Effects of ENSO off the southwestern coast of Mexico by using SST imagery: 19961999

1
Effects of ENSO off the south-western coast of
Mexico by using SST imagery 1996-1999

• R. Aguirre-Gómez1, O. Salmerón1 and R. Alvarez2
• 1.Instituto de Geografía 2. IIMAS,
• UNAM, México DF., MÉXICO.

2
Outline

• Introduction
• Objectives
• Study Area
• Methodology
• Climatological mean
• SST derived mean
• Results
• Conclusions

3
Introduction

• ENSO phenomenon is a natural oscillation of the
  atmosphere ocean system, appearing every 5 7
  years.
• During ENSO primary production importantly
  diminishes in the Eastern Pacific Ocean, from
  Peru to California.
• Studies based on Satellite imagery are scant
  along the Mexican Pacific Coast.

4
Objectives

• To analyse Sea Surface Temperature (SST)
  variations during a four year study, from 1996 to
  1999, using AVHRR database.
• To analyse SST anomalies by using a
  climatological mean, derived from 60
  years-database over the region.
• To analyse anomalies by using a SST mean derived
  by averaging satellite imagery.

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Study Area Location

• Southwestern coast of Mexico, between 16 to 23
  N and 101 to 112 W, which includes the coasts
  of the states of Sinaloa, Nayarit, Jalisco,
  Colima, Michoacán and part of Guerrero.

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Study area characteristics (1)

• Two main surface currents meet off the Mexican
  west coast (12 - 32N) The cold California
  Current (CC) and the warm North Equatorial
  Counter Current (NECC).

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Study Area Characteristics (2)

• CC is wide up to 800 km deep (? 500 m) slow
  (? 20 cm/s) N-S low salinity.
• NECC has temperate waters intermediate salinity
  (34.6 34.8 psu) poleward.
• The zone where both currents meet is know as The
  Transition Zone and is geographically variable.

8
Methodology (1)

• In order to assess the effect of ENSO (1997-1998)
  on the Southern Mexican Pacific, SST anomaly
  maps, derived from AVHRR images, prior, during
  and after the event were compared.
• November was chosen as representative because it
  is a transitional month between dry and rainy
  seasons and effects of ENSO are easily
  appreciated.

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Methodology (2)

• Monthly SST composites of November 1996, 1997,
  1998 and 1999 were used for generating thermal
  anomaly maps by two methods
• a) Composites were subtracted from a
  climatological SST mean of 25C (M1)
• b) SST maps of non-ENSO years were averaged to
  create a mean SST image (M2)
• A transept was chosen to evaluate thermal
  differences.

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Results M1November 1996

• CC shows negative anomalies (blue)
• GCC shows positive anomalies (orange)
• NECC warmer waters have higher positive anomalies
  (dark orange)
• Cape Corrientes has relatively colder waters

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M1 November 1997

• ENSO event fully developed
• Warmer conditions than those of the previous year
  are apparent, with positive SST anomalies over
  the entire area
• No difference between GCC and NECC waters

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M1 November 1998

• Period characterised by the onset of La Niña.
• Negative SST anomalies at the CC and NECC regions
• Higher SST anomalies are shown at the south of
  NECC
• Lesser ?T are manifest in the region of influence
  of the Gulf of California.

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M1 November 1999

• Negative SST anomalies are observable off the
  western side of BC associated to the CCS
• The area is under the influence of the GCC

14
M1 SST transept profiles
15
Transept description (Nov 1996)

• Nov 1996 Presence of colder waters around 110W
  (CCS) sharp pool of cooler waters at 106W (Cape
  Corrientes) Positive SST anomalies between 105
  and 102W (NECC)

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Transept description (Nov 1997)

• ENSO year
• Nov 1997 Nearly constant anomalies trend of
  about 4C except at 109W (Cape San Lucas). No
  cold waters at 106W.

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Transept description (Nov 1998)

• Nov 1998 Colder waters from CCS extended up to
  110W Cape Corrientes upwelling is observed as
  a sharp through Positive SST anomalies between
  105 and 102W (NECC waters).

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Transept description (Nov 1999)

• Nov 1999 between 110 and 108 W a mean SST
  anomaly of 2C Confluence of CCS and GCC beyond
  108C sharp depression at 106W NECC water
  between 105 and 102W.

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M1 Histograms of SST anomalies
20
Histogram description (Nov 1996)

• Nov 1996 dominance of positive anomalies (2 -
  4C) f up to 0.06 units. Smaller contribution
  of negative or zero SST anomalies.

21
Histogram description (Nov 1997)

• Nov 1997 dominance of high positive anomalies.
  Absence of negative SST anomalies and higher f (?
  0.1).

22
Histogram description (Nov 1998)

• Nov 1998 Presence of negative SST anomalies and
  a roughly bimodal positive SST anomalies
  distribution (? 0.08 r.u.)

23
Histogram description (Nov 1999)

• Nov 1999 larger presence of negative SST
  anomalies than before and positive SST anomalies
  show a trend for an even distribution

24
Results M2 November 1996

• Homogeneous distribution with slightly higher
  anomalies at Mazatlán and Cape Corrientes
• Positive SST anomalies of about 1C are seen near
  Manzanillo and L. Cardenas.
• Negative anomalies are randomly distributed off
  Mexican coasts.

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M2 November 1997

• ENSO year. SST anomalies are distributed both
  in the north and the south.
• Northern area dominated by positive SST anomalies
  whilst southern part has lower positive ones
  (even negatives). Important features are seen at
  San Lucas, Mazatlán and Cape Corrientes () and
  at Manzanillo y Lázaro Cárdenas (-).

26
M2 November 1998

• La Niña Year. Predominance of very small
  anomalies all over the map.
• The northern part is dominated by null anomalies
  (from Cape San Lucas to Cape Corrientes. The
  southern part has negative SST anomalies off
  Manzanillo and Lázaro Cárdenas.

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M2 November 1999

• SST anomalies distribution is rather homogeneous.
  
• Northern region has anomalies close to 0C (green
  tones) from Cape San Lucas to Cape Corrientes.
  Southern region shows small negative anomalies
  (-1C) from Cape Corrientes to Lázaro Cárdenas.

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M2 SST Transept profiles
29
Transept description (Nov 1996)

• Nov 1996. Colder Waters around 110W (CCS) a
  sharp pool of cooler water at 106W Positive SST
  anomalies at NECC region.

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Transept description (Nov 1997)

• Nov 1997. Higher SST anomalies area is located
  between 110 and 106W with a local minimum at
  110W and a local maximum off Baja California and
  near 106W. In the southern area anomalies are
  smaller.

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Transept description (Nov 1998)

• Nov 1998. Most remarkables areas are located at
  110W (CSL) and the zone located between
  Manzanillo and L. Cárdenas (? 104 W).

32
Transept description (Nov 1999)

• Nov 1999. SST values fluctuate around 0C and
  there is a slight difference between north and
  south (see peak at 108W).

33
M2 Histograms of SST anomalies
34
Histogram description (Nov 1996)

• Nov 1996. Dominance of positive SST anomalies
  (2- 4C) and smaller contribution of negative
  and zero anomalies.

35
Histogram description (Nov 1997)

• Nov 1997. Skewed bimodal distribution one
  centred at 0C (south) and the other centred
  about 1C (north). The amplitude of the
  distribution is wider.

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Histogram description (Nov 1998)

• Nov 1998. Gaussian-shaped profile centred about
  0C. Values are grouped between 2C and 2C.

37
Histogram description (Nov 1999)

• Nov 1999. The histogram expands between 2C and
  2C, with a central peak around 0C.

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Conclusions (1)

• Satellite images can show the effect of ENSO on
  the SST distribution off southwestern Mexico.
• Transepts and histrograms are valuable tools for
  assessing ENSOs effects

39
Conclusions First method

• Considering 25C as representative mean value has
  the following pros and cons
• 25C was obtained from SST data gathered over 60
  year of measurements on the region, hence is
  climatologically stable. This value has been
  traditionally used as representative of the
  region at least up to 20N.
• The main disadvantage is that it can under or
  overestimate SST anomalies in some important
  zones (e. g. off Cabo San Lucas)

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Conclusions First method

• Satellite images show an SST increment in the
  entire region during ENSO year 1997-1998, as
  compared to previous and susequent years.
• The inhibition of Cape Corrientes upwelling is a
  remarkable feature on ENSO year, due either to
  wind weakening or variations in the transition
  zone because of temperature increment.

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Conclusions Second method

• Using an average SST map has the following
  characteristics
• The main advantage is that this method considers
  global average values, allowing the creation of
  synoptic maps.
• The main disadvantage is that the set used for
  generating the average is still too small, hence,
  a significant number of images is necessary for
  having a climatologically representative SST map.