Upper Ocean Circulation

Transcription

1 Upper Ocean Circulation C. Chen General Physical Oceanography MAR 555 School for Marine Sciences and Technology Umass-Dartmouth 1

2 MAR555 Lecture 4: The Upper Oceanic Circulation The Oceanic Circulation Types The upper circulation The deep circulation The upper circulation The thermocline circulation (temperature) or the wind-driven circulation It principally exists in the upper few hundreds of meters and mainly driven by surface wind stress. The deep circulation The thermohaline circulation (salinity) prevails in the deep ocean and mainly driven by the buoyancy forcing associated with cooling (heated) by the cold (warm) air, and modified by sources and sinks of the fresh water. Observed currents: a result of the combined effects of wind- and buoyancy-driven forcings. 2

3 The Global Wind Field: sinking air sinking air rising air 30 o 60 o 90 o N Polar High easterlies Subpolar Low westerlies Subtropical High north-east trades 60 o heavy precipitation 30 o large evaporation rising air 0 o Equatorial Low 0 o heavy precipitation sinking air rising air 30 o 60 o south-east trades Subtropical High westerlies Subpolar Low easterlies Polar High 90 o S 60 o 30 o large evaporation heavy precipitation sinking air 3

4 ITCZ: Inter-tropical convergence zone 60 o E 90 o 120 o 150 o 180 o 150 o 120 o 90 o 60 o 30 o w 0 o 60 o 45 o 30 o ITCZ 15 o 0 o 15 o 30 o 45 o 60 o Wind Field on the northern hemisphere July 4

5 ITCZ Wind field on the northern hemisphere January 5

6 Wintertime upper layer circulation Subpolar gyre Subtropic gyre Kuroshio Gulf Stream N. Equatorial Current Equatorial Counter Current S. Equatorial Current The Antarctic circumpolar current 6

7 1. The Atlantic Circulation System In the northern hemisphere: Subtropic Gyre The Gulf Stream and Rings Subpolar Gyre In the southern hemisphere: Subtropic Gyre The Atlantic circumpolar circulation Equatorial current patterns are very similar in the Atlantic and Pacific Oceans, which will be described later. 7

8 40 o Subpolar Gyre Cape Hatteras Sargasso Sea North Atlantic Drift 40 o 20 o Subtropic Gyre North Equatorial Current 20 o Guyana Current 90 o 30 o 0 o The Gulf Stream: A strong narrow current flowing northward along the continental slope through the Florida Strait and leaves to the interior of the North Atlantic Ocean at Cape Hatteras. Bowing to popular usage, oceanographers also include its extension part between Cape Hatteras and Grand Banks as 8 continuous flow of the Gulf Stream.

9 Total wind-induced southward volume transport in the interior of the ocean is balanced by the a narrow northward flow on the western boundary West East Wind stress! x Gulf Stream Wind-induced subtropic gyre flow 9

10 Good light condition, mixed layer depth: m in summer, 400 m in winter Limit the upward nutrient flux from the deep ocean to the euphotic layer 10

11 Main Sources of nutrients supply in the mixed layer: 1) Nutrient recycling among the food web a nutrients-phytoplankton-zooplankton system; 2) Upward flux through the thermoclines turbulent diffusion and meso-or small scale eddy interaction. Sea Surface grazing Mixed layer P uptake NH 4 Z excretion Nutroclines NO 3 Turbulent diffusion High nutrients Deep NO 3 pool Vertical profile of NO 3 11

12 Global Distribution of Chl-a Concentration Biological Desert Subpolar gyre: mg/m2 In the divergence zone near the equator: mg/m2 Subtropic gyre: 5-25 mg/m2 12

13 The Gulf Stream: The warm and saline water; Steeply sloping isotherms (temperature) and isohalines (salinity); Cross-shelf~ 100 km; A vertical scale of ~ 1000 m; The surface current ~ 100 cm/s 100 km 20 Depth (m) Depth (m)

14 The position of thermal front of the Gulf Stream taken from the satellite images and surface tracking drifters Cape Hatteras 80 Sv 150 Sv 30 Sv 1 Sv (Sverdrup) = 10 6 m 3 /s 14

15 QS: Why does the Gulf Stream transport increase when it move northward along the slope and after it enters the interior North Atlantic Ocean? Recirculation and rings! 18 degree water 40 o Subpolar Gyre 20 o Cape Hatteras Sargasso Sea Subtropic Gyre Guyana Current North Atlantic Drift North Equatorial Current 40 o 20 o 90 o 30 o 0o 15

16 30 20 A direct current measurement on a north-south section of 55 o W (Hogg et al. 1986) 10 0 Surface Blue: The recirculation flow Zonal Velocity (cm/s) m 2000 m Red: The eastward flow Solid black line: Measurement uncertainty Recirculation increases with depth m o N 40 o 35 o 30 o Latitude (degree) At ~4000 m, it can reach ~10 cm/s 16

17 A chart of the topography of the 150 isothermal surface showing the Gulf Stream, cold-core rings and warm-core rings 17

18 Instability of the Gulf Stream front: eddy formation at the shelf break. Inner shelf Mid-shelf 0 Outer-shelf 50 km 100 Eddy scale: L ~ km Depth (m) o 24 o 22 o 20 o upwelling 18 o 16 o (km) Upwelling advects nutrients upward to the euphotic layer: producing a near-surface phytoplankton bloom. The euphotic layer depth: > 20 m in the outer shelf. 18