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On Interannual Variability and Climate Change in the North Pacific

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On Interannual Variability and Climate Change in the North Pacific ON INTERANNUAL VARIABILITY AND CLIMATE CHANGE IN THE NORTH PACIFIC By David Kurt Salmon RECOMMENDED: APPROVED: Dean, SchodjafLFisheries and Ocean Sciences Dean of me Graduate Schott Date ON INTERANNUAL VARIABILITY AND CLIMATE CHANGE IN THE NORTH PACIFIC A THESIS Presented to the Faculty of the University of Alaska Fairbanks in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY By David Kurt Salmon, B.A. Fairbanks, Alaska Abstract Long term changes in the atmospheric and oceanic environment of the North Pacific were investigated for the period 1946-1991. A climatology of North Pacific wind stress curl was developed because of the relevance of changes in wind stress curl to both oceanic and atmospheric variability. The dominant scales of spatial and temporal wind stress curl variability were determined and examined within the context of observed changes in North Pacific air temperature, sea surface temperature (SST), sea ice cover, oceanic mass transport and the occurrence of blocking anticyclones. Relationships between these variables and indices of tropical Pacific variability were also determined on interannual time scales. During 1976-1988, phase relationships were very strong between long term mean anomalies of wind stress curl, SST, air temperature, sea ice cover, The Pacific North American index, the Southern Oscillation Index (SOI), and tropical Pacific SST. Long term mean anomalies of these parameters did not change sign during 1976-88. These strong phase relationships did not occur amongst these variables during any other period of the record. The 1976-1988 period is characterized by intensified storminess, the decreased occurrence of blocking anticyclones, and decreased sea ice cover in the subarctic North Pacific. Intensified atmospheric circulation also occurred in the western Pacific subtropical anticyclone. Anomalously low SST occurred across the central and western North Pacific during this period while anomalously high SST was present in the eastern North Pacific adjacent to North America. Changes in the sign of the long term mean anomalies of wind stress curl, central North Pacific SST and the SOI suggest that this climate regime ended or relaxed after 1988. After 1975, long term changes in anomalies of the Southern Oscillation Index, tropical Atlantic wind stress, Sahel rainfall, and Greenland Sea ice cover have characteristics similar to those observed in the North Pacific. It is suggested that the climate anomalies observed in the North Pacific during 1976-1988 occurred as part of a hemispheric or global scale climate regime. Contents List of Figures........................................................................................ ix List of Tables ............................................................................................ xii Acknowledgments................................................................................ xiv Overview................................................................................ 1 1) A Climatology of North Pacific Wind Stress Curl ......... 5 Introduction ........................................................................................... 5 The Role of Wind Stress Curl in Forcing Oceanic Variability ......... 5 Previous Studies................................................................................... 9 Data and Computations ........................................................................... 12 Analysis ........................................................................................................ 15 Computed and Real Boundary Effects on Wind Stress Curl............ 15 Mean Pressure Field ............................................................................ 18 Annual Mean Curl ............................................................................... 20 Mean Fields and Variance .................................................................. 22 Annual and Semi-annual Harmonics and Spectral Energy ............. 32 Annual Cycle of Zonal Mean Curl ...................................................... 36 Annual Cycle of North Pacific Sverdrup Transport ........................ 41 Comparison with Previous Studies .................................................... 47 vi Empirical Orthogonal Function (EOF) Analyses .............................. 53 The EOF Method.................................................................................. 53 Experiment SI..................................................................................... 55 Experiment N Sl.................................................................................. 65 Comparison with Rienecker and Ehret ............................................ 65 Regional Interannual Variability of North Pacific Wind Stress Curl....................................................................................... 68 Regional Wind Stress Curl Series ..................................................... 68 Correlations and Coherence between Regional Curl Fields ........... 81 Correlations between Regional Curl and EOF Time Amplitude Functions............................................................................................ 83 Wind Stress Curl over the Bering And Chukchi Sea Regions 1981-1990..................................................................................... 85 The Seasonal Cycle .............................................................................. 85 Interannual Variability ....................................................................... 89 EOF Analysis ............................................................. 92 Oceanographic Effects of Interannual Wind Stress Curl Variability ............................................................................. 97 Wind Stress Curl and Observed Transport and Property Variations in the Northeast Pacific.................................................... 97 Wind Stress Curl and Ocean Transports in the Western Subarctic Pacific ................................................................................. 102 Wind Stress Curl and Planetary Waves in the Northeast Pacific ................................................................................ 104 Summary and Discussion ....................................................................... 107 2) Interannual Variability in the North Pacific Climate System ................................................................................... i l l Introduction............................................................................................. I l l Previous Studies................................................................................ 112 Data and Methods ................................................................................... 114 Analysis ..................................................................................................... 115 Regional Interannual SST Variability ............................................ 115 EOF Analysis of North Pacific SST ................................................. 121 Interrelationships between North Pacific Climate Variables 128 Relationships between the Large Scale Fields of SST and Wind Stress Curl............................................................................... 128 Mechanisms of Interaction Between SST and Wind Stress Curl... 135 Implications of Decadal Scale Climate Variability ...........................137 Interannual Variability of Wind Stress Curl, SST, and Subarctic AirTemperatures .............................................................. 140 Relationships between Wind Stress Curl and Sea Ice Cover 145 Interannual Variability in the Tropical Pacific Atmosphere: The Southern Oscillation Index and the Pacific North American Index ...................................................................... 149 Relationships between Tropical Variables, Extratropical SST and Wind Stress Curl ........................................................................ 154 Summary and Discussion ...................................................................... 162 3) Interannual Variability of North Pacific Blocking Activity and its Relation to Climate Change ..................... 167 Introduction............................................................................................. 167 Data .................................................... 169 Definition and Characteristics cf Marine Ridges .............................169 A n alysis....................................................................................................... 174 Comparison with White and Clark [1975] ...................................... 174 Marine Ridging Activity 1946-1990 .................................................. 174 Marine Ridging Activity as a Precursor to El Nino-Southern Oscillation Events ................................................................................176 Relationships between Marine Ridging and the Pacific North American Teleconnection ......................................................... 183 Decadal Scale Variability in Marine Ridging Activity ................... 186 Summary and Discussion...................................................................... 194 4) Conclusions.................................................................... 197 Relationships between Interannual Variations in the Pacific and Atlantic................................................................................ 198 R eferences...........................................................................
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