Severe Freeze in South America

While looking over the area average time series of the Amazon River basin produced by MERRA, a very cold event was apparent in July 1981. Figure 1 shows the time series of 850 mb temperature for the basin, and the event is several K colder than earlier in the reanalysis. The cold event starts around July 16 1981, and lasts a few days. Looking at the observation reports from this period, did not show any major problems with the input obs, yet the analysis increments of water vapor show a strong shift, starting with this event (Figure 2). So, the shift in the increments along with the temperature spike suggests, at first glance, a problem. However, the lack of rejected observations or increased error in the forecast says the observation and assimilation are OK.

While considering the problem, Austin Conaty used Google Scholar to search for cold events in the Amazon. It turns out that there was a strong system pushed northward across the continent bring cold air and a freeze to Brazil (Fig 3 from Fortune and Kousky, 1983). By looking at the Amazon Basin area average, we see the cold anomaly push into the basin. While it looks out of place, it is a real feature. We'll file this under, you learn something new every day.

Figure 1 Time series of Amazon basin average 1 hourly 850 mb temperature (K) for 1979 through 1981. The July 1981 cold anomaly stands out in this relatively short time series.

Figure 2 Time series of area averaged analysis increment of water vapor daily averages (dqvdt_ana, mm/day) for the Amazon basin from May 1981 through Sept 1981. As the cold air enters the region, the analysis increments shift (becoming smaller negative values).

Figure 3 Synoptic maps of the cold air event, brining a sever freeze to Brazil, copied from Fortune and Kousky (1983, MWR).