System Earth 4a

Oceans II

Misha Velthuis

m.velthuis@uva.nl

Tue 24 Sept 2024

Recap

ENSO

Termohaline circulation

Recap

What makes the oceans move?

Tidal energy

Wind stress

Thermohaline density differences

The four steps of wind stress

Step 1: Surface currents: at an angle of 20-45° of wind direction

Step 2: Ekman transport: net flow 90° of wind direction

Step 3: Geostrophic wind: perpendicular to the pressure gradient force

Step 4: Boundary currents: western an eastern

ENSO

El Niño Southern Oscillation

Irregular periodic variation in winds and sea surface temperatures over the tropical Pacific ocean.

ENSO: three phases of ocean-atmosphere interaction

Phase 1: normal conditions

Phase 2: strengthening of normal pattern: La Niña

Phase 3: weakening of normal pattern: El Niño

Phase 1: normal conditions (schematic)

Phase 1: normal conditions (visual)

Easterlies push water to the west.
Warm water accumulates in front of the coast of Australia/Indonesia.
Thermocline incline: deep in west, shallow in east.
Low pressure zone and convection in front of the coast of Australia/Indonesia.
Low pressure zone pulls in more air from the east.
At higher altitudes: air travels back (west → east) and subsides in front of the coast of Peru.
Shallow thermocline in front of coast Peru allows for cool (nutrient rich) upwelling: adding to the cooler temperature.

Thermocline: "an abrupt temperature gradient in a body of water such as a lake, marked by a layer above and below which the water is at different temperatures."

A thermocline reduces mixing (low density water floats on top, preventing water from below reaching the surface layer, almost like a layer of oil)
Strong heating at surface = generally strong thermocline
A shallow thermocline means a relatively thin upper layer of warmer, less dense water.

Phase 2: la niña (shematic)

Phase 2: la niña (visual)

Phase 3: el niño (shematic)

Phase 3: el niño (visual)

Zooming out: phase 1 (normal)

Zooming out: phase 3 (El Niño)

What causes the onset of an el niño

Several processes interact

See e.g.: Santoso, A., Hendon, H., Watkins, A., Power, S., Dommenget, D., England, M. H., … & Delage, F. (2019). Dynamics and predictability of El Niño–Southern Oscillation: An Australian perspective on progress and challenges. Bulletin of the American Meteorological Society, 100(3), 403-420. link

Ray, A., Rakshit, S., Basak, G. K., Dana, S. K., & Ghosh, D. (2020). Understanding the origin of extreme events in El Niño southern oscillation. Physical Review E, 101(6), 062210. link

Some details from Santoso et al. (2019):

Every ENSO event is unique.

Two archetypical El Niño events: one where the SST anomalies peaked in the central pacific (CP) and one where the SST anomalies peak in the eastern pacific (EP).

The type of El Niño (CP or EP) appears to be as important in explaining the impact (in Australia) as the strength of the El Niño event.

Theories:

The classical (1997) recharge-discharge oscillator theory: "periodical recharges and discharges of the zonal mean equatorial heat content, due to ocean-atmosphere interaction" Wiki

Quote from Ray et al. (2020)

"A close inspection of the system dynamics using several bifurcation plots reveals that a sudden large expansion of the attractor occurs at a critical system parameter via a type of interior crisis. This interior crisis evolves through merging of a cascade of period-doubling and period-adding bifurcations that leads to the origin of occasional amplitude-modulated extremely large events."

Impact El Niño

Impact on temperature

Currently

Thermohaline circulation

What makes the oceans move?

Tidal energy

Wind stress

Thermohaline density differences

Driven by density differences

Temperature

Salinity

Movement

Vertical: relatively dense water sinks to bottom

Horizontal: basin topogrophy + sloping isopycnals

Changes in density

Bottomwater formation

Downwelling and deep water bodies

Salinity-temperature-depth

Thermohaline circulation

Water that sinks down and flows away from point of downwelling …

… is replaced by surface currents

Thermohaline conveyor belt

Antarctic complexity

Gulf stream

Gulf stream is both wind driven, and part of the thermohaline circulation.

Around 40°0′N 30°0′W, it splits in two, with the northern stream, the North Atlantic Drift, crossing to Northern Europe and the southern stream, the Canary Current, recirculating off West Africa.

See p. 56 of this overview from IPCC:

"The Gulf Stream is part of two major circulation patterns, the Atlantic Meridional Overturning Circulation (AMOC) and the North Atlantic Subtropical Gyre"

btw ...

From Wiki:

Benjamin Franklin became interested in the North Atlantic Ocean circulation patterns. In 1768, while in England, Franklin heard a curious complaint from the Colonial Board of Customs: "Why did it take British packets several weeks longer to reach New York from England than it took an average American merchant ship to reach Newport, Rhode Island, despite the merchant ships leaving from London and having to sail down the River Thames and then the length of the English Channel before they sailed across the Atlantic, while the packets left from Falmouth in Cornwall?"

Franklin asked his cousin Timothy Folger, a Nantucket Island whaling captain, for an answer. Folger explained that merchant ships routinely crossed the current—which was identified by whale behaviour, measurement of the water's temperature, and changes in the water's colour—while the mail packet captains ran against it. Franklin had Folger sketch the path of the current on a chart of the Atlantic and add notes on how to avoid the current when sailing from England to America. Franklin then forwarded the chart to Anthony Todd, secretary of the British Post Office. Franklin's Gulf Stream chart was printed in 1769 in London, but it was mostly ignored by British sea captains. A copy of the chart was printed in Paris circa 1770–1773, and a third version was published by Franklin in Philadelphia in 1786.

Cold blob

Cold water appears to be sinking less.

System Earth 4a

Oceans II

Misha Velthuis

m.velthuis@uva.nl

Tue 24 Sept 2024

Contents

Recap

What makes the oceans move?

The four steps of wind stress

ENSO

El Niño Southern Oscillation

ENSO: three phases of ocean-atmosphere interaction

Phase 1: normal conditions (schematic)

Phase 1: normal conditions (visual)

Phase 2: la niña (shematic)

Phase 2: la niña (visual)

Phase 3: el niño (shematic)

Phase 3: el niño (visual)

Zooming out: phase 1 (normal)

Zooming out: phase 3 (El Niño)

What causes the onset of an el niño

Several processes interact

Impact El Niño

Impact on temperature

Impact on temperature

Currently

Thermohaline circulation

What makes the oceans move?

Driven by density differences

Movement

Changes in density

Bottomwater formation

Downwelling and deep water bodies

Salinity-temperature-depth

Thermohaline circulation

Thermohaline conveyor belt

Thermohaline conveyor belt

Antarctic complexity

Gulf stream

Cold blob

Gulf stream slowing down?

Gulf stream slowing down?