Lecture 15 General Circulation of the Atmosphere and Ocean

Transcription

1 Lecture 15 General Circulation of the Atmosphere and Ocean Non-rotating model of atmospheric general circulation Hadley (1735) Cell is induced by uneven distribution of solar heating. Conceptual models idealize to capture the essence (e.g., Earth is a solid sphere rotating around the axis between the poles). Numerical models are mathematical equations derived based on physical understanding and verified against observations (weather & climate prediction). 1

2 Three-cell model considers earth rotation effects and explains the easterly trades in the subtropics and prevailing westerlies in the mid-latitudes. General Circulation and Weather Raining near the equator Subtropical deserts like Sahara Precipitation in the midlatitude storm tracks 2

3 Westerly Jet Streams Maximum (> 60 m/s) over Japan & secondary max (40 m/s) on the US east coast. Responsible for longer return flights from North America. Wind velocity at 300 hpa in January, viewed from the North Pole A ring of air shrinks its radius as it moves poleward (at the level of tropopause), spinning faster than the earth, similar to a spinning skater pulling her hands inward. See movie at 3

4 Winter surface circulation (geostrophy + friction) Aleutian low and Pacific high are both a result of the Tibetan Plateau s blockage of the subtropical westerly jet. Summer surface circulation (geostrophy + friction) Thermal lows over continents and thermal highs over oceans in the NH. 4

5 Surface Pressure & Wind over the North Pacific Based on historical ship reports Winter Summer m/s Role of Ocean in Climate Cool temperature forms surface high over ocean. Oceanic highs produce dry/cool climate on the west coast and hot/humid climate on the east coast in summer, by advection. 5

6 Vertical Structure of the Ocean Warm water floating on cold deep water that sinks in higher latitudes. Thermocline is a thin layer with a rapid upward increases in temperature. Wyrtki & Kilonsky (1984, JPO) Tahiti Hawaii surface deep Much of the deep water is formed in the high-latitude North Atlantic during cold winter, and then spread throughout the global ocean. 6

7 Ekman flow Important in the top 10 m Surface flow in response to wind To the right of the wind Observable from iceberg drift Ekman Current Coriolis Wind Ekman (1905) explained Nansen s observation that icebergs moved to the right of the wind direction in the Arctic. Summer upwelling off California Northerly winds along the coast Offshore surface Ekman flow Deep cold water rises to compensate Fog formation on Californian coast High nutrients and good fisheries (Coastal water is not much warmer, but still warmer, in summer than winter.) 7

8 El Nino: abnormal warming in the central/eastern equatorial Pacific Ocean (including the Peruvian coast). Southern Oscillation: seesaw in sea level pressure between the western (Darwin, Australia) and central (Tahiti) Pacific. ENSO Why does sea surface temperature matter? It determines the location and intensity of atmospheric deep convection (cumulonimbus): e.g., minimum temperature for hurricane formation ~ 27 o C. wind (sea surface temperature changes) Key Words Scales of motion (mesoscale, synoptic & planetary) Wind shear, clear air turbulence Thermal circulation, land/sea breeze Monsoon, Tibetan Plateau Mountain/valley breeze, katabatic wind Chinook (Foehn) wind, Santa Ana wind General circulation, Hadley circulation Intertropical convergence zone (ITCZ), doldrums ITCZ s northward displacement Subtropical high, trade winds Westerlies, subtropical jet stream Aleutian low, Pacific high, Siberian high Icelandic low, Bermuda high Upwelling, Ekman flow El Nino, La Nina, Southern Oscillation Ocean-atmosphere interaction, positive feedback 8