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(Microsoft Powerpoint Circulación Oceánica y Clima Mecanismos y Patrones de la Circulación Oceánica profunda Antonio García-Olivares Institut de Ciències del Mar, CSIC, Barcelona [email protected] • La atm es continuamente inestabilizada al ser calentada desde abajo – Movimientos advectivos verticales – turbulencia • El océano es calentado desde arriba – generalmente estable y poco turbulento verticalmente La superficie mezclada versus el interior oceánico -Estratificación: Gradiente densidad potencial (densidad tras descontar la compresión) -Capa de mezcla: alta turbulencia, estratificación nula, alta T, baja densidad, alta mezcla -Termoclina: Alta estratificación, barrera a la difusión hacia arriba de CO 2, sales, T -Océano profundo: débilmente turbulento, débilmente estratificado Potential density • Movimientos oceánicos: – Corrientes de superficie (vientos y grad densidad): reparto desigual de la energía solar – Circulación profunda (turbulencia, vientos, grad densidad): idem – Mareas (energía gravitacional de la Luna) – Tsunamis (liberación de energía interna terrestre) La circulación vertical • If the stratification in the ocean was very strong and never disrupted, decomposition processes in the bottom waters would consume all of the oxygen and we would find only bacteria and viruses that can live without oxygen • It is evident that the bottom waters must be re- supplied with oxygen from the atmosphere from some large-scale water motion or circulation in the oceans. • De qué dependen: – La estratificación – Las tasas de circulación del agua profunda – El intercambio entre superficie y abismo? Trabajos de Sandström (1874-1947) - La inestabilidad provocada por un foco térmico frío en la superficie sólo provoca un llenado transitorio de agua fría hacia el fondo - Entre una placa cálida y otra fría en superficie sólo hay una débil circulación superficial, sobre un abismo frío = − α = − α −α < = − α −α > dW / dt ∫ dp ∫ ( 2→3 4→1)dp 0 dW / dt ∫ ( 4→1 2→3 )dp 0 C El área de un ciclo p(V) es la integral de p dV pero tb puede calcularse con v(p) y la integral de -v dp La dilatación debe producirse a mayor p (más profundidad) que la contracción térmica Océano actual - No muy diferente de lo previsto por advección horizontal - Isotermas e isopicnas de fondo en contacto con la superficie de latitudes altas S N Potential temperature section 25ºW (Atlantic) – WOCE A16 65ºN – 55ºS Pero: Se observa una vigorosa corriente profunda de unos 16 Sv en el N • Para que pueda haber una celda profunda activa bajo la superficial, tiene que haber una fuente de energía E igual a la E perdida por rozamiento viscoso • Una de esas fuentes es la producción de KTE. Una fracción γ no se disipa sino que intercambia cubitos de densidad diferente en la picnoclina, creando Ep Habría que entender la situación real como resultado de tres procesos : 1. La inestabilidad convectiva en latitudes altas ha llenado el abismo de aguas frías, a T = Tartic = 2 – 2,5ºC hasta cerca de la superficie 2. La turbulencia trae dV de alta ρ arriba y dV de baja ρ abajo, que puede ascender dejando un hueco para dV laterales de alta ρ (corriente profunda) 3. El W de vientos en los upwellings sube dV de alta ρ, dejando huecos análogos Nuevos hundimientos de agua profunda se pueden producir sólo si se producen esos huecos (“buoyancy”) gracias a 2 y 3 The effect of winds on the vertical movement of water Upwelling along the coast caused by Ekman transport of waters (waters move to the right of the wind). The waters moved offshore are replaced by waters from below. This brings cold, nutrient rich waters to the surface Downwelling caused by Ekman transport onshore (movement of water to the right of the wind direction). - Upwelling more frequent in coasts around gyres -Upwelling in equator: divergencia de vientos -Provocan afloramientos de agua subsuperficial Si sólo hay difusión: Modelo de Munk, 1966: -En estado estacionario de la picnoclina: -la advección de ρρρ hacia arriba debe igualar la difusión de ρ hacia arriba: - w y k son parámetros independientes, en este modelo - w y ρ en distintas z pueden medirse - ajustando a la ec. anterior por mínimos cuadrados, se puede estimar k -Se obtiene: w = 0.7 E -7 m/s (25 Sv ), k = 1 cm 2 /s -Medidas de microestructura y lanzamiento de trazadores pelágicos dan: kp = k / 10 - k >> k p se observan cerca de las plataformas, en islas, promontorios del fondo, y dorsales centrooceánicas HRP Highlights: Deep ocean mixing controlled by topography …. Causas de dependencia entre Q(t) y estado climático 1. En los estadiales, la banda de westerlies australes está corrida hacia el ecuador menor wind-stress sobre el paso de Drake menor upwelling de aguas profundas 2. En un océano muy estratificado (estadiales) la tasa de mezcla turbulenta del agua profunda hacia arriba debe disminuir, para una tasa de producción de KTE cte 3. En estadiales el V oceánico es menor mayor efecto de las mareas en la producción de KTE. Pero 1 + 2 >> 3 Explicación de (2): La tasa a la que la que la Ep debe ser añadida para subir agua de fondo que está siendo formada con caudal Q es: ≈ ∆ρ ρ E& p Q gh donde h es la altura típica del cambio de (picnoclina) = γε γ ≅ E& p donde (Ri) 2.0 es la mixing efficiency y ε es la producción y disipación global de KTE −1 - Si εεε = cte Q ≈ (∆ρ) -Análogamente, si hay un estado estacionario entre mezcla vertical y formación de aguas profundas Q, toda la subida de la picnoclina derivada de Q debe ser eliminada por una difusión de densidad hacia la superficie: Que escala como: w / h = k / h 2 O bien: k = w h = (Q / A) h . ≈ ∆ρ Por tanto: Q = k A / h Sustituyendo en (1): E& p Ak g CONCLUSIÓN: Si la potencia de KTE es cte (mareas y esfuerzos de viento) entonces las tasas de mezcla vertical deben ser inversamente proporcionales a las diferencias de densidad - Evidencias de aguas profundas peor ventiladas en el LGM Para la dependencia entre Q y k : análisis del caudal superficial que cierra el bucle u Geostrofía: L Aproximación “viento térmico”: v 1 1 g ∂ p = −(ρ + ρ)g ⇒ f∂ u = ∂ − ∂ p = − ∂ (∂ p) = ∂ ρ z 0 z z ρ y ρ y z ρ y 0 0 0 Que escala como: donde suponemos: Ly = L x = L Además, la profundidad D de picnoclina escala como: Y si suponemos isotropía horizontal: U / L = W / D La solución del sist de 3 ecs (en U, D y W) es: La situación es más compleja: Una parte de la energía la aporta el viento en el upwelling de latitudes del paso de Drake -En latitudes de Drake el agua superficial no tiene costa oriental sobre la que acumularse -No hay gradiente de p que devuelva atrás y hacia el continente el agua recirculación superficial del upwelling es más costosa que la profunda -Hay orografía bajo 2500 m -El agua puede volver en profundidad al S -El agua es extraida desde esa zona, y para retornar a esas profundidades, debe densificarse de nuevo hasta densidades de fondo -Lo consigue sólo tras pasar por el N (Klinger et al, 2004) Agua densa Un modelo realista debe Incluir: - Mezcla diapicna - Upwelling austral Gnanadesikan (1999): Upwelling Ts y mixing Tu conducen la convección Tn PROY : Mejorar el modelo de Gnanadesikan • Si hay elevación de agua densa por mezcla turbulenta y por upwelling, entonces puede haber descenso de agua densa que la sustituya. • Este descenso se producirá primero en los lugares con formación de aguas superficiales más densas Deep Water Circulation - Currently, deep waters are "formed" where the air temperatures are cold and where the salinity of the surface waters are relatively high . - The combinations of salinity and cold temperatures make the water denser and cause it to sink to the bottom. The Gulf Stream carries salt into the high latitude North Atlantic where the water cools. The cooling and the added salt (sea ice formation) cause the waters to sink in the Norwegian Sea. The formation of Atlantic Deep Water is 15 Sv. • En el Océano Indico las aguas superficiales son demasiado cálidas para hundirse: latitud • En el Pacífico Norte las aguas son frías, pero no suficientemente saladas para hundirse. • Causa: Los vientos medios que rodean al planeta son interceptados por las cadenas montañosas N-S de América (Rocosas y Andes) y descargan su humedad. La nieve y lluvia resultantes drenan hacia el Pacífico, devolviendole una gran parte del agua dulce. • En el Atlántico N las aguas son más salinas que la media. • Causas: – A) Una parte de la evaporación cruza el istmo de Panamá y precipita en aguas del Pacífico. – B) El Mediterráneo – C) Ausencia de cadenas montañosas alrededor de la cuenca vapor hacia Asia que abandona la cuenca Salinidad oceánica en superficie (en gramos de sal por kg de agua) Formación de aguas profundas (NADW) entre Groenlandia, Islandia y Noruega -El hundimiento se produce en: -Zonas de rotación cilónica, donde la picnoclina es succionada arriba por el bombeo de Ekman y la superficie hacia abajo -Aguas muy salinas del S que se mezclan con aguas del N procedentes de la formación de hielo -Regiones de vientos invernales fríos e intensos -Mar de Noruega (entre Greenland y Norway, al N de Iceland) Fram Strait Formación de hielo marino y salmuera sobre la plataforma antártica (mar de Wedell y mar de Ross) El agua se contrae entre 0º y -1.9º C Se enfría en superficie y se hunde. Sólo cuando toda la columna está a -1.9º C comienza la congelación por arriba. Esto sólo puede ocurrir en columnas poco profundas, no en mar abierto Dominante en los inicios de las eras glaciales.
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