The Open Economy effect is a reason for the negative relationship between Aggregate Demand and Price Level.

If the price level rises relative to other countries, domestically produced G&S become less competitive with those produced overseas (see Relative Inflation Rates). ⬇ Competitiveness causes ⬇Export Revenue domestic price level increases simultaneously meaning consumers & business firms will purchase more goods and services from foreign producers who now may provide a better value than domestic producers. Simply; More price level means ⬇ Net Exports