Natural decadal-multidecadal variability of the Indo-Pacific Warm Pool and its impacts on global climate

Vikram M. Mehta and Amita V. Mehta

The Center for Research on the Changing Earth System, Columbia, MD
The Joint Center for Earth System Technology, NASA/GSFC and Univ. of Maryland, Baltimore County, Maryland
USA

Importance of the IPWP in the global climate system

  • Warmest surface ocean water on the Earth
  • Annual-average SST>=28°C from approximately 90°E-180°, 20°S-20°N; pronounced annual cycle
  • Saturation vapor pressure non-linearly related to SSTdramatic increase in atmospheric moisture content and convection when SST >= ˜28.5°C
  • THE major source of heat for the global atmosphere
  • Numerous studies of possible mechanisms of maintenance of time-average state of the IPWP (Ramanathan and Collins, 1991; Wallace, 1992; Fu et al., 1992; Hartman and Michelsen, 1993; Waliser and Graham, 1993; Waliser, 1996; Sud et al., 1999)
  • Natural variability has not received much attention; influences ENSO and marine ecosystems at interannual timescales (Delcroix et al., 2000; Picaut et al. 2000)
Objectives of this study
  1. To quantify long-term variability of the IPWP
  2. To quantify its impacts on global climate
Indo-Pacific Warm Pool area and SST variations in binned and objectively-analysed sea-surface temperature observations

  • Area and warmest SST of Eastern Indian Warm Pool (EIWP) and Western Pacific Warm Pool (WPWP) from binned, ship-based SST observations 1909-1998; 10º longitude-10º latitude grid boxes; low-pass filtered time series shown as black line
  • Warm Pool area defined as area enclosed by the 28.5º C SST isotherm; average EIWP area 6 million sq. kms. and average WPWP area 12 million sq. kms.
  • Note that decadal-multidecadal departures from average EIWP area reach 100% of average, implying that the EIWP can disappear or double in size for decades
  • While not as large, WPWP area departures can reach 20-40% of average area for a decade or longer

  • Area and warmest SST of EIWP and WPWP from
    objectively-analyzed, ship-based SST observations 1903-
    1994 by seasons; 1º longitude-1º latitude grid boxes
  • EIWP area largest (11 million sq. kms.) in MAM and
    smallest (4 million sq. kms.) in DJF; WPWP area nearly
    constant
  • Similar absolute amplitudes of variability in all seasons

IPWP SST and area variability

  • Irregular oscillations in IPWP area, some times in phase on the EIWP and WPWP sides
  • SST oscillations largely in phase with area oscillations
  • Average oscillation period ~ 8-10 years; irregular on EIWP side
  • Warmest SSTs move along the SPCZ
  • SSTs in tropical North Pacific near the Central American coast oscillate in phase with WPWP
  • EIWP area highly variable; some times disappears completely, some times extends to the African coast
  • EIWP and WPWP some times expand-contract together or move east-west together
  • Equatorial and SPCZ arms of WPWP oscillate in phase

 

 

IPWP sea-surface temperatures from SODA: 1950-2001 Climatology + low-pass filtered


[Quicktime movie: 2.7MB]

Low-frequency, upper-ocean temperature variability in the eastern Indian and western Pacific Warm Pools; SODA; 1950-2001

  • Surface and subsurface anomalies of same sign, subsurface anomalies much larger than surface anomalies
  • Relationship between EIWP and WPWP anomalies not constant
  • Shallower anomalies in EIWP bacause of shallower thermocline in the eastern Indian Ocean?

 

Ocean-atmosphere variability in the WPWP region

  • Annual-average rain (Hulme), evaporation (NCEP), zonal stress (NCEP), SST (SODA), and upper-ocean heat content (SODA) anomalies, averaged over the WPWP region
  • Several multiyear episodes of a nearly-simultaneous increase in upper-ocean heat content, SST, zonal stress, and rainfall; with or without any increase in local evaporation

850-mb wind anomalies associated with decadal-multidecadal variability of the western Pacific Warm Pool: 1949-1998

  • Low-pass (>=8 years) wind anomalies; composited according to the phase of the low-pass filtered, western Pacific Warm Pool SST
  • Note the equatorial westerlies (1-2 m/s) when Warm Pool less warm(Fig. a) and in the following year (Fig. b)
  • Note the equatorial easterlies (1-2 m/s) when Warm Pool warmer(Fig. c) and in the following year (Fig. d)
  • Also, note coherent mid- and high-latitude wind anomalies in both phases, especially over the North Pacific and the North Atlantic, confirming previous results by Mehta et al. (2000) and Hoerling and Hurrel (2002)
Vertical temperature structure and upper-ocean heat content in WPWP and equatorial eastern Pacific: SODA, 1950-2001

  • Temperature anomalies from surface to 200m in the WPWP (shaded) and in the equatorial eastern Pacific (contoured) vary with almost-opposite phases at decadal-multidecadal timescales, implying slow variability of the equatorial thermocline
  • Largest temperature anomalies below 100m in WPWP and above 100m in EPac
  • Upper-ocean (surface to 125 m) heat content anomalies in the WPWP and Epac also vary with almost-opposite phases at decadal-multidecadal timescales

Evaporation anomalies associated with decadal-multidecadal variability of the western Pacific Warm Pool: 1949-1998

  • Low-pass (>=8 years) evaporation anomalies; composited according to the phase of the low-pass filtered, western Pacific Warm Pool SST
  • Note near-average evaporation in the WPWP and decreased evaporation in the North Pacific subtropics when WPWP less warm (Fig. a)
  • Note small area of increased evaporation south of the equator in the WPWP and increased evaporation in the North Pacific subtropics and in tropical central Pacific when WPWP less warm (Fig. b)
  • Is the subtropical North Pacific the major source of moisture for the rain over the WPWP?

Composite, annual rainfall anomalies:1950-1998
  • Uneven regional sampling in the Hulme rainfall data
  • Rainfall in the IPWP region above average (8-10% of the average) when the WPWP warmer than average
  • Rainfall in the IPWP region below average (8-10% of the average) when the WPWP less warm than average
  • Less (more) than average rainfall over northern South America when the WPWP warmer (less warm) than average
  • Local and distant rainfall variability appears to be related to Walker circulation variability
  • Seasonal composites suggest above- average rainfall in the Indian monsoon region when the WPWP warmer than average


Summary
  • SST, areal extent, and upper-ocean heat content of the IPWP undergo pronounced decadal-multidecadal variability; some times in phase on the EIWP and WPWP sides
  • Average oscillation period ~ 8-10 years; irregular on EIWP side
  • Largest temperature anomalies below 100m in WPWP; same sign of surface and subsurface temperature anomalies
  • SSTs in tropical North Pacific near the Central American coast oscillate in phase with WPWP ITCZ plays an important role?
  • EIWP area highly variable; some times disappears completely, some times extends to the African coast
  • Multiyear episodes of upper-ocean heat content, SST, zonal stress, and rain increases over the WPWP, with or without any increase in local evaporation
  • Thermally-direct variability of the Walker circulation associated with the IPWP variability
  • Variability in global atmospheric circulation associated with the IPWP variability
    modulates the North Pacific and North Atlantic Oscillations
  • Upper-ocean temperatures and heat content in the equatorial eastern Pacific vary with almost-opposite phase to that of the WPWP temperatures and heat content at decadal-multidecadal timescales modulates interannual ENSO variability (see poster)?
  • Decadal-multidecadal IPWP variability also appears to modulate impacts of interannual El Niño-La Niña events on global climate, especially on Australian and North American climates (other studies)


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