Has the AMOC Weakened or Strengthened during the Last Five Decades?

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Has the AMOC Weakened or Strengthened during the
Last Five Decades?
Chunzai Wang NOAA/AOML Miami, Florida
Shenfu Dong Ernesto Munoz UM/CIMAS Miami, Florida
Wang, C., S. Dong, and E. Munoz, 2009 Seawater
density variations in the North Atlantic and the
Atlantic meridional overturning circulation.
Climate Dynamics, in press. Available for
downloading at www.aoml.noaa.gov/phod/people/wang.
html
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Dickson et al. (2002, Nature) reported a
freshening of the deep ocean in the subpolar seas.

• Previous observational studies have focused on
  salinity and freshwater variability in the
  sinking region of the North Atlantic (NA).
• However, the AMOC is density-driven.
• Density depends on both T S.

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Temperature salinity (T S) difference between
1985-99 and 1956-69
Curry et al. (2003, Nature)
However, Curry et al.s approach did not
represent the AMO signal since the period does
not match the AMO phases.
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Based on very limited temporal sampling!
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Atlantic Multidecadal Oscillation (AMO)

• Previous observational studies of AMO focus on
  SST only.
• Does the upper ocean temperature of NA vary with
  AMO?
• Does salinity in NA vary with AMO?
• How does density in NA vary?

NOAA Atlantic Oceanographic Meteorological
Laboratory
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Datasets and Ocean Reanalysis Used in This Study

• Temperature and salinity (T S) dataset of the
  World Ocean Database 2005 (0-700 m and
  1957-2003).
• Three ocean reanalysis products (1) SODA (Simple
  Ocean Data Assimilation), (2) GECCO (German
  Estimating the Circulation and Climate of the
  Ocean) and (3) GFDL (Geophysical Fluid Dynamics
  Lab.).
• Given the T S data, we calculate potential
  density.
• E-P from the Southampton Oceanography Center
  freshwater flux climatology.

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Long-term trends (global warming) of upper ocean
T S anomalies

• Tropical and subtropical Atlantic Ocean become
  warm and salty.
• Subpolar NA basin is secularly cooled and
  freshened.
• Long-term trends are maximized in the Gulf Stream
  region.

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Long-term trend (global warming) of upper ocean
density anomaly

• Density in the subtropical NA Ocean is secularly
  decreased, but trend in the subpolar NA is small.
  
• Why?

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Zonally-averaged trends (global warming) of T S
anomalies

• Trends do not change sign vertically.
• Freshening in the subpolar NA can reach deeper,
  whereas increased salinity in the
  tropical/subtropical Atlantic is limited to the
  upper layer of 200 m.
• The Nordic Seas and Arctic Ocean become warmer
  associated with global warming.
• ? Melting ice in the Arctic Ocean?

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Zonally-averaged trend (global warming) of
density anomaly

• The opposite (compensating) effect of T S
  trends on density trend is obvious.
• The compensating effect makes density trend small
  in subpolar NA.
• Large decreasing trend of density around 40N is
  due to the warming-induced density in the Gulf
  Stream and its eastward extension region.

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How does the upper ocean vary with the AMO?
In the subpolar NA Ocean of 50N-75N, 60W-10E.

• The AMO signal is also manifested in the upper
  ocean temperature.
• Salinity anomalies vary with the AMO.
• However, density anomalies do not exactly
  coincide with the AMO.
• This occurs because density anomaly is dependent
  on both T S contributions.
• Density anomalies follow contribution by T more
  closely than that by S, indicating the dominant
  role of T.

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T S anomaly difference (AMO - AMO-) in the
upper ocean

• AMO (AMO-) is associated with a warm (cool)
  upper ocean of subpolar NA.
• AMO (AMO-) corresponds to a salty (fresh) upper
  ocean of subpolar subtropical NA.
• However, the Gulf Stream region is cool and fresh
  (warm and salty) in association with AMO (AMO-).
  
• Unlike SST, the upper ocean in the TNA and
  Caribbean Sea is cool (warm) associated with AMO
  (AMO-).
• Why?

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Zonally-averaged T S anomaly difference (AMO -
AMO-)

• Like previous studies used SST, the sea surface
  is warm in TNA.
• However, surface warming is accompanied by
  subsurface cooling.
• Thus, the average temperature over 0-700m is
  cool.
• Mechanisms The AMOC-induced variation through
  basin-scale thermocline adjustment by
  coastal/equatorial Kelvin Rossby wave
  propagations?

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Zonally-averaged density anomaly difference (AMO
- AMO-)

• Overall, the sea surface of NA is lighter and the
  subsurface becomes denser.
• The denser subsurface in TNA results from
  subsurface cooling effect.
• The denser subsurface at higher latitudes is
  mainly due to the increase in salinity.

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Relationship of T S anomalies in subpolar and
subtropical NA

• T S anomalies in subpolar NA lag those in
  subtropical NA by about 8-9 years.
• This suggests that anomaly signal in high
  latitude originates from low latitude.
• Provide an observational evidence for previous
  model studies that contend that northward
  advections of T S anomalies affect AMOCs
  variability (e.g., Latif et al. 2000 Krebs
  Timmermann 2007).

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Meridional density gradient between subpolar
subtropical NA

• In some cases, density anomalies in subpolar and
  subtropical NA tend to vary out of phase.
• Regardless of the out-of-phase relationship, a
  meridional density gradient can be formed.
• The meridional density gradient shows an upward
  trend, with larger values since the 1990s.
• We hypothesize that the meridional density
  gradient is related to the AMOC.

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Streamfunction from ocean reanalysis

• Each reanalysis behaves differently.
• But all show a clockwise circulation cell of the
  AMOC.

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Maximum streamfunction representing the AMOCs
strength

• All show upward trend.
• An increased rate of 0.5 Sv per decade.
• The result is opposite to Bryden et al. (2005,
  Nature).
• Other reanalysis?

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Relationship of the AMOC with the meridional
density gradient

• AMOC is positively correlated with the density
  gradient.
• Indicate that meridional density gradient is a
  driving force for the AMOC.

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Summary

• For the long-term trend (global warming),
  subpolar NA is becoming cooler and fresher,
  whereas subtropical NA is becoming warmer and
  saltier.
• Owing to opposite contributions by T S trends,
  density trend in subpolar NA is small.
• Both upper ocean T S anomalies vary with the
  AMO, but density anomalies do not precisely
  coincide with the AMO.
• These variations do not result in a density
  reduction in subpolar NA for slowing the AMOC
  down.
• Meridional density gradient between subpolar and
  subtropical NA suggests that the AMOC has become
  stronger.
• T S anomalies in subpolar NA lag those in
  subtropical NA by 8-9 years, suggesting that the
  anomaly signal at high latitude originates from
  low latitude.

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Implications

• A protocol for measuring the meridional density
  gradient between subpolar and subtropical NA
  Ocean may be useful for monitoring AMOC
  variability.
• Whether the AMOC is weakened under future global
  warming will probably depend upon the density
  variation patterns in NA basin, determined by the
  combined fluctuations of both ocean T S.

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Lozier et al. (2008, Science)
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ECMWF Ocean reanalysis (Balmaseda et al. (2007,
GRL)
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Contributions and Findings of Our Research

• Show co-variability of salinity, temperature and
  density on the long-term trend (related to global
  warming) and multidecadal variation (i.e., the
  AMO).
• Show different features of NA Ocean variability
  on the long-term trend and multidecadal
  timescales, and quantify the S T contributions
  to potential density.
• Show that the AMO signal can reach deeper than
  just SST manifestation.
• Show that the AMO signal is also manifested in
  the upper ocean salinity.
• Find that T S anomalies in the subploar NA lag
  those in the subtropical NA by about 8-9 years.
• The AMOCs strength is related to the meridional
  density gradient between the subpolar and
  subtropical NA Ocean.
• The AMOC may have strengthened during the past
  decades.

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Mean states in the sea surface of the North
Atlantic
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Zonally-averaged mean salinity, temperature and
density
The cool water due to the heat loss (to the
atmosphere) can sink and induce oceanic
convection.
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Mean salinity, temperature and potential density
over 0-700 m

• Unlike at the sea surface, ITCZ is not obvious,
  indicating that ITCZ is shallow.
• Maximum T is in the subtropical region.
• Low/high patterns reflect the subtropical and
  subpolar gyres.

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Density anomalies in subpolar and subtropical NA

• Density anomalies differ from the AMO though T
  S show the AMO.
• Density anomalies in the NA are dominated by T
  contributions.
• In some cases, density anomalies in subpolar and
  subtropical NA tend to vary out of phase.

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Density anomaly difference (AMO - AMO-) in the
upper ocean

• Opposite effects of T S on density are shown
  again.
• On average, density anomalies are positive
  (negative) in western (eastern) part of the basin.