Interannual Variability of Middle and Upper Tropospheric Dry Air and Its Impacts on

Tropical Cyclone Activity over the Atlantic BasinIsaac Hankes, Zhuo Wang, Gan Zhang, Cody Fritz, and Alan Marinaro

1Atmospheric Sciences, University of Illinois, Urbana, IL

ERA-Interim Reanalysis daily data were used to evaluate dry air

frequency over the Atlantic basin during the peak hurricane season

(Aug-Sep). Layer averaged RH is examined using EOF analysis for

the middle (600-850 hPa) and upper troposphere (300-500 hPa) from

1979-2010. The Best Track data were used to examine the genesis

locations and tracks of the storms. The dust optical depth dataset

developed by Amato Evan was used to evaluate to origin of dry air,

as well as the NOAA HYSPLIT model.

A dry event needs to satisfy the following three criteria:

RH <= RHc

RH is 0.5 standard deviation below the average

The above two criteria are satisfied continuously for at least one day.

RHc (critical relative humidity) is set as 50% for the middle

troposphere and 40% for the upper troposphere.

The presence of dry air at the midtroposphere in the tropics is well

known to affect tropical cyclone formation in all the ocean basins.

Some previous studies indicated/suggested the impacts of upper-

level dry air (Hopsch et al. 2010; Fritz and Wang 2013, Hankes et

al. 2014). In the Atlantic basin, the dry air may be associated with

the Saharan Air Layer (SAL) or mid-latitude frontal systems (Braun

2010). The interannual variations of the dry air frequency in the

tropical North Atlantic, however, remain unclear.

In this study, we examined 1) the interannual variations of dry event

frequency over the Atlantic Ocean, 2) their relationship with the

large-scale circulation, 3) possible impacts on the variability of the

Atlantic tropical cyclone activity, and 4) the origin of dry air.

Figure 1. EOF1 for the two layers,

and EOF2 for 600-850 hPa layer.

• The first EOF mode

explains 23 (27)% of the

observed variance for

600-850 hPa (300-500

hPa) layer, and the second

EOF mode explains 17%

of the observed variance

for the 600-850 hPa layer.

• A pattern of inter-

annual variability of dry

events is centered in the

central and

easternAtlantic alon and

north of 15N.

Conclusions

•When mid-level dry events occur more frequently, TC

activity tends to be reduced over the MDR and increased

over the subtropical western Atlantic, and many fewer

major hurricanes formed. When upper level dry events

occur more frequently, TC number is reduced, especially

in the MDR.

•The mid-level dry event frequency shows a weak

positive correlation to dust optical depth, but the upper

level dry event frequency is correlated negatively to the

dust optical depth over the MDR.

•Trajectory analysis shows that dry air in both layers

originates from the midlatitudes and West Africa. Dry air

is associated with an enhanced subtropical high, steering

flow toward the West Atlantic, and shear modulation is

related to the same large-scale pattern as dry air.

•Inter-annual variability of dry events is correlated to 850

hPa height anomalies over the Atlantic as well as to the

PNA with JUN-JUL lead time for predictability. These

height anomalies could be due in part to SST anomalies,

which are also correlated to the dry event EOF patterns.

For APR-MAY lead time, the height/SST anomalies are

consistent with the IOD which be important for African

convection, but this requires further study.

Figure 2. Total Tropical Cyclone ACE for 6 most positive and negative years of the 600-850 hPa EOF1 (left), 300-500 hPa EOF1 (middle), and 600-850 hPa EOF2

(right). Gray shading is equivalent to the ACE of a TC with depression to tropical storm intensity, blue is equivalent to the ACE of a category 1 to 2 hurricane, red is

equivalent to the ACE of a category 3 to 4, and black is equivalent to the ACE of a category 5 hurricane.

Figure 3. Composites of dust optical depth for the 5 strongest positive and negative phase years of all 3 EOF modes, and the mean dust distribution. Dashed contours

represent statistical significance to the 95% confidence interval in difference plots, and solid contours represent standard deviation for the mean plot (a) at .03 AOD

increments starting at .06.

Figure 4. 700 and 500 hPa RH from a representative dry air event for the

middle and upper troposphere. The box outlines a region within the core of

the dry air from which trajectories were calculated. 10-day backward

trajectories were then taken for the two layers.

Impacts

TC variability: Dry air occurrence reduces TC activity, and concentrates the activity near the US coastline.

Origin: Dry air shows a weak (600-850 hPa EOF1) or negative (other EOFs) relationship to the SAL, and

displays a connection to higher latitudes in association with subsidence around anticyclones.

Large-scale circulation: Dry air is associated with an enhanced subtropical high over the west (EOF1) and

central Atlantic (EOF2), implying stronger northerly flow from the midlatitudes in these regions.

Teleconnections: Enhanced ridging over the central Atlantic is observed during APR-MAY for the positive

phase of the 300-500 hPa EOF pattern, migrating westward through the season. Cold SSTs over the eastern

Atlantic also exist at this time, and persist through the season.

Email: ihankes@niu.edu

Introduction

Data and Methods

Results

Figure 5. Mean (a) and difference (b-d) plots for all 3 EOFs 850 hPa heights (contours) and 200-850 hPa vector

difference shear (shading). Dashed contours in the difference plots represent areas of negative difference between the

6 most positive and negative phase years.

Table 2. Correlations between various large-scale oscillations and EOF modes for three distinct time periods leading

up to and including the occurrence of the dry events.

Teleconnections AUG-SEP Corr. JUN-JUL Corr. APR-MAY Corr.

600-850 hPa EOF1 PNA -0.57 PNA 0.26 IOD 0.4

300-500 hPa EOF1 EA 0.36 PNA 0.5 PNA -0.23

600-850 hPa EOF2 AMM -0.32 SCA -0.37 NAO 0.29

TC count Middle EOF1 Upper EOF1 Middle EOF2

Pos 30 44 44

Neg 42 51 47

ACE Pos 240 462 473

ACE Neg 609 516 609

Table 1. Tropical Cyclone counts for the EOF

patterns and associated ACE.

Figure 6. Correlations between the 300-500 hPa EOF1 pattern and 850 hPa height anomalies (top) for AUG-SEP (left), JUN-JUL

(middle), and APR-MAY (right), as well as correlations to SST (bottom) over the same time periods.