NATS 101-06 Lecture 17 Air Masses. Supplemental References for Today’s Lecture Lutgens, F. K. and E. J. Tarbuck, 2001: The Atmosphere, An Introduction.
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Summer SST Along West Coast Ahrens Fig 7.24 Prevailing Winds Sea surface temperatures (SST) along West Coast are quite cold during summer, especially off Northern California. Due to upwelling of cold, nutrient rich water by prevailing N winds. Coastal Surface Water
Upwelling from Alongshore Winds Ahrens Fig 7.25 Wind pushes surface water southward. Coriolis force deflects water to the right. Cold water from below rises to surface. Fog persists over the cold water.
weather.unisys.com Upwelling Regions El Nino 3.4
El Nino-Southern Oscillation (ENSO) An important atmospheric-ocean feedback Normal conditions in tropical Pacific: -Warm SST, low SLP, and T-storms in W Pacific -Strong subtropical highs in E Pacific -Easterly winds and cool upwelling water along equator in East Pacific -Prevailing southerly winds off of Peru produce cold upwelling and excellent fishing
El Niño-Southern Oscillation (ENSO) Every few years (4-5 years): -Equatorial Central Pacific warms -Low SLP, T-storms shift to Central and East Pacific (the Southern Oscillation) -Trades and southerly winds off Peru weaken -Upwelling ceases off Peru, warming leads to massive kill off of fish. Typically occurs around Christmas (an El Niño event) Alters global patterns of wind, temp and rain
Walker Circulation Pushes water westward Darwin Tahiti Walker Circulation oscillates with a quasi-period of every few years. Oscillation is very evident in SLP records for Darwin and Tahiti. Aguado & Burt, p230
ENSO-Atmosphere Coupling Ocean temperature pattern SLP, winds, storms, etc.Ocean currents, upwelling By observing SST’s and surface winds in equatorial Pacific, we are able to forecast ENSO events with considerable skill (much better once they have started to form). ENSO forecasts lead to skillful seasonal forecasts for the US several months in advance (e.g. 1997-1998 winter).
Ocean and ENSO Summary Major Ocean Currents Driven by prevailing wind Upwelling Regions Occurs along west coasts of continents Cold water rises from below to surface Nutrient rich, excellent fishing regions
Summary El Nino-Southern Oscillation Occurs every few years Central equatorial Pacific warms Low SLP, T-storms move with warm water Upwelling weakens along Peru coast Can be predicted up to one-year in advance Modulates global patterns of wind, temp, rain
What is an Air Mass? Air Mass Large area (>1600 km by 1600 km) of air that contains relatively uniform, horizontal distributions of temperature and moisture.
How Air Masses Form If surface air resides in a region for a few days, it acquires the thermal and moisture characteristics of the underlying surface. Source regions for Air Masses are: Big in area [ >>(1600 km) 2 ] Dominated by persistent high pressure and light winds
Air Mass Source Regions Contrasting source regions are Continents versus Oceans Tropics versus Poles An Air Mass is designated in terms of its Source Region
Creation of mP Air Mass Lutgens & Tarbuck, p 230
Lake Effect Snows As cP air flows over the warmer, open Great Lakes, it is warmed and moistened. When the modified cP air flows onshore, prodigious snows of several feet can result. Lutgens & Tarbuck, p 230
Due Today Turn in 4” by 6” card, include Comments on Lecture SID NAME
Summary Air Masses Large (>1000 miles) regions with “uniform” temperature and moisture characteristics Classified by Source Region Continental (c) or Maritime (m) Polar (P) or Tropical (T) Source Regions Big in area (>>1600 km by 1600 km) Dominated by light winds (long resident times)
Assignment for Next Lecture Topic - Fronts Reading - Ahrens pg 212-219 Problems - 8.12, 8.13