Title: Observational Study of the Remote Forcing of the Pacific Subtropical Highs
1Observational Study of the Remote Forcing of the
Pacific Subtropical Highs
- Richard Grotjahn and Sheri Immel Department of
Land, Air, and Water Resources University of
California, Davis, CA, 95616-8627 U.S.A.
2Terminology and Abbreviations
- NP high North Pacific subtropical high.
- SP high South Pacific subtropical high.
- P precipitation
- SLP sea level presssure
- r rank correlation coefficient
- MA anomaly from averages for that month
- OLR outgoing longwave radiation
- EP flux Eliassen-Palm Fluxes
- b meridional derivative of Coriolis parameter
- v meridional component of velocity (lt0 is
southward) - MJO Madden-Julian Oscillation
- ENSO el nino Southern Oscillation
3Introduction
- NP high and SP high have different magnitude and
timing of seasonal change. See Fig. 1. - The highs are forced by local and remote
mechanisms that foster sinking above the high - Three remote mechanisms have been proposed.
(discussed separately below) - This work has modest goals (currently unfunded)
- Primary goal of this study to test if proposed
remote forcing mechanisms are likely or not
likely to play a role in subtropical high
strength using very simple tests.
4Current Mechanism
- Rising motion in deep tropical convection in
target region both Eastward and equatorward of
the highs sets up a pressure pattern and/or
circulation that fosters sinking on the east side
of the high. - That sinking enhances equatorward surface winds
from the proposed balance b v -dw/dz - Simplest test is to see if precipitation in
target region is associated with subtropical high
strength. - Relevant references Hoskins Rodwell (1995),
Hoskins (1996), Hoskins et al. (1999)
5Classical Mechanism
- Rising motion in deep tropical convection
Westward and/or equatorward of the highs leads to
Hadley and/or Walker type circulations with
sinking over the highs - Simplest test is to see if precipitation in
various target regions are associated with
subtropical high strength. - Relevant references extension of textbook
descriptions of zonal average Hadley
circulation to zonally-varying highs, e.g.
Grotjahn (1993). See also Chen (1999)
6Extratropical Forcing Mechanism
- Extratropical cyclones might
- suppress the areal extent of the subtropical
highs OR - have advection and divergent circulations that
enhance the highs. - Simplest tests look in storm track regions for
shifts of SLP and P fields. - Relevant references none? But, many references
find connections between midlatitude eddy forcing
and stationary wave properties particularly at
upper levels. Zonal mean cells e.g. Pfeffer
(1981). Lau Holopainen (1984) include surface
features for the winter NP high
7Analysis Procedures
- 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 shown here simple composites and 1-point
rank correlations. Significance tests are
bootstrap resampling and t- and D-statistics.
8Composite Maps (figs. 2, 3 6)
- Separate combinations of months with strongest
highs and months with weakest. Composites of
strong highs shown. Difference maps strong highs
minus weak highs also available. - Separate composites for SP and NP highs
- MA data shown use extended season months MJJAS
for NP high ONDJF for SP high. Total fields
available. - Boostrap resampling method used to test
significance.
91-Point Correlations (figs. 4 5)
- Rank correlations of 1 SLP factor versus 2-D map
of P. - Two significance tests used. Intersection of the
areas by both tests are shaded. These areas tend
to correspond closely with the 0.3 correlation
coefficient. - Factors tested include
- SLP at a specific grid point
- Longitude of SLP maximum in the high
- Peak value of SLP maximum
- Summertime SP and NP highs considered separately.
- Total field data tends to show larger areas and
more obvious dipolar pattern indicative of
shifting lines of precipitation. MA and total
results most similar along ICZ.
10Results (page 1 of 2)
- Main Caveats
- Cause and effect yet not examined
- Significance only approximately assessed on
composite maps - Significant correlations and composites of P are
found at remote locations - Tropical western Pacific and Indonesia
- ICZ immediately equatorward of the high
- Subtropical latitudes W of each high (e.g. SPCZ)
- Middle latitudes (storm track poleward and W of
each high) - Tendency for each remote factor to mainly affect
its side of the high seen in correlations for
both total and MA data. - Results for MA and total data similar except
fewer shifts (fewer dipole patterns) in P
fields with MA data.
11Results (page 2 of 2)
- P pattern is not opposite for weak highs in
some regions making correlation and composite
data differ. E.g. more rain occurs SE of NP high
for both weak and strong highs. - Total and MA data in composites and correlations
show SP high stronger with P deficit over
Kiribati region and excess over Papua N.G.
Suggests possible intraseasonal link to MJO
and/or interannual link to ENSO. - Evidence supporting the Current view is not
strong it is - weak in NH and
- unclear in SH
- Further works with daily data, improved
diagnostic analyses and model simulations are
needed to establish cause and effect and
illuminate the dominant mechanisms.
12References cited
- Chen, P., 1999 On the origin and seasonality of
the subtropical anticyclones and cyclones.
Submitted to J. Atmos. Sci. (Presented orally at
last AOFD meeting in NY, 6/99) - Grotjahn, R., 1993 Global Atmospheric
Circulations Observations and Theories, Oxford
Univ. Press, 390 pp. - Hoskins, B., 1996 On the existence and strength
of the summer subtropical anticyclones. Bul.
Amer. Met. Soc., 77, 1287-1292. - _____, and M. Rodwell, 1995 A model of the Asian
summer monsoon. Part I The global scale. J.
Atmos. Sci., 52, 1329-1340. - _____, B., R. Neale, M. Rodwell, G.-Y. Yang,
1999 Aspects of the large-scale tropical
atmospheric circulation. Tellus, 51A-B, 33-44 - Lau, N.-C., and E. Holopainen, 1984 Transient
eddy forcing of the time-mean flow as identified
by geopotential tendencies. J. Atmos. Sci., 41,
313-328. - Pfeffer, R., 1981Wave-mean flow interactions in
the atmosphere. J. Atmos. Sci., 38, 1340-1359.
13Future Work
- Data Work with daily data and other fields,
especially the divergent circulation and OLR.
Supplement observations with numerical model data
as needed. - Observations Refine and greatly expand the
analytical and statistical techniques. Examples
include lag/lead correlations with significance
tests, lag/lead composites with bootstrap
testing, observational evaluation of terms in key
equations (e.g. 3-D EP fluxes, as well as simple
composites of terms), trajectory analysis. - Modeling Design numerical model experiments such
as sensitivity tests for remote and local
forcing quantities, baseline studies to better
understand observed significant events. Consider
applications of local interest such as air
quality. Proposed model MM5.
14Signup/Comments
- Would you like more information? Please visit our
website http//www-atm.ucdavis.edu/grotjahn/Subh
i/ - A handout is available. If Im out of handouts,
please sign up below. - Name Email Postal Address
15Figure 1 NP and SP Climatology
- SLP maximum value during each calendar month in
the 19 year data record. Median value is blue
line, values between the red and blue are in the
3rd quartile, those below the yellow line are in
the 1st quartile. - Beneath are similar plots for latitude and
longitude location using grid point numbers. The
interval is 2.5 degrees in each direction and the
values increase towards the E from the Greenwich
meridian.
16 Figure 2 NP high Composites
- 6 strongest NP highs in May-Sept. periods of the
19 year record. 6 weakest in those periods as
well. MA data shown. - SLP significant anomaly regions differ between
strong and weak. They are not opposite. Strong
highs may have some association with stronger SP
high and SH extratropical lows (Fig. 6) - P significant anomaly regions Central Pacific
ICZ is closer and stronger for strong high, E.
Indonesian P stronger for strong high, roughly
opposite patterns for weak highs. E Pacific
shifted S for weak highs, but even further S for
strong highs. Central American P positive anomaly
for both composites. Midlatitude P over Aleutians
may be greater for stronger highs.
17 Figure 3 SP high Composites
- 6 strongest SP highs in Oct.-Feb. periods of the
19 year record. 6 weakest in those periods as
well. Total and MA data shown. - Significant SLP regions are opposite between
strong and weak high composites over Amazonia and
the far western equatorial Pacific. - Significant P regions entire Pacific ICZ is
stronger further away for strong high. Total
data show ICZ shift away for strong, towards for
weak SP high. MA data have significant regions
only where P is greater e.g. SPCZ stronger for
strong highs. - P in the Kiribati region (15 degrees N of Fiji)
is opposite for weak and strong highs less P
there (with ICZ SPCZ split) for strong SP high,
more P there for weak high. - P along extratropical storm track stronger for
strong highs. There may be greater spread for
weak highs. - Presumably MJO and ENSO can enhance or reduce the
convection in this region and thus could be
linked to SP high strength.
18 Figure 4 NP 1-pt Correlations
- Correlations (contoured) between 2-D field of P
and SLP at a single grid point (the key point).
Each key point is indicated by a circled
asterisk. Shaded areas indicate significance
gt95. Brown colors mean less P is associated with
higher SLP at the key point. Blue means greater P
occurs for higher SLP at the key point. H marks
the NP high JJA average location. - Caution H marks high center only in total data
not MA data. - Higher r values tend to occur at remote spots on
the same side of the high as the key point. - Little of the P near Central America is
significant, even for key points on the East side
of the high, contrary to the current view. That
which is is less not more. - Total data have much larger significant areas.
19 Figure 5 SP 1-pt Correlations
- Similar to fig. 4 except for SP high in DJF
- Total data have much larger significant areas
than MA data. r values from MA and total data
have opposite sign over E Amazonia for NE and E
key points. Another difference is that total data
have more dipolar patterns implying seasonal
shifts of the ICZ. Otherwise the r values from
the two datasets are quite similar. - Total and MA data both show W, SW, and S key
points have dipolar correlation pattern implying
poleward shift of extratropical storm track P for
higher SLP and vice versa. - Key points to the E, NE, N, and NW of the SP high
center correlate with less P in the Kiribati
region and vice versa, consistent with the
composites results.
20 Figure 6 wider area composites
- Wider views of the composites shown in figs 3 and
2 except that MA data are used. - Strong summer NP highs seem to have some
association with SH SLP. - Strong summer SP highs do not seem to have much
connection to NH fields of SLP and P.
21Figure 7 OLR vs P comparison
- Comparison of composite differences (Strong highs
minus weak highs) for two fields OLR (upper
plot) and P (lower plot). - Note Record length and months used differ
between the two plots as do the individual highs
chosen. - This plot did not reproduce well in the preprint
volume.