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Forcing of cool and warm subsurface water events

in Bahía Salinas, Costa Rica

Eric J. Alfaro

1,2,3

; https://orcid.org/0000-0001-9278-5017

Jorge Cortés

1,4

; https://orcid.org/0000-0001-7004-8649

1. Centro de Investigación en Ciencias del Mar y Limnología (CIMAR), Universidad de Costa Rica, 11501-2060, San

José, Costa Rica; erick.alfaro@ucr.ac.cr, jorge.cortes@ucr.ac.cr

2. Centro de Investigaciones Geofísicas (CIGEFI), Universidad de Costa Rica, 11501-2060, San José, Costa Rica.

3. Escuela de Física, Universidad de Costa Rica, 11501-2060, San José, Costa Rica.

4. Escuela de Biología, Universidad de Costa Rica, 11501-2060, San José, Costa Rica.

Received 27-I-2021. Corrected 29-IV-2021. Accepted 20-V-2021.

ABSTRACT

Introduction: Bahía Salinas, on the north Pacific coast of Costa Rica, is a seasonal upwelling area. Sea tem-

perature in Bahía Salinas could be modulated by synoptic and other large-scale systems. This region belongs

to the Central American Dry Corridor (CADC), a sub-region in the isthmus that is relatively drier than the rest

of the territory, which extends along the Pacific littoral from western Guatemala through northern Costa Rica.

Objective: To study the warm and cold events that could be inferred by studying the sea subsurface temperature

in Bahía Salinas, and also analyzing the large-scale conditions and synoptic systems of the historical sources

when they occurred in order to identify the atmospheric mechanisms that favored their appearance.

Methods: A Sea Subsurface Temperature Index was calculated using hourly data from seven stations located at

three different points in Bahía Salinas. Records range from June 19, 2003 to December 5, 2017. Additionally,

six meteorological stations, with hourly wind records, were used to create two wind indices. The Sea Subsurface

Temperature Index was used to identify the warmest and coldest events in the bay. Wind indices and monthly

meteorological bulletins were used to analyze the large-scale conditions and synoptic systems in which cold and

warm events occurred in Bahía Salinas.

Results: Mean sea temperature in Bahía Salinas is 25.2°C. Colder temperatures were observed in February-

March, below 21°C. There were two maxima in May-June and August-October with temperatures above 27°C.

In four of the five cold events studied, Northeasterly wind anomalies were observed in the Costa Rican North

Pacific, associated with trade wind reinforcements; meanwhile westerly anomalies were observed in all the

warm events, associated with weaker trade wind conditions.

Conclusions: The main seasonal climate driver in Bahía Salinas is the North Atlantic Subtropical High because

its latitudinal migration is associated with the strength of the trade winds over Central America. Seasonal upwell-

ing is modulated also by two synoptic scale climate features, the boreal winter arrival of cold front outbreaks and

the winter maximum of the easterly Caribbean Low-Level Jet. El Niño-Southern Oscillation is also an important

modulator of the sea temperature variability, since warm and cool events are related with positive and negative

sea temperature anomalies.

Key words: sea subsurface temperature; upwelling; cold fronts-outbreaks; ENSO; Central America.

Alfaro, E. J., & Cortés, J. (2021). Forcing of cool and warm

subsurface water events in Bahía Salinas, Costa Rica.

Revista de Biología Tropical, 69(Suppl. 2), S127-S141.

https://doi.org/10.15517/rbt.v69iS2.48315

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Bahía Salinas in Costa Rica is located on

the Pacific slope, northwest of the country at

the Costa Rica-Nicaragua border and has a

marked east-west axis (Fig. 1). This region

belongs to the Central American Dry Corridor

(CADC), a sub-region in the isthmus that is

relatively drier than the rest of the territory,

which extends along the Pacific littoral from

western Guatemala through northern Costa

Rica. The CADC is a mainly rural area charac-

terized by a marked precipitation seasonality,

climate change vulnerability and rich biodiver-

sity (Gotlieb, Pérez-Briceño, Hidalgo & Alfa-

ro, 2019; Quesada-Hernández, Calvo-Solano,

Hidalgo, Pérez-Briceño, & Alfaro, 2019).

Sea temperature in Bahía Salinas could

be modulated by synoptic trade wind and

other large-scale systems (Chelton, Freilich,

& Esbensen, 2000). Bahía Salinas is in an area

where the climate variability, product of coastal

upwelling (December-April), affects the behav-

ior of sea surface temperatures (Amador et

al., 2016; McCreary, Lee & Enfield, 1989;

Tisseaux-Navarro, Salazar-Ceciliano, Cambro-

nero-Solano, Vargas-Hernández, & Marquez,

2021; Vargas, 2004), which can descend to

17º C in the nearby Gulf of Papagayo (Alfaro

& Cortés, 2012). Notice that other factors that

influence this cooling is the latent heat transfer

by evaporative processes and the vertical mix-

ing by turbulent processes (Amador et al. 2016;

Tisseaux-Navarro et al., 2021). The upwelling

is associated with the trade wind strength over

the isthmus. Amador et al. (2016) indicate that

trade winds owe their origin to the north-south

temperature gradient between the poles and

the Equator. As the Earth rotates, the winds

turn to the right, reacting to the Coriolis force

in the North Hemisphere. Trades are observed

as north-easterlies in Central America. On the

Pacific slope of Costa Rica, the magnitude of

the trade wind is normally strongest during

boreal winter and spring (December to May),

and decreases during the summer and autumn,

between June and November (Alfaro, 2002;

Alfaro, Chourio, Muñoz, & Mason, 2018).

On the large scale, Maldonado, Alfaro

and Hidalgo (2018) and Durán-Quesada, Sorí,

Ordoñez and Gimeno (2020) described that

El Niño-Southern Oscillation (ENSO), as a

result of coupled ocean–atmosphere dynamics

in the tropical Pacific, is the leading mode of

interannual variability in Central America. The

two ENSO phases, El Niño and La Niña are

associated with warming and cooling events

and goes hand in hand with the basin-scale

tropical Pacific anomalies, including Bahía

Salinas. This agrees with Alfaro and Lizano

(2001), who found that ENSO signal domi-

nates the inter-annual variability in the Golfo

de Papagayo and Bahía Salinas region, finding

a correlation of 0.77 between Niño 3.4 index

and the SSTs of the Costa Rican North Pacific.

A warming in the Equatorial Tropical Pacific

could influence the appearance of warm condi-

tions in the Costa Rican North Pacific (Alfaro

et al., 2012). The study of these warm events is

important for the populations of existing coral,

as bleaching and mortality have been reported

in region during the occurrence of some of

these warm events (e.g. Jiménez, Cortés, León

& Ruiz, 2001). Additionally, a cold event in

the bay can occur by the influence of a cooling

in the Equatorial Tropical Pacific. However, if

warm Sea Surface Temperature (SST) anoma-

lies in the Eastern equatorial Pacific occurs in

concordance with cool SSTs in the Tropical

North Atlantic (TNA)-Caribbean, the mag-

nitude of the trade wind could be reinforced,

triggering a cold event in Bahía Salinas (Alfaro

& Cortés, 2012; Alfaro et al., 2012).

In the large scale, the most important trade

wind modulator is the North Atlantic Subtropi-

cal High (NASH; Alfaro et al., 2018; Taylor &

Alfaro, 2005) due to the easterly trades found

on its equatorward flank. With the onset of

boreal spring, for example, the subtropical

high moves offshore and trade wind intensity

decreases. The variation in the strength of the

trades is an important determinant of climate

throughout the year for Central America. Dur-

ing the onset of the rainy season, there is also

a weak trade inversion with altitude, the ocean

warms, and atmospheric moisture is abundant.

The region is consequently at its wettest in the

boreal late spring, during summer and early

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autumn seasons. So, this NASH’s trade wind

modulation, produces a seasonal change in

wind speed (e.g., strongest in boreal winter and

weakest in boreal spring trough autumn), been

one of the most relevant features in the Eastern

Tropical Pacific (ETP, Amador et al., 2016).

In addition, trade winds could be modulat-

ed by synoptic scale systems. Significant syn-

optic influences include the intrusions of polar

fronts of mid-latitude origin which modify the

dry winter and early summer climates of the

entire Caribbean and Central American region

(Chinchilla, Gutiérrez, & Zárate, 2017; Mal-

donado et al., 2018; Zárate-Hernández, 2013).

During winter-spring, cold fronts entering the

Caribbean and that reinforce trade winds over

Central America. As mentioned in Alfaro and

Cortés (2012) and according to Amador et al.

(2016), during the boreal winter, the incur-

sion of polar air masses moving toward the

south entering the Caribbean Sea, produces

strong pressure gradients between the Carib-

bean Sea and the ETP. This wind with eastern

component, is channeled through topographic

depressions in southern Mexico and Central

America. One of these steps is located between

the lowlands of central Nicaragua and northern

Costa Rica. The winds produced by this chan-

neling are commonly called “Papagayos” and

have Jet Stream strength (Amador et al., 2016).

Their name is due to the gulf of the same name

located in the North Pacific of Costa Rica, off

Bahía Salinas. The other important synoptic

system that modulates the trade wind strength

is the Caribbean Low-Level Jet (CLLJ; Ama-

dor, 2008). Analyzing the CLLJ annual cycle,

Amador (2008) found that this jet presents two

maxima, one in July, the greatest in magnitude

and a secondary maximum in February, which

also influences the eastern wind reinforcement

over the northern portion of Costa Rica. It has

been documented that the funneling of these

strong northeasterly winds through passages in

the mountains produces upwelling in the semi-

enclosed bodies in the Pacific coast of Costa

Rica (Alfaro & Cortés, 2012; Fiedler & Lavín,

2019; Legeckis, 1988), because strong winds

drive the surface water off a gulf or a bay, and

the displaced water is then replaced by cooler

water from the depths.

The objective of this work is both to study

the warm and cold events that could be inferred

by studying the registered Sea Subsurface tem-

perature (SSbT) in Bahía Salinas, and also to

analyze the large scale conditions and synoptic

systems of the historical sources in which they

occurred in order to identify the atmospheric

mechanisms that favored their appearance. We

consider the event relationship with sources of

known variability as El Niño-Southern Oscil-

lation (ENSO), Tropical North Atlantic (TNA)

and boreal winter cold front outbreaks. The

characterization and understanding of these

events could be used to study other biologi-

cal and chemical oceanographic aspects of

the system like natural coastal acidification,

species conservation and management (e.g.,

Arias-Godínez et al., 2019; Cordero-Umaña

& Santidrián-Tomillo, 2020; Eisele, Madrigal-

Mora, & Espinoza, 2020; Espinoza, Araya-

Arce, Chaves-Zamora, Chinchilla, & Cambra,

2020; Sánchez-Noguera et al., 2018; Valverde-

Cantillo, Robinson, & Santidrián-Tomillo,

2019). The information is of special interest for

local fishermen, since this activity has been one

of the most important socioeconomic sectors

in Bahía Salinas through its history (e.g. Díaz,

Mora, & Madriz, 2019).

MATERIALS AND METHODS

In this section we describe the character-

istics of the data files used and how the infor-

mation was analyzed. First, a Sea Subsurface

Temperature Index (SSbTI) was calculated

from hourly data from seven stations located

at three different points in the bay (Fig. 1),

using Hobotemp® sensors, Onset Computer

Corp. The records and depths of these stations

are variable and range from June 19, 2003 to

December 5, 2017. The detail of these stations

is shown in Table 1.

As the data were taken at different positions

and depths, each of the series was normalized

to make its records comparable, subtracting the

historical monthly average and dividing these

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anomalies by their historical monthly standard

deviation (Wilks, 2019). Subsequently, the

gaps of the normalized data were filled in each

of the 7 stations, since they presented miss-

ing data (Table 1), following the methodology

proposed by Ureña, Alfaro and Soley (2016),

which combines Auto Regressive models and

Principal Components. Then, the hourly data

of the seven stations were smoothed applying a

triangular moving average of 169 data (Soley,

1994), to filter high-frequency signals, as in

Alfaro and Cortés (2012). Finally, the average

of the seven smoothed records was estimated to

obtain the SSbTI. This temperature index was

subsequently used to identify the five coldest

and warmest events.

Six meteorological stations, with hourly

wind records, were used to create two wind

indices. The data from meteorological stations

were obtained from the Costa Rican National

Meteorological Institute (CRNMI). The detail

of these stations is shown in Table 2. First,

zonal and meridional component time series

were calculated for each record. Then, the

TABLE 1

Sea Subsurface Temperature stations located in Bahía Salinas, Pacific of Costa Rica

Station Latitude (N) Longitude (W) Depth range (m) Record dates (% missing data)

Salinas1 11º 03.026’ 85º 42.721’ 4.6-6.0 15/12/2003-05/12/2017 (7.0)

Salinas2 11º 03.026’ 85º 42.721’ 6.7-8.1 19/06/2003-05/12/2017 (10.4)

Salinas3 11º 01.616’ 85º 45.801’ 3.3-4.9 23/08/2003-05/12/2017 (18.2)

Salinas4 11º 01.616’ 85º 45.801’ 7.4-9.1 23/08/2003-27/09/2016 (17.6)

Salinas5 11º 01.556’ 85º 46.298’ 10.5-11.9 08/10/2003-05/12/2017 (24.3)

Salinas6 11º 01.556’ 85º 46.298’ 19.7-20.9 16/12/2003-05/12/2017 (12.8)

Salinas7 11º 01.556’ 85º 46.298’ 4.5-6.6 18/02/2004-22/04/2009 (18.8)

Fig. 1. Location of the sea subsurface temperature and meteorological stations.

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records were averaged. Finally, a triangular

moving average of 169 data was applied as

with the SSbTI, to estimate a zonal and meridi-

onal component wind index (hereafter ZCI and

MCI, respectively).

The monthly meteorological bulletins

(https://www.imn.ac.cr/boletin-meteorologi-

co), prepared by the CRNMI, were consulted to

analyze the large-scale conditions and synoptic

systems in which the five coldest and warm-

est events (based in the observed anomaly of

the SSbTI) occurred in Bahía Salinas as study

cases, in order to identify the atmospheric

mechanisms that favored their occurrence.

RESULTS

The annual and daily cycle of the wind

recorded at Liberia meteorological station

(Table 2) is presented in Figure 2. Liberia sta-

tion, located at Daniel Oduber International

Airport, is used to monitoring the synoptic

scale conditions observed in Guanacaste by

the CRNMI. The mean wind speed was 3.5

-1

(Fig. 2a). Wind magnitude is very strong

during January-March between 12:00-17:00

local time (-6 GMT), having its maxima in

February with a secondary maximum in July

(Fig. 2a). This is in concordance with the

CLLJ maxima described by Amador (2008).

Wind magnitude was weaker between May

and November at night and early morning

hours, i.e. 23:00-07:00, with a minimum during

October. Daily and annual cycle (Fig. 2a) agree

with the ones reported for Bahía Culebra by

Alfaro et al. (2012). Winds were westward all

around the year (Fig. 2b), been Northeasterly

from December to April and July. South east-

erlies were observed in May-June and August-

November. Mean values for zonal (u) and

meridional (v) wind components were -2.0 and

-0.36 ms

-1

, respectively.

TABLE 2

Meteorological stations used with wind records (red asterisks in Fig. 1).

Station Latitude (N) Longitude (W) Altitude (m.a.s.l) Record dates (% missing data)

Finca Basilia del Oro 10°58.983’ 85°20.833’ 380 01/01/2003-31/12/2017 (8.3)

Copalchi, Peñas Blancas 11°53.617’ 85°37.090’ 57 10/04/2013-31/12/2017 (6.8)

Santa Elena, La Cruz 10°55.200’ 85°36.633’ 270 10/11/2004-31/12/2017 (11.7)

Santa Rosa, ACG 10°50.467’ 85°37.167’ 315 01/04/2012-31/12/2017 (15.3)

Liberia, Airport 10°35.340’ 85°33.128’ 89 01/01/2003-31/12/2017 (2.2)

Isla San José, ACG 10°51.257’ 85°54.575’ 44 25/05/2008-17/08/2017 (31.2)

Fig. 2. a) Annual (x axis) and daily (y axis) cycle of the

wind magnitude for the Liberia meteorological station

record (Table 2, from hourly wind records). b) Annual

cycle of the zonal (u, blue line) and meridional (v, red line)

wind component.

The annual cycle of the SSbT records

listed in Table 1 is presented in Figure 3. Mean

temperature is 25.2 °C. Colder temperatures

were observed in February-March, below 21

°C with a secondary minimum in July. There

were two maxima in May-June and August-

October with temperatures above 27 °C (above

28 °C in September). March was the month

that presented highest temperature variability

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and the lowest was observed in September.

The annual cycle presented in Figure 3, agrees

with the annual cycle reported by Alfaro et al.

(2012) for the mean air surface temperature in

Bahía Culebra.

Figure 5 shows the ZCI and MCI calcu-

lated from wind records listed in Table 2. First

data is for January 4th, 2003 at 13:00 and the

last data is for December 28th, 2017 at 12:00.

ZCI ranged from -1.79 to 2.15 ms

-1

for May

10th, 2010 at 13:00 and March 18th, 2003 at

21:00, respectively (Fig. 5a). MCI ranged from

-1.97 to 2.14 ms

-1

for November 25th, 2008

at 02:00 and September 27th, 2010 at 05:00,

respectively (Fig. 5b). ZCI (MCI) time series

in Figure 5 presented a positive (negative) long

term trend of 1.06 x 10

-7

(-1.36 x 10

-6

) ms

-1

hour

-1

with a statistical significance of 99 %

(p-value < 0.01). There are positive monthly

correlations between ZCI and MCI with SSbTI

of 0.31 and 0.33, respectively, with a statistical

significance of 99 % (p-value < 0.01). Monthly

correlation between ZCI and MCI was 0.15

(p-value < 0.05).

Figure 4 shows the SSbTI calculated from

records listed in Table 1 (see previous section).

First data is for June 23rd, 2003 at 03:00 and the

last data is for December 12th, 2017 at 23:00.

SSbTI ranged from -3.61 to 2.13 standard devi-

ations for May 9th, 2004 at 03:00 and February

25th, 2004 at 23:00, respectively. Time series in

Figure 4 presented a small negative long-term

trend with a statistical significance of 99 %

(-1.9 x 10

-7

hour

-1

, p-value < 0.01), positive and

negative values represent warmer and cooler

conditions than the historical monthly average.

Correlation between monthly SSbTI and Niño

3 index (https://psl.noaa.gov/data/climateindi-

ces/list/) was 0.36 (p-value < 0.01).

Fig. 3. Dotted line is the monthly average of mean Sea

Subsurface Temperature records listed in Table 1 observed

in Bahía Salinas (2003-2017), Costa Rica. Upper and lower

dashed lines are ± one monthly standard deviation.

Fig. 4. Sea Subsurface Temperature Index calculated from

records listed in Table 1.

Fig. 5. Zonal-u (a) and Meridional-v (b) wind Component

Index. Calculated from records listed in Table 2.

Cold Events

Event 1: April - May 2004

The event started the 25/04/2004 at 18:00

and ended the 24/05/2004 at 19:00. The mini-

mum was founded the 09/05/2004 at 03:00 and

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reached a value of -3.614 standard deviations

(Fig. 6). Easterly wind condition was observed

during almost all the event, starting in April

26th at 23:00 and finishing by May 18th at

12:00. Meridional wind presented Southward

conditions from April 28th at 01:00 to May

16th at 04:00.

Quirós and Stolz (2010) reported that there

was a significant decrease in the air surface

temperature over Central America in December

2010. During this month, 6 cold front outbreaks

reached Central America. All of them increased

the atmospheric pressure in the region and,

therefore, the intensity of the winds over the

isthmus. Additionally, La Niña conditions were

still strong in the equatorial eastern Pacific. In

November, SSTs in the Pacific of Costa Rica

were around 27.0 °C, but decreased in Decem-

ber to 26.2 °C, that is, there was a cooling of

8 tenths of a degree. However, that cooling is

comparing with the SST climatology, the cool-

ing was 1.2 °C, greater than the one registered

during 1999 La Niña event. During November

2010-January 2011, continuous negative SSbTI

values were observed in Bahía Salinas (Fig. 3).

Event 3: September 2012

The event started the 08/09/2012 at 16:00

and ended the 21/09/2012 at 13:00. The mini-

mum was founded the 15/09/2012 at 01:00 and

reached a value of -2.157 standard deviations

(Fig. 8). Northeasterly wind condition was

observed during almost the whole event from

September 8

at 23:00 until the 19

at 01:00.

Morera (2012) observed that positive wind

anomalies were recorded in September 2012

over a large part of Central America reaching

6 ms

-1

(21.6 kmh

-1

). The wind direction was

almost zonal. Easterly winds were favored by

Fig. 6. Subsurface Sea Temperature Index (black line,

standard deviation) recorded in Bahía Salinas, from April

25th to May 24th, 2004. Blue and red lines are for Zonal

and Meridional Wind Component Indices (m/s).

According to Alvarado (2004), the month

of May 2004 marked the beginning of the rains

in the Pacific Slope and in the Central Val-

ley. Intertropical Confluence Zone atmospheric

instability increases in Costa Rica and its sur-

roundings, giving a large formation of cloudi-

ness and as a result the respective increase in

rainfall. Guanacaste recorded more than 300

mm this month. This event coincided with an

ENSO “neutral year”, this is a condition in

which La Niña or El Niño ENSO phase did not

predominate. March-May 2004 cooler condi-

tions in Bahía Culebra were identified by Alfa-

ro and Cortés (2012) and Alfaro et al. (2012),

associated with trade winds intensity ranged

from moderate to strong and several cold front

outbreaks intrusions into the Caribbean.

Event 2: December 2010

SSbTI started to decrease the 19/12/2010 at

24:00 and ended rising the 02/01/2011 at 17:00.

The minimum was founded the 29/12/2010 at

04:00 and reached a value of -2.212 standard

deviations (Fig. 7). Southwesterly wind condi-

tion prevailed during this event.

Fig. 7. Subsurface Sea Temperature Index (black line,

standard deviation) recorded in Bahía Salinas, from

December 19th, 2010 to January 2nd, 2011. Blue and

red lines are for Zonal and Meridional Wind Component

Indices (m/s).

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a relatively strong CLLJ. Additionally, meridi-

onal wind component showed negative values,

indicating the prevalence of northern wind.

Madden Julian Oscillation (MJO) contributed

also to the rain irregularity observed over the

country. Costa Rican North Pacific region

showed a negative precipitation anomaly that

reached 41.2 % less than monthly climatology

(e.g., -134.8 mm at Liberia station), in agree-

ment with stronger trade winds. In Guana-

caste, the rain presented an irregular temporal

distribution, since about 80 % of the total for

the month was accumulated in five days and

there were more than 15 dry days. The 2012

El Niño event reached a maximum in July, and

since August began to weaken, but warmer

SST anomalies, compared with TNA, were still

present in September, this condition favored

also stronger than climatology trade winds.

Event 4: July 2014

The SSbTI started the cooling the

04/07/2014 at 03:00 and ended the warming

the 28/07/2014 at 22:00. The minimum was

founded the 17/10/2017 at 17:00 and reached

a value of -1.903 standard deviations (Fig. 9).

Northeasterly wind condition was observed

during the whole event.

According to Chinchilla (2014), during

July 2014, the ENSO SST indices, Niño 3

and Niño 1+2, remained above 0.5 °C and the

Southern Oscillation Index (SOI) was nega-

tive, so the associated atmospheric and ocean

indicators of El Niño were coupled. A signifi-

cant cooling in the TNA was also present. The

consequence of this SST configuration was

a strong wind pattern throughout the month,

since important trade wind anomalies were

observed, particularly in the period from July

9th to 11th. Additionally, strong winds towards

the Pacific slope, inhibiting cloud formation,

preventing also the moisture entrance from the

Pacific associated with the southwest breeze

and a meteorological drought was observed in

Guanacaste. Central America presented scare

cloud coverage, which allowed drier condi-

tions. The month of July 2014 was extremely

dry in Guanacaste. The July rain accumulated

average was 155 mm, that is, it rained only

2 % of the normal, been the lowest value

since 1936.

Event 5: October 2017

The event started the 02/10/2017 at 12:00

and ended the 03/11/2017 at 05:00. The mini-

mum was founded the 17/10/2017 at 17:00 and

reached a value of -2.181 standard deviations

(Fig. 10). Northeasterlies were observed from

October 11th at 22:00 to 23rd at 10:00, but

westward zonal wind component continued

until the 27th at 12:00.

Naranjo (2017) reported that SST indices,

Niño 1+2 and Niño 3.4 showed a cooling dur-

ing September 2017 and both exceeded the

threshold of -0.5 °C, indicating La Niña like

conditions, that continued through October.

Fig. 8. Subsurface Sea Temperature Index (black line,

standard deviation) recorded in Bahía Salinas, from

September 8th to 21st, 2012. Blue and red lines are for

Zonal and Meridional Wind Component Indices (m/s).

Fig. 9. Subsurface Sea Temperature Index (black line,

standard deviation) recorded in Bahía Salinas, from July

4th to 28th, 2014. Blue and red lines are for Zonal and

Meridional Wind Component Indices (m/s).

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The MJO went through its convergent phase

during the second and third week of October,

causing a subsidence pattern in the country and

indirectly, increasing the intensity of the winds.

This month is the rainiest month of the year in

the regions of the Pacific slope; however, the

distribution of rain was irregular.

Warm events

Event 1: February 2004

The SSbTI started to increase the

19/02/2004 at 23:00 and ended the descend

the 03/03/2004 at 13:00. The maximum was

founded the 25/02/2004 at 23:00 and reached

a value of 2.131 standard deviations (Fig. 11).

Southwesterly conditions were observed from

February 20th at 06:00 to 29th at 03:00, but

Northward meridional wind conditions were

recorded during the whole event.

According to Alfaro (2004), February

2004 was a month characterized by trade winds

of weak to moderate strength. However, winds

in the upper troposphere, presented a nega-

tive anomaly over the southern part of Central

America, which means that westerly winds

were weaker than climatology. Additionally,

surface southeast winds were observed over

the region.

Event 2: February 2007

The event started the 09/02/2007 at 17:00

and ended the 22/02/2007 at 06:00. The maxi-

mum was founded the 15/02/2007 at 06:00

and reached a value of 1.766 standard devia-

tions (Fig. 12). Southwesterly conditions were

observed since February 14th to the end of the

event, but Northward meridional wind condi-

tions were recorded during the whole event.

Fig. 10. Subsurface Sea Temperature Index (black line,

standard deviation) recorded in Bahía Salinas, from

October 2nd to November 3rd, 2017. Blue and red lines are

for Zonal and Meridional Wind Component Indices (m/s).

Fig. 11. Subsurface Sea Temperature Index (black line,

standard deviation) recorded in Bahía Salinas, from

February 19th to March 3rd, 2004. Blue and red lines are

for Zonal and Meridional Wind Component Indices (m/s).

Fig. 12. Subsurface Sea Temperature Index (black line,

standard deviation) recorded in Bahía Salinas, from

February 9th to 22nd, 2007. Blue and red lines are for

Zonal and Meridional Wind Component Indices (m/s).

According to Sánchez and Stolz (2007),

February 2007 was an anomalous month, since,

as a result of the El Niño conditions, above

normal air surface temperatures patterns were

observed in most of the country, ranged from

0.4 ºC to 0.8 ºC above average. Additionally,

the observed atmospheric circulation was the

main factor that blocked the entry of cold fronts

to the country in this period. Warmer condi-

tions in Bahía Culebra were observed also by

Alfaro et al. (2012) during this month.

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Event 3: March 2008

The event started the 26/02/2008 at 16:00

and ended the 02/04/2008 at 03:00. There are

two temperature peaks with a difference of one

week between them. The first maximum was

founded the 07/03/2008 at 14:00 and reached

a value of 1.724 standard deviations. The sec-

ond maximum was founded the 16/03/2008 at

12:00 and reached a value of 1.618 standard

deviations (Fig. 13). Southwesterly wind con-

ditions dominated during this two-peak event.

America remained above average. Warmer

conditions were perceived in Guanacaste in

both, minimum and maximum temperatures.

Air surface temperatures rose between 1 °C

and 2 °C, particularly in regions of the Pacific

slope. A wide warming widespread over the

region was associated with an El Niño strong

event condition in conjunction with a warming

in the TNA region. Warm waters in the Pacific

nearby coast of Costa Rica, reached anomalies

of 2.0 °C.

Event 5: December 2015

The SSbTI showed that positive tem-

perature anomalies were observed for a peri-

od of approximately four months, i.e. from

09/09/2015 at 20:00 to 25/01/2016 at 12:00

(Fig. 15a), however the main event started the

11/12/2015 at 02:00 and ended the 22/12/2015

at 10:00 (Fig. 15b). The maximum was found-

ed the 17/12/2015 at 12:00 and reached a value

of 1.875 standard deviations. Southwesterly

wind conditions were observed from December

11th at 02:00 to 18th at 10:00.

According to Poleo (2015), during Decem-

ber 2015, El Niño very strong condition (most

intense event in the past 30 years, with anoma-

lies 3 °C above the climate average) caused a

warm pattern throughout the country, register-

ing positive air surface temperature anomalies

between 0.2 and 1.5 ºC in both, minimum

and maximum temperatures. Extreme air sur-

face temperatures presented positive anomalies

Fig. 13. Subsurface Sea Temperature Index (black line,

standard deviation) recorded in Bahía Salinas, from

February 26th to April 2nd, 2008. Blue and red lines are

for Zonal and Meridional Wind Component Indices (m/s).

Fig. 14. Subsurface Sea Temperature Index (black line,

standard deviation) recorded in Bahía Salinas, from March

7th to 25th, 2010. Blue and red lines are for Zonal and

Meridional Wind Component Indices (m/s).

Sánchez (2008) reported that during March

2008, La Niña conditions continued to weaken,

especially in the ETP, where sea surface tem-

perature anomalies in March reached positive

values, approximately 1 ºC above normal. All

the oceanic indices showed a significant warm-

ing in that region. This is in agreement with

Alfaro and Lizano (2001) who found a lead

time of 0-2 months between Niño 3.4 index

and the SSTs in the North Pacific region of

Costa Rica.

Event 4: March 2010

Starting the 07/03/2010 at 24:00 and end-

ing the 25/03/2010 at 06:00. The maximum was

founded the 13/03/2010 at 01:00 and reached a

value of 1.831 standard deviations (Fig. 14).

Southwesterly wind conditions were observed

from March 7th at 16:00 to 15th at 10:00.

According to Chinchilla (2010), dur-

ing March 2010, air temperature in Central

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from 0.5 to 2 °C above the average throughout

the country. In addition, the SST in the TNA

presented positive anomalies, favoring a sce-

nario of subsidence in the region in favor also

of the observed warming. The little cloudiness

values over the country favored a higher emis-

sion related with long-wave radiation, so the

outgoing long wave radiation anomaly indicat-

ed positive values over Costa Rica, a reflection

of the absence of cloudiness for most of the

month. The Pacific slope had drier conditions

when compared with climatology, with rainfall

deficits at the coastal region and an associated

SST anomaly of 2.5 °C in the Pacific Ocean

near Costa Rica.

DISCUSSION

Bahía Salinas has a marked east-west axis

(Fig. 1). Sea temperature in this bay, has a

defined seasonal cycle (Fig. 3), in which cooler

temperatures are normally observed from the

end of the boreal winter to the beginning of

the spring (December-April), and the warmer

months are normally expected from late spring

to autumn (May-November).

The main seasonal climate driver in Cen-

tral America is the North Atlantic Subtropical

High (NASH), since its latitudinal migration is

associated with the strength of the trade winds

over Central America (Alfaro et al. 2018; Tay-

lor & Alfaro, 2005). Trade winds are observed

trough the isthmus with a marked easterly com-

ponent all around the year (Fig. 2b). Stronger

wind magnitudes are observed from December

to April, decreasing in magnitude from May

to November (Fig. 2), in concordance with the

Southernmost and Northernmost positions of

the NASH, respectively (Alfaro et al., 2018;

Amador et al., 2016; Durán-Quesada et al.,

2020; Maldonado et al., 2018). Wind and sea

temperature seasonal cycle are related because

strong easterly winds in Bahía Salinas, drive

the surface water off the bay, and the displaced

water is then replaced by cooler water from the

depths, presenting a coastal upwelling during

the winter (Cortés et al., 2014). The seasonal

upwelling in Bahía Salinas is enhanced because

the zonal wind with easterly component, is

channeled through a topographic depression

located between the lowlands of southern Nica-

ragua and northern Costa Rica. The winds pro-

duced by this channeling are commonly called

“Papagayos”, and have Jet Stream strength

(Alfaro & Cortés, 2012; Amador et al., 2016).

In four of the five cold events studied in

this work, Northeasterly wind anomalies were

observed in the Costa Rican North Pacific,

associated with trade wind reinforcements (Fig.

6, Fig. 8, Fig. 9, Fig. 10); meanwhile west-

erly anomalies were observed in all the warm

events identified, associated with weaker trade

wind conditions (Fig. 11, Fig. 12, Fig. 13, Fig.

14, Fig. 15). Notice also that positive monthly

correlation between SSbTI and both wind indi-

ces, ZCI and MCI, suggested also that NE and

SW wind anomalies tend to be related with cool

and warm conditions in Bahia Culebra.

In addition, seasonal upwelling in Bahía

Salinas is modulated also by two synoptic scale

Fig. 15. a) Subsurface Sea Temperature Index (black

line, standard deviation) recorded in Bahía Salinas, from

September 9th, 2015 to January 25th, 2016. b) Subsurface

Sea Temperature Index (black line, standard deviation)

recorded in Bahía Salinas, from December 11th to 17th,

2015. Blue and red lines are for Zonal and Meridional

Wind Component Indices (m/s).

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climate features. First, the boreal winter arrival

of cold front outbreaks that enhance the zonal

easterly wind component (Chinchilla et al.,

2017; Maldonado et al., 2018; Zárate-Hernán-

dez, 2013), condition observed for example

during December 2010 (Fig. 7). Second, the

winter maximum of the easterly CLLJ (Ama-

dor, 2008), observed normally in February. The

CLLJ has a maximum also in July, responsible

of the secondary seasonal sea temperature

minimum observed in Bahía Salinas.

ENSO is also an important modulator of

the sea temperature variability in Bahía Sali-

nas, since warm (El Niño) and cool (La Niña)

ENSO events are related with positive and

negative SST anomalies in the ETP, respec-

tively. For example, warm events in February

2007, March 2010 and December 2015, were

observed under El Niño conditions and the cold

events of December 2010 and October 2017

under La Niña conditions. However, if warm

SST anomalies are observed in the ETP in

concordance with relatively cool anomalies in

the TNA-Caribbean basin, a pressure gradient

could be stablished trough the isthmus, enhanc-

ing the easterlies over Central America and

decreasing the sea temperatures in Bahía Sali-

nas. This was observed for example during the

cold events of September 2012 and July 2014.

Positive monthly correlation between SSbTI

and Niño 3 index suggested that cool and warm

ENSO events tend to be related with also cool

and warm conditions in Bahia Culebra.

Advances in understanding the sea temper-

ature variability in locations like Bahía Salinas

are important, due to the historical presence of

fishermen settlements and the observed posi-

tive and negative sea temperature anomalies

related with commercial fish captures in the

Costa Rican North Pacific (Moreno-Díaz &

Alfaro, 2018; Moreno, Moya & Alfaro, 2017).

Moreno et al. (2017) showed that when SST

increases, the amount of fishing decreases in

the region and Moreno and Alfaro (2018) found

that fishermen’s income was 53 % higher in

cold events than the one obtained in warm epi-

sodes of ENSO. Fishing activity has been one

of the most important socioeconomic sector in

Bahía Salinas through its history (Díaz, Mora,

& Madriz, 2019) and the bay belongs to the

Central American Dry Corridor (CADC), a

sub-region that is a mainly rural area charac-

terized by a marked precipitation seasonality,

climate change vulnerability, rich biodiversity,

entrenched poverty, food insecurity and outmi-

gration (Gotlieb et al., 2019; Quesada-Hernán-

dez et al., 2019).

Ethical statement: authors declare that

they all agree with this publication and made

significant contributions; that there is no con-

flict of interest of any kind; and that we fol-

lowed all pertinent ethical and legal procedures

and requirements. All financial sources are

fully and clearly stated in the acknowledge-

ments section. A signed document has been

filed in the journal archives.

ACKNOWLEDGMENTS

This work was partially supported by

the following University of Costa Rica proj-

ects: A5-037, B9-454 (VI-Grupos), B8-604 and

B9-609 (Fondo de Estímulo), EC-497 (FEES-

CONARE), C0-074, B0-810 and C0-610

(Fondo de Estímulo). To the UCR Schools of

Physics and Biology for giving us the research

time to develop this study. To the UCR research

centers CIMAR and CIGEFI for their logistic

support during the data compilation and analy-

sis. To Sebastián Ruiz for his assistance with

data and information processing and to Paula

M. Pérez-Briceño for his help with Fig. 1.

RESUMEN

Forzamiento de los eventos cálidos y fríos del agua

subsuperficial del mar en Bahía Salinas, Costa Rica

Introducción: Bahía Salinas, en el Pacífico norte de Costa

Rica, es un área de afloramiento estacional. La temperatura

del mar en Bahía Salinas puede ser modulada por sistemas

sinópticos de gran escala. Esta región pertenece al Corredor

Seco Centroamericano (CSC), una subregión del istmo

relativamente más seca que el resto del territorio, que se

extiende a lo largo del litoral Pacífico desde el oeste de

Guatemala hasta el norte de Costa Rica.

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Objetivo: Estudiar los eventos cálidos y fríos que se

podrían inferir al estudiar la temperatura subsuperficial del

mar en Bahía Salinas, y también analizar las condiciones y

sistemas sinópticos a gran escala de las fuentes históricas

en las que ocurrieron para identificar los mecanismos

atmosféricos que favorecieron su desarrollo.

Métodos: Se calculó un índice de temperatura subsuper-

ficial del mar utilizando datos horarios de siete estaciones

ubicadas en tres puntos diferentes en Bahía Salinas. Los

registros van desde el 19 de junio de 2003 al 5 de diciembre

de 2017. Además, se utilizaron seis estaciones meteoroló-

gicas, con registros de viento horario, para crear un índice

de viento zonal y otro meridional. Se utilizó el índice de

temperatura subsuperficial del mar para identificar los

eventos más cálidos y más fríos en la bahía. Se utilizaron

los índices de viento y boletines meteorológicos mensuales

para analizar las condiciones y sistemas sinópticos a gran

escala en los que se dieron los eventos fríos y cálidos en

Bahía Salinas.

Resultados: La temperatura media del mar en Bahía Sali-

nas es de 25.2 °C. Se observaron temperaturas más frías

en febrero-marzo, por debajo de los 21 °C. El ciclo anual

presentó dos máximos en mayo-junio y agosto-octubre con

temperaturas superiores a 27 °C. En cuatro de los cinco

eventos fríos estudiados, se observaron anomalías de los

vientos del noreste en el Pacífico Norte costarricense,

asociadas a refuerzos de los vientos alisios; mientras tanto,

se observaron anomalías del oeste en todos los eventos

cálidos, asociadas con condiciones de vientos alisios más

débiles.

Conclusiones: El principal forzante climático en Bahía

Salinas es la Alta Subtropical del Atlántico Norte ya que

su migración latitudinal está asociada con la fuerza de los

vientos alisios sobre América Central. La surgencia esta-

cional está modulada también por dos características cli-

máticas de escala sinóptica en el invierno boreal, la llegada

de frentes fríos y el máximo de la Corriente en Chorro de

Bajo Nivel del Caribe Oriental. El Niño-Oscilación del Sur

también es un modulador importante de la variabilidad de

la temperatura del mar, ya que los eventos cálidos y fríos

están relacionados con anomalías positivas y negativas de

la temperatura del mar.

Palabras clave: temperatura sub-superficial del mar; aflo-

ramiento-surgencia; frentes-empujes fríos; ENOS; América

Central.

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