Extratropical Highlights –July 2020

 

1. Northern Hemisphere

The 500-hPa circulation during July featured above-average heights over the eastern North Pacific Ocean, eastern Canada, the northeastern U.S., and western Europe, and below-average heights over Scandinavia (Fig. E9). The main land-surface temperature signals included above-average temperatures in both the southwestern and northeastern U.S., and in central Europe (Fig. E1). The main precipitation signals included above-average totals in the south-central and Gulf Coast regions of the U.S., eastern Scandinavia, and northern China, and below-average totals in portions of northern Europe (Fig. E3).

 

a. North America

The 500-hPa circulation during July featured above-average heights over eastern Canada and the northeastern U.S. (Fig. E9). These conditions contributed to exceptionally warm surface temperatures in the northeastern U.S., with large areas recording departures in the upper 90^th percentile of occurrences (Fig. E1).

 

b. Record Atlantic hurricane activity

The May-July portion of the Atlantic hurricane season produced a record nine named storms (7 tropical storms and 2 hurricanes). Five of these storms formed during July, with two (Hanna and Isaias) becoming hurricanes. Three of these five July storms made landfall in the U.S., including TS Fay (in New Jersey), H Hanna (in Texas), and H Isaias (in North Carolina).

During July, conditions in the Atlantic Main Development Region (MDR, which spans the tropical Atlantic Ocean and Caribbean Sea) were already setting up for what is predicted to be an above normal and potentially extremely active hurricane season. These conditions included well above-average sea-surface temperatures (Fig. T18), weaker tropical easterly trade winds (Fig. T20), below-average sea-level pressure (Fig. T19), and an enhanced subtropical ridge at 200-hPa (Fig. T22). All of these conditions are consistent with the ongoing warm phase Atlantic Multi-Decadal Oscillation (AMO) and with its associated enhanced West African monsoon system (Figs. T23, T24, E4).

Similar conditions have been present throughout the current high-activity era for Atlantic hurricanes which began in 1995. Since that time, almost 70% of Atlantic hurricane seasons have been above normal, and only four have been below normal. This activity contrasts markedly with the preceding 24-year low-activity era of 1971-1994, during which 50% of the seasons were below normal and only two were above normal. That period was associated with the cold phase of the AMO and with its associated suppressed West African monsoon system.

 

2. Southern Hemisphere

The 500-hPa height field during July featured above-average heights over southern Australia, New Zealand, and the high latitudes of the eastern South Pacific, and below-average heights over the central Atlantic and Indian Ocean (Fig. E15). This overall pattern contributed to anomalously dry conditions across western and southern Australia (Fig. E3), with many areas recording precipitation totals in the lowest 10^th percentile of occurrences.