Title: On Remote Weather Associated with the Subtropical Highs of Pacific Sea Level Pressure
1On Remote Weather Associated with the Subtropical
Highs of Pacific Sea Level Pressure
- Richard Grotjahn
- Atmospheric Science Program,
- Dept. of LAWR, Univ. of California
- Davis, CA 95616, USA
2Subtropical High climatology
North Pacific (JJA)
South Pacific (DJF)
3Remote Forcing Hypotheses for SP high
- 3 remote sources
- Some connections will be visible through the
divergent circulations. - P or OLR are proxy for rising motion.
- Simplest tests is SLP linked to P in target
regions? P intensity? P shifts? P timing?
- (1) Hadley and Walker circulations,
- (2) Rossby wave forcing from East, b v f dw/dz
- (3) traveling frontal cyclones and anticyclones
4Precipitation Climatology
JJA
DJF
5Divergent Circulations
- Vertical motion largest over Amazonia, but
- Stronger motions from SW, W, and N feed surface
divergence
H
H
Meridional x-section 100 W
Zonal x-section 20 S
6Physical Interpretation of Gills Model
Invisicid form
Form vorticity eqn
7Rossby Wave Mechanism deduced from Gills
Tropical Circulation Model
8Analysis Procedures (Monthly Data)
- Preliminary study to identify coincident
behavior. - Monthly NCEP/NCAR Reanalysis data (1979-97).
- Seasonal groupings, local summer emphasized.
- Total and monthly anomaly (MA) fields. (MA
defined as deviations from the average
constructed from all occurrences of that month). - Monthly data cannot distinguish cause from
effect. - Tools (significance test) shown here
- composites (bootstrap resampling)
- 1-point rank correlations (t- and D-statistics).
9 Bootstrap Resampling (part 1)
F2(nx,ny,nt) (P, OLR, DWS, VP, SLP,)
F1(nt) (SLP, MJO, SOI,)
- T1 target group chose based on a criterion.
Each member 2-D field of F1. - T2 similar to T1. Target group for field F2
using same times as for T1. - R1n nth random group drawn from field F1.
Times randomly chosen from the entire record with
replacement but no duplication. Sample size
matches target sample. Many random groups. (e.g.
1000). - R2n - similar to R1n except randomly choices from
F2. Times used differ from those for R1n - For each grid point compare the mean of the
target group vs the means of the random samples
at that grid point.
F1 F2
Composite of the target group for F2 is
Composite of nth random group for F1 is
Composite of nth random group for F2 is
Compare
against all the
10- Bootstrap Resampling (part 2)
- Significance
- Determine separately for each location
point - Distribution from random composites at
each pt. - Level determined by number at a tail times
2 - Distribution can be normal-like,
bimodal, etc - Significant if target composite lies at
either tail (2-tailed test) - Example At point (i,j) of an observed
distribution. The star indicates a significant
target composite T2
Figure II.2 example of null distribution. This null distribution was generated while assessing the significance of the 850 hPa mean temperature. This histogram refers to the grid point closest to Sacramento, and gathers 1000 random samples. The target value has been added and is shown by a star. 99 of the values stand between the two dashed lines. (i.e. 5 random to right tail, 5 to the left)
11Composites ONDJF Monthly Anomaly Data
- E and NE lower SLP (purple) more P (N of South
America) for strong high and vice versa. - N and NW More P and Northward shift of ICZ
- W More P (green) westward shift of SPCZ
- NW N MJO? ENSO?
- S and SW Dipole (P) storm track shift to S for
strong SP high. Tracks may be broader for weak SP
high.
- 6 strongest 6 weakest
- Green significant above (1)
- Purple significant below (1)
12Composites JJA Monthly Anomaly Data
- 6 strongest 6 weakest
- Green significant above (1)
- Purple significant below (1)
- SLP
- Highest when high is NW
- SP high coordinated
- Weak strong composites not opposite
- Precip
- Shift of ICZ southward
- Shift of midlat NW-ward
- Stronger over Indonesia
131-Point Rank Correlations
NHST (Null Hypothesis Significance Test) Given
that F2 at (i,j) is not correlated with F1 at
(M,N), what is the probability that the indicated
correlation could occur by chance? 1 chance
is shaded
141-pt correlations of Monthly Anomaly Data
- P shown, OLR similar
- Blue significant above (1)
- Orange significant below (1)
- Intersection of 2 signif. tests used shaded
tend to match 0.3 correlation. - correlation points respond to events on the same
side. - NE to E Pacific ICZ (shift) and more Amazonian P
- NW side to ICZ SPCZ shift
- E, NE, N, and NW of the SP high center correlate
with less P in the Kiribati area like composites
results - W, SW S Total and MA data both show dipolar
correlation pattern implies poleward shift of
storm track P for higher SLP
151-pt correlations of MA Data NP High
- P shown, OLR similar
- Blue significantly (2.5) more P for higher SLP
at - Brown significantly (2.5) less P for higher SLP
at - H is total data mean location
- Signif. R at remote spots on the same side of the
high as the correl. point. - P near Central America not compelling. For key
points on the East side of the high, less P for
stronger SLP. - Results consistent w/ composites
16Work with Daily Mean Data SP high only
- Data Source
- NOAA/CDC (Boulder CO, USA)
- NCEP/NCAR reanalysis data
- SLP, U, V
- Ud, Vd, Velocity Potential (VP) from NCL commands.
- Data record
- 90-day DJF periods shown (122 day NDJF similar)
- Drawn from 01/1990 through 08/2002
- Goal
- Prior work showed remote links now wish to
establish cause and effect by using lags and
leads.
17Lags and Leads (expressed as F1 Relative to
F2) Example 1 day lag
(1 day lead is similar but F1 leads F2)
18SLP lagged autocorrelationslag (L) and lead (R)
SLP _at_ pt-8 correlations (CW 8, 4, 0, -4, -12d)
19SLP lagged autocorrelationslag (L) and lead (R)
SLP _at_ pt-11 correlations (CW 4, 2, 0, -2, -4d)
20SLP 1-pt Autocorrelation animation
21Velocity Potential (VP) at 200 hPa lag (L) and
lead (R) SLP _at_ pt-8 correlations (CW 8, 6, 4, 2,
0, -2, -4,-6 d)
22SLP-Vel.Pot. 1-pt correlation animation
23Vd Meridional Divergent Windat 200 hPa SLP _at_
pt-11 correlations (CW 4, 2, 0, -2, -4d)
24Cross-correlation points for SLP VP
25VP cross-correlations for SLP on NE side
26DWS cross-correlations for SLP max
271-pt Correlations of Climate Indices
SLP is 2-D field, climate index is the point
value Red significant (1) positive
correlation Blue significant (1) negative
correlation Correlations between SOI and Nino
34 and monthly SLP Nino 34 tends to be
positive when the SOI is negative Both indices
correlate with SLP on equatorial side of SP
high Both indices have some like to opposite
change in midlat storm track. MJO results like
VP shown mainly correlation only on N NE side
of high
28Conclusions
- Equatorial and NE side of SP high highly
correlated with pressure in equatorial E
Pacific. Stronger SLP on N side of SP high is
followed by lower SLP over SE Asia. - Equatorial side of NP high correlated with ICZ.
Relation to precip over Central America
inconsistent with Rossby wave model. - Stronger SP highs are those SW of the mean
position reinforced by divergent winds from
midlatitude cyclones. - Stronger NP highs are those NW of mean position
reinforced by midlat cyclones and Indonesian
precip. - Expansion of Amazonian velocity potential (VP)
min. leads to stronger SP high when reinforced
by weaker E. Indonesian VP min. Both lead to
westward move of VP max over Pacific. - This last item leads a westward migration of
higher than normal SLP on equatorial side of SP
high. - For many points cross spectrum (not shown) has
strong frequency 40d. Presumably consistent MJO
correlations found (not shown).