及金乐 第五章： 大气环流中的纬向环流系统 5.3 Walker Circulation 授课教师：张洋 2022.12.08
第五章： 大气环流中的纬向环流系统 5.3 Walker Circulation 授课教师：张洋 2022. 12. 08
及图 Outline Introduction Features of walker circulation Seasonal variation of walker circulation ENSO and Walker circulation (inter-annual variation) The two-box model of Walker circulation Summary 授课教师：张洋2
授课教师：张洋 2 Outline n Introduction n Features of Walker circulation n Seasonal variation of walker circulation n ENSO and Walker circulation (inter-annual variation) n The two-box model of Walker circulation n Summary
Introduction ■ Walker Circulation was first introduced in 1966 by Prof.Jacob Bjerknes,referring to the large scale atmospheric circulation along the longitude-height plane over the equatorial Pacific Ocean. Alr Clrculaton in a vEdical pla由n atihe Bquat on? THE WALKER CIRGULATION frade w nds .Tahiti Named after Gilbert Walker,a key contributor Pacil c Cccan to Southern Oscillation. 授课教师：张洋3
授课教师：张洋 3 Introduction n Walker Circulation was first introduced in 1966 by Prof. Jacob Bjerknes, referring to the large scale atmospheric circulation along the longitude-height plane over the equatorial Pacific Ocean. Named after Gilbert Walker, a key contributor to Southern Oscillation
及图斯 Introduction High tropospheric isobaric surface Low tropospheric isobaric surface INDIAN PACIFIC ATLANTIC 0° 90°E 180° 90W 0° Longitude (Adapted from Webster 1983) Ar Clreulaton in avedical plabe athgatt THE WALKER CIRCULATION rade w nds ·Tahiti Pacil c Cccan
授课教师：张洋 4 Introduction WALKER CIRCULATION K-M Lau, NASA/Goddard Space Flight Center, Greenbelt, MD, USA S Yang, NOAA/NWS/NCEP, Climate Prediction Center, Camp Springs, MD, USA Copyright 2002 Elsevier Science Ltd. All Rights Reserved. doi:10.1006/rwas.2002.0450 Lau, K-M NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA Yang, S NOAA/NWS/NCEP, Climate Prediction Center, Camp Springs, MD 20746, USA Introduction The term Walker Circulation was first introduced in 1969 by Professor Jacob Bjerknes, referring to the large-scale atmospheric circulation along the longitude–height plane over the equatorial Pacific Ocean. The Walker Circulation features low-level winds blowing from east to west across the central Pacific, rising motion over the warm water of the western Pacific, returning flow from west to east in the upper troposphere, and sinking motion over the cold water of the eastern Pacific. Since Bjerknes’s introduction of the Walker Circulation, there have been reports of similar east–west circulation cells spanning different longitudinal sectors along the Equator. Today, the Walker Circulation generally refers to the totality of the circulation cells as shown in Figure 1. Bjerknes originally named the Pacific east–west circulation the Walker Circulation because he considered it the key part of Sir Gilbert Walker’s Southern Oscillation (see El Nino and the Southern Oscillation: Observation (0148)). He interpreted the Walker Circulation as an atmospheric circulation driven by the gradient of sea surface temperature along the Equator and suggested that the characteristics of the Walker Circulation were largely determined by the coupling between the tropical atmosphere and oceans. Bjerknes’s work on the Walker Circulation marked an important milestone toward our basic understanding of the dynamics of zonal atmosphere–ocean coupling along the equatorial Pacific Ocean. Although his results were based on very limited data, Bjerknes’s original conjecture that the year-to-year variation of the Walker Circulation is closely tied to that of the Southern Oscillation and El Nin˜o has been confirmed by a large number of observational and modeling studies during the several decades since his first report. Climatology and Variability Annual Mean Thanks to the advance in satellite observations and improved assimilation of observations into global general circulation models, we have now a much more detailed and quantitative description of the Walker Circulation. We know that the tropical wind is made up of rotational and divergent components. The former is directly related to the effects of the rotation of the Earth and the latter to the overturning circulation, driven by atmospheric heating processes. The Walker Circulation and associated overturnings in the equatorial plane should refer only to the divergent component of the wind. Figure 2A shows the annual climatology (the mean state of all months) of the overturning circulations along the equatorial plane as streamlines constructed from the divergent zonal and vertical winds. It can be seen that the major rising High tropospheric isobaric surface Low tropospheric isobaric surface INDIAN PACIFIC ATLANTIC 0° 90° E 180° 90° W 0° Longitude 0450-F0001 Figure 1 Schematic view of the east–west atmospheric circulation along the longitude–height plane over the Equator. The cell over the Pacific Ocean is referred to as the Walker Circulation. (Adapted from Webster (1983).) 0450-P0005 0450-P0010 0450-P0015 0450-P0020 WALKER CIRCULATION 1 rwas.2002.0450 26/4/02 13:03 Ed:: M. SHANKAR No. of pages: 6 Pgn:: seetharama (Adapted from Webster 1983)
及金病 Observed features ANNUAL MEAN GLOBAL SEA SURFACE TEMPERATURES 0 4 8 12 16 20 24 28 32 Temperature oc) 搅课致帅：张洋5
授课教师：张洋 5 Observed features
Observed features RH Combined uchi and w (5N-5S) DEC 1999-FEB 2000 Mean 100 200 80 00 6 40 400 85时 500 600 I1s 70 Abrupt variation 000 00E 120E 140E 160E 160 160M 140W 120W 19W 80W Adapted from Emanuel,2005 授课教师：张洋 6
授课教师：张洋 6 Observed features Abrupt variation Adapted from Emanuel, 2005
及图 Observed features great amount of convective cloud in the western pacific 1992010100 授课教师：张洋7
授课教师：张洋 7 Observed features great amount of convective cloud in the western pacific
Observed features Walker Circulation viewed as an air-sea coupled,self- maintained system Convective Loop Equator Thermocline 120°E 80'W Adapted from NOAA 授课教师：张洋8
授课教师：张洋 8 Observed features Walker Circulation viewed as an air-sea coupled, selfmaintained system Adapted from NOAA
ENSO and Walker Circulation EI Nino is defined by prolonged differences in tropical Pacific Ocean SST when compared with the average value.The accepted definition is a warming or cooling of at least 0.5 C averaged over the east-central tropical Pacific Ocean. Typically,this anomaly happens at irregular intervals of 2-7 years and lasts nine months to two years. The Southern Oscillation is the atmospheric component of EI Nino.This component is an oscillation in surface air pressure between the tropical eastern and the western Pacific Ocean waters.The strength of the Southern Oscillation is measured by the Southern Oscillation Index(SOl).The SOl is computed from fluctuations in the surface air pressure difference between Tahiti and Darwin,Australia. 授课教师：张洋 9
授课教师：张洋 9 ENSO and Walker Circulation n El Niño is defined by prolonged differences in tropical Pacific Ocean SST when compared with the average value. The accepted definition is a warming or cooling of at least 0.5 °C averaged over the east-central tropical Pacific Ocean. Typically, this anomaly happens at irregular intervals of 2–7 years and lasts nine months to two years. n The Southern Oscillation is the atmospheric component of El Niño. This component is an oscillation in surface air pressure between the tropical eastern and the western Pacific Ocean waters. The strength of the Southern Oscillation is measured by the Southern Oscillation Index (SOI). The SOI is computed from fluctuations in the surface air pressure difference between Tahiti and Darwin, Australia
及图 ENSO and Walker Circulation El Nifo Conditions Equator 。 Thermocline 120°E 80°W Adapted from NOAA 授课教师：张洋10
授课教师：张洋 10 ENSO and Walker Circulation Adapted from NOAA