of conditions, as Figure 5 and 6 demonstrate.
Figure 6 – US Drought Monitor map of drought conditions in Colorado as of June 14, 2011. Eighteen months ago, more than half of Colorado was drought-free. As you can see, the southeast part of the state has seen Severe or worse drought conditions for a long time now.
The US is not likely to see drought relief through March (drought predictions are accurate for ~3 months at a time) . A negative Arctic Oscillation (AO; Figure 7) is challenging the return to ENSO-neutral conditions, which should allow normal precipitation to fall over the US. The AO has been negative in previous winters and it has caused the severe winter storms that affected the northeastern US as well as UK (record wet year in 2012) and Scandinavia.
The lack of sea ice in the Arctic back in September is part 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 N/S 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.
Hence, the drought we see now over the US is causally linked to the Arctic Oscillation as well as the long-lasting, moderate La Niña (2010-2012). Both of the natural variations exist on top of the background climate, which we are warming (this is why there was record low Arctic sea ice in 2012). We will continue to see the climate modulate normal weather conditions until we stop emitting greenhouse gases. As I’ve written, that isn’t likely to happen any decade soon.