Decadal Variability of the Indo-Pacific Warm Pool and Its Association with Atmospheric and Oceanic Variability in the NCEP–NCAR and SODA Reanalyses

Hui Wang The Center for Research on the Changing Earth System, Columbia, Maryland

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Vikram M. Mehta The Center for Research on the Changing Earth System, Columbia, Maryland

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Abstract

Decadal variability of the Indo-Pacific warm pool (IPWP) sea surface temperature (SST) and its association with atmospheric and oceanic circulations are investigated with observed 50-yr (1952–2001) SST, and the NCEP–NCAR atmospheric and Simple Ocean Data Assimilation (SODA) oceanic reanalysis data. The decadal variability of the IPWP SSTs was analyzed by applying an empirical orthogonal function technique to low-pass-filtered SSTs. Two leading empirical modes (EMs) well represent the IPWP SST decadal variations. EM1 is an ENSO-like pattern with out-of-phase SST anomalies in the western Pacific and the Indian Ocean, whereas EM2 displays an in-phase relationship between SST anomalies in the two regions. Consequently, spatial evolution of EM1 is dominated by opposing changes in zonal and meridional dimensions and thus a strong deformation of the warm pool on decadal time scales. EM2 is dominated by changes in size and intensity of the warm pool.

Analyses of ocean thermodynamic fields associated with the two SST EMs indicate that decadal changes in the IPWP can extend down to 300-m depth. Oceanic processes may thus be involved in the IPWP decadal variability, including advections of mean temperature by both mean and anomalous ocean currents and effects of shallow tropical circulations (STCs) on the IPWP SST, which is consistent with some previous studies on tropical decadal variability. The results may also indicate the existence of both positive and negative feedbacks between the IPWP SST and the STCs.

Both December–January–February (DJF) and June–July–August (JJA) atmospheric circulations exhibit thermally direct responses to the two decadal IPWP SST EMs by altering the Hadley and Walker circulations. In addition, significant upper-level rotational flow anomalies in the extratropics are found to be associated with the decadal IPWP SST variability. Consistent with the upper-level flow anomalies and 850-hPa convergence–divergence patterns associated with the two SST EMs are rainfall anomalies over the United States. In DJF, the rainfall anomalies are mainly in Florida, the Gulf Coast, southern Texas, Arizona, and along the West Coast. In JJA, the rainfall anomalies are mainly in the Midwest and the Southeast. Since these rainfall anomalies are a significant fraction of seasonal-average rainfall and since these anomalies persist for many years, they potentially make a significant impact on U.S. water resources and agriculture. Further analysis of observations and modeling studies are required to understand the physics of the IPWP SST decadal variability and its impacts on global climate, and to assess its predictability.

* Current affiliation: Wyle Information Systems, and NOAA/Climate Prediction Center, Camp Springs, Maryland.

Corresponding author address: Vikram M. Mehta, The Center for Research on the Changing Earth System, 10211 Wincopin Circle, Suite 240, Columbia, MD 21044. Email: Vikram@crces.org

Abstract

Decadal variability of the Indo-Pacific warm pool (IPWP) sea surface temperature (SST) and its association with atmospheric and oceanic circulations are investigated with observed 50-yr (1952–2001) SST, and the NCEP–NCAR atmospheric and Simple Ocean Data Assimilation (SODA) oceanic reanalysis data. The decadal variability of the IPWP SSTs was analyzed by applying an empirical orthogonal function technique to low-pass-filtered SSTs. Two leading empirical modes (EMs) well represent the IPWP SST decadal variations. EM1 is an ENSO-like pattern with out-of-phase SST anomalies in the western Pacific and the Indian Ocean, whereas EM2 displays an in-phase relationship between SST anomalies in the two regions. Consequently, spatial evolution of EM1 is dominated by opposing changes in zonal and meridional dimensions and thus a strong deformation of the warm pool on decadal time scales. EM2 is dominated by changes in size and intensity of the warm pool.

Analyses of ocean thermodynamic fields associated with the two SST EMs indicate that decadal changes in the IPWP can extend down to 300-m depth. Oceanic processes may thus be involved in the IPWP decadal variability, including advections of mean temperature by both mean and anomalous ocean currents and effects of shallow tropical circulations (STCs) on the IPWP SST, which is consistent with some previous studies on tropical decadal variability. The results may also indicate the existence of both positive and negative feedbacks between the IPWP SST and the STCs.

Both December–January–February (DJF) and June–July–August (JJA) atmospheric circulations exhibit thermally direct responses to the two decadal IPWP SST EMs by altering the Hadley and Walker circulations. In addition, significant upper-level rotational flow anomalies in the extratropics are found to be associated with the decadal IPWP SST variability. Consistent with the upper-level flow anomalies and 850-hPa convergence–divergence patterns associated with the two SST EMs are rainfall anomalies over the United States. In DJF, the rainfall anomalies are mainly in Florida, the Gulf Coast, southern Texas, Arizona, and along the West Coast. In JJA, the rainfall anomalies are mainly in the Midwest and the Southeast. Since these rainfall anomalies are a significant fraction of seasonal-average rainfall and since these anomalies persist for many years, they potentially make a significant impact on U.S. water resources and agriculture. Further analysis of observations and modeling studies are required to understand the physics of the IPWP SST decadal variability and its impacts on global climate, and to assess its predictability.

* Current affiliation: Wyle Information Systems, and NOAA/Climate Prediction Center, Camp Springs, Maryland.

Corresponding author address: Vikram M. Mehta, The Center for Research on the Changing Earth System, 10211 Wincopin Circle, Suite 240, Columbia, MD 21044. Email: Vikram@crces.org

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