Area Climatology — Page 428, Lesson 389

I want to walk you through the summer climatology of the North Atlantic, focusing on how the pressure systems, the polar front, and the subtropical jet change from winter to summer. We'll also look at the resulting weather, visibility, and winds. Let's start with the Subtropical Jet. This jet stream is located close to the surface position of the Subtropical High. Over the North Atlantic, that high is the Azores High. The jet is caused by the temperature difference between two adjacent columns of descending air: the warmer Hadley cell to the south and the cooler Ferrel cell to the north. The wind is westerly, and it blows at the 200 hPa pressure level, which is about 39,000 feet. In winter, it's located between 25°N and 40°N. Over the North Atlantic in winter, it blows from New York to Morocco. Now, moving into Summer, the pressure systems change dramatically. The North American Low replaces the North American Winter High. The Icelandic "Statistical" Low is present, but it's less deep and split, with an average pressure of 1010 hPa. The Azores High intensifies to 1025 hPa and shifts further north, to about 35°N. We also see hurricanes in the Caribbean and Florida area. The polar front is still present but less active. The North American Winter High has disappeared, and with it, the east coast temperature contrast between land and sea. That part of the polar front therefore disappears in summer, and the western end of the front starts at Labrador, Newfoundland, and Eastern Canada. There, the advanced warm Gulf Stream sea current meets the receded cold Labrador Sea current. In the east, because the Azores High is intensified and further north, it pushes the Polar Front northwards toward Scotland. So, in summer, the average position of the Polar Front lies from Labrador/Newfoundland to north of Scotland to Norway. You can see this on Figure 22.10. Because the temperature differences across the front are less, frontal activity is less intense and less frequent. The weakened Icelandic "statistical" low is now split, with an average of 1010 hPa centred west of Greenland, over Iceland, and in the Baltic. Now let's look at Weather and Cloud. The New York winter snows are gone. London temperatures remain moderated by air flow from the Atlantic. Polar air is less cold, and the reduced temperature contrasts mean less convection cloud over the sea. From the Azores High, warm, moist mTw air — that's maritime tropical air — flows northwards over cooler seas. This causes advection fog, stratus, and stratocumulus clouds. This can widely affect the southwest English coasts in late spring and early summer. In the Caribbean, the northeast trade winds continue to cause orographic cloud and rain on windward slopes. Additionally, in summer, rainfall is increased by convection. For Visibility: Inland radiation fog is less likely in spring and summer. If it does form, early morning insolation — that's solar heating — causes quick clearance. Advection fog can form over the cooler seas and near southwest-facing coasts of the UK and France in late spring and early summer, caused by that mTw air from the Azores moving northeast. Near Newfoundland, widespread advection fog can form over the Grand Banks — that's approximately 45°N, 50°W — in May and June. This happens when advancing warm, moist air from the Mexican Gulf overruns the very cold Labrador Sea current. Finally, Winds: In mid-latitudes, surface winds are still generally westerly, but they are less strong than in winter. The same is true for upper winds, because the temperature differences are less. In the Caribbean, the northeast trade winds prevail at the surface. Let me also show you Figure 22.9, which illustrates the upper wind changes when crossing a polar front depression from east to west. That covers the key summer changes in the North Atlantic area climatology.