Seasonal and interannual variabilities in a model of the equatorial Pacific Ocean
Seasonal and interannual variabilities in a model of the equatorial Pacific Ocean
A general circulation model of the equatorial Pacific ocean is developed and used to investigate the seasonal and interannual variabilities of the ocean's upper layer in response to surface wind forcing. Three major simulations are performed: the annual mean, the seasonal cycle and the 1982/83 El Niñ. The surface wind data used to force the model is obtained from Florida State University (FSU). The winds for the annual mean experiment are ten year averages from 1961-1970 and the winds for the experiment of the seasonal cycle are the ten year averages for individual months. The monthly mean winds from January 1982 to December 1983 are used for the simulation of the 1982/83 El Niñ event. The results from the annual mean experiment suggest that although the total heat storage of the ocean basin has increased the ocean upper layer experienced considerable cooling. At the end of the 27 months' spin-up integration, the cold water tongue was enhanced and the warm water pool evident in the western Pacific at the beginning of the integration almost disappeared. Heat budgets for this experiment showed that the cooling was caused by horizontal advection, and there was a lack of surface heating to compensate for this heat loss. The effect of diffusion was small compared with advection. The results also showed that the model produced a too diffuse thermocline in the eastern half of the ocean basin, which has been a common problem of general ocean circulation models. The equatorial current system was reproduced well. The seasonal variability of the sea surface temperature is annual in the east and semi-annual in the central and west, which again is controlled by horizontal advection. The annual range of this variation is small: 2.5oC in the east and less than 1oC in the west. This may be attributed to the small seasonal variations of the surface winds. The seasonal variability of the major zonal currents can be summarised as follows: the South Equatorial Current (SEC) has a semi-annual signal being strong in winter and summer and weak in spring and autumn; the Equatorial Undercurrent (EUC) fluctuates in opposition to the SEC; the North Equatorial Countercurrent (NECC) and the North Equatorial Current (NEC) vary in phase and are most intense in autumn and almost disappear in spring. Meridional circulation is present throughout the year and is weakest in spring and gets its maximum strength in autumn. The simulation of the 1982/83 El Niñ is successful in reproducing the major features of the 1982/83 El Niñ such as the disappearance of the equatorial undercurrent, the pattern and magnitude of the sea surface temperature anomaly and the variations of the thermocline. However there is a delay of about 2-3 months in these events in the model compared with those observed. This is attributed to the wind stress forcing during 1982 in the eastern equatorial Pacific, which is more intense in the FSU wind dataset than in the National Meteorological Centre (NMC) data set.
University of Southampton
Jia, Yanli
74140e23-0366-4bd2-b483-53c7a43315cb
1989
Jia, Yanli
74140e23-0366-4bd2-b483-53c7a43315cb
Jia, Yanli
(1989)
Seasonal and interannual variabilities in a model of the equatorial Pacific Ocean.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
A general circulation model of the equatorial Pacific ocean is developed and used to investigate the seasonal and interannual variabilities of the ocean's upper layer in response to surface wind forcing. Three major simulations are performed: the annual mean, the seasonal cycle and the 1982/83 El Niñ. The surface wind data used to force the model is obtained from Florida State University (FSU). The winds for the annual mean experiment are ten year averages from 1961-1970 and the winds for the experiment of the seasonal cycle are the ten year averages for individual months. The monthly mean winds from January 1982 to December 1983 are used for the simulation of the 1982/83 El Niñ event. The results from the annual mean experiment suggest that although the total heat storage of the ocean basin has increased the ocean upper layer experienced considerable cooling. At the end of the 27 months' spin-up integration, the cold water tongue was enhanced and the warm water pool evident in the western Pacific at the beginning of the integration almost disappeared. Heat budgets for this experiment showed that the cooling was caused by horizontal advection, and there was a lack of surface heating to compensate for this heat loss. The effect of diffusion was small compared with advection. The results also showed that the model produced a too diffuse thermocline in the eastern half of the ocean basin, which has been a common problem of general ocean circulation models. The equatorial current system was reproduced well. The seasonal variability of the sea surface temperature is annual in the east and semi-annual in the central and west, which again is controlled by horizontal advection. The annual range of this variation is small: 2.5oC in the east and less than 1oC in the west. This may be attributed to the small seasonal variations of the surface winds. The seasonal variability of the major zonal currents can be summarised as follows: the South Equatorial Current (SEC) has a semi-annual signal being strong in winter and summer and weak in spring and autumn; the Equatorial Undercurrent (EUC) fluctuates in opposition to the SEC; the North Equatorial Countercurrent (NECC) and the North Equatorial Current (NEC) vary in phase and are most intense in autumn and almost disappear in spring. Meridional circulation is present throughout the year and is weakest in spring and gets its maximum strength in autumn. The simulation of the 1982/83 El Niñ is successful in reproducing the major features of the 1982/83 El Niñ such as the disappearance of the equatorial undercurrent, the pattern and magnitude of the sea surface temperature anomaly and the variations of the thermocline. However there is a delay of about 2-3 months in these events in the model compared with those observed. This is attributed to the wind stress forcing during 1982 in the eastern equatorial Pacific, which is more intense in the FSU wind dataset than in the National Meteorological Centre (NMC) data set.
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Published date: 1989
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Local EPrints ID: 461575
URI: http://eprints.soton.ac.uk/id/eprint/461575
PURE UUID: 46572fc4-0f22-4688-9bbf-25c3f43fa6c8
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Date deposited: 04 Jul 2022 18:50
Last modified: 04 Jul 2022 18:50
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Author:
Yanli Jia
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