Oceanography of the tropical Pacific Ocean Part 2 ... · on the tropical Pacific Ocean •...

Rarotonga, Oct 2011 1

Oceanography of the tropical Pacific Ocean ‐Part 2‐

Historical changes and future projections

A. Ganachaud1, A. Sen Gupta2, J. Brown3, L. Muir3,

with contributions from J. Orr, S. Wijffels, K. Ridgway, M. Hemer, C. Maes, C. Steinberg, A. Tribollet, B. Qiu, J. Kruger

1 Oceanographer,

Institut de Recherche pour le Développement,

Nouméa, New Caledonia2 Climate modeling expert,

Centre for Climate Change Research,

University of New South Wales,

Sydney, Australia3 Climate modeling experts,

Centre for Australian Weather and Climate Research,

CSIRO Wealth from Oceans National Research Flagship

Hobart, Tasmania, Australia


Temperature trends

Cravatte et al. 2009

Guyennon et al. 2011, submitted

trend (°C/50yr)

Time series from an in situ sensor in Noumea


Temperature trends

trend (°C/50yr) Extension of the Warm Pool (10yr average)

19601970198019902000



Salinity trends

trend (°C/50yr) Extension of the Warm Pool (10yr average)

1970198019902000



Outline

State of the Ocean1. Provinces2. Currents3. Warming and stratification4. Vertical structure 5. Nutrients and oxygen6. Nutrient supply processes7. Waves and Sea Level Rise

Observed and Projected changes


Ocean state: Trade Winds and Warm Pool

Classical view :The Trade Winds pile up warm

waters in the west


Ocean state: Oceanic provinces ("biomes")


Pacific Ocean circulation

Source: Fieux, M. 2010

Ocean currents transport nutrients, oxygen and fishlarvae


Ocean state: currents

• Winds create two broadwestward flows in the tropical Pacific

SouthEquatorial

Current

NorthEquatorial

Current




• ITCZ and SPCZ influence on the wind field give rise to two eastward countercurrents




• ITCZ and SPCZ influence on the wind field give rise to two eastward countercurrents

• Archipelagoes and coastslead to strong north‐southcoastal currents


Ocean state: TemperatureVertical Structure

Temperature acrossthe Equator

100m

500m

0m


Ocean state: TemperatureVertical Structure

Presence of stratification in the thermocline

100m

500m

0m

0m

250m

500m

0°C 10°C 20°C 30°CTemperatures


Ocean state: Nutrients

DE

PTH

Nutrients are mostly depleted in the euphotic zone

Replenishment by remineralization of marine snow

Dissolved nitrate at 100m

Oceanic upwelling or mixing isneeded to transfer them to the surface layer

Similar features for phosphate & silicate


Ocean state: Oxygen

DE

PTH

Oxygen is abundant near the surface and depletednear 400m

Replenishment by high latitude atmospheric input and subsurface transport by ocean currents

Dissolved Oxygen at 400m


Ocean state: how to supply the euphotic zone ?

1. Eddies2. Upwelling (vertical current; east

equator and some islands)3. Internal tides4. Mixed layer... against stratification

??


Ocean state: Eddies

Small‐scale circulation generatedspontaneously or by interaction between the large‐scale flow and land


Ocean state: Eddiesand land effects

Small‐scales generatedspontaneously or by interaction betweenthe large‐scale flow and land

Boundary currents



Boundary currentsUpwelling




Boundary currentsUpwellingVertical mixing for

internal tides



Seasonal variations of the mixed layer depthpumps deep nutrientstowards the sunlitzone

Ocean state: Mixed layer


Variability

Seasonal

El Nino

PDO

Eddies

Internaltides

Courtesy J. Lefèvre, IRD

Global Warming


Global Warming

What changes were detectedover the past ~50 years ?

What do IPCC projections suggest ?


•SEC strengthens in the south subtropical gyre•EAC strengthens•Little change in the north

Change in Sea Level (Roemmich, pers. Comm.)

Ocean currents

Observed changes to the circulation


•SEC weakens on the equator•EUC moves upward•Eastward SECC weakens•Little change in the subtropical gyres

Changes 2100/A2 versus 2000Average over 13 IPCC projections

Ocean currents

Projected changes to the circulation


Temperature: recent changes at the surface

Surface Temperature trend over past 50yr

Cravatte et al., 2009

Tropical warming of 0.1°C/decade and Warm Pool expansion

Climatological average

Warm Pool


Temperature: recent changes ... and surface projections

Multi-model projected temperature change(13 IPCC models; A2/2100 versus

1980-2000)

Warming ~2.5°CWeaker warming in the

southeast Pacific


Temperature: recent changes ... and surface projections

Multi-model projected temperature changeadded to 20th century observations

"Warm pool" as per 28°C definition

Interannual variability


Temperature: recent changes at the surface

Surface Temperature trend over past 50yr

Cravatte et al., 2009

Tropical warming of 0.1°C/decade and Warm Pool expansion

Climatological average

Warm Pool


Temperature: recent changes at depths

Temperature change over past 50yr(Durack & Wijffels, 2010)

Contours are average temperature

Warming ~1°C down to 100‐200m

Weaker warming or coolingbelow the thermocline

‐> Enhanced stratification


o Surface water becomes ‘lighter’ more than deep water, due to: Surface warming Surface freshening

o Increased stratification inhibits mixing

Observed change in density with depth

Warm, ‘light’ water

Cold, ‘heavy’ water

Nutrient supply recent changes ... and projections

Courtesy A. S. Gupta


Temperature: recent changes ... and projections

Multi-model projected temperature change(13 IPCC models; A2/2100 versus

1980-2000)

More warming ~2°C down to 80m

Weaker warming or cooling belowthe thermocline

Even more stratification


oOver the past 20 years: Only two time series

oOne suggests a decrease, the second one no trend

oToo few data to be conclusive ! (need continued observations!)

Nutrient supply recent changes

Phosphate concentration in the mixed layer(Watanabe et al. 2005)

Observed changes in nutrientconcentrations



Phosphate concentration in the mixed layer(Watanabe et al. 2005)

What controls nutrient concentration:

oBiological activityoSupply to the euphotic (sunlit)

zone from deep ocean:oStratificationoOcean currentsoTurbulence in the mixed layeroUpwellingoEddies


oStratification increases by 20-30%; especially in the Warm Pool

oThe winter mixed layer shallows by ~20m;

oEquatorial upwelling decreases, but region-wide 9°S-9°N upwelling remains constant.

oEquatorial UnderCurrent reinforced


Projections (2100/A2):

Suggest reduced nutrient supply and therefore biological activity

}Reinforcement possible in the PEQD(Polovina et al.)

}


o More oxygen data than nutriento Major decrease of dissolved oxygen in

the remineralization zone with westward extension of the oxygen minimum zone (OMZ)

Dissolved oxygen recent changes

Stramma et al. 2008


Dissolved oxygen concentration is expected to continue to decline due to high latitude ocean warming

Low oxygen areas are expected to expand

Dissolved oxygen recent changes ... and projections

Stramma et al. 2008


Two more actors: Waves and Sea Level


Wave climate

Projected increase (or decrease)to significant wave height Wang and Swail, 2006

•Too few observations to determine CC trends •The wave "climate" is related to ENSO and other climate signals•Only few projections; not resolved by IPCC models


Sea level rise

•As water warms it expands•As ice-sheets and glaciers melt they increase ocean volume

+20 cm over 100yrsIncreasing sea level:•Alters oceanic ecosystems/habitat•Changes shape of coastlines•Changes nature and extent of mangrove


Sea level rise

•New published estimate suggest IPCC AR-4 was too conservative: +80 cm to +1.4m possible•Regional deviations from global mean are of O(5cm)+20 cm


Conclusions on the tropical Pacific Ocean

• Ocean is warming, withenhanced signal in the upper 100‐200m

• More stratification limitsnutrient supply;

• Mixed layer reach is reduced• Some ocean currents

projected to change;• Equatorial divergence region

projected to shrink

• Ocean is warming, withenhanced signal in the upper 100‐200m

• More stratification limitsnutrient supply;

• Mixed layer reach is reduced• Some ocean currents

projected to change;• Equatorial divergence region

projected to shrink

• Dissolved oxygen decreases; ocean becomes more aciditic

• Sea level has risen by ~20 cm;

• Sea level could rise by another 80‐140 cm

• Dissolved oxygen decreases; ocean becomes more aciditic

• Sea level has risen by ~20 cm;

• Sea level could rise by another 80‐140 cm

Oceanography of the tropical Pacific Ocean Part 2 ... · on the tropical Pacific Ocean •...

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