OCS Study 

MMS 2000-028 

Remote Sensing St 

in the Northeastern 

idy of U pwe I I i n g 

Gulf of Mexico and 

the Effects of Hurricanes Earl and Georges 

Annual Report: Year 2 

U.S . Department of the Interior 

Minerals 1Vlanagement Service 

Gulf of Mexico OCS Region 
OCS Study 

MMS 2000-028 

Remote Sensing Study of U pwel I i ng 

in the Northeastern Gulf of Mexico and 

the Effects of Hurricanes Earl and Georges 

Annual Report: Year 2 

Authors 

Frank E . Muller-Karger 

University of South Florida (USF) 

Department of Marine Science (DMS) 

St. Petersburg, Florida 

Fred Vukovich 

SAIC 

Raleigh, North Carolina 

Robert Leben 

University of Colorado 

Boulder, Colorado 

Bisman Nababan 

USF/DMS 

St. Petersburg, Florida 

Chuanmin Hu 

USF/DMS 

St. Petersburg, Florida 

Doug Myhre 

USF/DMS 

St. Petersburg, Florida 

Prepared under MMS Contract 

1435-01-97-CA-30857 

by 

University of South Florida 

Department of Marine Science 

140 7th Avenue, South 

St . Petersburg, Florida 33713 

Published by 

U .S. Department of the Interior 

Minerals management Service New Orleans 

Gulf of Mexico OCS Region May 2000 

DISCLAIMER 

This report was prepared under contract between the Minerals Management Service 

(MMS) and the University of South Florida. This report has been technically reviewed 

by the MMS, and it has been approved for publication . Approval does not signify that 

the contents necessarily reflect the views and policies of the MMS, nor does mention of 

trade names or commercial products constitute endorsement or recommendation for use. 

It is, however, exempt from review and compliance with the MMS editorial standards. 

REPORT AVAILABILITY 

Extra copies of this report may be obtained from the Public Information Office (Mail 

Stop 5034) at the following address: 

U.S . Department of the Interior 

Minerals Management Service 

Gulf of Mexico OCS Region 

Public Information Office (MS 5034) 

1201 Elmwood Park Blvd. 

New Orleans, Louisiana 70123-2394 

Telephone: (504) 736-2519 

1-800-200-GULF 

CITATION 

Suggested citation : 

Muller-Karger, F.E., F. Vukovich, R. Leben, B. Nababan, C. Hu, and D. Myhre . 2000. 

Remote sensing study of upwelling in the northeastern Gulf of Mexico and of 

the effects of Hurricanes Earl and Georges . Annual Report: Year 2. U.S . Dept. 

of the Interior, Minerals Management Service, Gulf of Mexico OCS Region, 

New Orleans, La. OCS Study MMS 2000. 42 pp. 

iii 

TABLE OF CONTENTS 

LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii 

LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii 

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi 

I . SUMMARY OF THE SECOND-YEAR RESULTS . . . . . . . . . . . . . . . . . . . . . . . 1 

1 .1 . Sea Surface Temperature (SST) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

1 .2 . Sea Surface Pigment Concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

1 .3 . Altimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

1 .4 . Field Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

1 .5 . Other MMS-Related Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

II . THE SPRING 1998 NEGOM COLD WATER EVENT: REMOTE 

SENSING EVIDENCE FOR UPWELLING AND FOR EASTWARD 

ADVECTION OF MISSISSIPPI WATER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

2 .1 . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

2 .2 . Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

2 .3 . Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

2 .3 .1 . AVHRR SST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

2.3 .2 . Winds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

2.3 .3 . SeaWiFS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

2.3 .4 . Altimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 

2.4 . Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 

2.5 . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 

v 
III. HURRICANE IMPACTS ON THE NORTHEASTERN GULF OF MEXICO 

AS SEEN WITH THE NOAA AVHRR AND SEAWIFS . . . . . . . . . . . . . . . 27 

ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 

3 .1 . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 

3 .2 . Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 

3 .2 .1 . Sea Surface Temperature (SST) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 

3 .2 .2 . Sea Surface Pigment Concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 

3 .2 .3 . Sea Surface Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 

3 .3 . Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 

3 .3 .1 . Sea Surface Temperature Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 

3 .3 .2. Sea Surface Pigment Concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 

3 .4 . Summary and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 

IV . REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 
LIST OF TABLES 

Page 

2 .1 . Center latitude and longitude for 3x3 pixel boxes (103 km2) from which 

SST was extracted from AVHRR images (stations shown in Figure 2.1) . . . 13 

3 .1 . Sea surface temperature averages within an 8x8 pixel box centered 

at 28.59 °N, 89.19 °W, near the track of Hurricane Georges . . . . . . . . . . . . . . . 33 

3 .2 . Mean chlorophyll-a concentration (and standard deviation) within an 

8x8 pixel box centered at the location of lowest SST 

(28 .59 °N, 88.19 °W) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 

LIST OF FIGURES 

Page 

1 .1 . Loop Current Intrusion Index - A measure of the volume of Loop Current 

water east of 90°W determined from satellite altimetry . . . . . . . . . . . . . . . . . . 2 

1 .2. Integral Measures of the Loop Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

1 .3 . Mean Chlorophyll-a concentration during the NEGOM III cruise 

from (A) SeaWiFS data and (B) Fluorometer flow-through data . . . . . . . . . . . 6 

1 .4 . DOC fluorescence (A) and salinity (B) over NEGOM during 

NEGOM III cruise showing a strong negative correlation . . . . . . . . . . . . . . . . . . 6 

1 .5 . Sea surface height dynamic over NEGOM of (A) summer 1998 and 

(B) winter 1998 showing different pattern and location of high and low 

sea surface height dynamic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

1 .6 . Mean chlorophyll-a concentration during the NEGOM IV cruise 

from (A) SeaWiFS data and (B) Fluorometer flow-through data showing a 

positive correlation value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

1 .7 . Mean chlorophyll-a concentration (A) from SeaWiFS data during 

summer 1999 and (B) from SeaWiFS data during the NEGOM V 

cruise (Spring 1999) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

1 .8 . Mean sea surface temperature (SST) comparison over NEGOM between 

in-situ (A) and NOAA AVHRR satellite (B) showing a consistent 

correlation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

vii 
1 .9 . Salinity and DOC flow-through correlation from two different cruises 

showing highly negative correlation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

1 .10 .Chlorophyll-a concentration and DOC fluorescence correlation from 

three different cruises showing relatively highly positive correlation . . . . 9 

1 .11 . Chlorophyll-a concentration from in-situ and SeaWiFS 

data from three different cruise showing that SeaWiFS data 

in general overestimate in-situ data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

2.1 . Coastal zone of the NEGOM, encompassing the area between 26-31 .5 °N 

and 82-90 °W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

2.2 . AVHRR-derived SST on (1) 21 May 1995, (2) 17 May 1996, (3) 12 May 

1988, (5) 7 June 1988, and (6) 17 July 1988 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

2.3 . SST time series at one offshore station (Station 1 in Table 2 .1 and Fig . 2 .1) 

and 5 coastal NEGOM stations (Station 2-6 in Table 1) . . . . . . . . . . . . . . . . . . . . . . . . 16 

2.4 . SST time series at 30 .32 °N, 87 .75 °W, off Fort Walton Beach . . . . . . . . . . . . . . . . 17 

2.5 . Wind speed and direction (the direction wind blew from) from May 1 

through July 30, 1998, at NDBC Buoy 42040 (29.18 N, 88 .30 W; 

Mobile South, 64 nautical miles of Dauphin Island, Alabama; See 

Figure 2 .1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1 

2.6 . 1998 SeaWiFS imagery for (1) 24 April, (2) 1 May, (3) 3 May, (4) 11 May, 

(5) 13 May, (6) 18 May, (7) 15 June, (8) 5 July, (9) 27 August . . . . . . . . . . . . . . . . 22 

2.7. Geostrophic velocity and sea surface height in the Gulf of Mexico inferred 

from blended TOPEX/Poseidon and ERS altimeter data for mid-May and 

mid-July 1998 (courtesy of R. Leben, University of Colorado) . . . . . . . . . . 22 

3 .1 . Sea surface height anomaly of the Gulf of Mexico based on TOPER/ 

ERS-2 analysis. (a) when Hurricane Earl reached the NEGOM area 

(September 1, 1998) and (b) after Hurricane Earl left NEGOM area 

(September 5, 1998) (Leben, 1998) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 

3 .2 . Sea surface height anomaly of the Gulf of Mexico based on TOPER/ ERS-2 

analysis. (a) when Georges reached south of Mississippi coast (September 

28, 1998) and (b) after hurricane Georges left the Mississippi coast 

(October 6, 1998) (Leben, 1998) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 

3 .3 . Daily-averaged, near-surface currents for the period 29 August through 

7 September 1998 from a mooring located at the shelf break at 29.7 °N 

and 86.27 °W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 

Vill 
3 .4 . Composite NOAA infrared AVHRR SST images of the NEGOM prior to 

Hurricane Earl, Aug 29-31,98 (a) and after Hurricane Earl, Sep 3-4, 98 (b) 32 

3.5 . Daily-averaged, near-surface currents for the period 15 September through 

15 October 1998 from a mooring located at the shelf break at 29 .25 °N and 

88.50 °W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 

3 .6 . Composite (average) NOAA AVHRR SST image of the NEGOM (a) prior 

to Hurricane Georges, (b) after the Hurricane on October 1, (c) Oct 5-6, 

(d) Oct 9-10, (e) Oct 14-15, and (f) Oct 19, respectively . . . . . . . . . . . . . . . . . . . . . 35 

3 .7 . The lowest temperature in the intense cold lens developed by Hurricane 

Georges in the first 19 days in October 1998 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 

3 .8 . Composite (average) SeaWiFS image of the NEGOM (a) prior to Hurricane 

Georges, (b) after the hurricane's passage on Sep 30-Oct 2, (c) Oct 5-6, 

and (d) Oct 16-19 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 

ACKNOWLEDGMENTS 

This work was supported by the Minerals Management Service, Gulf of Mexico, OCS 

Region, under Contract Number 1335-01-97-CA-30857 . SeaWiFS data are property of 

Orbimage Corporation and were processed and reproduced for this report following 

guidelines outlined in the Dear Colleague Letter published by the National Aeronautics 

and Space Administration's (NASA) SeaWiFS Project at the Goddard Space Flight 

Center. Wind data were obtained from the NOAA National Data Buoy Center. This 

report benefited immensely from discussions held with Dr. William Schroeder (Dauphin 

Island Sea Lab, Alabama), to whom I am indebted for sharing his insight on dynamics of 

the circulation of shelf waters in the northeastern Gulf of Mexico area. We wish to thank 

Tom Berger and Peter Hamilton (SAIC) for their support in providing current meter data 

in the Northeastern Gulf of Mexico. We thank Dr. Doug Biggs and Dr. Norman 

Guinasso (TAMU) who help us coordinate field data collection using GYRE. We also 

thank for the GYRE crews and other participants during NEGOM cruises who provide 

help during field data collection. 

I. SUMMARY OF THE SECOND-YEAR RESULTS 

1.1 . Sea Surface Temperature (SST) 

For the period covered under this study (April 1997-December 1999), we have collected 

approximately 5,820 Advanced Very High Resolution Radiometer (AVHRR) SST 

images of the Gulf of Mexico (GOM). These images are stored in high (1x1 km2 pixel) 

and low resolution (2.8x4.1 km z pixel) . We have also computed monthly mean SST for 

the Gulf of Mexico for the period of 1993-1999. Near-real time and monthly mean SST 

images for the Gulf of Mexico can be accessed at: htcp ://paria .marine .usf.edu. 

During the period of May-July 1998, we observed unusual upwelling along the coasts of 

the Florida Panhandle in the northeastern Gulf of Mexico. This led to 3-6 °C lower SST 

than normal . This is discussed in Chapter II of this report . We also observed strong 

upwelling near the shelf break in the northeastern Gulf of Mexico after hurricanes Earl 

(September 2-4, 1998) and Georges (September 25- October 1, 1998) . This is discussed 

in Chapter III of this report . 

1 .2 . Sea Surface Pigment Concentration 

We have collected approximately 1,500 SeaWiFS images (September 1997- December 

1999) over the Gulf of Mexico and computed monthly mean of chlorophyll-a 

concentration over the Gulf of Mexico for the period September 1997 - December 1998. 

Near-real time SeaWiFS data can be accessed at : hctp ://paria .marine .usf.edu. In addition, the 

historical Coastal Zone Color Scanner (CZCS) were processed to pigment products 

representing the concentration of phytoplankton, and then averaged to derive a series of 

92 monthly pigment concentration means. 

We reprocess SeaWiFS images collected with our local antenna every time NASA 

introduces a new algorithm to calculate chlorophyll concentration . We also are modifying 

the processing algorithm for turbid waters using the NEGOM data. The NASA algorithm 

produces large errors in chlorophyll concentration in coastal waters . 

Using SeaWiFS data, we found higher pigment concentration in upwelling areas 

observed during the period of May-July, 1998, and after hurricane Georges. These 

findings are presented in Chapters II and III of this report. 

1 .3 . Altimetry 

At the University of Colorado, we continue to update the Gulf of Mexico near-real time 

sea surface dynamic height based on blended data from TOPEX/Poseidon (TOPEX) and 

the Second European Remote-Sensing Satellite (ERS-2). The Gulf of Mexico altimetry 

data are available on-line at the following addresses: 
" Near-Real-Time Altimeter Data: 

" Gulf of Mexico Near Real-Time Altimeter Data Homepage 

(http://wwwccar.Colorado.edu/-realtime/gom/gom-nrt .html) 

" Gulf of Mexico Near Real-Time Data Viewer (http://www-ccar.colorado.edu/ 

-realtime/gom-real-time ssh) 

" Gulf of Mexico Near Real-Time Geostrophic Velocity Vector Viewer 

(http://www-ccar.Colorado.edu/-realtime/gom-real-time vel) 

" Gulf of Mexico Along-Track Data Host (http://www-ccar.colorado.edu/ 

-realtime/gom data ssh/ssh.html) 

Gulf of Mexico Along-Track Visualization (One Day) (http://wwwccar.colorado.edu/-realtime/gom data ssh/sshpic.html) 

" Gulf of Mexico Along-Track Visualization (Multiple Day) (http://wwwccar.colorado.edu/ -realtime/gom data ssh/sshpic2 .htm1) 

" Latest Gulf of Mexico sea surface height maps (http://www-ccar.colorado.edul 

research/gom/html/latest nrt.html) 

" Historical Gulf of Mexico Altimeter Data: 

Gulf of Mexico Historical Data Viewer (http ://www-ccar.colorado.edu/ 

gom-historical ssh) 

" Gulf of Mexico Historical Geostrophic Velocity Viewer (http://ccar.colorado.edu/ 

gom-historical vel) 

A CD-ROM containing all of the historical data for the Gulf of Mexico from early 1992 

through the end of 1998 is now available. 

Dr. Leben has developed a Loop Current Intrusion (LCI) index based on altimetry data, 

which analogous in spirit to the ENSO indices used to monitor El Nino. Loop Current 

Intrusion index for the period of 1992-1999 is presented in Figure 1 .1 . 

E 

U 

M 

2 

ul 1 .5 

0 

E 

> 1 

x 10 ° Volume o1 EadernGOM Loop Current V'taier 

c7 

Ju1912 Jan43 Ju193 Jarfl4 Ju194 Jan95 Ju195 Jan96 Ju19B Jan97 JuK37 Jart98 Ju198 Jarfl9 

Figure 1 .1 . Loop Current Intrusion Index - A measure of the volume of Loop Current 

water east of 90°W determined from satellite altimetry. Estimates from 

historical data are shown in grey, from near real-time data in black. 

Several integral measures of Loop Current properties including estimates of Loop 

Current area, volume and circulation have been produced from the statistics of the 

historical data set. The Loop Current circulation, which is estimated by a line integral of 

2 
the velocity along the 17-cm sea surface height contour, is a proposed index for 

continuous monitoring of the Loop Current penetration and shedding (Figure 1 .2). The 

17-cm contour was chosen by experience and success tracking the Loop Current and its 

eddies in altimetry and with a data assimilative model. 

l 

v 

J 

lOs Area of Loop Current, bleaa =14?000 km' 

0. 

0 .6 

7an93 7u193 7aa% Ju191 7aa95 7u195 7aa96 7u196 7aa9i 7u197 7an98 7u198 7an99 

V1' 

. . . . . 

-, . . . 

.10" Volume apt Loop Current, Mean -?.1' 0000et13 ml 

U '~l 

. yr. . 

N 

U y ;y r r 

- Y 

RC 

1 .5 - -

. . . . . . . .1f . . 

7aa93 7u193 Jan94 Iv194 7aa95 7u195 7aa96 7u1% 7aa9i 7u197 7aa98 7u198 7aa99 

x 10' Curulatioa .V ong Lapp Curirat, Mean =1? i 1000 m' s 1 

. . . . . . . .. 

r . . . . . . . 

1 .8 

~ 1 .6 

.~ 

1 .~ 

1 .3 . . . . . . . . . 

a 

0.8 

0.6 

7aa93 7u193 7aa94 7u194 7an95 7u195 7au% 7u196 7aa9i 7u197 7aa98 7v198 7aa99 

~+ 

q' . 

. . . . . 

. . . . 

. 

t ., . . . . . , . . . . . . , . . 

r 

, , . . . . ., . . . ., . . . . . . ., . . 

. . . : 4, . . 

. . . 

V 

. . 

. . . . 

Y 

. . . . ~. 

Y 

Figure 1 .2 . Integral Measures of the Loop Current. Top Panel - Surface area found by an 

integration of the area within the 17-cm SSH contour. Middle Panel -

Volume of the Loop Current water found by integration of a reduced gravity 

approximation for the upper layer of the ocean within the 17-cm SSH 

contour. Bottom Panel - Circulation associated with the Loop Current found 

by line integration of the velocity component along the 17-cm SSH contour. 

In all panels, the estimates of the Current eddy shedding times by Sturges are 

shown in vertical lines (grey) (Sturges and Leben, in press) . 

Quantitative comparisons of altimetry-derived currents to currents estimated from 

Lagrangian drifters have been initiated. This work is being done in collaboration with 

Peter Niiler and Carter Ohlmann, who have supplied daily averaged horizontal velocity 

3 
components derived from the Sculp I, II and III data sets . The data sets used for the 

comparison are the Sculp I daily velocity estimates co-located with descending TOPEX 

tracks. Co-location was determined by identifying daily Sculp I values within a 1/4° cell 

that also contains a TOPEX ground track point. In all, 2770 co-located positions were 

identified, of which 286 were in water of depth greater than 1500 meters. The horizontal 

velocities were used at each co-located point to determine the component perpendicular 

to the TOPEX tracks. Crosstrack geostrophic velocities were computed from alongtrack 

sea surface slopes estimated by linear regression over data point windows of 23 points 

(125 km) along the TOPEX tracks . This allows direct comparison of altimeter-derived 

currents with surface currents from drifters without interpolating the altimetry to a regular 

grid. First, we need to examine at the altimeter/drifter crosstrack velocity correlation over 

the continental shelf/slope and the deep basin to determine the utility of the altimeter over 

those regions of the Gulf. Second, we need to examine the correlation of the 

altimeter/drifter crosstrack velocities as a function of time lag between the observations. 

Significant correlations are found over the shelf, slope and deep basin . Decorrelation 

times decreased from 15 days over the continental slope and deep basin to 3-5 days over 

the LATEX shelf. 

1 .4. Field Programs 

We have joined with Texas A&M University (MMS PM Worth Nowlin) to collect biooptical field data on oceanographic cruises to the NEGOM. For the period of 1998-1999, 

we participated in six cruises: NEGOM II (May4-15, 98), NEGOM III (Jul 26-Aug 8, 

98), NEGOM IV (Nov 12-24, 98), NEGOM V (May 14-28, 99), NEGOM VI (Aug 15-

27, 99) and NEGOM VII (Nov 12-22, 99) . 

For these cruises, we collected water samples for Gelbstoff Absorbance (ag), Particle 

Absorbance (aP), and Dissolved Organic Carbon (DOC) concentration . We also 

measured remote sensing reflectance (above and under water). In addition to the above 

parameters, we measured attenuation and absorption of light at nine wavelengths at the 

surface with a continuous-flow a/c-9 meter and DOC fluorescence with a flow-through 

fluorometer . For NEGOM VI and VII cruises, in addition to the above parameters, we 

measured optical fluorescence of surface waters continuously using a SAFIRE instrument 

in collaboration with Carlos del Castillo (USF) . We also participated on monthly 

ECOHAB cruises to the West Florida Shelf. During ECOHAB cruises, we collected 

similar observations. Our goals from these cruises are precise, calibrated underwater and 

above-water radiometric measurements to derive remote sensing reflectance, which are 

being compared to SeaWiFS data. We also are preparing a database of dissolved organic 

carbon concentrations to help interpret the ocean color signals in continental shelf waters . 

The spatial and temporal variations of bio-optical properties over the Northeastern Gulf 

of Mexico from the two-year observations from the ship and satellite are summarized as 

follows : Surface chlorophyll-a concentration (CHL), colored dissolved organic matter 

(CDOM) fluorescence (a proxy for concentration), salinity, and temperature data were 

collected continuously along ship tracks with flow-through systems. Water samples and 

optical data were also collected at discrete stations to calibrate the flow-through data and 

4 
to develop an ocean-color algorithm. The flow-through data document that freshwater 

from the Mississippi River spread eastward in the summers of both 1998 (Figure 1 .3) and 

1999 (Figure 1 .7), up to 27°N and 87°W, as indicated by low salinity (-23 psu) (Figure 

1 .4), high CHL (-2 mg/m3, compared with <0.1 mg/m3 for oligotrophic ocean) (Figure 

1 .3 and 1 .7), and high CDOM (absorption at 400 nm is -0.3 m 1, compared with <0.05 

m-1 for oligotrophic ocean) (Figure 1 .4) . The ocean color satellite imagery (SeaWiFS, 

Orbview II) shows that the spatial patterns of eastward entrainment of this river plume 

were similar in both summers . The eastward entrainment of this river plume was 

facilitated by the present of an anticyclonic eddy located south of the Desoto Canyon area 

(Figure 1 .5). In Spring and Winter, however, there is no significant eastward entrainment 

of the Mississippi River water (Figure 1 .6 and 1 .7B). Sea surface temperature (SST) from 

NOAA AVHRR satellite is in general consistent with SST from in-situ measurement 

(Figure 1 .8). Correlation between CDOM and salinity was higher (-0.85 or higher for 

more than 7500 data points) (Figure 1 .9) than the correlation between CDOM and 

chlorophyll-a (0 .783 or less for more than 7500 data points) (Figure 1 .10), suggesting that 

CDOM is a better indicator than CHL of river plume waters. CDOM can be a better 

indicator of river plume water than chlorophyll-a since most of the CDOM enhancement 

in ocean water come from river water (land) whereas chlorophyll-a enhancement in ocean 

water could occur due to an upwelling process. Along the coast from Tampa Bay to 

Mobile Bay, the relationship between CDOM and salinity is robust, suggesting that a 

regional algorithm may be developed to estimate salinity from space. Chlorophyll-a 

concentration values from SeaWiFS in general overestimate in-situ measurements 

(Figure 1 .11) . Correlation of chlorophyll-a concentration values between in-situ and 

SeaWiFS data during winter (Figure 1 .11B, NEGOM4) was better compared to spring 

and summer (Figure 1 .10 NEGOM 3 and 5), suggesting that the presence of gelbstoff 

produced larger errors in the estimation of chlorophyll-a concentration from SeaWiFS . 

1.5 . Other MMS-Related Programs 

We completed construction of an X-band antenna to collect MODIS data from Terra, 

launched by NASA on December 18, 1999. The antenna hardware was funded by 

NASA. Software is being developed partly using MMS support. The satellite data will 

support the various Gulf of Mexico MMS programs . 

5 
Figure 1 .3 . Mean Chlorophyll-a concentration during the NEGOM III cruise from (A) 

SeaWiFS data and (B) Fluorometer flow-through data. Red solid line on (A) 

shows the ship track. 

Figure 1 .4. DOC fluorescence (A) and salinity (B) over NEGOM during NEGOM III 

cruise showing a strong negative correlation. 

3 1 " 

P7" 

26'r 

-1 

30 N 30 -N 

29-N 29'N 

281 

I 

_7 27'N 2 

Ix 

WN -N 

-N 

U -w 

--N 

2 uu~ 

Sea surface height (cm) 

T 

-30 -27 -24 -21 -18 -15 -12 -9 -6 ,' 0 3 6 9 12 15 1 '8 21 24 27 30 

!N rv 

!B N 

!7'N 

!B'N 

,. N 

Figure 1 .5 . Sea surface height dynamic over the NEGOM of (A) summer 1998 and (B) 

winter 1998 showing different pattern and location of high and low sea 

surface height dynamic . 

6 
L I

Figure 1 .6 . Mean chlorophyll-a concentration during the NEGOM IV cruise from (A) 

SeaWiFS data and (B) Fluorometer flow-through data showing a positive 

correlation value. Red solid line shows ship track. 

Figure 1 .7 . Mean chlorophyll-a concentration (A) from SeaWiFS data during summer 

1999 and (B) from SeaWiFS data during the NEGOM V cruise (Spring 

1999) . Red solid line shows the ship track. 

Figure 1 .8 . Mean sea surface temperature (SST) comparison over NEGOM between insitu (A) and NOAA AVHRR satellite (B) showing a consistent correlation. 
A: NEGOM3 : Ju1.26-Aug.06, 1998 

35.0 

30.0 

bQ 

0 25.0 

3 

° 20.0 w 

;. ., . . 

I i 

1~ ' """"""" Salinity 

DOC flour I 

rZ = -0 .85, n=7562 ~ , 

15 .0 

0 

35 .0 

a 

30.0 

10.0 

8 .0 

6.0 

O 

4.0 

U 

O 2.0 Q 

0.0 

1000 2000 3000 4000 5000 6000 7000 8000 

Data index 

B : NEGOM4: Nov. 14-24, 1998 

!d".JJI .a+K `l*S)LL}"k .4i Y J~'V 

- `~(t,~ j 

- ~~l S h 

- ~ . . 

' . ; . ; 

Salinity 

DOC fluor 

- r2 = - .093,n=8200 

25 .0 

0 

~. 

3 20.0 0 

w 

15.0 

0 2000 4000 6000 8000 

Data index 

10.0 

.-

. ., 

8 .0 

6.0 

4.0 p 

I 

2.0 U O 

Q 

0.0 

Figure 1 .9 . Salinity and DOC flow-through from two different cruises showing highly 

negative correlation. 

8 
io 

03 

to 

O 

1 

3 0 

w 

10 

ct2 

M 

b 

bA 0 

0 

w 

1000 2000 3000 4000 5000 6000 7000 8000 

0.1 

0 

NEGOM4 Chl-a (mg/m3) 

r2=0.67 °°°°°° DOC Fluorescense 

II=g200 

i 

o. i 

-1000 1000 3000 5000 7000 

10 

CI 

03 

b 

of 

w 

0.1 

7VVV 

NEGOMS Chl-a (mg/m3) 

"""""" DOC fluorescense 

~t ~ { ;, ~I 

~9 I 

v~- w y~ 

4! 

I 11~lIILL I 

0 1000 2000 3000 4000 5000 6000 7000 8000 

Data index 

Figigure 1 .10 . Chlorophyll-a concentration and DOC fluorescence from three different 

cruises showing relatively highly positive correlation . 

9 
10 

- o NEGOM3 

-v ~ o 

o g o 

0 

3 C3 

C3 

o ~ o 

- o 

8 

~bA 

a 

0 

0 

s 

U 0 

0 

8 

a 

B 

00 

0 

0 .1 ` 

0 

Data index 

E 

E 

io 

T 1 

s 

a 

0 

0 

s 

U 

10 

MJ 

G 

CL 

O 

O 

U 

v.i 

-1000 1000 3000 5000 7000 9000 

Data index 

0.1 

0 1000 2000 3000 4000 5000 6000 7000 8000 

Data index 

Figure 1 .11 . Chlorophyll-a concentration from in-situ and SeaWiFS data from three 

different cruises showing that SeaWiFS data in general overestimate insitu data. 

10 
II. THE SPRING 1998 NEGOM COLD WATER EVENT: REMOTE SENSING 

EVIDENCE FOR UPWELLING AND FOR EASTWARD ADVECTION OF 

MISSISSIPPI WATER 

ABSTRACT 

Unusual upwelling in May-July 1998 along the coasts of the Florida Panhandle in the 

northeastern Gulf of Mexico led to 3-6 °C lower sea surface temperatures (SST) than is 

normal for these waters at this time of the year. Concurrently with the anomalous SST, 

substantial volumes of turbid Mississippi River water spread east of the Delta along the 

coast. The upwelling and eastward dispersal of Mississippi water were associated with 

both anomalous pulsating eastward winds and a large anticyclonic eddy that migrated 

into the northeastern Gulf of Mexico and reached the shelf break between June and 

August 1998 . We document the anomalous wind, upwelling, and river plume dispersal 

patterns using data from the Advanced Very High Resolution Radiometer (AVHRR), 

Sea-viewing Wide-Field-of-View Sensor (SeaWiFS), the Ocean Topography Experiment 

(TOPEX) and European Remote-Sensing Satellites (ERS) altimeter sensors, combined 

with wind data gathered at a National Data Buoy Center (NDBC) buoy (NDBC Buoy 

42040). The geostrophic current field inferred from the altimeter data suggests 

convergence near the Mississippi River Delta, eastward flow along the coast, and 

offshore flow in the vicinity of Cape San Blas. Eastward winds stronger than 3 m s-' 

occurred along the Alabama coast up to 32% of the time between May and August 1998, 

when this anticyclone was near the shelf, compared to only about 20-26% during the 

same months in 1996, 1997, and 1999. These eastward anomalies in the wind may in turn 

be associated with the warm anticyclonic eddy. Upwelling of cold water along the coast 

was enhanced by the presence of baroclinic structure associated with the eddy. Eastward 

transport of Mississippi water was facilitated both by eastward winds and the eddy as it 

interacted with shelf waters. Hypoxia off northern Florida in late June and early July 

1998 probably followed decay of local phytoplankton blooms after eastward winds 

relaxed, westward winds picked up, and the Mississippi River plume moved toward the 

coast, since this led to vertical stability in the water column. 

2.1 . Introduction 

In May 1998, the Remote Sensing Lab of the University of South Florida distributed a 

series of satellite images to various researchers involved in a Minerals Management 

Service program designed to study the Northeastern Gulf of Mexico, alerting them of a 

strong upwelling event along the coast of the Florida Panhandle. By June and July 1998, 

alarming reports appeared in the local press of "strange" water and "unusual" conditions 

along the northeastern shores of the Gulf of Mexico between Pensacola and Panama City, 

Florida (Figure 2.1) . Fishermen and the public reported massive fish kills and green or 

brown water filled with flocculates. News articles cited bottom-dwelling crab, shrimp, 

and small flounder swimming on top at the surface, and massive amounts of shellfish 

delivering spat . These reports also announced the sudden arrival of cold water at the 

shore, with one account suggesting a 9°C drop in temperature in about 24 hours. The 

11 
National Marine Fisheries Service (NMFS) reported low oxygen concentrations in waters 

at the inlet to St . Andrew Bay by Panama City (Collard et al., this issue), but the media 

reports led to the conclusion that hypoxia was widespread. These anecdotal accounts 

linked fish kills to cold, low oxygen, low salinity, and discolored water spreading along 

the coast . Unfortunately, it was not possible to coordinate resources fast enough to 

conduct detailed oceanographic ship surveys of the event. The local press from Louisiana 

to central Florida covered the event through the end of July 1998, and scientific and 

government institutions were called upon to explain the mysterious anomaly. 

It is hard to decipher what really happened in the northeastern Gulf of Mexico (NEGOM) 

during May, June, and July 1998 using the information available from these reports. The 

various facts don't seem to add up. For example, how abnormal are cold temperatures 

along the NEGOM coastline? Why would the cold water, which is derived from 

upwelling, have low salinity, be discolored, or even have low dissolved oxygen? Is it 

possible that both river water and upwelling affected the coast at about the same time? 

During the time of the event, various oceanographic satellites collected data in an 

operational mode over the affected area. Here we try to answer some of these questions 

using the remotely-sensed observations. We use these data to 1) assess the magnitude of 

the temperature anomaly observed along the NEGOM coast, 2) trace the source of the 

discolored water, and 3) evaluate the possibility of recurrence of the cold water and low 

oxygen phenomena. 

2.2. Methods 

To conduct our study, we used three primary satellite data sets. Sea Surface Temperature 

(SST) was derived from infrared (IR) observations collected by the Advanced Very High 

Resolution Radiometer (AVHRR) sensors flown on the National Oceanic and 

Atmospheric Administration's (NOAA) Polar Orbiting Satellite series. We used 4,077 

high resolution AVHRR images collected over the NEGOM between September 1993 

and July 1999 using the High Resolution Picture Transmission (HRPT) antenna located at 

the University of South Florida, in St. Petersburg, FL (http ://paria .marine.usf.edu) . All 

passes from all satellites were combined to build the time series (nightime and daytime 

passes from the NOAA 11 through 14 satellites) . The visible and near-infrared channels 

were calibrated to percent albedo, and the IR channels were calibrated to degrees Celsius . 

The navigation was manually corrected to compensate for errors in the satellite clock and 

orientation . SST was computed using the multi-channel sea-surface temperature 

(MCSST) algorithm developed by McClain et al. (1985) . The MCSST algorithm uses 

channels 4 and 5 to compute the temperature, with channels 2, 3, and 4 used to determine 

cloud cover. Through various comparisons (unpublished data) we have confirmed that the 

approximate root mean square (rms) error of the AVHRR SST retrievals are of the order 

of 0.5 C (see also McClain et al., 1983 ; Strong and McClain, 1984 ; Walton, 1988 ; Wick 

et al., 1992) . Finally, the SST fields were remapped to a Cyclindrical Equidistant 

projection covering the Gulf of Mexico at a spatial resolution of 2 .8x4.1 km2 . 

12 
90W 89W 88W 87W 86W 85W 84W 83W 82W 

30 N 

29 N 

28 N 

27 N 

30 N 

29 N 

28 N 

27 N 

90 W 89W 88W 87W 86W 85W 84 W 83W 82W 

Figure 2 .1 . Coastal zone of the NEGOM, encompassing the area between 26-31 .5 °N and 

82-90 °W. Squares show the six stations from which SST time series were 

constructed using the AVHRR images (position shown in Table 2.1). The 

filled circle shows location of NDBC meteorological buoy 42040, which 

provided wind observations. 

Table 2.1 . Center latitude and longitude for 30 pixel boxes (103 km2) from which SST 

was extracted from AVHRR images (stations shown in Figure 2.1) . 

Station Latitude Longitude Area 

1 27 .1913 °N 86.1251 °W NE Central Gulf of Mexico 

2 30.1367 °N 85 .8282 °W Off Panama City 

3 30.3200 °N 86.7500 °W Off Fort Walton Beach 

4 30.2636 °N 87 .2755 °W Off Pensacola 

5 29.4257 °N 85 .0489 °W Off Cape San Blas 

6 30.0859 °N 88.3517 °W West of Mobile Bay 

13 
We constructed local time series of SST by averaging all observations within boxes of 

30 pixels (103 km2) for each image. We extracted series for 6 boxes, 5 located near the 

coast in the NEGOM and one offshore (Table 2.1 ; Figure 2 .1) . 

Wind speed and direction (1995-1999) were obtained from the National Data Buoy 

Center's (NDBC) buoy No. 42040, located at 29.18 °N, 88 .30 °W, 64 nautical miles south 

of Dauphin Island, Alabama (Figure 2 .1) . 

Phytoplankton "chlorophyll-a concentration" fields were derived from the SeaWiFS 

(Sea-viewing Wide-Field-of-view Sensor; also known as Orbview II). SeaWiFS is flown 

jointly by NASA and Orbimage Corporation . The data were processed using the 

atmospheric correction algorithms described by Gordon and Wang (1994), and the biooptical algorithm described by O'Reilly et al. (1998). We are aware that there are 

unresolved issues that lead to severe errors in estimates of chlorophyll-a using SeaWiFS 

data in coastal zones, particularly in Case II waters (Morel and Prieur, 1977) where 

suspended sediments and colored dissolved organic matter significantly affect the ocean 

color signal . Nevertheless, here we use the SeaWiFS chlorophyll-a images primarily to 

infer the dispersal pattern followed by Mississippi River water into the area of concern . 

These discolored waters are very distinct in these images. We make no use of the data to 

determine actual phytoplankton concentrations . 

Finally, we inferred circulation patterns from satellite altimetry, using the Sea Surface 

Height (SSH) and geostrophic current approximation products generated by Robert 

Leben at the Colorado Center for Astrodynamic Research (University of Colorado ; 

http ://www-ccar.Colorado.edu/-leben/research .html) . These products incorporate data 

from the Ocean Topography Experiment (TOPEX) and European Remote-Sensing 

Satellites (ERS) satellites . 

2.3 . Results 

2.3 .1 . AVHRR SST 

We use the AVHRR imagery to examine the spatial structure of the coastal upwelling 

plume in May, June, and July of 1998, and to assess how it compared to conditions in 

other years from 1994 through 1999 . Figure 2 .2 shows examples of the imagery. Figure 

2.3 presents SST extracted at one offshore location (Station 1) and at various coastal 

stations (Stations 2-6 in Table 2 .1 and Figure 2.1). Figure 2 .4 illustrates interannual SST 

variability off Fort Walton Beach, FL (Station 3), where the coldest SST off the Florida 

Panhandle was repeatedly detected . For display, the station data have been smoothed 

using a 100-point running mean filter on a series containing up to 8 observations per day. 

The offshore station taken in the central NEGOM (Station 1) shows an annual 

temperature range of about 9-11 C, from -31 C in June-August to -20-23 C in FebruaryMarch (Figure 2.3). This region shows winter temperatures that are warmer than all 

coastal NEGOM locations . Warm winter anomalies are observed when the Loop Current 

or one of its warm eddies intrudes north of 27 °N, as in January and February of 1996 and 

14 
1998 (Figure 2.3) . In contrast, the coastal NEGOM features some of the coldest 

temperatures observed in the Gulf (-12 C) in January-February. After March, NEGOM 

waters undergo substantial warming. A cold water tongue originates in the NEGOM 

every winter and extends southward along the West Florida Shelf between approximately 

November and May. Examples of the structure of this plume are shown in Panels 2-3 of 

Figure 2 .2 . This structure is important for the regional ecology as it is associated with 

elevated phytoplankton concentrations and serves a mechanism for dispersal of river 

water along the West Florida Shelf break (Gilbes et al., 1996). 

Figure 2.2 . AVHRR-derived SST on (1) 21 May 1995, (2) 17 May 1996, (3) 12 May 

1988, (4) 4 June 1998, (5) 7 June 1988, and (6) 17 July 1998 . SST(C) has 

been coded according to the color bar included. Note that extremely high 

SST(>30°C) are shown in blue-grey. Clouds and missing data are shown 

in black, and land in is shown in grey. 

15 
30 

28 

26 

24 

22 

V 

F 20 

N 

N 

18 

16 

14 

12 

10 

1/1/94 7/1/94 1/1/95 7/1/95 1/1/96 7/1/96 1/1/97 7/1/97 1/1/98 7/1/98 1/1/99 7/1/99 

Time 

Figure 2.3 . SST time series at one offshore station (Station 1 in Table 2.1 and Fig. 2.1) 

and 5 coastal NEGOM stations (Stations 2-6 in Table 2.1). The offshore 

station (top black line) shows much warmer SST during each winter, while 

SST at all coastal stations (other lines) behaves similarly throughout the 

year. 

16 
30 - 

Top 

25 

~ 

y 

- 

U ! 

20 

~ 1a ' 

10 

J F M A M J J A S O N D 

Month 

-1993 -1994 1995 

---- 1996 -1997 1998 

-1999 100 per. Mov. Avg. (1998) 

29 

27 

25 

v 23 Arw 

cn 

N 21-

19 

17 

5 

M A M J J A 

Month 

100 per. Mov. Avg . (1998) 100 per. Mov. Avg . (1997) 

100 per. Mov. Avg . (1999) 100 per. Mov. Avg . (1996) 

Figure 2.4 . SST time series at 30.32 N, 87.75 W, off Fort Walton Beach. Top: Seasonal 

cycles for 1994-1999, with 100-point running mean overlaid for the 1998 

cycle . Bottom: Detail of the seasonal cycle off Fort Walton Beach, showing 

the running means for March-June 1996, 1997, 1998, and 1999. 

17 
The AVHRR images show that in May of most years, Mississippi River water that enters 

the ocean is colder than surrounding waters (Panels 1 and 3 in Figure 2 .2) . However, the 

river plume water warms up quickly and its temperature signal becomes indistinguishable 

from that of oceanic waters nearby . More significant cold water patches were seen in 

May of several years along the coast east of Mobile Bay, even though during this period 

SST rises at a rate of almost 0 .25 C per day (Figure 2 .4 bottom). In May 1995, SST off 

Mobile Bay reached -23 C, or about 1-2 degrees below surrounding waters (Panel 1 in 

Figure 2.2). In May 1996 (Panel 2 in Figure 2.2), a larger plume of about 23 °C 

originated in the coastal zone and extended to the southeast along the shelf break of the 

West Florida Shelf. Figure 2.3 shows that SST in the NEGOM in May (19-25 C) is 

always lower relative to SST in the central Gulf of Mexico (-25-27 °C) due to the 

cooling effected by these coastal upwelling foci . By June, temperatures throughout the 

NEGOM are usually more uniform, with both offshore and coastal waters above 25-27 

°C (Figure 2.3). 

Of the series of images examined from 1994 through 1999, those for 1998 showed the 

coldest and most intense springtime upwelling (Panels 3-6 in Figure 2.2). In May, June, 

and July 1998, waters along the Florida coast west of Cape San Blas were between 2 and 

5 °C cooler than SST for those months in 1996 and 1999 (Figure 2.4 bottom) . Additional 

cooling occurred in events during the first half of June and in mid-July 1998. During each 

of these months, the cold coastal wedge observed along the Florida Panhandle varied in 

width and over several SST degrees (Figure 2 .4 bottom), probably reflecting variation in 

the strength of the upwelling over scales of 2-10 days. 

Coastal upwelling, for example, intensified around 10 May 1998 . AVHRR imagery 

clearly showed a very narrow (QO km) band of cold temperatures along the coast 

between Cape San Blas and the entrance to Mobile Bay (Panel 3 in Figure 2 .2). SST as 

low as 18 °C were seen in the coldest patch, located between Pensacola and Fort Walton 

Beach, FL. By May 16, warm temperatures were seen along the Panhandle coast, but the 

cold wedge returned on May 19. From 20-24 May coastal waters warmed up again 

(Figure 2 .4 bottom). When present, the cold patches appeared stretched toward offshore 

and to the east (Panel 3 in Figure 2.2), and ultimately they appeared to disperse to the 

southeast along the shelf edge of the West Florida Shelf. 

Temperatures along the coast of the NEGOM rose in June and July 1998 (Figure 2.4 

bottom), but remained cooler than offshore waters (Panels 3-6 in Figure 2 .2, and Figure 

2.3). This indicated continuation and strengthening of the upwelling process. Indeed, 

June 1998 showed SSTs that were 1-4 °C colder than those in June 1996 and 1999 

(Figure 2 .4 bottom) . However, SST in May and June 1998 were very similar to those in 

1997 . Mid-July 1998 showed much colder (-24 °C) coastal temperatures than any July 

between 1994 and 1999 (>27 °C; Figure 4 top) . These cold temperatures were observed 

in spite of offshore temperatures at their seasonal SST peak of more than 29 °C. An 

example of the mid-July upwelling plume is shown in Panel 6 of Figure 2.2 . Throughout 

this event (May-July 1998), SST along the coast remained variable over a scale of days. 

18 
By end of July, coastal SST in the NEGOM had risen to its seasonal mean of about 27 

°C. 

2.3 .2 . Winds 

Figure 2.5 shows variation in wind speed and direction to the east of the Mississippi 

Delta between May and August 1998 . We assume that the wind observations at NDBC 

Buoy 42040 (Figure 2 .1) are representative of conditions over the shelf off the Florida 

Panhandle. The wind records for 1996, 1997, 1998, and 1999 at this location show that 

NEGOM winds are stronger than 3 m s-1 greater than 65% of the time between 1 May 

and 31 August in each of these years. However, winds had an eastward component (wind 

direction greater than 220 degrees) 32% of the time between May and August only in 

1998 . This is in contrast to 20% in 1996, 26% in 1997, and 22% in 1999 over the same 

time period . 

The wind at the beginning of May 1998 was about 7 m s-1 from the west, but it quickly 

decreased and reversed to reach about 3 m s-' from the east by May 6. It then reversed 

again, and by 10 May had reached over 9 m s-' blowing from the west. This condition 

was favorable for upwelling along the Florida Panhandle coast, and resulted in the cool 

coastal SST pattern observed in the AVHRR image for May 11-13 (Panel 3 of Figure 

2.2). Winds decreased afterwards and reached almost 1 m s-' in mid-May. Stronger winds 

(> 6 m s-') were experienced during May 27-29, 1998, but blowing from the ESE, which 

was not favorable for upwelling. Indeed, coastal waters warmed up rapidly in late May 

(Figure 2.4). 

In early June 1998, winds reversed again (Figure 2 .5), increasing to nearly 8 m s~1 from 

the WSW. This resulted in the upwelling seen on June 4-7 (Panels 4 and 5 in Figure 2.2), 

and which is also shown also as a dip in SST in Figure 2 .4. Winds then reversed and 

decreased in speed, and no upwelling was seen in AVHRR images for June 10. Another 

reversal after June 10 led to the strongest winds of the month by June 15, with winds over 

8 m s-1 from the west, and AVHRR data showed a small drop in SST (Figure 2.4 bottom) . 

The second half of June was very cloudy, which corresponded with a period of very low 

winds . A thin wedge of cool waters, however, could be seen again around 29-30 June, 

under moderate but upwelling-favorable winds (Figure 2 .5). 

The first 10 days of July 1998 showed moderate to weak winds (1-5 m s-1) from the SSW, 

but during July 10-19, winds were very strong (6-8 m s-1) and upwelling favorable as they 

blew from the west (Figure 2.5). This led to the unusual cooling observed in July 1998, 

with SST nearly 3 °C below those seen during July of other years (Figure 2.4 top). Panel 

6 of Figure 2.2 shows the spatial distribution, with nearshore SST along the entire coast 

of Florida west of Cape San Blas 3 to 6 °C lower than water farther offshore . 

2.3.3. SeaWiFS 

Figure 2.6 shows the chorophyll-a product derived from SeaWiFS ocean color imagery 

for selected dates between April and August 1998 . The sequence of SeaWiFS images 

19 
shown in Panels 2 through 6 of Figure 2.6 (1-18 May 1998) shows that a bolus of highly 

discolored and turbid Mississippi River plume water spread east of 88 °W over a period 

of about 10 days and then continued moving east along the coast. The image for May 3 

(Panel 3 of Figure 2.6) shows the bolus originated in the western sector of the Mississippi 

Delta (Mississippi Bight, located west of 89 °W) and that it rounded the tip of the delta, 

entering the eastern sector (east of 89 °W) . This eastward movement was likely driven by 

the strong (9 m s-') wind observed blowing from the west-southwest during the first week 

of May (Figure 2.5). The SeaWiFS image for 13 May (Panel 5 in Figure 2.6) also shows 

an area of over 100 km diameter with low pigment concentrations in the NEGOM 

centered around 27 .75 °N, 86 .25 °W. This is the signature of an anticyclonic feature 

lodged in the NEGOM. 

A portion of the Mississippi water that moved east in May 1998 was entrained in a small 

(-50 km diameter) cyclonic eddy located off the tip of the delta near 28 .25 °N, 88 .75 °W 

(Panels 4 and 5 in Figure 2 .6) . However, most of this turbid water continued to flow to 

the north and east, past the Chandeleur Islands . Over the period May 11-18 (Panels 4-6 in 

Figure 2.6), this turbid water spread in a narrow plume east of Mobile Bay along the 

coast (north of 30 °N and between 86-89 °W). On 18 May (Panel 6 in Figure 2.6), coastal 

turbidity reached very high levels along the coast west of 86 °W, and the SeaWiFS 

processing software masked coastal pixels as invalid (white mask). Farther to the east, 

prior to reaching Cape San Blas, the discolored plume turned offshore and to the 

southeast . 

The large amount of discolored water derived from the Mississippi River delta was 

trapped in the NEGOM during the second half of June and the early part of July 1998. 

Indeed, as winds reversed and became westward after June 10, the Mississippi water 

moved toward the west, was pushed against the Chandeleur Islands and the Mississippi 

Delta, and spread over a large area along the Alabama and Florida Panhandle coasts 

(Panels 7-8 in Figure 2.6). This water was dispersed (Panel 9 in Figure 2.6) after winds 

switched again to the east and gained in strengths in mid July (Figure 2.5). 

2.3 .4 . Altimetry 

The large anticyclone shed by the Loop Current early in 1998 moved into the NEGOM 

and was detected in the AVHRR and SeaWiFS images, and was most clearly outlined in 

the SSH and geostrophic current fields estimated from the TOPEX/Poseidon and ERS 

altimeters . The sequence of AVHRR-derived SST fields and the altimeter SSH fields 

show that this anticyclonic eddy separated from the Loop Current in March (series not 

shown here). After separation, the anticyclone remained in the NEGOM for about 3-4 

months, during which it dispersed warm water into the otherwise colder NEGOM (see 

warm peak between February and April 1998 in the offshore station SST cycle, Figure 

2.3). In April-July 1998, the Loop Current remained south of 25 °N. 

Figure 2.7 shows SSH fields for the Gulf of Mexico for mid May and mid July 1998 . The 

strong anticyclone maintained an ellipsoidal shape with its major axis oriented NW to SE 

throughout the June-August period . SSH values of nearly 20 cm were observed in the 

20 
10 

8 i 

.--, 

H 

G1 N 

2 ' I I I I 

I 

0 

w CO CO CO CO w CO CO CO CO 

o~ Q? all 01, all a~ a', a', a', 

N C~~ 

o 

~ 

0 0 0 0 0 

N c~~ ~ N Q 

LO LO LO (D CO CO f~ I~ ITime 

350 

300 

N 

d 250 : f ~ 

I 

l 

200 ~ 

0 150 I 

v 

100 

O 

50 - n 

00 CO ' 

all 01, 

OD 

01, 

C 0 

01, 

CO 

(1), 

00 00 CO 00 

01, all a', 

r 

N c\ O ~ 

O 

N 

O O O O 

22 ~ N c~YJ 

U) LO LO CO c0 cD I~ I~ f ~ 

Time 

Figure 2.5 . Wind speed and direction (the direction wind blew from) from May 1 

through July 30, 1998, at NDBC Buoy 42040 (29.18 °N, 88 .30 °W; Mobile 

South, 64 nautical miles of Dauphin Island, Alabama; See Figure 1) . 

21 
89 88 87 86 85 W 

30N 

29N 

28N 

I 

Imo :-~ r.~ 

Figure 2 .6 . 1988 SeaWiFS imagery for (1) 24 April, (2) 1 May, (3) 3 May, (4) 11 May, 

(5) 13 May, (6) 18 May, (7) 15 June, (8) 5 July, and (9) 27 August. Purple 

and blue show clear waters, and green, yellow and red show increasingly 

turbid waters. Extremely turbid waters (panels 2,3, and 6) and some clouds 

(SW to NE diagonal front in panel 2) are masked white . Other clouds and 

missing data are masked black. 

TC?EX: EMS 2 Analysis May 14 1998 

acx 

771 

Z4T 

7' T 

'8T 

TOF'EX ERS-2 Analysis Jul 15 1995 

iva 

7TN 

7n'N 

2~'N 

IpH 

Sra wrLnx ~r~p,1d k~mi Sin zurlrr Miy~t Ic rni--~~~ 

Figure 2.7 . Geostrophic velocity and sea surface height in the Gulf of Mexico inferred 

from blended TOPEX/Poseidon and ERS altimeter data for mid-May and 

mid-July 1998 (courtesy of R. Leben, University of Colorado). 

22 
central portion of the eddy, located farther offshore . The ellipsoidal eddy formed a SSH 

ridge that extended into the coastal NEGOM region between the Mississippi Delta and 

Cape San Blas . The SSH values here are likely to be less accurate because of problems of 

interpretation of altimeter data over the continental shelf, but the ridge shows lower SSH 

values than offshore, or values of 5-10 cm. The geostrophic current field inferred from 

this SSH distribution lead to convergence near the Mississippi River Delta, eastward flow 

along the coast, and offshore flow in the vicinity of Cape San Blas (Figure 2.7). 

2.4 . Discussion 

Chuang et al. (1982) and Schroeder et al. (1987) concluded that the nearshore circulation 

off eastern Louisiana, Mississippi, and Alabama is strongly wind-dependent, in spite of 

the potential for westward geostrophic flow generated by coastal fresh water input 

(Schroeder et al., 1987) . Chuang et al. (1982) also found that winds during summer were 

relatively mild, with bursts spaced about 10 days apart or longer, but that wind 

magnitudes rarely exceeded about 8 ms-1 . The Hellerman and Rosenstein (1983) 

climatological winds show that on the average, winds in the NEGOM around 29 °N, 89 

°W have a westward component on a year-round basis, with a southward component 

between January-March and September-December, and a northward component between 

April and August. The zonal component of these climatological winds shows a minimum 

in July, and the meridional component shows its minimum in August. 

However, if wind data are examined on an annual basis, each year may show a different 

mean orientation of the wind, as found by Chuang et al. (1982) . Indeed, as shown above, 

the wind records from NODC buoy No. 42040 show that winds in the NEGOM can be 

strong and have variable direction during summers. However, strong eastward winds are 

infrequent, reaching 32% of the time between May and August only in 1998 and less in 

1996, 1997, and 1999 . 

Clearly, inferences made based on Hellerman and Rosenstein (1983) wind climatological 

products are misleading because they average wind direction as well. What we may infer 

is that the shifting winds lead to an inconsistent summer circulation pattern on the shelf. 

Drifter trajectories (Muller-Karger et al ., 1998) generally have shown slow motions 

(order of 2-10 cm s-') within 50 km of the coast immediately to the east of Cape San Blas 

between March and June of 1996. 

However, there are specific instances of baroclinic forcing along the shelf break 

(Paluszkievicz et al., 1983) that lead to strong (>30 cm s-') currents along the shelf in the 

NEGOM. Chuang et al. (1982), for example, speculated that currents off Alabama in 

1976 showed a stronger cross-shelf component because of a northward intrusion of the 

Loop Current described by Vukovich et al. (1979) . The observations of Schroeder et al. 

(1987) also support earlier inferences that the Loop Current plays an important role in 

exchange of water along the shelf break in the region . Such stronger currents, effected by 

interaction of large-scale circulation features such as the Loop Current and eddies with 

shelf waters, are also evident in the Minerals Management Service SCLTLP drifters 

(Muller-Karger et al., 1998). 

23 
The strong anticyclone shed early in 1998 from the Loop Current drifted into the 

NEGOM in April and interacted with shelf waters over the narrow shelf and within 

DeSoto Canyon off the Florida Panhandle (Nowlin et al., 1999). Also, the winds between 

May and July 1998 (Figure 2.5) generated pulses of upwelling along the coast of the 

NEGOM between May and July 1998, as shown above. However, the outcropping of 

cold waters was probably facilitated by the presence of the anticyclone in the vicinity of 

the DeSoto Canyon (Nowlin et al., 1999). The walls of such eddies consist of a baroclinic 

structure in which isotherms are raised from the interior of the eddy toward its periphery. 

As the eddy neared the shelf break, the upwelling-favorable wind raised isotherms further 

and cold waters that are normally found at depths greater than 100 m racked the surface, 

cropping out along the coast . While upwelling-favorable winds are observed during most 

years, they have been less frequent than in 1998 and they act on more stratified waters in 

which the thermocline lies at greater depths . Therefore, if an anticyclone is not 

interacting with the shelf break in the NEGOM, there is less of a surface effect even 

during periods of eastward wind. 

The anticyclonic eddy seen in the NEGOM in 1998 led to a steep gradient in SST 

between the coast and offshore waters. Indeed, offshore NEGOM waters experienced 

significant warming in the first half of 1998 relative to other years. SSTs reached 26 °C 

as far north as 27 °N in February 1998, when normally minima of 20-22 °C should be 

observed there at that time (Figure 2 .3 ; at Station 1 in Table 2.1 ; see also Muller-Karger 

et al., 1991). In May-June 1998, waters at 27 °N were still 2-3 °C warmer than their 

seasonal average for that time (Figure 2.3, curve for Station 1). This area typically would 

experience the effect of the cold plume that originates in the coastal zones of the 

NEGOM (as in Panel 2 of Figure 2.2). However, the Loop Current anticyclone occupied 

this area and migrated far to the north, bringing very warm waters into the NEGOM early 

in the season. 

This early warming of the central/eastern and northeastern Gulf of Mexico may have 

contributed to the anomalous winds observed during the 1998 season . It is possible that 

the unseasonably warm water mass represented strong meteorological forcing which 

stimulated the higher frequency of eastward winds seen along the shelf in the NEGOM in 

1998 . Explicitly, normal meteorological conditions lead to variable winds along the coast 

of the Florida Panhandle at this time of the year. However, the large warm water mass 

associated with the eddy may have created a local atmospheric low pressure system 

which affected local winds. Indeed, the eastward winds observed by Chuang et al. (1982) 

over the shelf in 1976 may have been associated with a similar low pressure system, 

caused by the northward intrusion of the Loop Current identified by Vukovich et al. 

(1979) . 

The question remains, then : What caused the anomalous fish kills and discolored water in 

the NEGOM during June and July 1998? 

The bulk of the Mississippi water usually drifts west along the coast of Louisiana and 

Texas (Muller-Karger et al., 1991 ; Muller-Karger, 1993) . However, eastward movement 

24 
of Mississippi water is not unusual . For example, Coastal Zone Color Scanner (CZCS) 

satellite images had shown that Mississippi River water can drift east and south, and 

reach Florida Strait during late summer every year (see Muller-Karger et al., 1991, and 

references therein). Other reports of such eastward movement abound (see, for example, 

Schroeder et al., 1987) . The SeaWiFS data show that a large volume of Mississippi 

plume water moved from the central Louisiana coast, around the tip of the Mississippi 

Delta, and eastward into the NEGOM in May and June 1998 . This discolored water was 

trapped in the coastal NEGOM by during the second week of June and through about 

July 10 by weak and westward winds (Figure 2 .5) . The discolored water moved east 

again after July 15 and eventually cleared the area in late July, after winds became strong 

and eastward. 

In the Mississippi Delta bight, the peak in discharge of nutrient-rich Mississippi and 

Atchafalaya Rivers, combined with the lack of strong winds in summer, leads to 

substantial phytoplankton blooms. These sink and decompose, causing seasonal hypoxia 

in bottom waters because vertical mixing is inhibited (Walsh, 1988 ; Turner and Allen, 

1982; Turner and Rabalais, 1991 ; Justic et al., 1993 ; Atwood et al., 1994 ; Rabalais et al., 

1996). The oxygen depletion begins in late spring, reaches a maximum in mid-summer, 

and disappears in the fall . The area affected has grown to covers approximately 18,000 

km2 every year, but has not been yet reported to extend into the eastern sector of the 

Mississippi Delta. A similar, but much more modest condition seems to occur within 

Mobile Bay. The problem occurs in the middle of an important commercial and 

recreational fisheries area and is considered a threat the economy of this region of the 

Gulf. Clearly, this phenomenon is of great concern because it leads to massive annual fish 

and shrimp mortality . 

The documentation of the massive eastward transport of Mississippi plume water around 

the tip of the Delta in May and June 1998 is therefore significant. When Mississippi 

River bight water moved to the east and spread along the Florida Panhandle coast in May 

1998, it likely had a strong stabilizing effect on the water column . The AVHRR image 

for 12 May 1998 (Panel 3 of Figure 2 .2) shows a newly upwelled water patch along the 

coast. The upwelling of deeper waters would displace any low salinity and discolored 

water present near the coast farther offshore . The SeaWiFS images for 18 May and 5 July 

(Panels 6 and 8 in Figure 2.6) confirm separation of the discolored plume from the coast 

in a southeastward direction, eastward of about 86.5 °W . This also followed the pattern of 

currents inferred from the altimeter data, and allowed for a wedge of cold water to break 

the surface from below along the coast. 

The wind reversals then repeatedly moved the river water back toward the coast to cause 

the periodic masking of the upwelling in the AVHRR data. However, as the river water 

was advected toward the coast, local downwelling, also driven by the westward wind, 

caused submergence of phytoplankton blooming along the coast. These processes are 

accompanied by inhibition of vertical mixing and of oxygenation. This is the likely cause 

for depletion of dissolved oxygen in nearshore bottom waters during July 1998 off the 

Florida Panhandle. 

25 
2.5 . Conclusions 

Unusual upwelling and river plume dispersal patterns observed in the NEGOM in MayJuly 1998 were in great measure the result of the rare northward migration of an 

anticyclonic eddy shed by the Loop Current . This large and warm anticyclone led to 

regional warming in excess of 3 °C relative to normal conditions, which in turn may have 

contributed to anomalous meteorological conditions over the NEGOM. The baroclinic 

structure of the walls of the eddy placed cold waters over the shelf, and the eastward 

winds that were observed repeatedly during this period facilitated outcropping of the 

thermocline along the coast. Through combined action of strong winds from the west and 

the motion of waters in the periphery of the anticyclone lodged in the NEGOM, 

Mississippi plume waters moved east of the delta between May and June 1998. 

Mississippi waters were maintained offshore during upwelling periods, but were advected 

toward shore as winds relaxed or became westward, and eventually became stagnant 

along the coast in late June and early July. This stagnation, after local downwelling of 

coastal, biologically productive waters, is the likely cause of the hypoxia observed during 

this period in coastal waters. By late August, most of the Mississippi water had been 

flushed out of the NEGOM, and had dispersed between the NEGOM and the Florida 

Keys and beyond. 

It is unclear whether these phenomena were related to the strong El Nino of 1997-1998 or 

not . At this stage, there is no evidence for such a link. There are concerns that the 1998 

cold water and hypoxic condition may occur again in the future. It is unclear that we can 

predict recurrence, however, but it appears that we can anticipate substantial 

meteorological and oceanographic impact from errant anticyclones in the Gulf of Mexico . 

Clearly, routine monitoring of the entire northern Gulf of Mexico using a suite of nearreal-time, complementary satellite-based sensors is viable today and helps determine 

whether processes are underway that can lead to anomalous situations in deep water as 

well as continental shelf environments of the Gulf of Mexico. 

26 
III. HURRICANE IMPACTS ON THE NORTHEASTERN GULF OF MEXICO 

AS SEEN WITH THE NOAA AVHRR AND SEAWIFS 

ABSTRACT 

Sea-surface effects of hurricanes Earl and Georges in the Northeastern Gulf of Mexico in 

1998 were studied using the National Oceanic and Atmospheric Administration/ 

Advanced Very High Resolution Radiometer (NOAA/AVHRR) and the Sea-Viewing 

Wide Field-of-View Sensor (SeaWiFS) satellite data and in-situ current meter data. In 

each case, a region of low SSTs was detected to the right of the hurricane's track by the 

NOAA/AVHRR. For Earl, the sea surface temperature (SST) decreased by as much as 

3 .6 C, and for Georges, by as much as 4.2 C relative to that prior to the passage of the 

hurricanes. The lowest SSTs were found near or at the shelf break in each case, 

suggesting that the shelf slope may have enhanced upwelling associated with hurricanes. 

In the case of Georges, SeaWiFS detected a doubling in pigment concentration where the 

lowest SST associated with the hurricane was observed. SeaWiFS also detected a region 

with discolored water (i .e ., large pigment concentrations) immediately south of the shelf 

break near the Mississippi Delta region, but it was located principally to the left of the 

hurricane's path . The spatial mismatch between the SST and ocean color patterns 

indicated that the discolored water was due to offshore advection of Mississippi River 

water, rather than in-situ phytoplankton growth. For Georges, both the SST and the 

turbid features drifted to the southwest and dissipated over a 3- to 4-day period. 

3.1 . Introduction 

Sea-surface temperature (SST) changes associated with the passage of hurricanes are well 

documented. They range from -1 .0 °C to -6.0 °C (Jordan, 1964; Leipper, 1967 ; Wright, 

1969 ; Fedorov et al., 1979 ; Pudov et al., 1979; Smith, 1982; Cornillon et al., 1987 ; 

Nelson, 1996) . Such changes are generally centered from 20 km to 150 km to the right of 

the hurricane's path; that is, on the side of the hurricane where the most intense winds are 

found (i .e ., the right-hand quadrant) and where upwelling, induced by the hurricane, 

affects the first 200-300 m of the sea (Price, 1981 ; Brooks, 1983; Brink, 1989; Black et 

al., 1988). The data presented by Price (1981) and Withee and Johnson (1976) have 

suggested that the SST changes associated with hurricanes are functions of the 

hurricane's translation speed and intensity, the latter being defined by the central 

pressure . The magnitude of the SST response increased with decreasing translation speed 

and decreasing central pressure. A -6.0 C SST departure was noted with hurricane Hilda 

when its translation speed was about 3 m/s and its central pressure was 930 mb (Leipper, 

1967). Most of the data upon which these results rely were obtained through 

hydographic surveys, which unfortunately are not synoptic. Furthermore, the upwelling 

induced by a hurricane will not only affect SST, but may bring nutrients to the surface, 

which should affect the surface phytoplankton distribution . However, to our knowledge, 

the literature on enhanced phytoplankton concentrations in oceanic waters after a 

hurricane is small to non-existent. 

27 
In this paper, we examine the influence of the passage of hurricanes Earl and Georges, 

which took place in the late summer and early fall of 1998, on the synoptic SST and 

phytoplankton biomass distribution in the Northeastern Gulf of Mexico. The SST 

distribution was determined from the National Oceanic and Atmospheric Administration 

(NOAA) 12 and NOAA 14 Advanced Very High Resolution Radiometer (AVHRR) data, 

and the biomass distribution from the Sea-Viewing Wide Field-of-View Sensor 

(SeaWiFS) data, for the period immediately following the passage of these hurricanes. 

Mean near-surface currents from moorings in the Northeastern Gulf of Mexico were used 

to examine flow characteristics in the vicinity of the cold and pigment patches detected 

using the satellite data and produced by the hurricanes . 

3.2. Methods 

3.2.1 . Sea Surface Temperature (SST) 

The SST was derived from infrared (IIZ) observations collected by the AVHRR sensors 

placed into orbit on the NOAA Polar Orbiting Satellite series. The high-resolution 

AVHRR images that were used for this study were collected over the Northeastern Gulf 

of Mexico using the High Resolution Picture Transmission (HRPT) antenna located at the 

University of South Florida, in St. Petersburg, Fl. All passes (nighttime and daytime 

passes) from each NOAA satellite were combined to build the time series. The visible 

and near-infrared channels were calibrated to percent albedo, and the IR channels were 

calibrated to degrees Celsius. The navigation was manually corrected to compensate for 

errors in the satellite clock and orientation . The SST was computed using the multichannel sea-surface temperature (MCSST) algorithm developed by McClain et al. (1985) . 

The MCSST algorithm uses channels 4 and 5 to compute the temperature, with channels 

2, 3, and 4 used to determine cloud cover. Through various comparisons (unpublished 

data), we have confirmed that the approximate root mean square error of the AVHRR 

SST retrievals are of the order of 0.5 °C (see also McClain et al., 1983 ; Strong and 

McClain, 1984; Walton, 1988 ; Wick et al., 1992). 

Because of extensive cloud cover during summer in the Gulf of Mexico, we averaged 

AVHRR SST images collected during both day and night time over periods of 1-2 days. 

Pixels with cloud cover were excluded during this binning (compositing) process. After 

compositing images, the SST minima were located in the vicinity of the hurricane tracks. 

The difference in SST values before and after a hurricane's passage was calculated for 

these regions of SST minima using 8x8 pixel boxes . 

3.2 .2 . Sea Surface Pigment Concentration 

Phytoplankton "chlorophyll-a concentration" fields were derived from the SeaWiFS (Seaviewing Wide-Field-of-view Sensor; also known as Orbview II). SeaWiFS is flown 

jointly by NASA and Orbimage Corporation. SeaWiFS is a component of NASA's Earth 

Observing System (EOS) and builds on the legacy of the Coastal Zone Color Scanner 

(CZCS). SeaWiFS has been operational since September 18, 1997, and has provided 

global estimates of oceanic chlorophyll-a and other bio-optical quantities to the 

international community routinely to date (Hooker et al., 1992, McClain et al., 1998). 

The data over the Northeastern Gulf of Mexico were processed using the atmospheric 

28 
correction algorithms described by Gordon and Wang (1994), and the bio-optical 

algorithm described by O'Reilly et al. (1998). We are aware that there are unresolved 

issues that lead to severe errors in estimates of chlorophyll-a using SeaWiFS data in 

coastal zones, particularly in Case II waters (Morel and Prieur, 1977) where suspended 

sediments and colored dissolved organic matter significantly affect the ocean color 

signal. Nevertheless, here we use the SeaWiFS chlorophyll-a images primarily to infer 

the dispersal pattern followed by Mississippi River water into the area of concern. These 

discolored waters are very distinct in these images. 

During the presence of a hurricane, the Northeastern Gulf of Mexico was largely covered 

by clouds . SeaWiFS images collected during these periods and for several days after 

hurricane passage were nevertheless processed. Chlorophyll-a concentration before and 

after hurricane passage was measured within 8x8 pixel boxes centered on the location 

used for SST comparisons . 

3.2 .3 . Sea Surface Current 

Daily mean currents at 29 .7 °N, 86 .27 °W and 29 .25 °N, 85.5 °W and 12 m depth were 

computed based on ADCP data that were collected as part of the MMS DeSoto Canyon 

Program . The daily means from the current meter data were calculated by averaging the 

hourly u- (positive to the east) and v- (positive to the north) components of the current for 

each day. 

3.3 .Results 

3.3 .1 . Sea-Surface Temperature Changes 

Earl became a hurricane when a quasi-stationary tropical depression intensified in the 

Northeastern Gulf of Mexico around noon on 2 September 1998. That afternoon, the 

central pressure in the hurricane fell to about 986 mb and Earl became a Category 2 

hurricane for about 6 hours, with sustained winds reaching 85 mph. This hurricane then 

began to move to the northeast at an average speed of about 19 km/hr, making landfall 

near midnight in the Florida Panhandle region . By mid-day on 3 September, it was 

located north of 32 °N and was again designated a tropical depression . 

Earl had a major influence on the near-surface flow around the shelf break near the 

DeSoto Canyon on 3 September, as suggested by a Minerals Management Service current 

meter located at 29 .7 °N and 86 .3 °W (Figure 3.3). Prior to the passage of the hurricane, 

the near surface flow was to the south-southwest and probably was associated with the 

warm and cold rings located near the DeSoto Canyon (Figure 3.la,b) . Strong northward 

near-surface currents (daily-averaged speed of about 30 cm/s) were observed, however, 

on 3 September. As the hurricane continued northeastward and moved inland on 3 

September, the near-surface current speed decreased and the flow turned eastward. 

From late June through October, the northern boundary of the Loop Current was located, 

for the most pan, south of 25°N, and therefore, had no direct influence on the 

29 
Northeastern Gulf of Mexico. By early September, the center of a major warm core ring 

that had separated from the Loop Current in the eastern Gulf of Mexico in June 1998 was 

located west 93°W and it also had no direct influence on the Northeastern Gulf of 

Mexico. There were, however, a small cold ring and a small warm ring located in the 

Northeastern Gulf of Mexico at the time hurricane Earl and Georges passed through. The 

small warm ring was located in the DeSoto canyon region (i .e ., 28.5 °N, 87.5 °W) from 

early September through October (Figure 3 .1a) . This small ring broke into two smaller 

warm core rings which moved toward the southeast after Hurricane Earl passed through 

the area (Figure 3 .1b). Two small cold rings were also evident in the Northeastern Gulf of 

Mexico area (at 27 °N, 87.6 °W and at 27 .5 °N, 85 .7 °W) prior to Hurricane Earl (Figure 

3 .1a) in early September. These eventually drifted westward, out of the region of interest 

(Figure 3 .2a,b) . 

On 3-4 September, the AVHRR detected a small, isolated region of low SSTs to the right 

of the hurricane track in the vicinity of the shelf break (Figure 3.4) . The lowest 

temperatures were seen at the shelf break, with SSTs as much as 3 .6 °C lower than those 

observed there prior to the passage of Earl . The near-surface flow (Figure 3 .3) near the 

area of very low temperatures was strong to the east late on 3 September and on 4 

September, across isobaths. The SeaWiFS data showed no major changes in the nearsurface phytoplankton concentration distribution in this region, however. 

Hurricane Georges developed in the Tropical Atlantic Ocean and moved west-northwest 

across Cuba into the Gulf of Mexico, where it turned and moved to the northwest. It 

entered the Gulf of Mexico on 25 September 1998 and moved rapidly to the northnorthwest at an average speed of 18 km/hr. By early morning on 28 September, it 

reached the Mississippi River Delta region and moved inland. Between 25-28 

September, Georges was classified as a Category 2 hurricane, with sustained winds of 95 

mph and central pressures as low as 961 mb. 

The hurricane affected the near-surface flow at the shelf break near the Mississippi Delta 

on 27 September. Strong west-southwestward near-surface currents (i .e ., daily-averaged 

speed of about 54 cm/s) were observed on 27 September at another Minerals 

Management Service current meter located at 29 .25 °N and 88 .50 °W (Figure 3 .5). The 

current speed was about a factor of two greater than observed over the ten days prior to 

the hurricane. As the hurricane continued northward and moved inland on 28 and 29 

September, the near-surface current weakened (daily-averaged speeds of 12 to 27 cm/s to 

the north-northeast), but was still strong compared to before the hurricane . After the 

hurricane passed, the near-surface flow oscillated from southwest/west-southwest to 

north/north-northeast . The flow to the southwest and west-southwest was generally 

stronger . The average flow in the 10-day period after the hurricane passed was to the 

southwest, at about 5 cm/s. 

30 
30T1 

27N 

2dN 

21N 

27"N 

24'N 

21"N 

1 B'N 18'N 

96'W 93"W 90'W 87'W 84'W 81"W 96'W 93"W 90"W 87'W 84'W 81"W 

Sea surface height (cm) 

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 

Figure 3 .1 . Sea surface height anomaly of the Gulf of Mexico based on TOPER/ ERS-2 

analysis. (a) when Hurricane Earl reached the NEGOM area (September 1, 

1998) and (b) after Hurricane Earl left NEGOM area (September 5, 1998) 

(Leben, 1998). 

30N 

27N 

24'N 

21N 

30'N 30N 

2TN 27'N 

24'N 24TI 

21"N 21"N 

I"N 

"N 

N 

'N 

1H'N 1 8-N 18N 

9fi'W 93"W 90'W 87'W 84-W 81"W g6'W 93'W 90"W 87'W 84'W 81'W 

R 18-N 

uSea surface height (cm) 

-30 -25 -20 -15 -10 5 0 5 10 15 20 25 30 

Figure 3 .2 . Sea surface height anomaly of the Gulf of Mexico based on TOPER/ ERS-2 

analysis. (a) when Georges reached south of Mississippi coast (September 

28, 1998) and (b) after hurricane Georges left the Mississippi coast (October 

6, 1998) (Leben, 1998) . 

96"W 93'W 90'W 87"W 84"W 81-W 

30"N 

31 
Figure 3.3 . Daily-averaged, near-surface currents for the period 29 August through 7 

September 1998 from a mooring located at the shelf break at 29.7 °N and 

86.27 °W . North is toward the top of the page. 

1 

!_1 

Figure 3 .4 . Composite NOAA infrared AVHRR SST images of the NEGOM prior to 

Hurricane Earl, Aug29-31, 98 (a) and after Hurricane Earl, Sep3-4,98 (b). 

SST mean value of 8x8 pixel area centered at 28 .59 °N, 88 .19 °W is 29.2 °C 

(a) and 25.6 °C (b) before and after huricane passage respectively . The 

lowest SST located at the shelf break to the right of the hurricane path. The 

thick solid white line indicates the track of hurricane Earl. SST has been 

color-coded according to scale shown in insert . Land is shown in white and 

clouds are shown in black. Two hundred meter continental shelf outline 

shown as thin white line . 

32 
On 1 October, after hurricane Georges passed, the AVHRR detected a broad region of 

low SSTs in the eastern Gulf of Mexico (Figure 3.6b). Clouds obscured the eastern and 

southern portions of the area, but the region affected by the hurricane was evident. The 

cold patch was oriented northwest to southeast, and located largely to the right of the 

hurricane's track, with SST minima (23.1 °C) along the track of the hurricane. As in the 

case of hurricane Earl, the largest SST changes took place in a broad region near the shelf 

break, but in this case near the Mississippi Delta. Sea surface temperatures there were as 

much as 4 °C lower than those prior to the passage of Georges. The near-surface current 

meter located immediately to the north-northwest of the cold patch (Figure 3 .6b), showed 

that flow was to the north-northeast (i .e ., across isobaths ; Figure 3 .5) and relatively 

strong just prior to 1 October. Sea surface temperatures along the track the hurricane 

(Figure 3.6), returned to pre-hurricane values within about 4 days (Table 3 .1). 

Table 3 .1 . Sea surface temperature averages within an 8x8 pixel box centered at 28.59 

°N, 88 .19 °W, near the track of Hurricane Georges . Values in parentheses 

show standard deviation. 

Image Date Sep 24-25 Oct 1 Oct 5-6 Oct 9-10 Oct 14-15 Oct 19 

SST Mean (SD) 27 .6(0 .1) 23.1(0 .1) 26 .6(0 .1) 26.7(0 .1) 26.8(0 .1) 27 .1(0 .1) 

Composite AVHRR images centered around 5-6, 9-10, 14-15, and on 19 October 

(Figures 3.6c,d,e,f), indicated that the SST feature created by hurricane Georges moved 

to the southwest over a three-week period. The near-surface currents at 29.25 °N, 88 .50 

°W (Figure 3 .5) indicated an average 5 cm/s flow to the southwest. The southern extreme 

of the SST pattern seen on 1 October, however was not evident in any of the images after 

1 October, suggesting that temperatures south of 27 °N warmed up quickly . Shay et al. 

(1992) found that for hurricane Gilbert, the near-inertial currents created by the hurricane 

in the mixed layer converged to the storm track after about three days. If this occurred 

after hurricane Georges, the accompanying downwelling also contributed to the 

dissipation of the cold feature. Figure 3.7 suggests that the temperature in the feature 

increased to a stable value (i .e ., - 25 °C) in about three days. This coincides with the 

inertial periods found by Shay et al. (1992). The total change in temperature in those 3 

days was about 2.5 °C. The exponential behavior of the data suggests that the 

temperature increased by about 1 °C in less than one day. 

33 
20 cm/s 

15 20 25 30 5 10 15 

Sept. Oct. 

Figure 3 .5 . Daily-averaged, near-surface currents for the period 15 September 

through 15 October 1998 from a mooring located at the shelf break at 

29.25 °N and 88 .50 °W. North is toward the top of the page. 

3.3 .2 . Sea-Surface Pigment Concentration 

After hurricane Georges passed through the eastern Gulf of Mexico between September 

26-28, 1998, SeaWiFS detected new discolored features at the sea-surface immediately 

south of the shelf break near the Mississippi Delta region, where none were present prior 

to the hurricane's passage (Figure 3 .8). However, even though a portion of this discolored 

feature was located near the hurricane's track, precisely where the lowest temperatures 

were found at that time, the bulk of the colored feature was located west of the track 

(Figure 3.8b) . 

At the location where the lowest temperatures were found, chlorophyll-a was about 1 .25 

mg/m3 on September 30-October 2, or more than twice that observed prior to the passage 

of Georges (0 .59 mg/m3 on September 14-24 ; Table 3 .2) . However, by October 5-6 

chlorophyll-a concentrations had decreased to about half the value observed prior to the 

hurricane (Table 3 .2). This suggested that the hurricane enhanced surface nutrient 

concentrations, but that this effect was very short-lived. This effect was similar to the 

short-lived (2-3 days) phytoplankton bloom seen after Hurricane Bob in 1991 in Waquoit 

Bay, Cape Cod, Massachusetts (Valiela et al., 1998) . 

34 
.~ .~~ 

. . 

\, , l 

. M~I 

'~q J 

'. 

Y 

v x . 

ra 

~'g. . 

.. ~~ 

~~

. 

Figure 3 .6 . Composite (average) NOAA AVHRR SST images of the NEGOM (a) prior to 

the Hurricane Georges, (b) after the hurricane on October 1, (c) Oct 5-6, (d) 

Oct 9-10, (e) Oct 14-15, and (f) Oct 19, respectively . SST of 1 October 1998 

image indicates a large upwelling area close to the shelf break. SST mean 

value taken from 8x8 pixel area centered at 28 .59 °N, 88.19 °W. The thick 

solid line white indicates the track of Hurricane Georges. SST has been 

color-coded according to scale shown in insert . Land is shown in white and 

clouds are shown in black. Two hundred meter continental shelf outline 

shown as thin white line. 

Table 3.2 . Mean chlorophyll-a concentration (and standard deviation) within an 8x8 

pixel box centered at the location of lowest SST (28.59 °N, 88 .19 °W). 

Image Date Sep14-24 Sep30-Oct2 

Chl .-a 0.59(0.25) 1 .25(0.23) 

Oct 5-6 

0 .33(0 .13) 

~L 

Oct 16-19 

0.19(0.06) 

35 
26 

25 

`+ 

N 

24 

F 

23 

22 

E 

i 

--- - " --- - ...-- 

+ 

I 

f 

{ 

E 

+ f 

i 

0 2 4 6 8 10 12 14 16 18 20 

Day 

Figure 3.7 . The lowest temperature in the intense cold lens developed by Hurricane 

Georges in the first 19 days in October 1998 . The dotted line represents an 

exponential curve fit of the temperature data. 

The highest pigment concentrations after passage of the hurricane were observed west of 

the cold patch, to the left of the hurricane's path (Figure 3.8) . The strongest near-surface 

flow, which occurred of the 27`h of September (Figure 3 .5), was to the west southwest. 

This suggests that these colored patches were directly associated with hurricane-induced 

upwelling. Rather, this suggests that the cyclonic hurricane winds transported discolored 

shelf and river water offshore, into the deeper Gulf of Mexico. SeaWiFS images for 5-6 

October (Figure 3.8c) and 16-19 October (Figure 3 .8d) show that this feature drifted to 

the southwest over the next 18 days, gradually losing its turbid color. 

3.4. Summary and Discussion 

Changes in both sea-surface temperature and ocean color associated with hurricanes Earl 

and Georges were observed in the Northeastern Gulf of Mexico in 1998. Shortly after 

each hurricane passed, sea surface temperature minima were detected near or at the shelf 

break. These features were most likely associated with intense vertical mixing and 

upwelling. Black (1983) suggested that vertical mixing at the base of the mixed layer 

may account for over 80 percent of the temperature decrease at the surface found in the 

wake of hurricanes . For Earl, SST decreased as much as 3 .6 °C, and for Georges, as 

much as 4.2 °C relative to conditions prior to each hurricane. The SST minima near the 

shelf break in each case suggested that cross-isobath flow associated with the near 

inertial currents generated by the hurricane might have acted to intensify the upwelling 

36 
in those regions. For both Earl and Georges, the near-surface flow in proximity to the 

shelf break at the time of the observation of the relatively lowest SSTs was across 

isobath . It should be noted that centers of low temperature like those noted near the 

shelf break might be produce by the near-inertial currents alone (Black, 1983). 

Figure 3.8 . Composite (average) SeaWiFS image of the NEGOM (a) prior to Hurricane 

Georges, (b) after the hurricane's passage on Sep 30-Oct 2, (c) Oct 5-6, and 

(d) Oct 16-19. Chlorophyll concentration mean derived in 8x8 pixel area 

centered at 28 .59 °N, 88.19 °W. Chlorophyll concentration has been colorcoded according to scale shown below the images. Land and clouds are 

shown in black. 

37 
While pigment concentrations doubled at the location where the SST minima were seen 

after passage of Georges, SeaWiFS detected the largest pigment concentration increases 

immediately south of the shelf break near the Mississippi Delta region . Indeed, the 

current meter data showed strong near-surface flow from the shelf into the deep Gulf in 

this region on 27 September 1998, which suggests the pigment feature was advected from 

the shelf. 

After Georges, both the SST and the pigment features drifted to the southwest and 

dissipated . Current meter data suggested an average southwestward drift in the region of 

about 5 cm/s. The temperature in the cold lens increased by 2 .5 °C in about 3 days to a 

relative steady value of about 25 °C. The The pigment feature was no longer visible in 

the SeaWiFS data after about 3-4 days. This time scale (three days) coincides with the 

period for inertial currents found by Shay et al. (1992) for hurricane Gilbert. Thus, it is 

possible that downwelling associated with these inertial currents, as well as mixing and 

surface warming, contribute to the dissipation of hurricane effects within 2-3 days in the 

water column of the Gulf of Mexico. 

38 
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40 
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41 
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42 
QPQ~~,ENT OpTy~' The Department of the Interior Mission 

o`' 4 .i yp As the Nation's principal conservation agency, the Department of the Interior has responsibility for 

most of our nationally owned public lands and natural resources. This includes fostering sound use of 

our land and water resources; protecting our fish, wildlife, and biological diversity; preserving the 

_ p9 environmental and cultural values of our national parks and historical places; and providing for the 

'9RCH 9 ~0 enjoyment of life through outdoor recreation . The Department assesses our energy and mineral 

resources and works to ensure that their development is in the best interests of all our people by 

encouraging stewardship and citizen participation in their care. The Department also has a major 

responsibility for American Indian reservation communities and for people who live in island territories 

under U.S . administration . 

o, The Minerals Management Service Mission 

As a bureau of the Department of the Interior, the Minerals Management Service's (MMS) primary 

responsibilities are to manage the mineral resources located on the Nation's Outer Continental Shelf 

~ ~ 

(OCS), collect revenue from the Federal OCS and onshore Federal and Indian lands, and distribute 

`Aq _- ~ 

7< ~gV 

those revenues. 

~ ~7nAGeM~t Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program 

administers the OCS competitive leasing program and oversees the safe and environmentally sound 

exploration and production of our Nation's offshore natural gas, oil and other mineral resources. The 

MMS Royalty Management Program meets its responsibilities by ensuring the efficient, timely and 

accurate collection and disbursement of revenue from mineral leasing and production due to Indian 

tribes and allottees, States and the U.S. Treasury . 

The MMS strives to fulfill its responsibilities through the general guiding principles of : (1) being 

responsive to the public's concerns and interests by maintaining a dialogue with all potentially affected 

parties and (2) carrying out its programs with an emphasis on working to enhance the quality of life for 

all Americans by lending MMS assistance and expertise to economic development and environmental 

protection .