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UW ATMS 587 - Climate Dynamics

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DARGAN M. W. FRIERSON UNIVERSITY OF WASHINGTON, DEPARTMENT OF ATMOSPHERIC SCIENCES DAY 4: 10-7-13 Climate Dynamics (PCC 587): Water Vapor & the Hydrologic CycleWater Vapor and the Hydrologic CycleWhat’s Required for Rain? ! To get rain, you need water vapor and rising motion ¡ Condensation (water vapor turning to liquid water) happens when moist air cools ÷ And this cooling almost always happens with rising motion ! Let’s take a look at precipitation on a global scale…Why does it rain where it does? ! For reference, the precip climatology from GPCPWhat Causes Primary Precipitation Features? ! Tropics: ¡ Rising motion & rain over warmest temperatures ¡ Diurnal & seasonal precipitation ! Midlatitudes ¡ Rising associated with high/low pressure systems ¡ Storm track location/intensity is keyAverage Precipitation ! Precipitation (mm/day): Rainiest spots on global scale are narrow bands near the equator Many of the driest places on Earth are over the ocean! Most deserts are around 30 degrees latitudeAir rises above the warmest ocean surface The Hadley Circulation The circulation takes water vapor away from the deserts at 30 degrees and brings it into the tropical rainy regionsZonally averaged precip and evap ! Hadley cell is key to converging moisture towards the equator ¡ From the dry subtropics into the deep tropics Evap and precip annual means (NCEP Reanalysis 2)Seasonal March of Tropical Rains ! Rain shifts northward in Northern summer (JJA), southward in Southern summer (DJF) “Convergence zone”: where winds come together “Inter-tropical convergence zone” (ITCZ): the tropical band. The ITCZ follows the warmest ocean temperatures as they shift with the seasonsRainfall and Vegetation ! Vegetation follows the seasonal march of rainfallMonsoon Circulations ! Land heats up in the summer, leads to rising/rain hotter colder In India, Africa, Australia, etc all experience strong monsoons El Nino cycles greatly modify the location/intensity of monsoons in India & AustraliaPrecipitation Changes with El Niño ! Warmer water & rainier over central Pacific during El Niño Wetter El Niño sea surface temps Warmer East-west sloshing of warm water/precipitationMidlatitude Precipitation ! Midlatitude precipitation is associated with “baroclinic eddies” ¡ Rising motion is generated in particular regions of the weather systems“Storm Tracks” ! Precipitation (mm/day): Midlatitude precipitation is in storm tracks: preferred locations for stormsNorthward Moisture Flux ! Annual and zonal mean northward moisture flux in the atmosphere: Equatorward moisture flux in the tropics Poleward moisture flux in the extratropicsZonally averaged precip and evap ! Midlatitude storms take moisture out of subtropics and transport it poleward Evap and precip annual means (NCEP Reanalysis 2)Moisture and Horizontal Temperature Gradients ! Next: moisture also strongly influences pole-to-equator temperature gradients ¡ If moisture evaporates at low latitudes, but condenses at higher latitudes, this is exactly like a poleward transport of heat ! Let’s examine atmosphere and oceanic energy transportsEnergy Transports ! Climate system transports energy polewards (from hot to cold) Total (atmosphere plus ocean) flux Latitude Northward flux in NH, Southward flux in SHAtmospheric and Oceanic Energy TransportsBack to Observed Energy Transports ! Separated into atmospheric and oceanic components: Ocean flux Atmospheric flux Total (atmosphere plus ocean) flux Latitude Atmospheric flux is larger in midlatitudes, oceanic flux is larger in deep tropicsAtmospheric Energy Fluxes ! Let’s take a closer look at the atmospheric energy fluxes ¡ Dry static energy flux = internal + potential energy flux ¡ Latent heat transport = moisture flux Total atmospheric transport Dry static energy transport Latent heat transportMoisture flux ! Annual and zonal mean moisture flux in the atmosphere: Equatorward moisture flux in the tropics Poleward moisture flux in the extratropicsHadley Cell Energy Fluxes ! Equatorward moisture flux & poleward dry static energy flux in the tropics are due to the Hadley cells Moisture near the surface is converged equatorward by Hadley cells High dry static energy air aloft causes total transport to be away from the equatorHadley Cell Energy Transports ! Large dry static energy fluxes within Hadley cell ensure that total transport is poleward ¡ High potential energy air being moved poleward Total atmospheric transport Dry static energy transport Latent heat transportMidlatitude Moisture Flux ! Poleward moisture flux occurs in midlatitudes ¡ Primarily accomplished by eddiesMoisture Flux in Midlatitudes ! Poleward moisture flux occurs in midlatitudes ¡ Primarily accomplished by eddiesMoisture Flux as an Energy Flux ! Poleward moisture flux acts to flatten temperature gradients just like heat fluxes: ¡ When the moisture condenses at higher latitudes, it warms those latitudes Warming Cooling Cooling WarmingExtratropical Energy Fluxes ! Comparison with dry and total flux: ¡ Dry static energy flux = = flux of internal energy + potential energy Total transport Dry static energy transport v(cpT + gz)Extratropical Energy Fluxes ! Comparison with dry and total flux: ¡ Moisture flux is roughly 50% of the total transport in midlatitudes Total atmospheric transport Dry static energy transportWater Vapor and Global Warming ! With global warming, atmospheric moisture content will increase ¡ 20% increase with 3 K global temperature increase ! What effects will the increased moisture content have on the Earth’s climate? ¡ More moisture flux => flatter temperature gradients in midlatitudes ¡ This should weaken dry static energy transportsEnergy Fluxes in IPCC Simulations ! Change in moisture flux in slab ocean global warming simulations: • Increase in poleward flux in extratropics • Increase in equatorward flux in tropics From Held and Soden (2006)Energy Fluxes in IPCC Simulations ! Energy fluxes in slab ocean global warming simulations: Change in moisture flux Change in total flux Change in dry static energy flux ~70% compensation From Held and Soden (2006)Moisture and Horizontal Temperature Gradients ! Moisture plays major


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