NOAA Technical Memorandum ERL PMEL-86 OBSERVATIONS OF LONG ROSSBY WAVES IN THE NORTHERN TROPICAL PACIFIC William S. Kessler Pacific Marine Environmental Laboratory Seattle, Washington March 1989 UNITEO STATES NATIONAL OCEANIC AND Environmental Research OEPARTMENT OF COMMERCE ATMOSPHERIC ADMINISTRATION Laboratories Robert A. Mosbacher William E. Evans Joseph O. Fletcher Secretary Under Secretary for Oceans Director and Atmosphere/ Administrator NOTICE Mention of a commercial company or product does not constitute an endorsement by NOAA/ERL. Use of information from this publication concerning proprietary products or the tests of such products for publicity or advertising purposes is not authorized. Contribution No. 1113 from NOAA/pacific Marine Environmental Laboratory For sale by the National Technical Infonnation Service, 5285 Port Royal Road Springfield, VA 22161 11 CONTENTS PAGE ABSTRACT .... 1 INTRODUCfION. 1 1. DATA COLLECTION AND PROCESSING .. · 3 A. Introduction . · 3 B. Description of bathythennograph data sets. .4 C. Quality control. · 7 D. Gridding of data . · 8 E. Wind data. .... · 9 Table 1 . .11 Chapter 1 figures . .12 2. A SIMPLE MODEL OF LOW-FREQUENCY PYCNOCLINE DEPTH VARIATIONS . .17 A. Introduction............................. .17 B. The simple model. ............................ .17 C. The effect of mean zonal geostrophic currents: the non-Doppler effect. .21 Chapter 2 figures. ........................ .27 3. THE MEAN AND THE ANNUAL CYCLE OF THE DEPTH OF THE OF THE 20·C ISOTHERM IN THE TROPICAL PACIFIC. .32 A. Introduction . .32 B. The mean thennal structure of the tropical Pacific. .. .32 C. The annual cycle of 20·C depth ............ .35 D. The annual cycle of wind stress curl. ......... .37 E. Comparison of the simple model with observations . .38 Chapter 3 figures . .44 4. INTERANNUAL VARIABILITY OF 20·C DEPTH .. .77 A. Introduction . .77 B. Interannual variability on the eastern boundary .77 C. El Nino-related variability of the wind field ..... .79 D. Comparison of the simple model with observations. .80 Chapter 4 figures. ..... .87 5. SUMMARY AND CONCLUSIONS .108 6. ACKNOWLEDGMENTS . .112 7. REFERENCES. ...... .113 iii APPENDIX 1. A RAY THEORY MODEL OF THE RESPONSE TO PULSE-LIKE EASTERN BOUNDARY FORCING .118 A. Introduction . .118 B. Ray theory . .119 C. Ray results for individual frequencies .. 126 D. The total response to pulse-like forcing: a summation over many frequencies. .131 Appendix 1 figures. ........... .139 APPENDIX 2. MEAN VERTICAL/ZONAL AND VERTICAL/MERIDIONAL TEMPERATURE SECTIONS. .151 Appendix 2 figures. 152 LIST OF FIGURES 1.1 XBT data distribution .......... .12 1.2 MBT data distribution .......... .13 1.3 Seasonal distribution of MBT data. ... .14 1.4 Mean XBT-MBT 20°C depth difference. .15 1.5 Total observations in each 2° by 5" box . .16 2.1 Internal Rossby radii (first baroclinic mode) . .27 2.2 Internal long gravity wave speed (first baroclinic mode) . .28 2.3 Rossby dispersion diagrams for various mean background flows . .30 3.1 Mean 20°C depth in the tropical Pacific . .44 3.2 Mean 20°C depth in the Philippine Sea . .45 3.3 Mean temperature at 400 m depth .............. .46 3.4 Standard deviation of 20°C depth (annual and interannual) . .47 3.5 Percent of 20°C depth variance in the annual cycle ........ .48 3.6 Amplitude and phase of the first annual harmonic of 20°C depth . .49 3.7 Meridional profiles of observations, ULT and Ekman pumping. .50 3.8 Phase of the annual harmonic of20°C depth along the equator .... .51 3.9 Amplitude and phase of the first annual harmonic of wind stress curl. .52 3.10 Idealized traveling wave with spatially independent forcing.. .53 3.11 Correlation of ULT with 20°C depth at various wavespeeds. .54 3.12 Rossby wave speed estimates as a function oflatitude. .... .55 3.12b Rossby speed estimates with various choices ofc. ...... .56 3.13a Annual cycle observations, ULT and Ekman pumping at 4°N. .57 3.13b As 3.13a but at 6°N .. .58 3.13c As 3.13a but at 8°N.. .59 3.13d As 3.13a but at looN. .60 3.13e As 3.13a but at 12°N . .61 3.13f As 3.13a but at 14°N . .62 3.13g As 3.13a but at 16°N . .63 3.13h As 3.13a but at 18°N ....................... .64 3.14a Average 20°C depth, ULT and Ekman pumping during January .65 3.14b As 3.14a but during February . .66 3.14c As 3.14a but during March . .67 iv 3.14d As 3.14a but during April .68 3.14e As 3.14a but during May. .69 3.14f As 3.14a but during June. .70 3.14g As 3.14a but during July. .71 3.14h As 3.14a but during August .72 3.14i As 3.14a but during September. .73 3.14j As 3.14a but during October .. .74 3.14k As 3.14a but during November. .75 3.141 As 3.14a but during December. .76 4.1 Time series of 20'C depth on the eastern boundary .87 4.2 Lag correlation ofeastern boundary 20·C depths .88 4.3 Lag correlation ofequatorial 20·C depths ..... .89 4.4 Zonal wind stress on the equator . .90 4.5 EOF 3 of zonal wind stress. ............ .91 4.6 Wind stress curl at 12'N and 20·C depth at the eastern boundary . .92 4.7 Lag correlations of 20°C depths with 160'W at lOON and 200 N. .93 4.8 Correlation of ULT with 20°C depth at various wavespeeds. .94 4.9 Correlation of ULT and Ekman pumping with 20'C depth . .95 4. lOa Interannual observations, ULT and Ekman pumping at 8°N . .96 4. lOb As 4. lOa but at WON . .97 4.10c As 4. lOa but at 12°N . .98 4.10d As 4. lOa but at 14°N . · .99 4. We As 4.10a but at 16°N . .100 4.10f As 4.10a but at 18°N . · 101 4. 109 As 4.10a but at 200 N. .102 4.lOh As 4. lOa but at 22°N .. .103 4.11 Observed 20°C anomalies after the 1972 El Nino . .104 4.11 Continued . .105 4.12 20°C depths at BON, 125"W and at the equator, 80 0 W .106 4.13 Lag correlation between the time series in 3.12 .107 Al.1 Frequency distribution of gaussian pulse . .139 Al.2 Schematic Rossby wavenumber diagram with boundary . .140 Al.3 Rossby ray paths for 1/2-year period waves ........ · 141 A1.4 Ray variables for I-year period waves before the caustic . .142 Al.5 Ray variables for I-year period waves after the caustic .... .143 Al.6 Ray variables for 9-month period waves with a sloping coast. .144 Al.7 Ray variables for 2-year period waves with a sloping coast. .. .145 Al.8 Time evolution of a Rossby pulse leaving the eastern boundary. .146 Al.8 Continued . .147 Al.9 Time history of a Rossby pulse at various latitudes ..... ...... .148 Al.I0 Time history of a Rossby pulse 15,000 km west of the boundary at 15"N. .149 Al.11 Time history of a Rossby pulse at the eastern and western boundaries .150 0 A2.1a Mean temperature at 90 W during 1970 through 1987. .' · 152 A2.1b Mean temperature at llOo W during 1970 through 1987 . .153 A2.1c Mean temperature at 1300 W during 1970 through 1987 . · 154 A2.1d Mean temperature at 1500 W during 1970 through 1987 . .155 A2.1e Mean temperature at l700 W during 1970 through 1987 . .156 A2.lf Mean temperature at 170 0 E during 1970 through 1987 .157 v A2.1g Mean temperature at 1500E during 1970 through 1987 .158 A2.1h Mean temperature at 1300E during 1970 through 1987 .159 A2.2a Mean temperature at 25"N during 1970 through 1987 . .160 A2.2b Mean temperature at 200N during 1970 through 1987 .. 161 A2.2c Mean temperature at 15°N during 1970 through 1987 .... .162 A2.2d Mean temperature at WON during 1970 through 1987. ... .163 A2.2e Mean temperature at 5°N during 1970 through 1987 .... .164 A2.2f Mean temperature at the equator during 1970 through 1987. .165 A2.2g Mean temperature at 50S during 1970 through 1987 .. .166 A2.2h Mean temperature at 100S during 1970 through 1987 . .167 A2.2i Mean temperature at 15°S during 1970 through 1987 . .168 A2.2j Mean temperature at 200S during 1970 through 1987 . .169 LIST OF TABLES Table 1 Summary of the disposition of the BT profiles during quality control ...... .11 VI Observations ofLong Rossby Waves in the Northern Tropical Pacific William S. Kessler* ABSTRACT. Long baroclinic Rossby waves are potentially important in the adjustment of the tropical Pacific pycnocline to both annual and interannual wind stress curl fluctuations. Evidence for such waves is found in variations of the depth of the 20'C isotherm in the northern tropical Pacific during 1970 through 1987. 190,000 bathythermograph profiles have been compiled from the archives of several countries; the data coverage is dense enough that westward-propagating events may be observed with a minimum of zonal interpolation. After extensive quality control, 20'C depths were gridded with a resolution of 2' latitude, S' longitude and bimonths; statistical parameters of the data were estimated. A simple model of low-frequency pycnocline variability allows the physical processes of Ekman pumping, the radiation of long (non-dispersive) Rossby waves due to such pumping in mid-basin, and the radiation of long Rossby waves from the observed eastern boundary pycnocline depth fluctuations. Although the wind stress curl has very little zonal variability at the annual period in the northern tropical Pacific, an annual fluctuation of 20'C depth propagates westward as a long Rossby wave near 4'-6'N and 14'-18'N in agreement with the model hindcast.