Lecture 13. Global Wind Patterns and the OceansGlobal Wind Patterns and the Oceans• Drag from wind exerts a force called ‘wind stress’ on the ocean surface in the direction of the...• The currents in the upper ocean are driven by the wind• In general, they tend to flow in a similar direction to the prevailing wind• Poleward currents are warm; equatorward currents cold, so ocean currents move heat from the tro...• This heat transport helps warm the climate of higher latitudes, while cooling the tropics.• In the winter, strong temperature contrasts occur between the warm Gulf Stream and the cold int...Current Sea-Surface TemperaturesUpwelling• Along the Pacific coast in the summer, ocean temperatures stay quite cold, particularly off N C...• This is due to upwelling of cold (and nutrient-rich) water induced by the prevailing N winds.How Alongshore Winds Make Upwelling• Wind ‘stress’ pushes surface water southward.• Coriolis force deflects surface water to the right (offshore).• Cold subsurface water upwells to take its place.• Fog or persistent low cloud often forms in the chilled overlying air.Equatorial upwelling• Well below the ocean surface, it is icy-cold even in the tropics. A thermocline separates the w...• Prevailing tropical easterly winds tend to pile up the warm surface waters on the west side of ...• Easterly winds also induce ‘equatorial upwelling’ since- the easterly winds push on the ocean surface.- surface currents adjust so Coriolis force on the water balances wind stress.- In the northern hemisphere, the resulting current is northward- In the southern hemisphere, the resulting current is southward- To replace the diverging surface water, cold water is upwelled at equator.- This lowers SST along the equator.• If winds change to westerly, there is equatorial downwelling and no source of cold water, so SS...• If the thermocline is shallow (E Pacific), upwelling easily cools SST.El Nino / Southern Oscillation (ENSO)A coupled tropical atmosphere-ocean phenomenonNormal conditions• Warmest water, low surface pressure and persistent T-storms in W Pacific• Strong subtropical highs in E Pacific• Easterly winds and cool upwelled water along the equator in E Pacific• Southerly prevailing winds off the Peruvian coast produce cold upwelling, good fishing.• La Nina is an intensification of the east winds, upwelling and equatorial cooling.Every few years- an ‘El Nino’ or ‘warm event’:• Central equatorial Pacific ocean warms• Storms and low surface pressure move to central and east Pacific (the Southern Oscillation).• Trade winds and coastal southerly winds weaken• Off Peru, upwelling diminishes and ocean surface warms (an El Nino event), leading to fish dieo...• The changed tropical Pacific wind and pressure patterns influences the entire tropics and much ...The mega-El Nino of 1997-1998Ocean 3-5°C warmer than normal along equator in E Pacific, off Peru.The La Nina of 1998-1999• Ocean 2°C colder than normal along equator in central Pacific.ENSO over the last 20 yearsThe ENSO Ocean-Atmosphere FeedbackWith monitoring of the temperature in the upper ocean in the equatorial Pacific and the atmospher...Lecture 13. Global Wind Patterns and the OceansEOMGlobal Wind Patterns and the Oceans• Drag from wind exerts a force called ‘wind stress’ on the ocean surface inthe direction of the wind.• The currents in the upper ocean are driven by the wind• In general, they tend to flow in a similar direction to the prevailing wind• Poleward currents are warm; equatorward currents cold, so ocean currentsmove heat from the tropics toward the poles, doing a comparable amount ofheat transport as occurs in the atmosphere.• This heat transport helps warm the climate of higher latitudes, while coolingthe tropics.• In the winter, strong temperature contrasts occur between the warm GulfStream and the cold interior of N America. This produces a strong jetstream and a ‘storm track’(region of particularly intense storms) along theAtlantic coast of N America.Current Sea-Surface TemperaturesUnisys WWW siteUpwelling• Along the Pacific coast in the sum-mer, ocean temperatures stay quitecold, particularly off N California.• This is due to upwelling of cold(and nutrient-rich) water inducedby the prevailing N winds.EOMHow Alongshore Winds Make Upwelling• Wind ‘stress’ pushes surface water southward.• Coriolis force deflects surface water to the right (offshore).• Cold subsurface water upwells to take its place.• Fog or persistent low cloud often forms in the chilled overlying air.EOMEquatorial upwelling• Well below the ocean surface, it is icy-cold even in the tropics. A ther-mocline separates the warm surface waters from the cold deep waters• Prevailing tropical easterly winds tend to pile up the warm surface waters onthe west side of ocean basins, so the equatorial W Pacific thermocline isdeep (200 m) while the E Pacific thermocline is shallow.TAO WWW sitex’s denote TAO buoyocean temperaturemeasurements.• Easterly winds also induce ‘equatorial upwelling’ since- the easterly winds push on the ocean surface.- surface currents adjust so Coriolis force on the water balances wind stress.- In the northern hemisphere, the resulting current is northward- In the southern hemisphere, the resulting current is southward- To replace the diverging surface water, cold water is upwelled at equator.- This lowers SST along the equator.• If winds change to westerly, there is equatorial downwelling and no sourceof cold water, so SST remains warm even near equator.• If the thermocline is shallow (E Pacific), upwelling easily cools SST.Current (SH)Current (NH)CFCFWind stressWind stressEl Nino / Southern Oscillation (ENSO)A coupled tropical atmosphere-ocean phenomenonNormal conditions• Warmest water, low surface pres-sure and persistent T-storms in WPacific• Strong subtropical highs in EPacific• Easterly winds and cool upwelledwater along the equator in EPacific• Southerly prevailing winds offthe Peruvian coast produce coldupwelling, good fishing.• La Nina is an intensification of the east winds, upwelling and equatorialcooling.TAO WWW siteEvery few years- an ‘El Nino’ or ‘warm event’:• Central equatorial Pacific oceanwarms• Storms and low surface pressuremove to central and east Pacific(the Southern Oscillation).• Trade winds and coastal south-erly winds weaken• Off Peru,